MODE-S MULTILATERATION SYSTEM
FOR SURFACE SURVEILANCE
Definition
What is a Multilateration system?
Multilateration is a proven technology that was developed for military purposes to accurately locate
aircrafts, many of which did not wish to be seen, by using a method known as Time Difference of
Arrival (TDOA)
Multilateration employs a
number of ground stations
placed in strategic locations.
These units listen for
replies (Mode A/C/S). The
aircraft replies are received
by each station at fractionally
EPC
different times. These
individual time differences
allow an aircrafts position to
be precisely calculated in a
central processing unit
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How it works
A multilateration system needs a subsystem to
date (time-stamp) responses with high precision.
Normally it is used a combination between a GPS
signal, a special signal emitted by a transmission
station and complex algorithms of data
processing.
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The solution of the equations based in the
measurement of TDOAs are a group of
hyperboloids whose intersection indicates the
actual position of the target that sent a response.
The Target can be any element equipped with an
emitter (for example a transponder)
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Principle of Multilateration
Multilateration is an independent cooperative surveillance
INDEPENDENT: the position is calculated in the ground system, independent from avionics.
COOPERATIVE: It requires a transmitter (transponder) on board.
Co-operative Independent surveillance:
A number of antennas receive a signal from an aircraft.
The Central processing unit calculates the aircrafts position using the Time Difference of Arrival
(TDOA) of the signal at the different receivers.
The TDOA between two antennas corresponds with a hyperboloid, a 2D surface in a 3D space.
At least four antennas are required for a 3D position calculation
If altitude is known already, three antennas are sufficient.
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Flavours of MLAT
Aircrafts carry already many different antennas for the purpose of
communication, navigation and surveillance.
Some of these are useful for a MLAT system, other are less useful
or not useful at all
Usefulness is determined by the possibility of:
Aircraft identification
Signal availability
Update rate
Detection performance
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Flavours of MLAT
TYPES OF SIGNALS
PRIMARY (PSR)
+ Availability
No aircraft identification
Different frequency bands for each radar. It is
more difficult to construct a universal receiver.
Sensitive to clutter
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Flavours of MLAT
TYPES OF SIGNALS
SSR
+ Very high availability
+ Aircraft Identification through Mode A code
Mode A code is not always unique
Difficulty to distinguish between Mode A-C code.
Problems with interference in high density traffic
areas.
Update rate depends on radar configuration.
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Flavours of MLAT
TYPES OF SIGNALS
Mode S signal
+ Increasing Availability (high in the near future)
+ Unique identification through aircraft address
+ Provides 25-ft altitude accuracy measurements
Problems with interference in high density traffic
areas.
Update rate depends on radar configuration
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Flavours of MLAT
TYPES OF SIGNALS
Mode S Acquisition squitter
+ High Availability
+ Unique identification through aircraft address
+ Spontaneous emission so no interrogator is
needed
+ Update rate 1Hz
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Flavours of MLAT
TYPES OF SIGNALS
Mode S Extended squitter
+ First global ADS-B datalink for international
commercial flight
+ Unique identification through aircraft address
+ Spontaneous emission so no interrogator is
needed
+ Update rate up to 6.2 Hz
Traffic density may limit useful range
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Flavours of MLAT
TYPES OF SIGNALS
VDL Mode 4
+ ADS-B datalink that will be available in certain
regions
+ Designed to prevent transmission collisions
+ Allows unique aircraft identification
Low bandwidth of the signal makes TDOA
accuracy poor.
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Flavours of MLAT
TYPES OF SIGNALS
Universal Access Transceiver UAT
+ ADS-B datalink to be used for general aviation in
the US.
+ Allows unique aircraft identification
+ Update rate about 1Hz
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Flavours of MLAT
TYPES OF SIGNALS
NAVIGATION SIGNALS
Do not provide any form of aircraft identification
Radio altimeter:
+ It is used by the aircrafts during the approach for
determining its ground clearance. It only transmits radio
pulses in a narrow beam in vertical direction to the
ground, so its no useful for MLAT.
DME signals:
+ High availability, update rate 15 to 150Hz, Designed for
en-route guidance up to 250NM.
Lack of identification limits the usefulness for MLAT.
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Flavours of MLAT
TYPES OF SIGNALS
Communication Signals
No aircraft Identification
Signal availability is limited and very irregular
(only when a pilot communicates by VHF com).
