Bruno COLLARD

2014

ANE06 - Surveillance

Surveillance overview
SURVEILLANCE
OVERVIEW

1. Surveillance concepts
2. Separation minimum
3. Surveillance architecture
4. sensors
5. Strategy 1
 Surveillance concepts

2
 Surveillance concepts

ATC function
 Provision of safe and expeditious sequencing of traffic
 Provision of safe horizonal and vertical séparation distances
3 (safe meaning : NO COLLISION)
 Surveillance functions
• Surveillance with COM and NAV is an essential element of
integrated ATM operations
• Provides the necessary information to ensure safe and efficient
execution of air traffic control
• Real time elaboration of air situation :
– Position and identification of moving targets
– Detection and alerts for conflict situation
• Appropriate data processing, distribution and displays to the users
– Ground: Working controller position
– Airborne ( pilot awareness)
 Surveillance concepts
CNS : Communication / Navigation / Surveillance

Operational needs:
- CONTROL aircraft trajectories by giving pilots instructions
 COMMUNICATION

- provide pilots means to follow the requested trajectories on their

own
 NAVIGATION

- know where aircraft are at a given instant

- know where they are going to
 SURVEILLANCE
5
 Surveillance concepts - definitions
Definition
Surveillance : Ground based technique for the timely detection of targets
and determination of their position (and, if required, the acquisition of
supplementary information relating to targets) and the timely delivery of this
information to users in support of the safe control and separation of targets
within a defined area of interest.

Target: Any object that may collide with an aircraft

aircraft (including balloons, gliders..)

Ground vehicle (ground surveillance)

obstacle

6
 Surveillance concepts - definitions

4D position
 Geographic coordinates
 Geographic coordinates : Lat, Long, height
(WGS84 coordinates, earth model GRS80)
 Cartesian coordinates : X,Y,Z or FL : projection of
positions on a plan tangent to a given point of the
earth
 polar/spherical coordinates centred on the sensor
r (slant range), Q (azimuth from North), Z or FL :
raw coordinates (radar sensor)

+TIME : time when the mobile was located at the position

7 estimated by the surveillance system.
 Surveillance concepts - definitions

Other information
 identification
 Mode A
 Mode S address
 Call Sign
 Flight ident
 Registration number
 Flight parameter
 Heading
 Horizontal speed (ground, air)
 Vertical speed (climbing rate..)
8
 Traffic situation picture Traffic situation picture
required by ATC provided by Surveillance
controllers (perfect world) (real world)

Display of all useful target under Possible Miss of detection

responsibility

No false target Possible false alarm

Display of exact position of Accuracy  0

target
Probability of Large deviation

Full identification of all target Depend on operating sensors

and target

Availability of all aircraft Depend on sensors technology

parameters and target equipment

9
Air situation picture - History
Air situation picture - History
PPI : Plan Position Indicator

12
13
14
 Dependent Surveillance

Dependent independent
Target informing the ground system or other Position calculated by the receiving system
target of its own position calculated on and not dependant on position data
board. transmitted by the aircraft.

Cooperative Surveillance

Cooperative Non Cooperative

Mobile equipped with a transponder which Independent surveillance and no
responds to interrogations transmitted from deliberate interaction in the aircraft with any
a ground system (basic data link). active component such as transponder

15
Surveillance concepts-classification 1
 ATM Ground Based Surveillance categories
Non-Cooperative Independent Surveillance:
Calculates the (2D) position without reliance on aircraft avionics
– Primary Surveillance Radar (PSR)

Cooperative Independent Surveillance:

Provides the calculated aircraft 3D position and processes other aircraft avionic data
(Mode A/C/S, DAPs or ADD)
– (Monopulse) Secondary Surveillance Radar (SSR), SSR Mode S,
– Airport Multilateration (MLAT) and Wide Area Multilateration (WAM)

Cooperative Dependent Surveillance:

Provides the aircraft derived position (GPS or INS) and other aircraft avionic
data (including ADD) to broadcast “air-ground” and “air-air”
– ADS-B, (ADS-C)
 Surveillance concepts – classification 2
Ground and Air specific surveillance systems:
- Ground surveillance for flying aircrafts Air Surveillance

- Ground surveillance for taxiing aircrafts  Surface Movement Guidance

and Control System (SMGCS)

- Airborne surveillance for flying aircrafts  Airborne Separation

Assistance System + Traffic Information Service- Broadcast (ASAS +
TIS-B)

- Airborne surveillance for taxiing aircrafts  SMGCS, TIS-B

17
 Surveillance concepts – The 3 pillars of Air
Traffic Control
1- Detection

* without surveillance sensor  radiocom

* with surveillance sensor : Probability of detection

2 - Identification

* without surveillance sensor  radiocom

* with surveillance sensor : dependant surveillance

3- Separation

* without surveillance sensor  ATC procedures

- High separation minima > 10 NM

18
* with surveillance sensor  reduced separation minima
 Separation Minimum

19
 Separation minimum
Rules and procedures influenced by:
 Type of Aispace
 Aircraft characteristics
 Wake turbulence
 Airborne equipment
 Precision of flight control
 Navigation ground infrastructure
 Communication systems
 Surveillance systems
 weather
20
 Separation minimum
Rules and procedures influenced by:
 Category of Airspace
 controlled/non controlled

 A to E categories of airspaces ( ICAO classification)

 ATC service regarding separation or traffic information

 Aircraft characteristics
 Wake turbulence
 Airborne equipment
 Precision of flight control
 Navigation ground infrastructure
 Communication systems

21  Surveillance systems
 weather
 Separation minimum

Development of separation minima is complex

but
Rule of application very simple for control:

Do not give authorisations that lead to manoeuvres which would

reduce the separation between two aircraft to a distance lower
than the minimum applicable distance in the considered
conditions.

22
 Vertical separation minimum

2000 ft

FL410

2000 ft
1000 ft

FL290

1000 ft

23 usual airspace RVSM airspace

 Horizontal separation minimum
Fast and safe
Radar information telecommunication

Possible reduction of ICAO separation

minima

3 points must be distinguished :

Determination of the lowest possible
regulatory minima
Conditions for strategic application
 Conditions for tactical application

24
 Separation minimum: what is separation?

Physically, collision occurs only when 2 aircraft are located exactly at the same position.

Is it the same for radar calculated position?

Does collision occurs only when plot symbols are superimposed?

25
 The plot centre is not the true position of the aircraft.
It is the centre of an ellipse where the true position is likely to be.

Likely = 95% chance that the true position is

observed within the ellipse

Aircraft and plots separated for sure plots separated – ellipses in contact

Aircraft separated
True positions

Collision True positions

26
 Air Traffic Control  anticipate collision  predict trajectories and estimate position
within next seconds.

The further you anticipate, the poorer the accuracy of the predict position is.

position +
speed
errors

Calculated
speed
vector at tn
c
Speed accuracy

tn+2
tn-1 tn Turn
tn+m

position + speed +
mode of flight
errors

27
Separation minimum: anticipation
 ATC components
infrastructure
procedures

separation
minimum

PSR
technology
SUR 60/80
system Nm
related
Az: 0.15°
precision
D < 60
m
MSSR 250 Nm
ATC hum F
Az:
0.05°
training COM system D <40
related m
airspace /procedures 450 A/C
on-board components
pilot hum factor

aircraft inertia

SEPARATION STANDARDS DISTANCE LAYERS MODEL

 Separation minima: Conditions for strategic application
It is accepted that the various margins or "distance layers" (other than the
precision of the radar system) that are taken into consideration in the
calculation of the minima fixed by ICAO, are applicable.

The study needed for directions concerning separation minima before a radar
system is put into service is reduced to an analysis of all factors that may affect
the precision of data:

 Precision
 Assess average precision (compliance with Eurocontrol standard)

 Assess large precision deviations

measure the obsolescence of the radar information presented to the

controller

 analyse degrades operating modes that may affect precision

29
define a method for monitoring the system in operation
 Radar separation minima evolution

June 77 10 NM for multiradar tracks

5 NM "edge/edge" for analogical monoradar plots

February 79 10 NM for secondary monoradar tracks

Mai 90 8 NM for multiradar

5 NM for monoradar

Since january 99 5 NM multiradar (Eurocontrol CIP objectives, ICAO basic

distance)

30
 Radar separation minima evolution
3 NM if radar possibilities permit

2,5 NM on the same final approach path if:

• Appropriate radar resolution + 12 scan/mn
• Runway occupation time < 50 s
• Controller able to observe the runway and taxiway (visually or with SMR)
• Compliance with wake turbulence separations requirements ( 2,5 NM = 1 mn of
flight at 150 kts)

2,5
NM
 Radar separation minima evolution
2 NM on parallel approach with interdependent runway (< 1525 m)
1 NM on parallel approach with independent runway (> 1525 m)
• ILS or MLS approach
• Appropriate radar resolution + 12 scan/mn

2,5
NM

1 or 2 NM

32
 Oceanic Separation minima

North Atlantic South Pacific

On the same track 10 ' 15 '  100 NM

Between tracks 60 NM 80 NM

North Atlantic South Pacific

On the same track 30 NM 50 NM or 30 NM (ADS)

33
Between tracks 30 NM 50 NM
 Separation minima: practical application by control
The controller must always be ready to take measures in order to avoid aircraft
becoming separated by a distance that is less than the prescribed minimum. However,
there are "real time" operational situations when the controller may consider that the
level of risk is higher than the acceptable one that was used to establish the minima,
and she or he may decide temporarily to apply higher distances.
 Relative configuration of traffic and aircraft performance
 Workload context
 Loss of automatic identification
 congestion of radio frequencies or communication channels
 weather conditions
 degradation of radar information

The practical application of a radar separation results from a compromise between

safety and capacity felt by a controller in a given context.
34 (Average spacing practised : 4 to 4,5 NM when 3NM separation applied.)
 Surveillance Architecture

35
 target
Interrogation Spontaneous Ground to air
reply reports target situation

Target acquisition Ground to air

(Sensors) situation processor
Sensor data Data request GA status
data
Surveillance data
FP
Sensor data

processor Flight plans

(Multiradar tracker)
Processed Data
Alerts target data request
systems Adjacent
Service manager surveillance
functions
Status of
Service Situation
surveillance
request picture
alerts function

Surveillance users (HMI,..)

36
 Surveillance architecture: Surveillance Data Processor
Input : target reports from different sensors

Output : Air traffic situation picture updated periodically (set of tracks based on the user
service definition)

By pass: direct output from sensors to HMI in case of SDP failure

Main component : Multisensor tracker

Merge position information from several sensors and deliver the best estimate of the target position at a given
instant
Air picture includes:
- Aircraft horizontal position and history Best estimate at T+1
- Aircraft vertical position
- Aircraft identification T
- Mode A special code (7700, 7600, etc..)
- Ground speed
- Status of track head (P, S, P+S, extrapolated) T T+1

37
 ATC Center Large Airports

MSSR MSSR PSR

Flight Plans Radar Data

Processing Processing Local Radar Data
Processing

Control working Position- CWP CWP

 target
Interrogation Spontaneous Ground to air
reply reports target situation

Target acquisition Ground to air

(Sensors) situation processor
Sensor data Data request GA status
data
Surveillance data
FP
Sensor data

processor Flight plans

(Multiradar tracker)
Processed Data
Alerts target data request
systems Adjacent
Service manager surveillance
functions
Status of
Service Situation
surveillance
request picture
alerts function

Surveillance users (HMI,..)

