Global Positioning System
UNIT-I
Introduction
-By
GVNSK Sravya
Assistant Professor
ECE Dept.
GNITS

Contents
▶ Basic concept
▶ SystemArchitecture
▶ GPS and GLONASS Overview
▶ Satellite Navigation
▶ Time and GPS
▶ User Position and V
elocity Calculations
▶ GPS Satellite Constellation
▶ Operation Segment
▶ User Receiving Equipment
▶ Space Segment Phased Development
▶ GPSAided Geoaugmented Navigation (GAGAN) Architecture.
2

Basic Concept of GPS
▶Navigation refers to the art of determining the
current location of an object which could be in
space, in the air, on land, on or under the surface of
a body of water, or underground.
3

Contd…
▶ Navigation Modes In order to get from A to B there are
basically five basic navigation modes:
▶ Pilotage
▶ Celestial Navigation
▶ Dead Reckoning
▶ Radio Navigation
▶ Inertial Navigation
4

Contd…
▶ Satellite Navigation or Satnav System is a system that uses
satellites to provide autonomous geo-spatial positioning.
▶ It allows small electronic receivers to determine their location to
high precision using time signals transmitted along a line of sight
by radio from satellites.
5

Contd…
▶ GNSS stands for Global Navigation Satellite System, and is the standard
generic term for satellite navigation systems that provide autonomous geo-
spatial positioning with global coverage. This term includes e.g. the GPS,
GLONASS, Galileo, Beidou, IRNSS and other regional navigation systems.
6

GPS
▶GPS The GPS is part of a satellite-based navigation system
developed by the U.S. Department of Defense in 1995.
7

Contd…
▶ Official name of GPS is Navigational Satellite Timing And
Ranging Global Positioning System (NA
VSTAR GPS).
▶ Global Positioning Systems (GPS) is a form of Global
Navigation Satellite System (GNSS).
▶ Consists of two dozen GPS satellites in Medium Earth Orbit
(The region of space between 2000km and 35,786 km)
8

Contd…
known as a satellite
▶ T
wo dozen satellites working in unison are
constellation
States Air
▶This constellation is currently controlled by the United
Force 50th Space Wing
▶ It costs about $750 million to manage and maintain the system per
year
▶ Mainly used for navigation, map-making and surveying
9

Working of GPS
▶ Each satellites broadcast radio signals with their location and time.
▶ GPS receivers receives radio signals, and used these data to calculate its distance from at least
four satellites.
▶ Distance=Speed * Travel time
▶ GPS radio signals are travel at the speed of light .
▶ Both satellites and receiver generate the same pseudocode signals.
▶ Difference b/w the 2 signals is the travel time.
▶ Then the receiver uses Trilateration method to define its exact location on Earth.
10

TrilaterationProcess
▶ If we know the distance b/w the satellite and the receiver for: 1
satellite, the receiver’s location is known within a sphere.
11

Contd…
▶ 2 satellite, the receiver’s location is known within 3D ring.
12

Contd…
▶ 3 satellite, the receiver’s location is somewhere on at most two
3D regions.
13

Contd…
▶ 4 satellite, the region gets smaller because of the sphere of the
new satellite.
14

Architecture of GPS
▶ Architecture of GPS consists of three segments:
 Space segment(SS)
 Sontrol segment(CS)
 User segment(US)
15

Contd…
16

Space Segment
▶ Minimum of 24 satellites
(currently 32) in orbit
around earth at altitude
20,000 km.
▶ It transmit radio-
navigation signals, and
store and retransmit the
navigation message sent
by the control segment.
17

Control Segment
control station,
▶ Combination of a master
four
dedicated ground antennas
and six dedicated monitor
stations.
▶ Responsible for the proper
functioning of all the
operation of GPS such as
changing unhealthy satellite
with a healthy one.
18

Components of Control Segment 19

User Segment
▶ Comprises of thousand of
military users who uses the
secure GPS Precise Positioning
service, and millions of civil,
commercial and scientific users .
20

GLONASS
GLONASS
▶ Asecond configuration for global positioning is the Global Orbiting
Navigation Satellite System (GLONASS), placed in orbit by the
former Soviet Union, and now maintained by the Russian Republic.
20

