What is GPS?; Definition of GPS; GPS; Global Positioning System; How GPS works?; Working principle-GPS; One way ranging; Determining the position; The clock problem; PRC amplification; Pseudo-range; Different types of GPS locations; NAVSTAR System; Accuracy of GPS; Applications of GPS; Advantages of One-Way Ranging; Pseudo Random Code; PRC;

1. Prof. Samirsinh P Parmar
Asst. Prof. Dept of Civil Engg., Faculty of Technology,
Dharmsinh Desai University, Nadiad, Gujarat, India.
GIS for Earth Scientists

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 What is GPS
 How GPS Works
 One way ranging
 Determining Position
 The clock problem
 PRC amplification
 Pseudo-Range
 Different types GPS locations
 Where to Get Differential Corrections
 Future of GPS

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Definition- GPS
• GPS (Global Positioning System) is a remarkable technology that
enables precise location determination using signals from satellites
orbiting Earth.

4. What is
GPS?
 The original intent of the Global Positioning
System was to develop an all-weather, 24-
hour, truly global navigation system to support
the positioning requirements for the armed
forces of the U.S. and its allies.
 First satellite launched in 1978
 The total investment by the U.S. military in the
GPS system to date is well over $10
BILLION!
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5. What is
GPS?
Although the primary goal is to provide
positioning capabilities to the U.S. armed
forces and its allies, GPS is freely
available to all users.
The number of civilian users is already
far greater than the military users, and
the applications are growing rapidly.
The U.S. military however still operates
several "levers" with which they control
the performance of GPS.
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6. How GPS
Works:
 GPS relies on a network of 24 to 32 satellites orbiting the
planet.
 Each satellite broadcasts signals containing information about
its position and the current time.
 A GPS receiver on the ground picks up signals from multiple
satellites.
 By analyzing the time it takes for signals to travel from
satellites to the receiver, the GPS device calculates the
distance to each satellite.
 Using trilateration, the receiver determines its precise location
based on the distances from at least three satellites.
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7. Components
of GPS:
• Satellites: The heart of GPS, these satellites
transmit signals.
• Ground Control Stations: Monitor satellite health
and adjust their orbits.
• User Receivers: Devices (like smartphones or
navigation systems) that process satellite signals.
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8. How GPS
Works
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One Clock
Two Clocks
Consider two ways of determining ranges:

9. Advantages
of One-Way
Ranging
Receiver doesn’t have to generate signal, which
means
 We can build inexpensive portable receivers
 Receiver cannot be located (targeted)
 Receiver cannot be charged
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10. Determining
Range
(Distance)
 Measure time it takes for radio signal to
reach receiver, use speed of light to convert
to distance.
 This requires
 Very good clocks
 Precise location of the satellite
 Signal processing over background
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11. Determining
Position
 To determine position in 3-D, we need 3 satellites for
triangulation
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Once we have position narrowed to 2 possible points, we
can usually throw one away as “nonsense”

12. The Clock
Problem
 To measure distance from speed of light we need a VERY
accurate clock (clock error of 1/100 sec = distance error of
1820 miles!).
 GPS Satellites have very accurate atomic clocks.
 Our receivers do not have atomic clocks, so how can we
measure time with necessary accuracy?
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13. Psuedo-
Random
Code
 GPS satellites and receivers communicate
via pseudo-random-code (PRC) signals.
 PRC has three advantages:
1. Enhances signal over background
2. Allows synchronization of satellite and receiver
clocks
3. Military can change the code and switch of
system if necessary
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14. PRC Signal
Amplification
Uses correlation of
peaks between
generated random
signal and truly
random background
noise to enhance
signal
Allows receiver to
work without a big
satellite dish!
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15. PRC
Synchronization
GPS receiver
generates the same
PRC as satellite, i.e.
they start “counting” at
the same time.
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• By determining how far off the satellite and
receiver are in their counting, determines
difference in time it took for signal to reach
receiver.