39
 Surveillance architecture: Surveillance Services
Surveillance service : delivery of specified information from the
surveillance function to the user

Service Definition Request (SDR): specifies the

characteristics or level of the service required by the user
 Set of data Items (position, identification, speed, heading…)
 Performance characteristics (resolution, accuracy..)
 Volume of airspace

Type of SDR
 Configuration SDR (static)  basic ATM service
 Dynamic SDR
 Request of any data already available
 Request of additional or more recent information 
parameter extraction (Enhanced Mode s, ADS C)
40
 Surveillance architecture: Services Manager
Service manager function : Interface between the users and
the Surveillance Function. It receives services request from users and
returns a traffic situation picture comprising a set of system tracks based
on the service request.
SDP

Track reception Service

Systems status manager
area

Traffic situation Track Flight

Service controller labelling plans
picture preparation

Service Track distribution

requests

USERS
41
 target
Interrogation Spontaneous Ground to air
reply reports target situation

Target acquisition Ground to air

(Sensors) situation processor
Sensor data Data request GA status
data
Surveillance data
FP
Sensor data

processor Flight plans

(Multiradar tracker)
Processed Data
Alerts target data request
systems Adjacent
Service manager surveillance
functions
Status of
Service Situation
surveillance
request picture
alerts function

Surveillance users (HMI,..)

42
 Operators - controllers
 Oceanic and En route ATM centres

 TMA/approach, Airport/tower Units

 Air defence centres

 Airline Operation centres (AOC)

ATM Systems
Surveillance
 Flight data processing systems (FP correlation)
architecture:
 Flow management systems and traffic prediction
users -
 Alerts systems
customers
Other
 Recording and replay facilities

 targets

 Aircraft Noise monitoring systems

 Airport flight information systems Airline Operation centres (AOC)

43
 Search and Rescue Units
 Surveillance architecture: Alert systems (1)

Aircraft/aircraft collision avoidance systems

 Ground based system : Short Term Conflict Alert (STCA) or safety
net

2 mn

c
2 mn
c

 Airborne systems : ACAS

Horizontal Vertical plan

plan
44
 Surveillance architecture: Alert systems (2)

Ground/aircraft collision avoidance systems

 Ground based system : Minimum Safe Altitude warning (MSAW)

 Airborne systems : (Ground Proximity warning System ) GPWS and E-

GPWS or Ground Collision Avoidance system (GCAS)

45
 Surveillance architecture: Alert systems (3)
Protection of restricted area
Protection of a clearance
Cleared FL
Protection of an air space
(e.g. military area)

Runway protection (SMGCS)

Runway protection area

46
 Surveillance concepts: exclusions
What is not in the scope of "pure"
surveillance
 Conflict alert systems (STCA, MSAW, TCAS, ACAS…)
 Displays (presentation of information to the controller or pilot)
 Flight plan information system (FDPS) although labelling
(identification) is surveillance
 Time source (time stamping of data)
 Weather information
 Airport flight information system
 Noise monitoring system
 recording
47
 Surveillance sensors

48
 Sensors - types
Radar
 Primary surveillance radar (PSR)

 Secondary Surveillance Radar (SSR)

 Classical SSR
 Monopulse SSR (MSSR)
 Mode S MSSR
 Combined PSR + SSR

 Airport Surveillance Detection Equipment (ASDE) or SMR (Surface Movement Radar)

Multilateration
 Passive or Active

 Airport or Wide area Multilateration (WAM)

Automatic Dependant Surveillance (ADS)

 Broadcast (-B)

 Contract (-C)
 Height

Polar co-ordinates
- centre : radar station
- azimuth angle /geographic North
- Elevation angle / local horizontal
+ Time

F
O Q

50 Sensors – radar position estimation

 Range measurement: TELEMETRY

The target is found from the time it takes for the wave front to travel to the target and back

R = c Dt/2
Dt = 2R/c

Instant of
t
Reply or Echo
"interrogation"
transmission

51 Sensors – radar position estimation

 Angle measurement: directive antenna

The angular location of the target is found with a directive antenna to sense
the angle of of arrival of the echo signal.
To be detected, a target must be within the beam. If I know which sector I
scan, I know the sector where the target is.

Azimuth
52
 Sensors – multilateration position estimation X
z Position measurement: triangulation using TDOA Y
Z
Resolution of an equation related to the
intersection of Hyperboloids
+ Time
 Cartesian coordinates of the intersection
point = estimation of the aircraft position
Squitter or reply
TOA2

TOA3 TDOA12 TOA1

TDOA23

TDOA13

x
53
 Position (geographic coordinates) calculated on board using GNSS and altimeter

Squitter
Latitude (WGS84)
Longitude (WGS84)
FL
+ Time

54 Sensors – ADS position estimation

 Radio Detection And Ranging

55
Sensors - Radar
 Initial need: aircraft detection

Radar history
Primary Surveillance Radar

Detection of
Useful echoes: aircraft False alarms: clutter

Rain Estimation
Friend Foe Fixed echo rain echo

Identification cancellation cancellation

PSR
Weather
Doppler Circular
channel
filtering polarisation
Secondary Surveillance Radar
IFF Weather
Rain falls
hazard
Sensors – Radar history
56 estimation
estimation
 Surveillance means : History

Raw video

57
 Sensors: Primary Surveillance Radar and ASDE

Based on the principle of écho

Advantage Detection of all objects reflecting Type of Surveillance

EM waves  detection of all
aircraft or mobile Non cooperative
Drawback Lot of false alarm due to 
reflexions on useless objects
Independent
No altitude information

58
 P1 Mode A P3 P1 Mode C P3

transponder 8 ms 21 ms
Interrogations SSR

Reply SSR (12 bits)

F1 F2

20,3 ms

Basic datalink
Advantage Far less false targets than PSR Type of Surveillance
Provide Altitude (mode C)
cooperative
Identification (Mode A)
Independent for ranging
Drawback No detection, no information in
Dependent for FL and
case of absence of XPDR,
identification
XPDR failure or turned off

cf. ICAO ANNEXE 10 Vol. IV

59 Sensors: Secondary Surveillance Radar

 P1 P2 P6 : data bits
Mode S transponder Mode S Interrogations

Mode S Reply

F1 P6 : data bits

Advantage Idem SSR

+ improvement of SSR
performance (garbling,
fruit) Type of Surveillance
Idem SSR
+ solution to shortage of
code A
+ elementary surveillance cf. ICAO ANNEXE 10 Vol. IV
+ Enhanced surveillance
+data link capacity Sensors: Mode S Secondary
Drawback Idem SSR Surveillance Radar
60
 2 types:
- active MLAT
- passive MLAT  listening of existing surveillance signal
z

Type of Surveillance

Generally cooperative If active

Independent for X,Y position
(sometime Z coordinate)
Dependent for FL and identification (if TOA2
available)
y

TOA3 TDOA12 TOA1

TDOA23

TDOA13

Sensors – multilateration
61
 Advantage Light installations,
no need of any interrogator

Drawback Same as SSR

Fleet equipment XPDR
level N"e" Type of Surveillance

cooperative
dependent

Sensors: ADS
62
 Sensors : data output definitions
PLOT: measured position, time stamped at the time of detection of the target. It contains at minimum
range and bearing raw information and, in addition, Mode 3/A identity code and Mode C decoded
Altimeter height value, @mode S etc.. derived from the target.
r
North q
time
Mode A
q Mode C
Mode S
r

TRACKED PLOTS: succession of raw positions (plots) associated to an aircraft. At each antenna
revolution, a specific algorithm (tracker) links the plots that belong to the same trajectory
Raw position
+
track number
+
Or chained plot or target report
Speed vector

Rn-1 Rn True trajectory

63 Rn-5 Rn-4 Rn-3 Rn-2
 Sensors : data output definitions

TRACK: Tracked plots which position has been smoothed (correction of random errors using
Kalman filter).

Smoothed position
+
track number
+
Speed vector

Rn-1 Rn True trajectory

Rn-5 Rn-4 Rn-3 Rn-2

DATA format: Eurocontrol ASTERIX Standard

- PSR, SSR : asterix cat 01/02
- Mode S : asterix cat 34/48
- ASDE : asterix cat 10
- ADS-B : asterix cat 21
- MLAT, WAM : asterix cat 20

64
Non-Cooperative Independent Surveillance:
Calculates the (2D) position without reliance on aircraft avionics
 Primary Surveillance Radar (PSR)

Cooperative Independent Surveillance:

Provides the calculated aircraft 3D position and processes other aircraft avionic data
(Mode A/C/S, DAPs or ADD)
 (Monopulse) Secondary Surveillance Radar (SSR), SSR Mode S,
 Airport Multilateration (MLAT) and Wide Area Multilateration (WAM)

Cooperative Dependent Surveillance:

Provides the aircraft derived position (GPS or INS) and other aircraft avionic
data (including ADD) to broadcast “air-ground” and “air-air”
 ADS-B, (ADS-C)
 Surveillance sensor synthesis
PSR SSR/Mode S WAM/MLAT ADS B

Detection All aircraft Aircraft equipped Aircraft equipped Aircraft equipped

with xpdr only with xpdr only with xpdr
(poor detection on embarking ADS B
A/C xpdr) capacity only

False Alarm Some Very few Very few Very very few

Accuracy Good Very good - better very good inside Very good
than PSR – grid, poor outside
degrading with range Depends on density
of beacon

Coverage From 60 NM to Up to 250 NM depends on number 250 NM

200 NM of beacons

Update rate 12 s to 4 s 12 s to 4 s 2s to 1s 0,5 s

Availability Partly redundant Partly redundant Redundant Partly redundant

Price From 3 to 6 M€ 1 M€ 50 k€ per beacon 0,1 k€

 Surveillance Strategy

67
 Present ECAC strategy

Eurocontrol agency role : Harmonisation and integration of

the surveillance infrastructure in order to achieve the separation
minima

 EATCHIP project (1990  2000) + ECIP

 Eurocontrol standards (surveillance
standards, Asterix standard, common
specifications, Implementing Rules -IR)
 SESAR : WG 15.4.1
68
 Radar Coverage requirements :

En route Air Space Major Terminal Area

Permanent Duplicated SSR Duplicated SSR

(PSR?) + PSR

Duplicated : Duplicated coverage means that for a given point in space, the
radar data used by an ATS unit for the surveillance function are derived from at
least two independent Surveillance Radar sources working simultaneously.

Permanent : Permanent coverage means that the duplicated coverage is

available 24h hours a day. To avoid lack of double duplicated coverage during
radar maintenance period or in case of radar outage, a triple coverage is
needed. Permanent means : +1

69
Eurocontrol Coverage requirements
 Max IFR FL

30 NM SSR duplicated coverage 30 NM

6000 ft
Min cruising FL
SSR mono
coverage 3000 ft

Great Major TMA

TMA TMA 1 SSR
1 SSR + 1500 ft
2 SSR+
1 PSR 1 PSR

70 French Coverage requirements

 PRIMARY
SURVEILLANCE
RADAR

(PSR)
 Contents

PSR Principle
Radar equation
Radar Cross Section (RCS)
Radar architecture, components and functions
Radar types
 Contents

PSR Principle
Radar equation
Radar Cross Section (RCS)
Radar architecture, components and functions
Radar types
 Definition

A radar is a system that lights a given portion of space with

electromagnetic waves and receives the echoes returned by reflecting
objects located within this volume, which allows to detect them and to
estimate some of their characteristics such as their position.
 Initial need: aircraft detection

Radar history
Primary Surveillance Radar

Detection of
Useful echoes: aircraft False alarms: clutter

Rain Estimation
Friend Foe Fixed echo rain echo

Identification cancellation cancellation

PSR
Weather
Doppler Circular
channel
filtering polarisation
Secondary Surveillance Radar
IFF Weather
Rain falls
hazard
5 estimation
Radar history estimation
 working
principle

Basics pulsed radar

parameter

Time of transmission (1) Time of transmission (2)

Tr : pulse repetition period (ms) Tr =1/PRF
P : Pulse peak
power (kW) Listening period (reception) Listening period
6
T : pulse duration (ms)
 Detection principles

Useful signal
K : Detection threshold
Useful signal
noise

Detection False alarme Missdetection

Detection rules:
1- Received signal > K Detection
2- Received signal < K No detection
 Ranging principle
Range measurement: TELEMETRY
The target is found from the time it takes for the wave front to travel to the target and back

R = c Dt/2
Dt = 2R/c

t
Instant of Reply or Echo
"interrogation"
transmission
Dt = 1 µs  R = 150 m
Remarkable figures Dt = 1 ms  R = 150 km
8 Dt = 12,34 µs  R = 1 NM
 Height

Referential and co-ordinates

Polar co-ordinates
- centre : radar station
F
- azimuth angle /geographic North O Q

- Elevation angle / local horizontal

 Angles measurement
The angular location of the target is found with a directive antenna to sense the
angle of arrival of the echo signal.
To be detected, a target must be within the beam. If I know which sector I scan, I
know the sector where the target is.