GPS and GLONASS Overview
GPS Orbits
▶ The fully operational GPS includes
32 or more active satellites
approximately uniformly dispersed
around six circular orbits with four
or more satellites each.
GLONASS Orbits
▶ GLONASS has 24 satellites, distributed
approximately uniformly in three orbital
planes (as opposed to six for GPS) of
eight satellites each (four for GPS).
21

Contd…
GPS Orbits
▶ The orbits are inclined at an angle
of 55° relative to the equator and
are separated from each other by
60°.
GLONASS Orbits
▶ Each orbital plane has a
nominal inclination of 64.8°
relative to the equator, and the
three orbital planes are
separated from each other by
multiples of 120° right
ascension.
22

Contd…
GPS Orbits
▶ The orbits are non geostationary
and approximately circular, with
radii of 26,560 km and orbital
periods of one-half sidereal day
(≈11.967 h).
GLONASS Orbits
▶ GLONASS orbits have smaller
radii than GPS orbits, about
25,510 km, and a satellite
period of revolution of
approximately 8/17 of a
sidereal day.
23

Contd…
▶ Theoretically, three or more GPS satellites will always be visible
from most points on the earth’s surface.
▶ Four or more GPS satellites can be used to determine an observer’s
position anywhere on the earth’s surface 24 h/day.
24

Contd…
GPS Signals
▶ The GPS system uses Code
division multiplexing of
independent satellite signals.
GLONASS Signals
▶ The GLONASS system uses frequency-
division multiplexing of independent
satellite signals.
25

Contd…
GPS Signals
 Each GPS satellite transmits two
spread spectrum, L-band carrier
signals on two of the legacy L-
band frequencies—an L1 signal
with carrier frequency f1 = 1575.42
MHz and an L2 signal with carrier
frequency f2 = 1227.6 MHz.
GLONASS Signals
▶ Its two carrier signals corresponding to
L1 and L2 have frequencies f1 = (1.602 +
9k/16) GHz and f2 = (1.246 + 7k/16)
GHz, where k = −7, −6, . . . 5, 6 is the
satellite number. These frequencies lie in
two bands at 1.598–1.605 GHz (L1) and
1.242–1.248 GHz (L2).
26

Contd…
GPS Signals
▶ These two frequencies are integral
multiples f1 = 1540f0 and f2 = 1200f0
of a base frequency f0 = 1.023 MHz
▶ The L1 signal from each satellite uses
binary phase-shift keying (BPSK),
modulated by two pseudorandom
noise (PRN).
GLONASS Signals
▶ The L1 code is modulated by a C/A-
code (chip rate = 0.511 MHz) and by a
P-code (chip rate = 5.11 MHz). L2
code is presently modulated only by
the P-code.
27

Contd…
GPS Signals
▶ The initial satellite configuration used
SA with pseudorandom dithering of
the onboard time but this was
discontinued on May 1, 2000.
GLONASS Signals
▶ GLONASS does not use any form of
SA.
28

GPS Satellite Constellation 29
Fig. GPS Satellite Constellation

Contd… 30
Fig. GPS Satellite Constellation Planar Projection

Contd…
Orbits and Satellite Constellation
▶ GPS Satellite Constellation consists of 24 operational satellites in 6 orbital planes.
▶ Orbital period is of 1 sidereal day(11 hr. 58 mins.).
▶ Angle of Inclination is 55 degrees w.r.t equator.
▶ Equally spaced around the equator at a 60 degrees separation. Orbital radius
26,600km.(distance from center of the earth to sat.)
▶ Satellite geometry to provide good observability to users throughout the world
31

Contd…
Orbits and Satellite Constellation
▶ This geometry is measured by a parameter called DoP(Dilution Of Precision).
▶ At least 4 SV’s in each orbit.
▶ Orbital period is 12 hours.
▶ Receives information from CS.
▶ Broadcasts one-way ranging signals.
32

Space Segment Phased Development
▶ The continuing development of the control and space segments has
been phased in over many years, starting in the mid-1970s.
▶ This development started with a concept validation phase and has
progressed to several production phases.
▶ The satellites associated with each phase of development are called a
block of satellites.
33