16. PRC
Synchronization
How do we assure
satellite and receiver
start counting at same
time, i.e. clocks are
synchronized?
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• The trick is to use a 4th satellite to over-
specify position.
• This allow timing to be corrected by the
receiver

17. Pseudo-
Range (2-D
Example)
 If clocks were
perfect, 2 satellites
would locate
position
 If clocks are off,
range is off
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18. Pseudo-
Range
 We add a 3rd satellite
to over-specify position
 There is only one
combination of “wrong”
times for which all 3
ranges converge.
 Receiver varies clock
times until all satellites
agree
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19. Satellite
Position
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• Must know position of satellite to determine
receiver location
• Satellites are put in precise orbit
• Satellite's orbit or "ephemeris“ is monitored by
DOD and transmitted to satellite

20. Atmospheric
Correction
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• Must correct for atmospheric effects with modeling
GPS signal slowed down through the charged particles of the
ionosphere and then through the water vapor in the troposphere

21. GPS
Constellation
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Must have at least 4 satellites overhead to determine position

22. NAVSTAR
Current GPS System is NAVSTAR.
There are 4 GPS satellite constellations
in existence:
 Block I satellites were the experimental satellites
launched between 1978 and 1985 used to test the
system. Eleven (11) were launched, none
functioning.
 Block II satellites comprise the first nine spacecraft
of the operational series.
 The Block IIA satellites comprise the second 19
spacecraft of the operational series.
 The Block IIR satellites comprise the replacement
series.
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23. Where are
the
Satellites?
Orbit is high enough
to avoid earth gravity
perturbations, low
enough to pass
correction stations 1
per day.
Orbital period of GPS
satellites is ~12 hours
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ellipse
orbital
of
axis
major
-
semi
a
e
a
GM
T
:
Law
Third
s
Kepler'



3
2

24. Satellite
Overhead
Schedule
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25. Orbital
Period and
Altitude
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Orbital period and satellite altitude.
Semi-major axis
a ( km )
Altitude
( km )
Period
( min )
Comment
6700
7200
10600
12800
16800
26600
42300
300
800
4200
6400
10400
20200
35900
90
100
180
240
360
718
1436*
remote sensing satellites
GPS satellites
geostationary satellites
* length of a sidereal day
http://liftoff.msfc.nasa.gov/RealTime/JTrack/3D/JTrack3D.html

26. Accuracy :
• Standard GPS: Accurate to within a few meters.
• Differential GPS (DGPS): Corrects errors,
achieving sub-meter accuracy.
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27. GPS
Accuracy
Between 1st and 3rd May 2000, the National Geodetic
Survey/NOAA compared the accuracy of GPS determined
navigation positions at its Continuous Reference Station
with and without Selective Availability.
 With SA turned on, 95% of solutions were within a radius of 45
meters
 With SA turned off, 95% of the estimated (horizontal) positions
were within 6.3 meters.
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"The decision to discontinue Selective Availability is the latest measure in an ongoing
effort to make GPS more responsive to civil and commercial users worldwide…This
increase in accuracy will allow new GPS applications to emerge and continue to
enhance the lives of people around the world.“
President Bill Clinton, May 1, 2000

28. Accuracy
of GPS
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Autonomous Accuracy 15 - 100 meters
Differential GPS
(DGPS)
Accuracy 0.5 - 5 meters
Real-Time
Kinematic Float
(RTK Float)
Accuracy 20cm - 1 meter
Real-Time
Kinematic Fixed
(RTK Fixed)
Accuracy 1cm - 5 cm

29. GPS
Signals
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GPS satellites broadcast on three different frequencies, and each
frequency (or career wave) has some information or codes on it. You
can think of it as three different radio stations broadcasting several
different programs. The table below lists the signals and the contents:
L1 Career L2 Career L3 Career
19 cm wavelength 24 cm wavelength
Data not available
1575.42 M Hz 1227.6 M Hz
C/A Code P Code
Navigation Navigation Message
•P Code : Reserved for direct use only by the military
•C/A Code : Used for rougher positioning
•For Single frequency use only L1 career is used
•For Double frequency, L1/L2/L3 career is used
•The navigation message (usually referred to as the ephemeris) tells
us where the satellites are located, in WGS-84.