Azimuth Elevation
 Echoes
Target lighted during a period equal to:
Rev speed*beam aperture.
During this period several pulses are
transmitted
DQ = Rev speed*repetition period
Because of antenna revolution
each new echo coming from the
same target shifts to the right. Q3dB = Beam
aperture

Raw representation on PPI display:

Target = "banana"
Q3dB wide
pulse duration deep.
 PPI : Plan Position Indicator

12
PPI
14 Primary Secondary
 Pulse Repetition Frequency (PRF)
Time of transmission (1) Time of transmission (2)
Tr : pulse repetition period (ms)

Listening period (reception) Listening period

PRF = 1/Tr (unit = Hz)

Must give time to echoes coming from the expected range (Rmax) of the radar
to travel back to the radar :
2 Rmax
TR 
c
c
PRF 
2Rmax
15
 Transmition / receiption phases
Transmitted pulses
Receive Transmitter

Pulse repetition time

time TR
The pulse transmission rate can also be
expressed as the Pulse Repetition Frequency
PRF=1/TR
Received echoes / replies
r

Round trip time

Dt
 Hits per scan
1 aircraft = N echoes in the main beamwidth
 3dB
Dwelltime 
6  VRot
units
 3dB  3dB  PRF
number of hits per scan   • beam in degrees
6  VRot  TR 6  VRot • Vrevolution in revolution/minute
• TR in seconds

Antenna position
upon the first hit Possible replies

Antenna position
upon the last hit
Beamwidth
 Resolution
Ability to separate two close targets

Q3dB Q3dB

Targets Separated in azimuth Targets not Separated in azimuth

Q3dB Q3dB

Targets Separated in range Targets not Separated in range

 Resolution
Range resolution: Range resolution proportional to
pulse duration:
DRmin ~ cT/2

Dt = 2DR/c

Dtmin = 1/Df ~ T

T T
Angle resolution equal to
Angle resolution: beam width: DQmin = Q3dB

D
R
Cross range resolution:
Q
DDmin = R Q3dB
 Resolution /sample volume
Volume defined by the different resolution length.
- Targets belonging to this volume have no chance to be separated.
- All scatterers that belong to it participate to the signal at a given instant.

Civil PSR
Resolution volume
No vertical
discrimination

No altitude
estimation
 Contents

PSR Principle
Radar equation
Radar Cross Section (RCS)
Radar architecture, components and functions
Radar types
 Useful echoes received power vs threshold

Useful signal
K : Detection threshold
Useful signal
noise

Detection False alarme Missdetection

Detection rules:
1- Received signal > K Detection
2- Received signal < K No detection
 The radar equation

Many factors affect the power of the received echo

– Transmitted power (Pr proportional to Pt)

– Target range (Pr ↘ rapidly as the target range ↗)
– Antenna gain (measure of how much the antenna
concentrates the transmitted signal into a narrow beam)
– Target RCS  special discussion on this later
– Antenna effective area

23
 The radar equation

Derivation of the radar equation

PT G   2 2
PR  L
4  R
3 4 S

Note : For practical reasons, dB are often used in calculations

24
 One word about « losses » (1/2)

Microwave plumbing losses

– Transmission line loss
– Duplexer loss

Antenna losses
– Beam-shape / scanning losses
– Radome loss

Signal processing losses (filtering,…)

25
One word about « losses » (2/2)

Ls (dB)
 Range attenuation – one way
Incident wave attenuation
P/1010

P/108

P/106 Pi  2
R
1 km 10 km 100 km

1000 t 1 kg 1g 0,01g
 The radar equation vs Noise figure
Pr is proportional to R-4
Down to which power can the receiver correctly process (not just detect) the
signal ?
 This will be determined by the SNR (Signal to Noise
Ratio)

Signal

28
 Noise and Attenuation
Received Power: Pr

- high power transmitted: MW to kW

- very low power received: pW

Pr = / Dt4

S/N min for detection

Noise level

maximum
range Delay: Dt=2R/c
 The radar equation vs Noise figure

The radar equation can then be written as :

PT G  LS
2 2
PT G  LS 2 2
4
R  
max
4  Smin 4   SNRmin  kT0 FDB
3 3

 Determine maximum range

30
 Equipments related to radar equation parameters

transmitter antenna
target

PT G  LS
2 2
R 4

max
4   SNRmin  kT0 FDB
3

Detection performances
receiver

31
 Range proportional to pulse energie
The radar equation can then be written as :

PT G 2 2LS 1
R 4
 with DB 
max
4   SNRmin  kT0 FDB
3 T
Pulse duration

Pulse Energy
PT TG  LS 2 2
R 4

max
4   SNRmin  kT0 F
3

32
 detection performance  SNR

Detectable SNR as a function

- target fluctuation
- number of echos integrated
- Pd
-Pfa

Non fluctuating target

 The radar equation
The radar equation takes into account many parameters…

34
Map of dependencies in radar

35
 Coverage and horizon
maximum range

maximum
height radar horizon

Aircraft flying under the radar horizon R (NM) h(m)

can ’t be detected.
10 27
20 108
Depending on the distance to the radar, 50 676
there is a FL limit under which nothing 80 1730
100 2704
is detected. 150 6084
200 10815
 Definition of coverage
The radar coverage is the volume of space
where all specified targets (radar cross
section) may be detected and located with
the required performances:
- Detection probability on a given RCS
- False alarm rate
- Accuracy
- Resolution

The limit of the coverage volume is the

maximum range calculated using the
radar equation at every direction
(azimuth and elevation)
 Pt 500 kW

G 33 dB

Exercise : TA10 MTD f0 3 GHz

SER 1 m²

Rmax 100 km

tau 1 µs

(4)3 33 dB

k.To -204 dBJ

1) Compute the SNR for an echo from

a target located at Rmax

2) Assuming Pfa = 10-6 , what is the

expected Pd ?

3) Conclude !
38
 Contents

PSR Principle
Radar equation
Radar Cross Section (RCS)
Radar architecture, components and functions
Radar types
 radar range / RCS

target

PT G  LS
2 2
4
R 
max
4   SNRmin  kT0 FDB
3

40
 What is RCS?

It is the measure of the size of the target « as seen by the radar »

It is determined by
– how much of the incident signal is intercepted by the
target
– how well the target reflects radar waves
– how much of the reflected signal is directed back to the
radar antenna

While the RCS is expressed as an area (m²), the RCS does NOT correspond
to the geometric cross-sectional area

41
 Factors influencing RCS
– Size
– Geometry and shape

– Orientations of surfaces relative to incident field

– Composition, material
– Frequency, wavelength
– polarisation

42
 Cylinder RCS versus orientation and polarisation

H Polarization
V Polarization

10 20 30 40 50 60 Angle d’aspect
 RCS of a sphere versus its size with respect to 
Radar Cross Section / Physical Cross Section

Rayleigh region Mie or resonance region Optical region

σsphere / πr2

2πr/λ<1 1 < 2 π r / λ < 10 2 π r / λ > 10

44 Circumference / Wavelength = 2 π r / λ
r = sphere radius
 The RCS of some simple-shaped objects

Formula for calculating RCS

Sphere max = .R²

Cylinder max = 2Rh²/

Flat metal plate max = 4(ab)²/²

45
RCS of an aircraft
 RCS : Exercise

Exercise : compute the RCS of:

1) a sphere whose radius is 0,5 m

2) a 1 m square flat plate seen by a radar whose transmitted frequency is :
a) 1 GHz
b) 10 GHz

47
 Contents

PSR Principle
Radar equation
Radar Cross Section (RCS)
Radar architecture, components and functions
Radar types
 Modulated received
echoes / replies
Rotating joint
Modulated
transmitted pulses

Radar Receiver

architecture Raw video

Transmitter
Signal processing

Presences

Extractor
Plots

Local tracker

Tracks
49
Output
 Modulated received
echoes / replies
Rotating joint
Modulated
transmitted pulses

Receiver
Transmitter Raw video
Transmitter
Signal processing

Presences

Extractor
Plots

Local tracker

Tracks
50 Output
 Radar architecture : the transmitter

• Transmitter characteristics :

 Peak power, mean power, Energy transmitted

 Bandwidth, frequency stability
 Efficiency, cost, life expectancy

51
 Transmitted / received signals
Transmitted pulses
HF Carrier
Transmitter

frequency f0

Pulse repetition time Peak power Pt

time TR

 Peak power influences Range

t
 Pulse duration influences both
- Range (cf. energy Pt )
- Range resolution Duration 

Pulse
 Range proportional to pulse energie

Pulse Energy
PT TG 2 2LS
4
Rmax 
4   SNRmin  kT0 F
3

53
 Pulsed radars vs CW radars : CW radars

May be used for simpler applications than pulsed radars

The Tx transmits continuously
– Large ranges can be achieved w/o the high peak-power levels
required in pulse radar (Max range is a function of the
AVERAGE Tx power)
– CW radars are generally simpler, less costly and more compact
These radars can easily determine the relative speed of a target using the
Doppler effect
 Range Resolution

Range resolution proportional to

pulse duration (only for pulse modultated
by constant frequency):

DRmin ~ cT/2

Dt = 2DR/c

Dtmin = 1/Df ~ T

True formula : DRmin ~ c/2Df With Df = pulse spectral width

 Transmitter devices

Device Frequencies Peak power Operating Bandwidth Stability Other factors

voltage

Magnetron 400 MHz to 30 High ~ 20 kV Low Poor Cheap, compact

GHz (50 kW to 5 MW)

Klystron 300 MHz to 30 High ~ 100 kW Medium Good Costly, shielding

GHz (1 kW to 30 MW) required

Solid state Up to 5 GHz Low ~ 50 V High Very good Limited peak power
(up to 50 kW)

56
 The radar RANGE is dimensioned by the ENERGY
T included in the transmitted pulse

Pp Energy = Pulse duration . Peak Power

Solid State Technology:

Vacuum tube technology:

Pp = 1 MW Range Pp = 10 kW
E=1J E=1J
T = 1ms 100 NM T = 100ms

resolution = 150 m Resolution resolution = 15 km

 Pulse compression

• Pulse compression is a signal processing technique to augment the

distance resolution as well as the signal to noise ratio. This is achieved by
modulating the transmitted pulse
 Far-range detection : pulse compression
 Technique used to reduce the range resolution of a long pulse
to the one of a short pulse

Range Resolution: DR = c/2DB where DB is the spectrum width of the signal

Case of a pulse modulated by a fixed frequency:

DB  1/ so DR  c/2

Case of a pulse modulated by a linear frequency (LFM):