Satellite Block Development
▶ Five satellite blocks have been developed to date.
▶ The initial concept validation satellites were called Block I.
▶ The last remaining prototype Block I satellite was disposed of in late 1995.
▶ Block II satellites are the initial production satellites, while Block IIA refers to upgraded
production satellites.
▶ Block IIR satellites, denoted as the replenishment satellites.
▶ Modified Block IIR version of satellites is denoted as Block IIR-M.
▶ Block IIF satellites are referred to as the follow-on or sustainment satellites.
▶ Since satellites are launched only as replacements for a satellite failure, their scheduling is
difficult to predict, especially when most satellites have far outlived their design lifetime.
34

Block I, Navigation development
Satellites
▶ Five satellite blocks have been developed to date.
▶ Eleven satellites of this kind were launched between 1978 and 1985.
▶ The SelectiveAvailability (S/A) was not implemented.
▶ They weighed about 845Kg and had a planned average life of 4.5 years,
although some of them lasted up to 10.
▶ They were capable of giving positioning service for 3 or 4 days without
any contact with the Control Centre.
35

Block II and IIA, Operational Satellites
▶ They consist of 28 satellites in total that were launched from 1989 on and
many are still operating.
▶ They weigh about 1500 Kg and have a planned average life of 7.5 years.
▶ Since 1990, an improved version was used, Block IIA (advanced), with
capability of mutual communication.
▶ They are able to supply positioning service for 180 days with no contact
with the control segment.
▶ However, under normal operating mode, they communicate daily.
36

Block IIR, Replacement Operational
Satellites
▶ From 1997, these satellites are being used as spares for Block II.
▶ Block IIR is formed by a set of 20 satellites, although it could be
increased by 6 more.
▶ They weigh about 2000Kg and have a planned average lifespan of 10
years.
▶ These satellites can determine their orbits and compute their own
navigation message autonomously.
37

Block IIR-M, Modernized Satellites
▶ They include a new military signal and the more robust civil signal.
▶ There will be eight satellites in the Block IIR-M series.
▶ The first satellite of this block was launched on September 26, 2005.
38

Block IIF, Follow-on Operational
Satellites
▶ The first satellite (SVN62) was launched on May 28th 2010.
▶ These satellites include the third civil signal on the L5 band.
▶ Their theoretical average life is about 12 years, and they will have inertial
navigation.
39

Block III (GPS III)systems
▶ The new generation of GPS satellites introduces significant
enhancements in navigation capabilities, by improving interoperability.
▶ They provide the fourth civil signal on L1 band.
▶ The first launch is expected as of 2017.
40

Navigation Payload Overview
▶ The navigation payload is responsible for the generation and transmission
of ranging codes and navigation data on the L1, L2 and L5 frequencies to
the user segment.
▶ Control of the navigation payload is taken from reception of the predicted
navigation data and other control data from the CS via the tracking,
telemetry, and control (TT&C) links.
41

Satellite Navigation Payload 42

Contd…
▶ Atomic frequency standards (AFSs) are used as the basis for generating
the extremely stable ranging codes and carrier frequencies transmitted by
the payload.
▶ The navigation data unit (NDU), known as the mission data unit contains
the ranging code generators that generate the C/Acode and P(Y) codes.
43

Contd…
▶ The combined baseband ranging signals are then sent to the L-band
subsystem where they are modulated onto the L-band carrier frequencies
and amplified for transmission to the user.
44

Contd…
▶ The L-band subsystem contains numerous components, including the L1
and L2 transmitters and associated antenna.
▶ The NDU processor also interfaces to the crosslink receiver/transmitter
for intersatellite communication, as well as ranging on Block IIR and
later versions.
▶ This crosslink receiver/transmitter uses a separate antenna and feed
system.
45

Control Segment (Operating Segment)
▶ CS Comprises of 2 Master control stations(MCS), 16 monitoring stations
and 12 ground antennas.
Monitoring Station
and their
▶ Check position, speed, altitude and health of tracked SV’s.
▶ Collects GPS signals, navigation data, atmospheric data
variations
▶ Sends collected information to MCS.
46

Contd…
Master Control Station
▶ Computes space coordinates of SV’s.
▶ Evaluates health of SV’s.
▶ Generates navigation data.
▶ Performs satellite maintenance.
▶ Resolves satellite performances.
▶ Maintains GPS constellation.
▶ Through Ground Antennas, MCS provides commands, keeps control and uploads navigation
messages and other data to the GPS satellites.
47