30. Different
types GPS
locations
 Autonomous Positions
(C/A signal, 5-15 m accuracy)
 Real-Time Differential GPS
(C/A signal, 0.5-5 )
 Real-Time Kinematic (RTK) Float
(C/A and Carrier, 0.2-1 m)
 Real-Time Kinematic (RTK) Fixed
(C/A and Carrier, 1-5 cm)
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31. Differential
GPS
(DGPS)
 Error due to signal transmission through the
atmosphere can be corrected using DGPS
 Atmospheric errors are the same over short
distances.
 Error in base station, can be removed from remote
(roving) receiver position, and code phase signal.
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32. Code vs.
Carrier
Phase
 Satellites generate Code Phase and Carrier Phase
signals.
 Code phase is used by hand-held GPS
 Carrier phase used by surveying instruments,
navigational systems
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33. Where to
Get
Differential
Corrections
 The United States Coast Guard and other
international agencies are establishing reference
stations all over especially around popular harbors
and waterways.
 Anyone in the area can receive these corrections and
radically improve the accuracy of their GPS
measurements. Most ships already have radios
capable of tuning the direction finding beacons, so
adding DGPS will be quite easy.
 Many new GPS receivers are being designed to
accept corrections, and some are even equipped with
built-in radio receivers.
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34. Differential
Code GPS
(Navigation)
Differential corrections may be used in
real-time or later, with post-processing
techniques.
Real-time corrections can be transmitted
by radio link. The U.S. Coast Guard
maintains a network of differential
monitors and transmits DGPS
corrections over radio beacons covering
much of the U.S. coastline.
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35. RTK
(Differential
Carrier
GPS)
 RTK is based on using many (~5 satellites) to
resolve timing.
 Produces very accurate measurements because
using carrier phase.
 Requires advance tracking of satellites, and better
signal resolution (bigger antennae and more
power)
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36. Future of
GPS
 Soon, the U.S. Federal Communications
Commission will require location determination
technology in cellular phones for use in
emergencies as part of their enhanced 911
service.
 Future plans for improving the accuracy of
GPS include the launching of eighteen
additional satellites that are awaiting launch or
are currently in production.
 Two new signals will be broadcast from the
satellites by 2005, to help bypass any
distortion from the ionosphere.
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37. Challenges:
 Signal Obstruction: Tall buildings, dense forests,
or tunnels can weaken signals.
 Multipath Interference: Signals bouncing off
surfaces cause inaccuracies.
 Selective Availability (SA): Previously intentional
degradation by the U.S. military (now turned off).
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38. Applications
of GPS
1.Navigation:
1. GPS is crucial for navigation. Whether you’re driving, hiking, or
sailing, GPS devices guide you to your destination by providing
accurate location information.
2.Mapping and Surveying:
1. Surveyors use GPS technology to create detailed maps. GPS
devices allow precise marking of points of interest and
recording travel routes.
3.Recreation:
1. Geocaching: A fun activity where participants use GPS
coordinates to find hidden caches.
2. Outdoor Sports: Hikers, cyclists, and runners use GPS for
tracking routes and measuring performance.
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39. Applications
of GPS
4. Asset Tracking:
Businesses attach GPS trackers to assets like vehicles, containers,
or personnel. These trackers monitor speed, direction, and safety.
Techniques like geofencing create virtual barriers.
5. Commercial Applications:
1. Agriculture: GPS helps manage operations by collecting data on
soil composition, weather conditions, and guiding farmers in the
fields.
2. Logistics and Transportation: Logistics companies use GPS for
accurate movement management and real-time traffic
information.
3. Mining: GPS tracks workers, equipment, minerals, and haul
trucks, improving productivity.
4. Utilities: Utility companies use GPS for network maintenance,
outage response, and repair locations.
6. Precise Timing:
4. GPS provides highly accurate timing for applications like traffic
signal synchronization, cell phone base station coordination, and
more 1
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40. Special
Thanks to
IIRS,
Dehradun
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For an expert lecture,
contact me on
spp.cl@ddu.ac.in