Choose frequency shift DB so that DR = expected value

 Pulse compression benefits

• Long range performance from long pulses

• Good resolution equal to short pulse equivalent
• Low peak powers
 Compatible with solid state Tx
 Limit the need for high voltages, waveguide pressurisation
• Disadvantages
 Can affect minimum range
 Generates time sidelobes – can be overcome though

60
 Frequencies used in radar

PSR (en-route) SSR

PSR (approach)

PSR (ground)
 Transmitters

L band S Band
 Modulated received
echoes / replies
Rotating joint
Modulated
transmitted pulses

Antenna and
Receiver
micowave Raw video
devices Transmitter
Signal processing

Presences

Extractor
Plots

Local tracker

Tracks
63
Output
 Antennas

L Band – 1 GHz

S Band – 3 GHz
 Surface radar antennas

X Band – 9GHz

Ku Band - 12 GHz

65
 Antennas
The antenna performs the following functions :

 Interfaces Tx output and space

 Interfaces space and RX input
 Provides the necessary beam shape
 Rejects ground clutter
 Provides the required target update rate
 Measures the azimuth and / or elevation of the
target

66
Antenna radiation pattern
Cone of silence
Antenna radiation pattern

Gain
Spill over
G
Side lobes

3dB beamwidth
3dB

Elevation Azimuth
68
 Antenna elevation pattern
Max gain
Cosecant pattern

Gain fall rate

toward ground Cone of silence
Antenna horizontal radiation pattern

With linear illumination :

Max gain, min beamwidth
High sidelobes

Tapering illumination at edges at

reflector
Reduces sidelobes…
… at the expense of gain &
beamwidth
 Vertical lobing

Minima

Maxima

Lobing of the radiation pattern in elevation due to ground reflections

71
Parameters influencing antenna performances

parameters Antenna Radar

characteristic performance
Gain Power budget 
Shape, size and detection capacity /
illumination range
Beam width Azimut resolution

Wavelength (frequency) influences size.

Radar coverage figure vs elevation antenna pattern

PT G 2LS
2
R( ) max ( )1/ 4
4   SNRmin  kT0 FDB
3
 Rotary joint

A rotary joint allows one section of

a waveguide to rotate with respect
to the other with:
 low reflection
 negligible power leakage

74
Circulator
 Modulated received
echoes / replies
Rotating joint
Modulated
transmitted pulses

Receiver
Receiver Raw video
Transmitter
Signal processing

Presences

Extractor
Plots

Local tracker

Tracks
76
Output
 Radar architecture : receiver
•Receiver functions :
 Noise setting  Low Noise Amplifier (LNA)
 amplification
 STC (sensitive Time control)
 demodulation, frequency transposition

•Receiver characteristics :
 Sensitivity
 Noise
 Dynamic range
 Bandwidth, local oscillator stability

77
 Receiver: big picture

RF IF
RF IF
STC M.F. PSD
Amp Amp

LO LO
(COHO)

78
 Sensitive Time Control

• Also known as Time Variable Base

Clipping

• Also known as Swept Gain

• Allows signal processing (roughly)

independently from the range

79
 Modulated received
echoes / replies
Rotating joint
Modulated
transmitted pulses

Signal Receiver

processing Raw video

Transmitter
Signal processing

Presences

Extractor
Plots

Local tracker

Tracks
80 Output
 Resolution volume

- All scatterers that belong to it participate to the signal at a given instant.

No vertical
discrimination
 Different kinds of clutter

discrete diffuse

Buildings,
fixed Ground
mountains

Birds, wind Seaa, Rain

mobile
turbines Storm

• An analysis of the clutter has to be made before choosing a processing

technique
• Clutter processing can be made at different stages (not only in the SP unit)
82
 Clutter rejection technics at antenna level
- High/low coverage
- circular polarisation

83
 High beam

Low beam

Close Close Far

fixed echo aircraft aircraft

Received signal on HB
t
LB reception

Received signal on LB
t
HB/LB switch

HB reception

High/low
beam
Processed signal
t
Circular polarisation RHCP LHCP

=a +b
Elliptical

Right-handed CP

LHC RHC
Weather channel

Aircraft channel

Left-handed CP
Advantages
Drawbacks
- Weather clutter cancellation
- Additional 3 dBloss
- Weather hazard localisation
85
Circular polarisation

CP efficiency :

Cancellations in excess of 30 dB have

been achieved from dense rain clouds

Cancellations of only 15 dB or less

are obtained from non-spherical
precipitation (eg large wet snowflakes)

86
 Clutter rejection technics using signal
processing and Doppler effect

87
 Clutter rejection

Useful targets / Useless targets

88
 Clutter rejection
What do
I need?
To sort useful echoes (aircraft)
and useless echoes (clutter)

Which criterion
make the difference
between both type of targets? Movement:
1-All aircraft move.
2- most of clutter is fixed

How can I estimate

movement?
Doppler effect
 Doppler effect (a.k.a. Doppler shift)

Vtan
The Doppler effect (or Doppler shift), named
after the Austrian physicist C. Doppler, is the V
change in frequency of a wave (or other
periodic event) for an observer moving
relative to its source. Vrad

This shift is proportional to the speed of the

target relative to the radar – which is the
radial speed !

90
When a wave hits a target, the frequency of the returned echo shifts
with respect to the radial speed of the target. The radial speed is the
component parallel to the wave propagation direction.

Dfd = 2Vr/

Vr=0 f = cte

Vr 0 toward the radar: f

Vr 0 from the radar: f

 Rejection Rule
Rejection of all return signals with Dfd = 0

2 mistakes
No Rejection of moving clutter
Rejection of useful targets
(tangential trajectories)
Dfd  0

Dfd = 0
 Doppler effect : its effect on PSR echoes

93
 Doppler effect : Moving target indicator (MTI)

94
 Doppler in receiver

RF (+ fd) IF (+ fd) fd
RF IF
STC M.F. PSD
Amp Amp

LO LO
(COHO)

95
 Signal processing : big picture

Bipolar video
Signal processing
I channel
Unipolar video
Plots
IF
PSD (I2+Q2)1/2 Detector Extractor

Signal processing
Threshold
Q channel

96
 f0 +/- fd

I&Q f0 Targets
demodulation

I(t)
Transmitter LP filter I channel

90°
Local
oscillator f0 Q(t)
LP filter Q channel
97
 Threshold 1
Threshold 2

Threshold 7
CFAR 1
CFAR 2
PSD Doppler filter bank

CFAR 7
Extractor

ZVF

Signal processing Clutter map Detector

Threshold
 MTD : Doppler filter bank

Theoretical / practical

100
 N hits with PRF1
SR LR

N=8
SR LR

SR LR

N hits with PRF2

SR LR

N=8

SR LR

SR LR

101
 Discretized coverage

Each individual cell is filled with

enough hits per scan to allow
processing.

The size of the cell roughly

correponds to the resolution
surface (range / azimuth)

102
 Modulated received
echoes / replies
Rotating joint
Modulated
transmitted pulses

Receiver
Extractor Raw video
Transmitter
Signal processing

Presences

Extractor
Plots

Local tracker

Tracks
103
Output
 Extraction : from the presence to the plot
Set of présences or preplots Plots making

extraction

Signal processing (MTD) output

Plot (range , azimuth) A PSR plot report contains:

– Measures: slant range & azimuth
– Time Stamp
To tracker – Doppler radial speed
– Amplitude or signal quality
– …

Extractor output
 Modulated received
echoes / replies
Rotating joint
Modulated
transmitted pulses

Receiver
tracker Raw video
Transmitter
Signal processing

Presences

Extractor
Plots

Local tracker

Tracks
105 Output
 Tracker : from the plot to the track

track
plot ?
Plot/trajectories association

tracker
? ?

Extractor output
?
Tracker output
 tracker

Trajectory identification
(one number per track) 495

Ground speed estimation

( ≠ radial speed !)

Ultimate clutter elimination stage

 Contents

PSR Principle
Radar equation
Radar Cross Section (RCS)
Radar architecture, components and functions
Radar types
(Air PSR, ASDE, Weather radar)
 Radar types and uses
L Band S Band

frequencies 1,215 GHz - 1,550 GHz 2,7 GHz - 2,9 GHz

Wave length 23 cm 10 cm

range 80 à 200 NM 50 à 80 NM

Revolution speed 4 à 6 rev/mn (En route) 12 à 15 rev/mn

12 à 15 rev/mn (Approach)

Antenna size 12 m wide (En route) 5 m wide

9 m wide (Approach)

Tower height > 25 m > 12 m

Use En Route / approach Approach

Financial estimate 5 + 1 M€ 3 + 0.8 M€

 Performances
Range accuracy  50 m

Azimuth accuracy:  0,15 °

Equivalent cross distance at 80  400 m
NM:

Range resolution power 200 m

Azimuth resolution power 3°

Equivalent cross distance at 80 8 km
NM :
Probability of detection 95%

False track rate 1 false track per scan

Q RQ
 air radar/surface radar
Air PSR Surface radar (ASDE)

Coverage TMA (thousands of km3) airport (few km2)

Scanned area sky ground

Range 80 NM 5 km

Target behaviour Ponctual echo non ponctual echo

Separation standard 3 NM 50 m

resolution 4 km 20 m

Distance accuracy 400 m 10 m

Refresh period 4 ou 5 s 1s
 Surface radar- ASDE
PSR (air) ASDE Consequences
Coverage - Volume - 105 km3 Surface - some Lower Pulse energy (Pc*T) 
range 60 to 200 NM km2 - 5 km equipment size ↓

Scanned area Air Ground surface Fixed echoes and signal

processing: useful targets are
doppler free
Separation 3NM 50 m resolution
Resolution r 150 m 2m Pulse duration (<100ns)
Resolution Q 2° 20 m Antenna directivity size 
higher frequencies (bande X)
Precision 400 m 10 m

Update rate 4 ou 5 s 1s High speed at take off /landing

targets ponctual Non ponctual Specific extraction methods

Surface radar- ASDE
 A few figures

Approach 1 Approach 2 Approach 3 Ground

TA 10 TRAC 2000 STAR 2000 Astre
Frequency 2700 / 3100 MHz 1250 / 1350 MHz 2760 / 2860 MHz 15,7 / 16,7 GHz
Frequency band S L S Ku
Wavelength 10 cm 23 cm 10 cm 2 cm
Peak power 0,4 à 1 MW 10 kW 30 kW 30 kW
Pulse width 1 µs 1 µs + 60 µs 1,4 µs + 75 µs 40 ns
Pulse Repetition Interval 1 ms 1,5 ms 120 µs
Antenna gain & 36 dB 35 dB 34 dB 41 dB
beamwidth 1,5 ° 2° 1,4 ° 0,3 °
Rotation speed 15 rpm 12 à 15 rpm 12 rpm 60 rpm
Operational range 40 NM 80 NM 80 NM 5 NM

114
Weather radar information coming from
meteorological services

PSR weather channel

The pilot will detect weather hazard
– visually
– using on board weather radar
The pilot will ask the controller for heading changes in order to avoid the
storm

Weather information
needs
 But, unfortunately:
– Modern radar are very efficient for weather clutter cancellation
– Controllers do not know why the pilot ask for trajectory changes

Need:
To detect and position weather hazards
To display them on the control position
 Rain echo power PSR antenna elevation aperture too
estimation using PSR wide to guaranty that the
scattering volume is homogeneous.
antenna

- vacuum
- rain
- aircraft
- ground echoes

Rain fall estimation

not reliable
 h: rain volume reflectivity estimation (dBZ)
=f(drops diameter, drops concentration)

Rain fall
estimation
Weather radar Pr= Kh/R2

Pr estimation R = cDt/2

Dt = 2R/c
Pr
 Weather radar Characteristics
- Rain fall estimation from received power
- No detection procedure
- narrow beam (scattering volume homogeneity)
- slow update cycle for weather radar images
- general characteristics:

Frequency : 2,7-2,9 (S) / 5,6 - 5,8 (C) GHz

wave length : 10,7 / 5,3 cm
range : 150 km
beam width : 1,3 °
revolution speed : 1 revolution/mn
refreshment period : 1 image every 5 mn
 Circular polarisation
= +b
elliptic
CR CL

Circular Right

CR CL
(all met echoes)
Weather channel

aircraft echoes
 component of
(aircraft channel)
==>3dB loss

Circular Left

Advantages Draw back

- met clutter rejection -3 dB loss on aircraft
- weather hazard localisation channel
 PSR Weather Channel Characteristics
- Weather hazard level triggering (No Rain fall estimation)
- Weather hazard localisation
- same geographical reference as aircraft channel
- quicker refreshment cycle for hazard images: <1 per mn
- general characteristics: same as PSR

Weather hazard level (6)calculation

and localisation

Pr estimation R = cDt/2

Dt = 2R/c
Pr
 Conclusion on PSR

Strong points Limitations

Measurement provides slant range & azimuth Detection of wanted returns in clutter

Unwanted false plots

Only suitable tool to detect non-cooperative
aircraft / vehicles (or weather phenomena)
No identification

Very complex signal processing

No (or limited) height information

Secondary Surveillance Radar
(SSR)
 SSR Content

- SSR Principles
• Positionning
• Mode A/ mode C interrogations and replies
- SSR Antenna
• horizontal, elevation patterns
• Monopulse
• Side lobes - ISLS
- SSR limitation
• Garbling
• Fruit
 Why ?
Primary radar limitations :
– Tracks / flights not identified
– No altitude information (civil radar)
– Detection of useless objects

Once a target is detected, how can one identify it ?

– 2nd World War technique of IFF (Identification, Friend or Foe)  Has led to
current SSR
– SSR functions :
• Aircraft / flight Identification (this is what SSR is for)
• Try to get more information from the aircraft (Altitude?)
• False Alarm suppression
 SSR principle
SSR functions :
- Detection
- Ranging (distance measurement) Same as PSR
- Azimut measurement
- Target identification Need active cooperation of the target
- Altitude report  aircraft must embarked a specific equipment

transponder xpdr

SSR is a basic datalink between a radar and a transponder :

- Uplink from Interrogator to xpdr using frequency f1 =1030 MHz
- Downlink from xpdr to radar receiver using frequency f2 =1090 MHz

Pb: connection is very short. It lasts the time the aircraft is lighted by the radar antenna
 SSR principle

Cooperative system  Equipped aircrafts

Localization +
– Identification
– Level
– …

Transponder

5146
350 ICAO standardized : annexe 10,
Volume IV
Radar station
 Operation of a Secondary Radar
Detection is performed by recognizing the structure
of a SSR reply transmitted by a transponder. It supposes that
 aircraft is equipped with xpdr
 Xpdr is turn on and functionning properly
5146
Advantage : no unwanted detection (false alarm)
Drawback : Xpdr failure makes aircraft undetectable
Az

330

5146
ID ?
Flight D
330
Level ?
 Display of SSR informations

leader 7000 Mode A

35 Mode C + tendency
Speed vector

Current position Past positions (6, here)

 Height

Position estimation

Polar co-ordinates
- centre : radar station
F FL
- Range O Q

- azimuth angle /geographic North

- Fligh Level (pressure altitude)
P1 P3
Slant range measurement

travel time : t

F1

Tpdr proc. time

tr = 3μs
F1
travel time : t

Total time: 2t + tr c.(t  tr )

R
2
 Returned position information
True North

Az

5146
330
Identification?
FL ?

5146
c.(t  tr )
R
330
R
2
Az a / c  Azantenna _ axis
Z  Flight Level  altitude

t = 1 µs  R = 150 m
t = 1 ms  R = 150 km
t = 12,34 µs  R = 1 NM
 Frequencies used

Worldwide SSR frequencies :

- Uplink frequency f1 = 1030 MHz

- Downlink frequency f2 = 1090 MHz

Use of 2 differents frequencies makes

uplink and downlink independants:
 Avoid any backscattered interrogation
to be processed and detected by the
radar receiver
 Radar equation : uplink/downlink link budget

.Gradar Gtransp .i .Ls

radar 2
Pc
  MTLxpdr  69dBm
transpondeur
Pr
4.  .R
2 2

.Gradar Gtransp .r .Ls

transp 2
Pc
  MTLradar  85dBm
radar
Pr
4.  .R2 2
 SSR Content

- SSR Principles
• Positionning
• Mode A/ mode C interrogations and replies
- SSR Antenna
• horizontal, elevation patterns
• Monopulse
• Side lobes - ISLS
- SSR limitation
• Garbling
• Fruit
 Interrogation types

Pulse spacing distinguishes various « questions » or « modes of interrogations »

Military Civil

mode 1 : 3 µs Mode A : 8 µs
Military ident Identification
P1 P3
mode 2 : 5 µs Mode B : 17 µs
Military ident Not used
S Spacing
Pc
Mode 3 Mode C : 21 µs
t
same as civil mode Fligh level
Mode 4 and 5 Mode D : 25 µs
Not used
 working
principle

Basics SSR parameters

Time of transmission (1) Time of transmission (2)

Tr : pulse repetition period (ms) Tr =1/PRF

P : Pulse peak
power (kW) Listening period (reception) Listening period

Mode A interrogation Mode C interrogation

 replies
Target lighted during a period equal to:
Rev speed*beam aperture.
During this period several interrogations
are transmitted
Q = Rev speed*repetition period
Because of antenna revolution
each new reply coming from the
same target shifts to the right. Q3dB = Beam
aperture

Raw representation on PPI display:

Target = "banana"
Q3dB wide
pulse duration deep.
 Pulse Repetition Frequency (PRF)
Time of transmission (1) Time of transmission (2)
Tr : pulse repetition period (ms) Tr =1/PRF

Listening period (reception) Listening period

17
Mode A interrogation Mode C interrogation

PRF = 1/Tr (unit = Hz)

Must give time to replies coming from the expected range (Rmax) of the radar
to travel back to the radar :
2 Rmax
TR 
c
c
PRF 
2Rmax
 Hits per scan
1 aircraft = N echoes in the main beamwidth
 3dB
Dwelltime 
6  VRot
units
 3dB  3dB  PRF
number of hits per scan   • beam in degrees
6  VRot  TR 6  VRot • Vrevolution in revolution/minute
• TR in seconds

Antenna position
upon the first
interrogation Possible replies

Antenna position
upon the last interrogation
Beamwidth
 The reply (1)
• Frequency: fr=1090Mhz
F1 C1 A1 C2 A2 C4 A4 X B1 D1 B2 D2 B4 D4 F2 E G SPI

t
450ns

1,45μs
20,3μs 4,35μs

F1 and F2 framing pulses

SPI : special pulse identification

 The reply (2)

Data pulses : , Bi and Ci with i=1,2 or 4

Data pulse Ai present means : Ai = 1

Data pulse Ai absent means : Ai = 0

To be decoded data pulses are grouped by 4 sets of 3 pulses. Each

set encodes one digit from 0 to 7.

1st digit A4 A2 A1
2nd digit B4 B2 B1
3rd digit C4 C2 C1
4th digit D4 D2 D1
 Mode A reply
F1 C1 A1 C2 A2 C4 A4 X B1 D1 B2 D2 B4 D4 F2 E G SPI

A = 20*A1+21*A2+23*A4 = 1*A1+2*A2+4*A4 = 5
 Specific mode A codes

- 7500 : Hijacking
- 7600 : Radio failure
- 7700 : Emergency
- 7777 : parrots / far field monitor
- 1000 : Mode S flight
- 7000 : VFR flight
 Flight level (mode C code)

High pressure Standard Low pressure

isopressure atmosphere

FL
True altitude
Hr>N True altitude
Hr<N

1013 Sea level,

true altitude = 0
 Secondary radar link - synthesis

Reply 1090 MHz

F1 + 12 bits + F2

Interrogation
1030 MHz P1-P3

P1 P3 P1 P3
Interrogations SSR Mode A Mode C
8 ms 21 ms

F1 F2
Replies SSR
20,3 ms
 SSR Content

- SSR Principles
• Positionning
• Mode A/ mode C interrogations and replies
- SSR Antenna
• horizontal, elevation patterns
• Monopulse
• Side lobes - ISLS
- SSR limitation
• Garbling
• Fruit
SSR Antenna
 SSR Antenna patterns
SSR operations require 3 radiation pattern types:

pattern transmission reception

S Main directive - sigma yes yes
W Control - omega yes yes
 Monopulse - delta no yes

 Monopulse pattern used only if SSR is monopulse type

 The only way of operating different radiation patterns using the

same antenna is to use a phased array antenna
Distribution
Elevation radiation pattern
Cosecante pattern

Cone of silence
Transmition Azimuth pattern

Main lobe

control lobe

Side lobes
 SSR Content

- SSR Principles
• Positionning
• Mode A/ mode C interrogations and replies
- SSR Antenna
• horizontal, elevation patterns
• Monopulse
• Side lobes - ISLS
- SSR limitation
• Garbling
• Fruit
 Azimuth measurement
Classic SSR Estimated azimut of the
N replies with N ~number of hits aircraft = average of
replies azimuth
North

Poor accuracy
Less that accuracy provided by PSR

Monopulse SSR Technic that allows precise

Only one reply azimuth estimation using only
North
one reply

Better accuracy than classic SSR and PSR

Mandatory for operating Mode S
Monopulse SSR ==<for Mode S
 Monopulse azimuth measurement
• Improving azimuth measurement accuracy
• Measurement of the O.B.A. (Off-Boresight Azimuth)
• 2 antenna patterns (S et )

pattern S

pattern 

Az aircraft  Az axis _ antenna   OBA

 Sigma Delta patterns

Monopulse function
uses the central part of
the antenna Delta and
Sigma patterns where
gain variations are
linears:
Dynamic < 0,5°
 Monopulse principle
Antenna gain

S beam  Gain = max

gain (about constant)

- gain + gain
Signal on  
Q oba  f( )
Signal on  

Q = Azimut shift / antenna main beam axis

 A Monopulse principles
B

F1 F2
S
Aircraft A

- + SPI
S

Aircraft B

 SSR Content

- SSR Principles
• Positionning
• Mode A/ mode C interrogations and replies
- SSR Antenna
• horizontal, elevation patterns
• Monopulse
• Side lobes – ISLS
- SSR limitation
• Garbling
• Fruit
 Sidelobes

Exercice : power budget on side lobe

radiation.
- Antenna max gain : 27 dB
- Side lobe level : -20 dB below max
6320 - Transmitter output peak power : 1 kW
- Losses : 1 dB
- XPDR antenna gain : 0 dB
- Aircraft distance : 100 km.