Contd…
GroundAntennas
▶ Collects, stores data from MCS.
▶ CS provides commands and keeps control on satellites.
▶ Uploads to the GPS satellites using S band signals(2-4 GHz)Computes space coordinates of
SV’s.
48

User Segment(User Receiver Equipment)
▶ The user segment of GPS system
consists of a GPS receiver.
▶ Processes the L band signals
transmitted from satellites to determine
PVT.
▶ GPS receivers fundamentally consist of
3 basic constituents- antenna, GPS
receiver and a controller.
49

GAGAN(GPS aided GEO Augmented
Navigation) Architecture
50
Fig.Architecture of GAGAN

Contd… 51
▶ It is an implementation of regional satellite based augmentation
system(SBAS) jointly developed by ISRO and AAI to provide the best
possible navigational services over Indian FIR (Flight Information Region)
with the capability of expanding to neighboring FIRs.
Ground Segment
▶ Developed to provide accuracy of GNSS receiver
▶ GAGAN consists of 3 basic components- Space segment
and User Segment

Contd… 52
Space segment consists of
(i) 3 GEO satellites
(ii) GPS constellation

Contd… 53
Ground segment consists of
▶ INRES(Indian Reference Stations)
▶ INMCC(Indian Master Control Stations)
▶ INLUS(Indian Navigation land earth uplink stations)

Contd… 54
GAGAN User Segment Consists of
▶ GAGAN-enabled GPS receivers, with the same technology as
WAAS receivers, capable to use the GAGAN Signal-in-Space
(SIS).

Satellites of GAGAN 55
▶ GSAT-8 is an Indian geostationary satellites, which was
successfully launched on 21 May 2011 and is positioned in
geosynchronous orbit at 55 degrees E longitude.
▶ GSAT-10 is augmented to carry 12 Ku Band, 12 C Band and 12
Extended C Band transponders and a GAGAN payload.

Contd… 56
▶ The spacecraft employs power handling capability of around 6kW
with a lift off mass of 3400kg. GSAT-10 was successfully launched
on 29 September 2012.
▶ GSAT-15carries 24 Ku band transponders with India coverage
beam and a GAGAN payload, was successfully launched on 10
November 2015.

Time and GPS
Coordinated Universal Time Generation
▶ Coordinated Universal Time (UTC) is the time scale based on the atomic
second, but occasionally corrected by the insertion of leap seconds, so as to
keep it approximately synchronized with the earth’s rotation.
▶ The leap second adjustments keep UTC within 0.9 s of UT1, which is a
time scale based on the earth’s axial spin.
▶ UT1 is a measure of the true angular orientation of the earth in space.
Because the earth does not spin at exactly a constant rate, UT1 is not a
uniform time scale.
57

Time and GPS
GPS System Time
▶ The timescale to which GPS signals are referenced is referred to as GPS
time.
▶ GPS time is derived from a composite or “paper” clock that consists of all
operational monitor station and satellite atomic clocks.
58

Time and GPS
GPS System Time
▶ Over the long run, it is steered to keep it within about 1 µs of UTC, as
maintained by the master clock at the U.S. Naval Observatory, ignoring the
UTC leap seconds.
▶ At the integer second level, GPS time equaled UTC in 1980. However, due
to the leap seconds that have been inserted into UTC, GPS time was ahead
of UTC by 14 s in February 2006.
59

Time and GPS
Receiver Computation Of UTC
▶ The parameters needed to calculate UTC from GPS time are found in
subframe 4 of the navigation data message.
▶ These data include a notice to the user regarding the scheduled future
or recent past (relative to the navigation message upload) value of
the delta time due to leap seconds, together with the week number
WNLSF(Week number Leap second future) and the day number DN
at the end of which the leap second becomes effective.
60

Time and GPS
Receiver Computation Of UTC
▶ The latter two quantities are known as the effectivity time of
the leap second. “Day 1” is defined as the first day relative to
the end/start of a week and the WNLSF value consists of the
eight least significant bits (LSBs) of the full week number.
61

