Global Navigation Satellite
System (GNSS)
Global Navigation Satellite System (GNSS)
GNSS (Global Navigation Satellite System) is a satellite system that is
used to pinpoint the geographic location of a user's receiver anywhere in
the world.
• NAVSTAR GPS - Unites State of America
• GLONASS - Russia
• Galileo European Union
GNSS will be used primarily for global, all weather, continuous, real-time
positioning and for marine, air, and land navigation.
Global Navigation Satellite System (GNSS)
• Each of the GNSS system employs a constellation of orbiting satellites
working in conjunction with a network of ground stations.
• Basically, the users of these systems simultaneously measure ranges to
several satellites, and receive a broadcast data message containing
information on satellites s positions.
• If these position are known, in an earth-centered, earth-fixed (ECEF)
coordinate system the users position can be computed either in
Cartesian geocentric coordinates or, equivalently, in geographic
coordinates
• The ECEF- referenced satellite positions computed from the received
data message depend on the configuration of the satellite orbits and the
way the orbital data represented in the message.
Global Navigation Satellite System (GNSS)
• On February 22, 1978, the first GPS prototype satellite was launched
into orbit by United States, starting a new era in satellite navigation.
• Four and a half years later on October 12, 1982, the first GLONASS
satellites were places in orbit by Soviet union.
• Since then the two satellite navigation systems have been built up
slowly. Now the world will have two separate and independent tool for
navigation and positioning of unprecedented accuracy and reliability.
NAVSTAR GPS
NAVSTAR GPS
• NAVigation by Satellite Timing And Ranging (NAVSTAR)
• The United States Department of Defense (DoD) has developed the Navstar
GPS, which is an all-weather, space based navigation system to meet the
needs of the USA military forces and accurately determine their position,
velocity, and time in a common reference system, any where on or near the
Earth on a continuous basis (Wooden, 1985).
• Started in February 22, 1978
• Initially developed as a military navigation system to assist US military
later extended to civilian users.
• 24 Satellites orbiting the earth
• One Way communication
Segments of GPS
Space segment: The Space Segment of the system consists of the GPS
satellites; These space vehicles (SVs) send radio signals from space.
- Control segment: The Control Segment consists of a system of
tracking stations located around the world.
- User segment: The GPS User Segment consists of the GPS receivers
and the user community. GPS receivers convert space vehicle (SV)
signals into position, velocity, and time estimates.
Satellite orbits
• Four GPS satellite will be unevenly distributed in each of six orbital planes.
• These planes are inclined to the equator by 55°, and are separated from each
other by 60° in longitude.
• The satellites orbit are circular with a radius of about 26,500 kilometres.
• Keplers third law relates the orbital radius to orbital period, the time
needed by satellite to travel full circle in its plane.
• GPS orbital period is exactly one half of a sidereal day. (A sidereal day is
rotation period of the earth, and is equal to a calendar day minus four
minutes).
• Therefore, after one sidereal day the geometric relationship between fixed
spots on the earth and the satellites repeats.
• For an observer on the earth, all GPS satellite reappear in same part of the
sky day after day, always four minutes earlier each day.
Calculate orbital period
Kepler's Third law of planetary motion
P2/a3=4 2/GM
• where P is period
• a is orbital radius
• GM is geocentric gravitational constant (GM) = 3.986005*1014 m3/s2
Satellite signal
• The GPS user equipment receives the sum of the signals broadcast by
all visible satellites. A particular signal can be tracked with a radio
frequency channel in the GPS receiver by looking for the satellites
unique pseudo random noise (PRN) codes modulation, thereby
rejecting all signals with a different code.
• The procedure of separating the total incoming signals into the
components transmitted by different satellites is called code division
multiple access (CDMA).
• In order to decode the signal, the receiver must be aware of the PRN
codes for each satellite.
Satellite signal cont.
• All GPS satellites transmit the two carrier signals at the same L-band
frequencies and modulates them with satellite-specific C/A-codes and
P-codes.
• L1: 1575.42 MHz (C/A-code and P-code)
• L2: 1227.60 MHz (P-code)
These dual frequencies are essential for eliminating the major source of error, i.e. the ionosperic
refraction.
• For positioning and timing, GPS provides two levels of services:
• Standard Positioning Service (SPS) with access for civilian users.
• Precise Positioning Service (PPS) with access for authorized users.
Satellite signal cont.
• The First code is the coarse/acquisition (C/A) code, which is available
for civilian use.
• The C/A-code is designated as the standard positioning service (SPS).