Is xpdr triggered by radar interrogation

Radar antenna radiated on side lobe?
 Sidelobes

These two a/c receive

the interrogation

North
Radar antenna 6320

5660
6320

5660

This a/c does not receive

the interrogation
Solution : ISLS A
P1 P3

S Pc

t
P2

t
2 µs
Pc P1 = PcP2 = 1,6 KW
P1 P3
B P2
A

C
B

W Diagram C
Transmission patterns
Sigma, Omega
 SSR Content

- SSR Principles
• Positionning
• Mode A/ mode C interrogations and replies
- SSR Antenna
• horizontal, elevation patterns
• Monopulse
• Side lobes – ISLS
- SSR limitation
• Garbling
• Fruit
 SSR Range Resolution
Garbling

Q3dB Q3dB

Targets Separated in range Targets not Separated in range

 Garbling

R1

R1~R2 R2

P1 P3
Replies not decodable
Radar Tx

Radar Rx
Reply a/c 1 Reply a/c 2
 Monopulse degarbling process

F1a F2a
F1 and F2 hard to
Frame1on S
identify on S signal
Frame1 on 
F1b F2b
Frame2 on S

Frame2 on 

F1a F1c F1b F2a F2c F2b

Frame garbled on S
Frame garbled on 
 Monopulse but garbling

F1a F2a
Frame1on S Overlapped pulses
Frame1on  (fully or partially)
F1b F2b
Frame2 on S

Frame2 on 

F1a F1b F2a F2b

???
 SSR Content

- SSR Principles
• Positionning
• Mode A/ mode C interrogations and replies
- SSR Antenna
• horizontal, elevation patterns
• Monopulse
• Side lobes – ISLS
- SSR limitation
• Garbling
• Fruit
SSR Limitations : FRUIT

F1 F2

False Reply Unsynchronised with

Interrogator Transmission F1 F2

Reception of a reply not triggered

by the green radar
 FRUIT elimination : staggered PRF

d d d

Tr1

Replies from the aircraft to Radar 1 interrogations . . .

d1 d2 d3

Tr2
On-board
equipment

SPI
Transponder antenna(s)
SSR MODE S
 Mode S Content

- Mode S advantages
- Mode S radar principle
- Elementary Surveillance (ELS)
• Radar sensors
• ATM systems
• Airborn systems
- Enhanced surveillance (EHS)
 Mode S

• Mode S Technique
• SSR limitations
• Mode S Surveillance

• Elementary surveillance (ELS)

• Radar
• ATC systems
• Airborn systems - regulations

• Enhanced surveillance (EHS)

 Mode S radar standards

ICAO standards:
– SARPS Mode S ( ground station et
transponder) Annexe 10 edition 1987 +
revision 1993, 1996, 1998 ,2002, 2006, 2008)
– SARPS Mode S network published 1993,
revised 1997
– Specifics services manual (doc 9688)
edition May 1997, revised 2002, 2006
Industry standard
– Eurocaé /ARINC /RTCA concerning
transponders Mode S

5
 Why Mode S is essential?
– to fix SSR drawbacks (garbing, Fruit)  has been invented for
that
– Improvement of tracking and Flight plan correlation
– Reduce RF 1030 – 1090 pollution
– Mode A code Shortage
– Increase radar capacity (technlogy)
 European radar density

- 80 civil SSR
in the FABEC area
- As many military IFF
 SSR limitations SSR : Mode A shortage and integrity

Number of Mode A is limited:

- 12 bits = 4096 codes available
- Not sufficient in dense trafic areas

Mode A integrity
- Received Mode A value not checked
- Mode A encoding not robust to interference

ATC systems using Mode A

- multi-radar tracker : plot/track correlation STR (pb of duplicated Mode A)
- Track servers : Flight Plan/ track correlation

5
RadmS
 Use of Mode A for Flight Plan correlation
SSR1 SSR2 SSR3
Mode A Mode A Mode A
X1 X2 X3
Y1 Y2 Y3
Z1 Z2 Z3
Track data Flight plan data

Mode A
ACID
Multi-Radar Surveillance Mode A
X/Y/Z
Tracker Server Departure FDPS
Arrival
Heading
...
Speed

ACID, position, heading, speed,

departure, arrival airport
Informattion displayed to ATCO
 Data processing capacity

6
RadmS
 RF pollution - Mode A/C radar RF occupancy
– A mode A/C radar interrogates an aircraft 10 to 20 times per antenna
revolution
– ICAO annexe 10 specifies the rate of occupancy of the transponder to
500 replies per sec
– In airpaces where aircrafts are interrogated by 50 radars, this rate can
be reached
 Loss of detection

– Aircrafts reply to all A/C interrogation.

– The number of replies transmitted by transponders is proportional to the
number of radar and to the density of Air trafic
 In the Core area : 1090 MHz frequency is sometimes saturated

Mode S radars interrogate an aircraft only ONCE per antenna revolution,

6 which reduce the transponder occupancy and 1090 utilisation.
European SSR Overlap
9 Light Green
From10 to 14 Bright Red
From 15 to 19 Bright Yellow
From 20 to 24 Dark Blue
From 25 to 29 Light Green
From 30 to 34 Pink
From 35 to 39 Light Blue
From 40 to 42 Orange

overlap FL 100
 SSR Limitations : The solutions
Real need for improving the performances of SSR and ATC systems

SOLUTION for that is Mode S Surveillance and therefore Mode S radar

Mode S specifications are:

- Interoperability with existing surveillance equipments (same frequency)
- Allocation of a unique 24 bits address to each aircraft (@ Mode S) in order
to interrogate each aircraft individually (SELECTIVE interrogation) 
GARBLING Suppression
- Allocation of an identity code for each interrogator (16 II codes ou 63 SI
codes) in order to identify each interrogator. The ID code will be part of both
radar interrogation and aircraft reply  FRUIT suppression
- Development of a special protocol for acquiring @ Mode S of aircraft
entering the coverage of the radar

6
 Mode S  new capacities

 Mode S Improves radar detection and ranging performances

- Data security improvement, check of received data integrity (use of code
correcting technics)
- Improvement of altitude encoding  FL – Mode C LSB = 25 ft
- Technology changes so that data processing must be able ti manage 900
aircraft in the radar coverage

 Mode S provides new functionalities

- Extra data available for identification: @ 24 bit and aircraft ID (ACID) 
usable for replacing Mode A and fixing mode A shortage issue
- Important number of avionic parameters may be send down to the ground
(Ground Speed, Mode of Flight, Roll angle, heading, wind, …. )

6
 Mode S Content

- Mode S advantages
- Mode S radar principle
- Elementary Surveillance (ELS)
• Radar sensors
• ATM systems
• Airborn systems
- Enhanced surveillance (EHS)
 Mode S principles

Selective interrogation

– Shall help us to get rid of garbling phenomena

– Shall allow to downlink parameters derived from on-board
avionics systems

Monopulse detection (prerequiste for Mode S)

– One aircraft reply = one hit per scan only

Interoperability between existing SSR systems

– “Backwards compatibility”
 Mode S principles
Purpose : avoid miss understanding, jamming,
cacophony
→ Create an adressed data link between a radar
interrogator and a given aircraft (Selective Mode)
Mode S Adress (@modes) – 24 bits - unique

Interrogator Adress :
- II : Interrogator Identifier(16)
- SI: Surveillance Identifier (64)
 Mode S technique

3 types of interrogations : All Call Mode S (AC), Roll Call (RC), All Call mode AC

Mode S aircraf entering

the coverage Mode S aircraft locked

Mode A/C aircraft

Acquisition
@Mode S selective Interrogation
Of a @Mode S

AC ACac RC
 operating mode S
Mode S General Call - Each aircraft has an unique identifier (24 bits)
- Each radar has a given identifier. Take care not to allocate
or All Call the same identifier to two radars which coverages are not
separated.

The first step is a basic surveillance action which consists in

Aircraft entering the - Detecting the aircraft entering the radar coverage
radar coverage for - Locating the aircraft
the first time And, more specific Mode S action,
- looking after the Mode S address of this unknown aircraft
in order to operate mode S selective interrogation with
First scans of him
antenna For Sensor 05 - Add this @modeS to the list of the Mode S xpdr managed
by the radar
I am C0123F
Here Sensor 05 ,
Identify yourself
Interrogator 1
Interrogator 2
…..
@modeS 1
05 I locate the
@modeS 2
position of
….. the
C0123F
….
C0123F
 Selective Interrogation (Roll Call) & Lock-out
2 conditions for operating selective interrogation:
- The radar must know the MODE S Addres (0xC0123F ) of the aircraft he will
interrogate selectively (1 All Call is normally enough)
After 3 or 4 - The radar has to be able to predict the position of the aircraft at the next antenna
scans of scan (3 positions are necessary to calculate a speed vector and a tracking window 
3 All Call)
antenna Here C0123F When Conditions are OK for operating Selective interrogation, the aircraft must
Here C0123F 2200 stop replying to All Call because it is no more usefull and, overall, beacause
replying to All Call generates FRUIT and GARBLING and pollutes 1090 frequency.
330  The Radar-Sensor will give the aircraft an order of locking itself with the radar.
The aircraft is locked only for this Radar Sensor .
Interrogator 1
Here Sensor 05 :
Interrogator 2 Here Sensor 05 : For 0xC0123F
….. For C0123F Lock out your Addresse
Lockout your Address Mode S!
05 locked
Mode S ! Mode C ?
Mode A ?
@modeS 1
I will keep on to
The Mode S Transponder @modeS 2
interrogate the
0xC0123F is going to lock itself . ….. next positions
It will answer just for Selectif
of the
Answer.
…. 0xC0123F
C0123F
 Lock-out process
Locking a Mode S transponder (@ 0xC0123F ) with a given interrogator (II=05) means :
- It will reply only to Selective interrogations.
- In other word : It will no more reply to All Call coming from interrogator II=05

Issue : What happen if the aircraft is lost by the radar (ex. Technical pb, aircraft masked by a mountain
etc..)?
 After 3 antenna revolutions without detection, the radar cancels the track and remove @ 0xC0123F from
the @modeS list.
 If the aircraft is back into the radar coverage and become detectable :
 It will not be interrogated using selective mode because it is unknown for the radar
 It will be interrogated using All Call which is the normal process for unknown aircraft : BUT,
because it is locked with the interrogator, it is not allowed to answer to All Call from 05  the
aircraft becomes UNDETECTABLE by Radar II =05

SOLUTION :
- The locked is maintained only during 18 seconds
- A timer is launched when the transponder locks with the radar.
- The timer is reset at each selective interrogation
- If no selective interrogation is received during 18 s, the transponder is unlocked with radar II=5

This is always the case when the aircraft lives the radar coverage  allows another radar, with the same II
code to acquire the aircraft.
 We know the Address Mode S 0xC0123F , so we know the position of this aircraft ,
After the fourth Scan So we can predict his position at the next scan of antenna with a uncertainty slot
coming from the calculus of tracking process . The radar will have to take account of
this uncertainty and will wait the reply in a delay that we will call : “answer windows”

A good tracking involve a short “answer windows” .

Here C0123F
330 The radar at the reception of the answer , verifies with the Mode S Address that it is
the correct aircraft . This phenomena remove the FRUIT.

We can remark that the radar don’t ask systematically the Mode A. It changes rarely ,
Here C0123F and this minimize the FRUIT and the GARBLING. A . The Radar is called upon in the
2200 case of a change of Mode A . The request is done by the aircraft itself.

In this kind of surveillance , we will get the Mode A (12 bits) et the Mode C ( 25 feet of
accuracy).

Here Sensor 05 ,
For C0123F
Question about
your fly level ?

Selective Call
Here Sensor 05 ,
For C0123F
Question about
your Mode A?
1st selective call
XPDR capacity are asked by the radar only once, each time the aircraft is
acquired by a new radar.

We can remark that the radar don’t ask systematically the ACID. It changes
Here C0123F rarely , and this minimize the FRUIT and the GARBLING.
AF450 In case of ACID change, the transponder informs the radar that ACID has
changed. When receiving this notice, the radar ask for the new ACID during
the following.

Here C0123F
XPDR lever,
Datalink Here Sensor 05 ,
Capacity, For C0123F
BDS Question about
XPDR capacity ?