Satellite Navigation
▶ The GPS is widely used in navigation. Its augmentation with
other space-based satellites is the future of navigation.
Navigation Solution (Two-Dimensional Example)
 Antenna location in two dimensions can be calculated by
using range measurements.
62

Symmetric Solution Using Two
Transmitters on Land.
 In this case, the receiver and
two transmitters are located
in the same plane, as shown
in Fig. with known positions
x1, y1 and x2, y2.
63
Fig. Two transmitters with known two
dimensional positions

Contd…
 Ranges R1 and R2 of two transmitters from the user position are calculated as
R1 = c AT1
R2 = c AT2
Where c = speed of light (0.299792458 m/ns)
AT1 = time taken for the radio wave to travel from transmitter 1 to the user
AT2 = time taken for the radio wave to travel from transmitter 2 to the user
(X , Y )= user position
64

Dilution of Precision
 The accuracy with which a position can be determined using GPS
depends on the accuracy of the individual pseudo range measurements
and on the other hand it also depends up on the GPS satellite
geometry.
 This is termed as the Dilution of Precision(DOP).
 Satellite Geometry can affect the quality of GPS signals and accuracy
of receiver trilateration.
65

Contd…
 There are 5 distinct kinds of DOP.
• GDOP Geometric Dilution of Precision
• PDOP Position Dilution of Precision
• TDOP Time Dilution of Precision
• VDOP Vertical Dilution of Precision
• HDOP Horizontal Dilution of Precision
66

Contd…
 There are 5 distinct kinds of DOP.
• GDOP Geometric Dilution of Precision (Latitude, Longitude, Altitude and
clock)
• PDOP Position Dilution of Precision (Latitude, Longitude, Altitude )
• TDOP Time Dilution of Precision (Clock)
• VDOP Vertical Dilution of Precision (Altitude)
• HDOP Horizontal Dilution of Precision (Latitude and Longitude Positions)
67

Contd… 68

Computation of DOP Values
 As a first step in computing DOP, consider the unit vectors from the receiver
to satellite i
Where,
(x,y,z) represents unknown position of the receiver
(xi,yi,zi) represents known positions of the satellite
69

Computation of DOP Values
 Formulate the matrix, A, which (for 4 pseudo range measurement residual
equations) is
70

Computation of DOP Values
 The first three elements of each row of A are the components of a unit vector
from the receiver to the indicated satellite. The last element of each row
refers to the partial derivative of pseudo range w.r.t. receiver's clock bias.
 Q, as the covariance matrix resulting from the least-squares normal matrix
 elements of Q are
71

Computation of DOP Values 72
 PDOP, TDOP and GDOP are given by
 GDOP is the square root of the diagonal elements of the matrix Q.

Contd… of DOP 73
 DOP gives the geometric orientation of the satellites w.r.t the antennas.
 Values of the DOPs are used for the GPS measurement quality
 Smaller values of DOP gives the better satellite geometry and accurate user
positions, values greater than 5 suggest poor satellite geometry and least
accurate user positions.
 Ideal satellite geometry has one satellite directly above the antenna and
remaining three satellites are spread by 120 degree apart.

Satellite Geometry 74
a) Ideal Satellite Geometry

Satellite Geometry 75
b) Poor Satellite Geometry

User Position Calculations With No Errors
 Position calculation with no errors is given as
ρr = pseudorange (known),
x, y, z = satellite position coordinates (known),
X, Y, Z = user position coordinates (unknown),
where x, y, z, X, Y, Z are in the earth-centered, earth-fixed (ECEF) coordinate system.
76

Contd…
 Squaring both sides yields
 where r equals the radius of earth and Crr is the clock bias correction.
77

Contd…
 The four unknowns are (X, Y, Z, Crr ).
 position (x, y, z) is calculated from ephemeris data.
 For four satellites, the above Eq becomes
78

Contd…
With four unknown state vectors X, Y, Z and Crr
We can rewrite the four equations in matrix form as
79

Contd…
Then we pre multiply both sides of above equation with M-1
80

User Velocity Calculations With No Errors
The equation in this case is given as
81

Contd… 82

Contd…
The above equation is w.r.t one satellite,
Similarly the equation for 3 satellites is given as
83

Contd…
Uv is the User velocity
N is the Matrix
Pseudo range rate
84