• The C/A-code is presently modulated upon L1 only and is purposely
omitted from L2. This omission allows to deny full system accuracy to
non-military users.
• The second code is the precision (P) code, which has been reserved for
US military and other authorised users.
• The P-code is designated as the precise positioning service (PPS).
• The P-code is modulated on both carriers L1 and L2.
Satellite signal cont.
A GPS signal contains three types of data
• Unique pseudorandom noise (PRN) codes:
The pseudorandom code is simply an I.D. code that identifies which satellite
is transmitting information.
• Satellite ephemerides data :
which is constantly transmitted by each satellite, contains important
information about the status of the satellite (healthy or unhealthy), current
date and time. This part of the signal is essential for determining a position.
• Almanac data :
The almanac data tells the GPS receiver where each GPS satellite should be at
any time throughout the day. Each satellite transmits almanac data showing
the orbital information for that satellite and for every other satellite in the
system.
GPS Time
• GPS Time is based on an atomic time scale.
• The GPS clock data are transmitted in terms of clock offset, clock
frequency offset, and clock frequency rate, and allow the calculation
of the difference between the individual GPS satellites time and the
GPS system time.
• Coordinated Universal Time as maintained by the U.S. Naval
Observatory (UTC (USNO) is also based on an atomic time scale,
with an offset of an integer number of seconds with respect to GPS
system Time.
• GPS system Time does not count leap seconds, and therefore an offset
exists between UTC (USNO) and GPS system Time.
GPS Datum
Datum is a set of parameters (translations, rotations, and scale) used to
establish the position of a reference ellipsoid with respect to points on
the Earths crust.
• The satellite ephemerides broadcast by the GPS satellites contain the
parameters of the satellite orbit in terms of a linearly varying ellipse,
plus small correction terms accounting for irregularities in the orbit.
The ephemeris data are updated every hour.
• From these, users can compute ECEF coordinates of the satellite for a
particular measurement time using well-known equations. The
resulting ECEF coordinates are referenced to the World Geodetic
System 1984 (WGS-84).
Control Segment: US DoD Monitoring
• The control segment consists of five Monitoring Stations (Hawaii, Kwajalein [West Pacific], Ascension
Island [South Atlantic], Diego Garcia [Indian Ocean], Colorado Springs), three Ground Antennas
(Ascension Island, Diego Garcia, Kwajalein).
• The Master Control facility is located at Schriever Air Force Base (formerly Falcon AFB) in the State of
Colorado, USA.
• They measure the distances of the overhead satellites every 1.5 seconds and send the corrected data to
Master control.
• Orbits are precisely measured. Discrepancies between predicted orbits (almanac) and actual orbits are
transmitted back to the satellites
User Segment
• It consists of receivers that decode the signals from the
satellites.
• The receiver performs following tasks:
- Selecting one or more satellites
- Acquiring GPS signals
- Measuring and tracking
- Recovering navigation data
Sources of GPS Signal Error
• Satellite geometry
• Satellite orbits
• Multipath
• Atmospheric effects
• Clock
• Selective Availability (S/A)
Error : Satellite Geometry
• Satellite Geometry refers to the relative position of the satellites at any
given time.
• Ideal satellite geometry exists when the satellites are located at wide angles
relative to each other.
• Poor geometry results when the satellites are located in a line or in a tight
grouping.
• The best geometry, and least error, occurs when the satellites are equally
distributed.
• Satellite geometry error occurs when the satellites are concentrated in on
quadrant or in a line.
• Satellite geometry error is not measureable, it tends to enhance other errors.
Ideal Satellite Geometry
N
W E
S
Good Satellite Geometry
Good Satellite Geometry
Poor Satellite Geometry
N
W E
S
Poor Satellite Geometry
Poor Satellite Geometry
Error : Satellite Orbits
• Even though the satellites are positioned in
very precise orbits, slight shifts are
possible due to the gravitational influences
of the sun and moon or solar wind.
• These Minor disturbances in satellite orbits
also known as ephemeris errors.
• Orbit errors can be as high as 2 meters.
Error: Multipath
• Multipath errors are occurred when GPS signal is reflected off
of objects before it reaches to the GPS receiver.
• Increment in travel time of signal due to reflection causes the
errors.
• Increase chance of occurrence when around tall buildings.
• Satellite signals require a direct line to GPS receivers.
• The more satellites a GPS receiver can "see," the better the
accuracy.
• Buildings, terrain, electronic interference, or sometimes even
dense foliage can block signal reception, causing position
errors or possibly no position reading at all.
