01-Oct-19

Introduction to
Global Positioning System (GPS)

Fundamental Problem
 How to know my location precisely ?
– in any condition
– at any time
– everywhere on earth
 How to locate a landmark or target precisely ?
-- Guidance or Navigation

How far or
which route?

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Navigation Types
• Landmark-based Navigation Easily recognizable
– Stones, Trees, Monuments
Limited Local use
• Celestial-based Navigation
– Stars, Moon
Complicated, Works only at Clear Night
• Sensors-based Navigation
– Dead Reckoning
Gyroscope, Accelerometer, Compass, Odometer
Complicated, Errors accumulate quickly
• Radio-based Navigation
– LORAN, OMEGA
Subject to Radio Interference, Jamming, Limited Coverage
• Satellite-based Navigation (GNSS)
Global Coverage with high positioning accuracy

Global Navigation Satellite System (GNSS)

o GNSS refers to a constellation of satellites providing signals from
space transmitting positioning and timing data.
o It provides global coverage.
o GNSS receivers determine location by using the timing and
positioning data encoded in the signals from space.

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Current GNSS Technologies

GPS Global Positioning System USA (1960)
GLONASS Global Navigation Satellite System Russia (1982)
BDS BeiDou Navigation Satellite System China (2000)
Galileo European Union
(2020)
QZSS Quasi-Zenith Satellite System Japan (2018)
DORIS Doppler Orbitography and Radio France (1990)
positioning Integrated by Satellite
GAGAN GPS Aided GEO Augmented Navigation India (2016)

What is GPS?
The Global Positioning System (GPS) is a precise worldwide
radio-navigation system, and consists of a constellation of
satellites and their ground stations, operated and maintained by
the US Department of Defense (DoD)
Using GPS, anywhere on Earth, we can obtain
1. Exact location (longitude, latitude and height
co-ordinates) accurate to within a range of 20
m to approx. 1 mm.
2. Precise time (Universal Time Coordinated, UTC)
accurate to within a range of 60ns to approx. 5ns.

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Use of GPS
GPS receivers may used both by:
 Individuals (e.g. for leisure activities, such as trekking,
balloon flights and cross-country skiing etc.)
 Companies (surveying, determining the time, navigation,
vehicle monitoring etc.)

GPS Satellites and Orbit

o There are currently 28 operational satellites orbiting the Earth at a
height of 20,180 km on 6 different orbital planes.
o Orbits are inclined at 55° to the equator,
– such constellation are designed to ensure at least 4 satellites are
in radio communication with any point on the planet

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Transit Time of GPS Signal

• By comparing arrival time of radio signal from GPS satellites
with the on board clock time the moment the signal was
emitted, it is possible to determine the transit time of that signal
• Transit time helps to calculate the distance of the satellite to the
GPS receiver

Determining a Position in 3-D Space

• If distances to three satellites are known, all possible positions of
GPS receiver (user) are located on the surface of three spheres
whose radii correspond to the distance calculated.
• The position of GPS receiver will be at the point where all three
surfaces of the spheres intersect
Here we assume:
Signal transit time can be precisely
measured Impossible
Velocity is reduced due to
atmospheric interference
since clocks on board all three satellites are
synchronized, transit time measurements for
all is inaccurate by same amount

If transit time is out by just 1μs, a

positional error of 300m is occurred

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Determining a Position in 3-D Space

If time measurement is accompanied by a constant unknown error,
there will be four unknown variables in 3-D space:
o longitude (X)
o latitude (Y)
o height (Z)
o time error (Δt)

So, four satellites are needed to determine

an exact position in 3-D space

GPS Segments
GPS comprises three segments:
• The space segment (all functional
satellites)
• The control segment
 master control station
 monitor stations
 ground control stations
• The user segment (all civil and
military GPS users)

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Space Segments
o consists of satellite constellation and their orbits
o These satellites transmit the following Navigation Messages
to the control segments:
• Satellite time and synchronization signals
• Precise orbital data (ephemeris)
• Time correction information to determine the exact
satellite time

Control Segments
The control segment (Operational Control System) consists of:
• a Master Control Station located in the state of Colorado
• five Monitor Stations equipped with atomic clocks that are
spread around the globe in the vicinity of the equator
• three Ground Control Stations that transmit information to the
satellites.

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Functionalities of Control Segments

most important tasks are:
 Observing movement of satellites and computing orbital
data (ephemeris)
 Monitoring satellite clocks and predicting their behavior
 Synchronizing on board satellite time
 Relaying precise orbital data received from satellites in
communication
 Relaying approximate orbital data of all satellites
(almanac)
 Relaying further information, including satellite health,
clock errors etc.

User Segments
• This is the user with the GPS receiver,
• receiver receives satellite signals and convert them
into altitude (Z), longitude (X) and latitude (Y)
• It is the total of user and supplier community, both civilian
and military.
• consists of all earth-based GPS receivers.
• Receivers vary greatly in size and complexity
GPS receiver components:
o antenna and preamplifier
o radio signal microprocessor
o control and display device
o data-recording unit
o power supply
UBlox GPS
Garmin GPS

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Sources of Errors in GPS measurement

several causes may contribute to the overall error:
• Satellite clocks: although each satellite has four
atomic clocks on board, a time error of just 10 ns
creates an error in the order of 3 m
• Satellite orbits: The position of a satellite is generally
known only to within approximately 1 to 5 m
• Speed of light: the signals from the satellite to the user
travel at the speed of light. This slows down when
traversing the ionosphere and troposphere and can’t
be taken as a constant.

Sources of Errors in GPS Measurement

Measuring signal transit time: The user can
only determine the point in time at which
an incoming satellite signal is received to within
a period of approx. 10-20 ns, which corresponds
to a positional error of 3-6 m. The error
component is increased further still as a result
of terrestrial reflection (multipath).

Satellite geometry: The ability to

determine a position deteriorates
if the four satellites used to take
measurements are close together.
The effect of satellite geometry on
accuracy of measurement is referred
to as GDOP (Geometric Dilution of
Precision)

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Source of Error in GPS measurement

Selective Availability

Position Accuracy and Selective

Availability

Differential GPS
Differential Global Positioning System (DGPS) is an
enhancement to Global Positioning System that provides
improved location accuracy, from the 15m nominal GPS
accuracy to about 1-3 cm in case of the best
implementations.

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How Differential GPS Works

DGPS uses a network of fixed ground-based reference stations to
broadcast the difference between the positions indicated by the GPS satellite
systems and the known fixed positions. These stations broadcast the difference
between the measured satellite pseudoranges and actual (internally computed)
pseudoranges, and receiver stations may correct their pseudoranges by the
same amount. The digital correction signal is typically broadcast locally over
ground-based transmitters of shorter range.

User Reference
Station

GPS Observation
Static Observation
 Antenna is fixed at a point
 Gives higher accuracy since observation is done for
long time period
 A few meters level accuracy
Kinematic Observation
 Antenna is moving
 Just a few or single observation at a particular point
 Accuracy is lower
 Sometimes error is too large (few hundreds of meters)

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