Here Sensor 05 ,
For C0123F
Question about
ACID?
Specific selective Call
Mode S technique –radar coverage change

II1 All Call

II2 Roll Call

@ 24 bits
 Enhancement

Fruit cancelling thanks to:

– Less interrogations
– Transponder reply containing the interrogator code

Garbling cancelling thanks to:

– Roll Call mecanism so that replies do not overlap
 FRUIT Suppression

Data field including

the interrogator ID
code

?
?
no

yes

Track update

7 The Mode S reply encompasses the ID code of the interrogator that has asked the question
RadmS
 Garbling reduction

Roll Call

Interrogation reply Interrogation Reply

Time

The 2 interrogations are sequenced (the 2nd is delayed if necessary)

so that the replies are not superimposed
 Surveillance Mode A/C Experimental Scenario

Surveillance Mode S

7
RadmS
 XPDR XPDR XPDR
Mode S interrogation types Mode A/C Mode S Mode S
non locked locked
(@X) (@Y)

reply
Mode A No reply No reply
All Call P1 P3 P4
Mode A

reply
All Call

All Call
Mode C Mode C No reply No reply
P1 P3 P4

All Call reply No reply

Mode S
P1 P2 P6 (UF11) No reply Mode S
(with @X)
Demande Reply DF4
Mode C (with Mode
P1 P2 P6 (UF4 pour @Y) No reply No reply
Roll Call

C)

Demande Reply DF5

Mode A No reply No reply (with Mode
P1 P2 P6 (UF5 pour @Y) A)
 All Call and Roll Call periods (1/2)

AC RC AC RC AC RC AC RC AC RC AC RC
time
– All Call (AC) periods help
monitor Mode A/C aircraft and
acquire Mode S aircraft coming
under the coverage
– Roll Call periods enable
selective interrogation of Mode
S aircraft already acquired
using the AC periods
 Roll call and all call content
Interrogation Interrogation Réponse Interrogation Réponse Réponse
@1 @2 @1 @3 @3 @2
P1 P2 P6 P1 P2 P6 P1 P2 P6

time

Data in P6 are :
•type of interrogation (UF
Mode S Interrogation field)
•Mode S @ of the selected
P1 P2 P6 (data bits) aircraft
AC RC •Locking command
Mode S reply • probability of reply
• Airborn parameter
Data bits request

P1 P2 P6 P1 P3 P4 Listening period (waiting for replies)

8 All Call All Call Réponses des avions Mode A/C time
1 RadmS
Mode S (UF 11) Mode A/C et des nouveaux avions Mode S
 Roll Call Interrogations types

Transpondeurs Transpondeurs
Mode A/C Mode S

Reply by mode A/C

P1 P2 P6 mode S only No Reply (If Selected and
(short) Addressed )
Mode S

mode S only No Reply transactions

P1 P2 P6 long
(long) by Data-Link

RQ : Normalisation of Mode S Segment in ICAO Annexe 10

 Interrogation Mode S (P1, P2, P6)

information of P5
control SLS

P1 P2 P6

Phase inversion 1st bit last bit

synchro

• P2 is used to block the Mode A/C basic transponders and avoid them to answer to a
Mode s interoogation
• P5 is transmitted on the channel W for removing the answers from the interrogations on
the secondary lobes by a Mode S Radar.
• P6 contains the bloc of 56 bits (In this case only surveillance data) or of 112 bits (with
56 or 80 bits of useful data : messages of data link)
• DPSK Modulation (Uplink rate bit = 4 Mega bits/sec)
 Modulation of P6 et ISLS (Interrogation)
0,8 µs

ISLS
P5 on Ω > P6 for the secondary lobe of Σ
P5 P5 masks the synchro-bit

First inversion Margin inerval

first 0,5 µs
of phase => Synchro Last
Element bit Element bit
250 ns
P6
P1 P2
1,25 µs 0,5 µs
1 1 0 0 1 0 1
2 µs 2,75 µs
16,25 or 30,25 µs

HF Signal : Interrogating Frequency 1030 Mhz ±10 Khz

The principle of modulation DPSK is to ’inverse or not the HF signal HF every 0,25 µs
1 => Inversion of the HF signal
0 => Non-Inversion of the HF signal
Mode S Interrogation /
uplink format
 Uplink Format
(bloc of Data in Short P6)

Format
N° 1 6 56
4 UF:00100 PC : 3 RR : 5 ID: 3 SD : 16 AP : 24 Requête Mode C
1 6 56
5 UF:00101 PC : 3 RR : 5 ID: 3 SD : 16 AP : 24 Requête Mode A

1 6 56
All-Call Mode S
11 UF:01011 PR : 4 II : 4 19 (Pading) AP : 24
seulement
 Mode S messages
(UF Number) MESSAGE LENGTH MODE S INTERROGATION REQUEST

0 Short (56 bits) Short air-air surveillance (ACAS)

1, 2 and 3 - Not defined

4 Short (56 bits) Surveillance, altitude request

5 Short (56 bits) Surveillance, identify request
6, 7, 8, 9 and 10 - Not defined
11 Short (56 bits) Mode S-only all-call

12, 13, 14 and 15 - Not defined

16 Long(112 bits) Long air-air surveillance (ACAS)
17, 18 and 19 - ADS

20 Long (112 bits) Comm-A, Comm-B altitude request (SLM *)

21 Long (112 bits) Comm-A, Comm-B identify request (SLM *)
22 and 23 - Not defined
24 (2-bit field) Long (112 bits) Comm-C, Comm-D (ELM **)
 Data link : Interrogation (Segment in P6)

Field that contains

the data-link message
for the upper level of
exchange (Packet Mode S ,
Packet X25 …… )
Format
N° 1 6 112
segment data-link
20 UF:10100 PC : 3 RR : 5 ID : 3 SD : 16 MA : 56 AP : 24 standard + req. Mode C
1 6 112
segment data-link
21 UF:10101 PC : 3 RR : 5 ID: 3 SD : 16 MA : 56 AP : 24 standard + req. Mode A
1 6 112
segment data-link
24 UF:11 RC : 2 NC : 4 MC : 80 AP : 24 extended
1 3 112
 All Call P6 structure

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

UF=11 PR IC CL libres AP = @appel général + Parité

identification interrogateur adresse appel général = 24 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
code II 0 0 0 code II de 0 à 15

poids 0 0 1 code SI de 1 à 15
faibles 0 1 0 code SI de 16 à 31
du code 0 1 1 code SI de 32 à 47
SI 1 0 0 code SI de 48 à 63

Probabilité de réponse

0 0 0 0 répondre avec Pr=1

0 0 0 1 répondre avec Pr=1/2
0 0 1 0 répondre avec Pr=1/4
0 0 1 1 répondre avec Pr=1/8
0 1 0 0 répondre avec Pr=1/16
0 1 0 1 non assigné
0 1 1 0 non assigné
0 1 1 1 non assigné
1 0 0 0 ne pas tenir compte du verouillage et répondre avec Pr=1
1 0 0 1 ne pas tenir compte du verouillage et répondre avec Pr=1/2
1 0 1 0 ne pas tenir compte du verouillage et répondre avec Pr=1/4
1 0 1 1 ne pas tenir compte du verouillage et répondre avec Pr=1/8
1 1 0 0 ne pas tenir compte du verouillage et répondre avec Pr=1/16
1 1 0 1 non assigné
1 1 1 0 non assigné
1 1 1 1 non assigné
 Roll Call P6 structure

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

UF PC = 0 RR = 0 DI SD AP = @Mode S + Parité

Non pris en compte si:

interrogation de surveillance
ou CommA avec DI=3

0 à 15 pour demander une réponse

au format surveillance DF4 ou 5

interrogateur II

0 0 0 code II 0

interrogateur SI

0 1 1 code SI LS 0 0 0 0

0 aucun changement de l'étatde vérouillage

1 commande vérouillage
Mode S Reply /
Downlink format
 Data Link : Reply

Préambule Bloc de données

Bit 1 Bit 2 Bit 3 Bit 4 Bit N-1 Bit N
1 0 1 0 1 0 1 0 1 0 1 0

Durée = 8 µs Durée = 56 ou 112 µs

Format
N°
segment data-link
20 DF:10100 FS : 3 DR : 5 UM : 6 AC : 13 MB : 56 AP : 24 standard + Code C
1 6 112
segment data-link
21 DF:10101 FS : 3 DR : 5 UM : 6 ID : 13 MB : 56 AP : 24
standard + Code A
1 6 112

24 DF:11 1 KE : 1 ND : 4 MD : 80 AP : 24 segment data-link

1 3 112 étendu
 All Call reply

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

DF=11 CA AA = @Mode S PI = code interrogateur + Parité

Capacité du transpondeur pour la surveillance

0 0 0 surveillance seulement (pas de communication) - code 7 de CA non géré

0 0 1 réservé
0 1 0 réservé
0 1 1 réservé
1 0 0 capacité comm A et B et code 7 positionnable au sol
1 0 1 capacité comm A et B et code 7 positionnable en vol
1 1 0 capacité comm A et B et code 7 positionnable en vol ou au sol
1 1 1 avion an alerte ou SPI (FS actif) ou DR non nul
 Roll Call altitude reply
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56

DF=4 FS DR = 0 UM AC AP = @Mode S + Parité

info altitude

Statut du vol

0 0 0 ni alerte, ni SPI, en vol

0 0 1 ni alerte, ni SPI, au sol
0 1 0 alerte (7500, 7600,7700), pas de SPI, en vol
0 1 1 alerte (7500, 7600,7700), pas de SPI, au vol
1 0 0 alerte (7500, 7600,7700) et SPI
1 0 1 SPI sans alerte
1 1 0 non assigné
1 1 1 non assignés

Demande descendante

0 en surveillance

Code altitude

M Q M: en feet Q: précision 25ft ou 100ft

0 0 0 0 0 0 0 0 0 0 0 0 0 pas d'altitude disponible

C1 A1 C2 A2 C4 A4 0 B1 0 B2 D2 B4 D4 altitude en pieds, precision 100 ft codage mode C code Gray

z 0 z 1 z altitude en pieds, precision 25 ft, codage sur 11 bits de N tel que

Z= (25N-1000)+/-12,5ft

z 1 z altitude en mètres
 Performances of a Mode S radar
(Eurocontrol specifications)
• Data processing capacity 900 Aircraft-tracks
• Probability of detection of 99%
• Accuracy :
– Random Error in Range 15 m en Mode S et 30 m en SSR.
– Random Error in azimuth : less than 0.068°
• The station Radar can support a rate of average of 11000/s in the lobe
of 3 dB
• Protection of data by a Parity Control (CRC)
 Mode S Content

- Mode S advantages
- Mode S radar principle
- Elementary Surveillance (ELS)
• Radar sensors
• ATM systems
• Airborn systems
- Enhanced surveillance (EHS)
 Elementary surveillance

ELS operations rely on 3 basements :

- Mode S interoogator (radar)

- ATC systems

• Multiradar radar tracker

• Correlation function

• Fall back picture → label disply

- Airborn equipment (transponder)

 Elementary surveillance
Tracker aspect

 Use of the 24 bits address for merging informations related to the same
aircraft coming from several radars sources
• advantage: @ 24 bits is far more reliable than Mode A
because it is related to unique aircraft and so, it identifies it
without any doubt

• Statistics : only 2 or 3 Mode S addresses duplicated detected

each year

 Improvement of FL tracking (tracking in vertical plan) thanks to the division

by 4 of the FL encoding précision (100 ft to 25 ft)

9
 Elementary Surveillance
Flight Plan correlation aspects – step 1
MS1 MS2 MS3
1
@MS @MS @MS
Mode A Mode A Mode A
AF123 AF123 AF123
XYZ1 XYZ2 XYZ3

Tracks info FP info

AF123 AF123
@MS Mode A
Mode A Departure
XYZ airport
MRT Surveillance FPDP
heading Arrival airport
speed picture ...
Server
MS@