• GPS units typically will not work indoors, underwater or
underground.
Error : Atmospheric effects
• Satellite signal travels at the speed of light in space, but
slowed down as its passes through the atmosphere.
• Atmospheric errors are separated in two categories: the
ionospheric effect and the tropospheric delay.
• The ionospheric effect is frequency dependent and it is
caused by the region of the atmosphere between 50 and
1000 km above the surface of the Earth.
• The tropospheric delay is frequency independent, and it
is caused by the lower part of the atmosphere, between
the surface and 50 km.
• The majority of this effect can be eliminated by the
receiver.
• The GPS system uses a built-in model that calculates an
average amount of delay to partially correct for this
type of error.
Error : Clock
• Slight fluctuations in atomic clocks.
• In spite of the synchronization of the satellite and receiver clocks, and
small amount of inaccuracy in timing remains.
• This can result in errors up to 1 meter.
• To keep clock errors to 1 meter or less, the time error must be limited
to 20-30 nanoseconds.
• A receiver's built-in clock is not as accurate as the atomic clocks on-
board the GPS satellites. Therefore, it may have very slight timing
errors.
Error : Selective Availability (S/A)
• Selective Availability (SA) is an intentional degradation of the signals
by time varying bias once imposed by the U.S. Department of
Defence.
• SA was intended to prevent military adversaries from using the highly
accurate GPS signals.
• The US DoD turned off SA in May 2000, which significantly
improved the accuracy of civilian GPS receivers.
• S/A could be reactivated at any time by the US DoD.
Accuracy – Sources of Error
Source Amount of Error
Ionosphere effect ± 5 meters
Troposphere effect ± 0.5 meters
Multipath distortion ± 1 meter
Satellite clock drift ± 2 meters
Ephemeris errors ± 2.5 meters
Numerical errors ± 1 meter
Total GPS accuracy 15 meters
How Does GPS Work?
GPS Receiver : Basic Principle
• The receiver collects, decodes and processes the satellite signals.
• The basic receiver does not include a transmitter.
• The receiver determines its location by trilateration.
• Trilateration works by using the distance from known positions.
• Triangulation works by using the angles from known positions.
Trilateration Triangulation
Calculating a Position
• A GPS receiver calculates its position by a technique called satellite
ranging, which involves measuring the distance between the GPS
receiver and the GPS satellites it is tracking.
• The range (the range a receiver calculates is actually a pseudo range,
or an estimate of range rather than a true range) or distance, is
measured as elapsed transit time.
• The position of each satellite is known, and the satellites transmit their
positions as part of the "messages" they send via radio waves.
• The GPS receiver on the ground is the unknown point, and must
compute its position based on the information it receives from the
satellites.
Measuring Distance (Pseudo Range) to Satellites
• Measuring the distance between the GPS receiver and a satellite requires measuring the time it
takes for the signal to travel from the satellite to the receiver.
• Once the receiver knows how much time has elapsed, it multiplies the travel time of the signal
times the speed of light (because the satellite signals travel at the speed of light) to compute
the distance.
• Distance measurements to four satellites are required to compute a 3-dimensional (latitude,
longitude and altitude) position.
• In order to measure the travel time of the satellite signal, we have to know when the signal left
the satellite AND when the signal reached the receiver. Presumably our receiver "knows"
when it receives a signal, but how does it know when the signal left the satellite?
• GPS satellites generate a unique pseudo random noise (PRN) codes that actually repeat every
millisecond. The trick is that the GPS satellites and our receivers are synchronized so they're
generating the same code at exactly the same time.
• So, when a GPS receiver receives codes from a satellite, it looks back to see how long ago it
(the receiver) generated the same code. The time difference is how long the signal took to get
from the satellite to the receiver. In other words, the receiver compares how "late" the
received satellite code is, compared to the code generated by the receiver itself.
Pseudo Random Noise Code
Time
Difference
Satellite PRN
Receiver PRN
Measuring Distance to Satellites cont.
• Distance is calculated using the velocity equation.
Distance
Velocity =
Time
• Rearranging the equation for distance:
Distance = Velocity x Time
• If the system knows the velocity of a signal and the time it takes for
the signal to travel from the sender to the receiver, the distance
between the sender and the receiver can be determined.
Measuring Distance to Satellites cont.
• The signals from the GPS satellites travel at the speed of light3,00,000
kilometers/second.
• How far apart are the sender and the receiver if the signal travel time was
0.23 seconds?
Distance (ft) = Velocity (km/sec) x Time (sec)
km
= 3,00,000 x 0.7 sec = 2,10,000 km
sec
It should be clear that this system requires very accurate measurement of
time and synchronization of clocks.