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Flavours of MLAT / Overview
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Flavours of MLAT / Overview
Format
Information
Mode A
Identification (A)
Mode C
FL (C)
Availability
On any SSR A-C interrogation (over
100/s by TXP in central
European airspace)
Quality
12 bits
Step : 100 ft
spontaneous
Mode S squitter
@ Mode S
Mode S extended
squiter
@ Mode S ; Position lat-long; FL
; FI (Flight Ident)
Short ACAS
@ Mode S ; FL
Long ACAS
@ Mode S ; FL ; coordination
air/air
Mode S Elementary
surveillance
@ Mode S ; (A or C) ; FL
Mode S Enhanced
surveillance
(aprox. 1 S)
24 bits
24 bits WGS
spontaneous
84 Step 25 ft
Over interrogation ACAS
24 bits 25 ft
Over interrogation Mode S
25 ft
@ Mode S ; (A or C) ; FL ; Data
link
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Flavours of MLAT
CONCLUSION
ALL CURRENT MLAT SYSTEMS ARE BASED ON 1090 MHz
TECHNOLOGY BECAUSE:
- Wide availability of the technology as part of the airborne
infrastructure.
- Extensive experience with this technology in terms of
surveillance.
- The properties of these signals to be used in a MLAT
system (aircraft identification and accurate TDOAs
measurements).
Other signals could be possible, but more research and
development is needed before they reach the same level of
maturity.
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MLAT architectures
ARCHITECTURE SUMMARY
Common clock systems
Transponder synchronisation
Standalone GNSS synchronisation
Common view GNSS synchronisation
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MLAT architectures
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Active versus Passive System
Active:
One or more transmitting antennas for interrogation
Three or more receivers
Independent to trigger transmission from aircraft
Passive:
Only receivers
Required signals limitted
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Active and passive MLAT systems
PASSIVE MLAT SYSTEMS
No transmitter means:
- No transmitter licence
- No 1030 spectrum usage
- No increase to 1030/1090 SSR FRUIT
- Reduced cost/complexity
Aircraft detection requires:
- The aircraft is Mode S equipped or
- The aircraft is within existing MSSR coverage or
- The aircraft is within range of a TCAS equipped
aircraft
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Active and passive MLAT systems
ACTIVE MLAT SYSTEMS
Independent of existing infrastructure
- Improved detection of Mode A/C - only aircraft
Simpler and cheaper than MSSR interrogator
- Fixed antenna
- Low power interrogator option
- Sectored interrogation option
Suitable for Mode S surveillance
Care must be taken not to generate excess of 1030/1090
FRUIT
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MLAT OUTPUT
MLAT ASTERIX CATEGORIES
CAT 10: Monoradar Surface Movement Data (Part 7)
CAT 19: Multilateration System Status Messages
CAT 20: Multilateration Messages
CAT 21: ADS-B Messages
CAT 242: Version number exchange
Types of Output currently implemented
- Data driven output (from every measurement)
- Periodic output smoothed.
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Advantages of a Multilateration system
High redundancy
To calculate the correct target position
the system selects the best four
receptor to make the calculation. In case
of fail or degradation of one receptor
the system selects another one that is
working properly to calculate the
position
High accuracy
The accuracy on interest zones can
be better than five meters.
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Advantages of a Multilateration system
Low fail probability
Because there are no mechanical elements
High update rate
The system must not wait for any antenna rotation. When any target transmits a response signal,
it is detected by the multilateration system. As multilateration works with extended squitter and
it can be sent as quick as six per second (data input), the system could have an update rate of
one second (data output)
Low consumption
The system hasnt got mechanicals elements and need not a high power transmission to reach
far away.
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Advantages of a Multilateration system
EPC
Expansion capability
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A Multilateration system can be easily expanded with
a low cost because reception stations are very cheap
compared with a common SSR station
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Low environmental impact
Multilateration receptors/transmisors are
smaller than SSRs or PSRs. The
electromagnetic noise generated by
multilateration is negligible comparing
with other radar technologies
SSR
MLAT
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Advantages of a Multilateration system
Easy installation
Common radar technologies need a
complex installation process
Including ADS-B
Because of its own definition,
multilateration can work with Mode A/C/S
and ADS-B signals, so it decodes any of
these signals and sends the appropriate
codification data to the Control Centre or
Tower.
Coverage detailed
Because Multilateration uses a lot of
receptors, users can define the desire
coverage to minimize shadow zones
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An example of a Multilateration System
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CONCLUSIONS
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MLAT does not introduce any new functions
Effectiveness based on immediate surveillance
integration.
No new airborne equipment.
Usage of existing signals
Creation of A-C interrogation coverage at very low
repetition frequencies.
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CONCLUSIONS
New continuous identification of moving vehicles
Development of high performance functions (alarms,
guidance, routing)
Essential for SMGCS applications
Fast development and deployment
HIGH INTEGRATION WITH ADS-B
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The value of anticipation
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