Flight N° , Position, heading, Information delivered

speed, Departure, arrival
to ATCO

RadmS
 Flight Plan correlation aspects - step 2

Pure ELS FP Corrélation  to be applied on Mode S flights once ELS fully

operational (radar Mode S + systems ELS compliant + Mode S transponder).
What is a Mode S flight ?
 Flight flying through sectors that are all encompassed into Mode S
Airspaces :
• Double Mode S radar coverage
• ATC Système ATC full ELS capable
 Flight Plan field : equipement A/C =« equipped »

→Flight tagged Mode S

→Automatic allocation of mode A = 1000 (for all Mode S flights)
FP correlation no more performed using Mode A
FP correlation is performed using ACID and then maintained using the 24bits @

1
 Save Mode A codes and fixes the Mode A shortage issue
RadmS
 Get rid of Mode A codes
MS1 MS2 MS3
MS@ MS@ MS@ Flight Plan
A1000 A1000 A1000 Processing
AF123 AF123 AF123 System
XYZ1 XYZ2 XYZ3

Mode A
MS@
mode
AF123 Flight Plan Data
Tracks A1000
Function Airport
doing the Departure
AF123 correlation Airport
XYZ with the flight
Correlation Item Arrival ...
headig plan data
Aircraft Identification
speed
MS@

N° Fligth Plan
Position
Heading
Speed Available INFORMATIONS
Advantage : only one mode A code Departure For CONTROLER
for all Mode S flights Airfield
Arrival
Airfield
Elementary Surveillance: fall back picture improvement

In fall back mode (mono radar data)  use of ACID received from
Mode S replies

Flight number = ACID delivered to the controler for identification

instead of Mode A

Flight level

A5312 FL280 AFR2130 FL280

identification 160 160
Ground speed
Non Mode S label Mode S label
 Mode S Content

- Mode S advantages
- Mode S radar principle
- Elementary Surveillance (ELS)
• Radar sensors
• ATM systems
• Airborn systems
- Enhanced surveillance (EHS)
Mode S transponders
 Avionic integration

altimeter

ACID Mode A Mode C

Transponder
Box avionic
Mode s
Enhanced surveillance data
and RA (ACAS)

@mode S
 Transponder registers

Mode S Transponder Avionic

255 registers BDS

Avionic data:
20 Flight ID
10 Data link

Speed, heading,
Lock out

roll angle etc..

MB

Com A interoogation
UF=20 protocol RR = Id interrogator N° BDS CRC + @mode S
ou 21 17 ou requested
18
Com B Reply
DF = 20 Flight 0 UM Altitude (25ft) BDS value CRC + @mode S
ou 21 or Mode A
status
 BDS 20: Callsign

FIELD FIELD
1 33 MSB
2 34
3 35 CHARACTER 5
4 BDS Code 2 0 36
5 37
6 38
7 39 MSB
8 40
9 MSB 41 CHARACTER 6
10 42
11 CHARACTER 1 43
12 44
13 45 MSB
14 46
15 MSB 47 CHARACTER 7
16 48
17 CHARACTER 2 49
18 50
19 51 MSB
20 52
21 MSB 53
22 54 CHARACTER 8
23 CHARACTER 3 55
24 56
25
26
27 MSB
28
29 CHARACTER 4
30
31
32 8 characters available
 Mode S Transponders : Level 1

ICAO defined 5 levels :

Level 1 : Same functions as Mode A/C XPDR with FL report

- Able to process All Call mode S interrogations as well as selective interrogations

(Roll call) and deliver mode S identification (@mode s) and Altitude

- Able to manage lock out process

- Report of flght status data (alert, ground)

- Short Squitter / anti collision service

→ level not operated and not recommended by ICAO

 Mode S Transponders : Level 2

Level 2 = Level 1 +

Management of short message communication protocol (Comm A & Comm B)

Report of datalink capability

Flight Identification (ACID)

→ First level operated by ICAO

→ Requested in European regulations

→ Level compliant with Elementary and Enhanced surveillance

 Mode S Transponders : Level 3 and 4

Levels 3 & 4 = Level 2 +

Management of extended message communication protocol (Comm C et Comm D)

 Uplink only : level 3

 Uplink and downlink : level 4
→ Level 3 : Useless

→ Level 4 : Full datalink

level 5 = level 4 + Extra Communications capabilities

ADS-B capabilities – Extension « e »

extended Squitter l -ELM

Latitude (WGS84)
Longitude (WGS84)
FL
 SI Capability – Extension « s »

Only 16 codes II...

→ II code shortage in Europe

SI code operations :

→ 64 codes SI (drawback : suppression of

some datalink functions)
 Shortage of II/SI codes

A solution : Mode S radar clusters

 Different Address II or SI for the two Radar
All Call
II2
Roll Call

New Aircraft arriving

In the coverage area !

Coverage of Radar 2
Coverage of Radar 1
 All Call
Same II Address for the two Radar Sensor
Roll Call
II1 Forbidden  detection losses
New Aircraft arriving
In the coverage area !
The Aircraft is not kept watch on
in this aera

Coverage of Radar 2
Coverage of Radar 1
 The clusters : the cure ! All Call
You send the list of the aircraft to the adjacent Radar Roll Call
II1
New Aircraft arriving
In the coverage area !

Transmission of the @Mode S and position

via a telecom/Network Coverage of Radar 2
Coverage of Radar 1
 Synthesis ELS capabilities

An aircraft that is ELS compliant meets the following functionalities :

• 24 bit aircraft address
• SSR Mode 3/A
• Altitude reporting in 25ft increments
• Flight Status (airborne/on the ground)
• Data Link Capability Report (BDS 10 hex)
• Common Usage GICB Capability Report (BDS 17 hex)
• Aircraft identification (BDS 20 hex)
• ACAS Active Resolution Advisory (BDS 30 hex) if ACAS equipped
• The aircraft operator has to ensure that the aircraft reports a unique 24 bit
aircraft
 Mode S Content

- Mode S advantages
- Mode S radar principle
- Elementary Surveillance (ELS)
• Radar sensors
• ATM systems
• Airborn systems
- Enhanced surveillance (EHS)
 Enhanced surveillance

Enhanced Surveillance aims at supplementing surveillance data using

downlinked airborne parameters:
 To get knowledge of the instantaneous state vector of the flight (heading,
speed)
 To improve the prediction of the flight path (climbing rate, intention
parameters)
Two enhanced surveillance services have to be considered :
 CAP service (Controller Access Parameter)
 Magnetic heading, selected altitude, IAS,…
 SAP service (System Access Parameter)
 Selected altitude, ground speed, true track angle, roll angle, vertical
rate,…
 EHS : parameter extraction

Mode S specific services are used :

- Periodic or on duty extraction : service GICB (ground initiated COMM-B)

- Extraction on event : dataflash application

- Broadcast – air- ground or ground air short messages

 All the B.D.S Registers
Register No. Assignment Minimum update rate Register No. Assignment Minimum update rate
00 Not valid N/A
16
3016 ACAS active resolution advisory
01 Unassigned N/A
16 3116-3F16 Unassigned N/A
02 16 Linked Comm-B, segment 2 N/A 4016 Aircraft intention 1.0 s
03 Linked Comm-B, segment 3 N/A 4116 Next way-point identifier 1.0 s
16
04 Linked Comm-B, segment 4 N/A 4216 Next way-point position 1.0 s
16
05 Extended squitter airbone position 0.2 s 4316 Next way-point information 0.5 s
16
4416 Meteorological routine air report 1.0 s
06 Extended squitter surface position 0.2 s
16
4516 Meteorological hazard report 1.0 s
07 Extended squitter status 1.0 s
16 4616 Reserved for flight management system Mode 1 To be determided
08 Extended squitter identification and type 15.0 s 4716 Reserved for flight management system Mode 2
16 To be determided
09 Extended squitter airbone velocity 0.2 s 4816 VHF channel report 5.0 s
16
0A Extended squitter event-driven information variable 4916-4F16 Unassigned N/A
16

0B Air/air information 1 (aircraft state) 1.0 s 5016 Track and turn report 1.0 s
16
5116 Position report coarse 0.5 s
0C 16 Air/air information 2 (aircraft intent) 1.0 s
5216 Position report fine 0.5 s
0D -0E Reserved for air/air state information To be determided
16 16 5316 Air-referenced state vector 0.5 s
0F Reserved for ACAS To be determided 5416 Way-point 1 5.0 s
16
10 Data link capability report < 4.0 s 55 Way-point 2 5.0 s
16 16
11 -16 Reserved for extension to data link capability report 5.0 s 5616 Way-point 3 5.0 s
16 16
17 5.0 s 4716-5E16 Unassigned N/A
16
Common usage GICB capability report
5F16 Quasi-static parameter monitoring 0.5 s
18 16-1F 16 Mode S specific services capability report 5.0 s
6016 Heading and speed report 1.0 s
20 Aircraft identification 5.0 s Extended squitter emergency/priority status
16 6116 1.0 s
21 Aircraft registration number 15.0 s 6216-6F16 Reserved forxtended
e squitter
16

22 Antenna positions 15.0 s 7016-E016 Unassigned N/A

16
23 Reserved for antenna position 15.0 s E116-E216 Reserved for Mode S BITE N/A
16
E316-F016 Unassigned N/A
24 Reserved for aircraft parameters 15.0 s
16
F116 Reserved for military use To be determined
25 Aircraft type 15.0 s
16 F216-FF16 Unassigned N/A
26 -2F Unassigned N/A
16 16

Refer to ICAO Annex 10

 EHS carriage regulation
An aircraft is considered to be EHS capable if the full list of 8 Downlink Aircraft Parameters
(DAP), set out below, can be supplied :

BDS register Downlink Aircraft Parameters

BDS 4,0 Selected altitude
BDS 5,0 Roll angle
Track angle rate
True track angle
Ground speed
BDS 6,0 Magnetic heading
Indicated Airspeed or Mach Number
Vertical rate (barometric or inertial rate of clim or descend)

To enter an EHS declared airspace, aircrafts are required to be EHS capable.

Ex. Amsterdam FIR > FL245 or UK
 Use of BDS

BDS register Downlink Aircraft Parameters

BDS 4,0 Selected altitude

 Level Bust alerts

Selected level

RadmS
 Use of BDS
BDS register Downlink Aircraft Parameters
BDS 5,0 Roll angle
Track angle rate
True track angle
Ground speed

 Improvement of tracker behaviour

True trajectory Tracked/filtered

trajectory
 EHS carriage regulation

BDS register Downlink Aircraft Parameters

BDS 6,0 Magnetic heading
Indicated Airspeed or Mach Number
Vertical rate (barometric or inertial rate of clim or descend)

 Raw and accurate date

 High Update rate
 Save VHF load when operating approach control
 Example of BDS

BDS 4,0 BDS 5,0 BDS 6,0

 Example of EHS : the GICB service
Dynamic
Enhanced
Surveillance
Climbing rate
The controler wants Interrogation Transponder
to check the climbing Request manager with BDS 6,0
rate of the aircraft request 255
register 6,0
BDS
BDS 6,0
included in MB
(DF20 ou DF21)
Parameter server

Preset of the parameters to be

BDS 6,0 reported automatically extracted by the
in the surveillance radar station
output
Static Enhanced
Surveillance
 EHS Label

Speed vector
Position

FL
(climbing, descending)

ACID AFR2130 F280

160 310 Selected altitude
Ground Speed
070 2500 120

Magnetic heading Indicated speed

Vertical speed
École Nationale de l’Aviation Civile
7 avenue Édouard Belin
BP 54005
31055 Toulouse cedex 4
FRANCE

Civil Aviation University of China

N°2898 Jinbei Road
Tianjin
CHINA