These time errors limit the precision of this system.
GPS : Trilateration/Triangulation
• Each satellite knows its position
and its distance from the center of
the earth.
• Each satellite constantly broadcasts
this information.
• With this information and the
calculated distance, the receiver
calculates its position.
• Just knowing the distance to one
satellite doesnt provide enough
information.
GPS : Trilateration/Triangulation cont.
The receiver is
• When the receiver knows its somewhere on
distance from only one satellite, this sphere.
its location could be anywhere on
the Represented by the earths
surface that is an equal distance
from the satellite.
• Circle in the illustration.
• The receiver must have additional
information.
GPS : Trilateration/Triangulation cont.
With signals from two satellites, the receiver
can narrow down its location to just two points
on the earths surface.
Were the two circles intersect.
GPS : Trilateration/Triangulation cont.
• Knowing its distance from three
satellites, the receiver can
determine its location because
there is only two possible
combinations and one of them
is out in space.
• In this example, the receiver is
located at b.
• The more satellite that are used,
the greater the potential
accuracy of the position
location.
GPS : Trilateration/Triangulation cont.
• GPS satellites transmit time information derived from high accuracy atomic
clocks but the GPS receivers cannot afford such high precision clocks.
• The first three measurements narrow down our position. In practice, a
fourth measurement is needed to correct for clock error.
• A fourth satellite must be applied correction to compensate for the
difference between the satellite and receiver clocks.
• There are four GPS satellite signals that are used to compute positions in
three dimensions and the time offset in the receiver clock.
• The next four slides show graphically how the timing offset correction
works.
A
B
4 seconds
6 seconds
Note: The explanation of correcting for timing offset will be shown in two dimensions for illustration. Remember that in
reality it takes three measurements to locate a point in three dimensions.
In an ideal situation there would be no timing error. Let's say we're 4 seconds from satellite A and 6 seconds from satellite
B: our position is where the 2 circles intersect.
A
B
4 seconds
5 seconds 6 seconds
(wrong time)
7 seconds
(wrong time)
If the receiver clock is one second fast (it's ahead one second from the satellite clock) the receiver will "think" the distance
from satellite A is 5 seconds and the distance from satellite B is 7 seconds. And it "thinks" our position is where the two
dotted circles intersect.
A
B
4 sec o n ds Back to the ideal situation with no
6 se co n ds timing error: if we have accurate
clocks, and we add a third
measurement, all three circles
intersect at the correct point,
8 se co n ds because the circles represent the
C true ranges from the three
satellites.
But with inaccurate clocks, the circles cannot
intersect: there is no point that can be 5
A seconds from A, 7 seconds from B and 9
B
seconds from C.
5 se c o nds
(w ron g tim e)
When the receiver gets a series of
7 se c o nds measurements that cannot intersect at a
(w ron g tim e)
single point, it finds the adjustment to all
measurements that lets the ranges go through
one point. In this example, subtracting 1
9 se c o nds second from all three measurements makes
(w ron g tim e)
the circles intersect at a point.
C
So, by adding one extra measurement we can
cancel out any consistent clock error the
receiver might have. Remember that in 3
dimensions this means we really need 4
measurements to cancel out any error.
4 Measurements: 4 Variables
• GPS has made a considerable impact on almost all positioning, navigation,
timing and monitoring applications. It provides particularly coded satellite
signals that can be processed in a GPS receiver, allowing the receiver to
estimate position, velocity and time (Hofmann-Wellenhof et al., 2001).
• In order to determine a position in GPS-only mode the receiver must track a
minimum of four satellites, representing the four unknowns of 3-D position
and time.
• If only 3 satellites are present, the receiver will guess at elevation and
compute latitude and longitude.
• Distance measurements to 4 satellites are required to compute a 3-D
position (Latitude, Longitude and Altitude).
S2(x2,y2,z2)
S3(x3,y3,z3)
S1(x1,y1,z1)
S4(x4,y4,z4)
GPS Receiver
4 Measurements: 4 Variables
• Latitude (X)
• Longitude (Y) POSITION DETERMINATION
WITH PSEUDO RANGES
• Altitude / Elevation (Z) = {(x1-x)2+(y1-y)2+(z1-z)2}
1
• Time (T) 2 2 2
2 = {(x2-x) +(y2-y) +(z2-z) }
2 2 2
3 = {(x3-x) +(y3-y) +(z3-z) }
Therefore, you need information
from four satellites.
