Global Positioning System (GPS) is a satellite-based navigation system that provides accurate location and time information to users around the world. GPS was originally developed by the U.S. military but is now used widely in civilian applications like cell phones and automotive navigation. The GPS network consists of 24 satellites in medium Earth orbit that transmit signals used to calculate a receiver's position via triangulation. The satellites circle the globe twice a day and their positions are carefully monitored to ensure accurate location data for GPS users anywhere on Earth.

1. Global Positioning System

2. What is GPS
Global Positioning System (GPS) is a satellite based navigation system that can provide people
who use it with their exact position on Earth, tell them how to get to another location, how fast
they are moving, where they have been, how far they have gone, what time it is.
GPS was originally designed to help the U.S. military with finding the accurate location of their
soldiers, vehicles, planes and ships around the world. Now, GPS is used in cellular phones,
navigation and map making.

3. GPS Segment
GPS: Theory, Practice and Applications
Frederic G. Snider, R.P.G.

4. Space segments
GPS satellites fly in medium Earth orbit (MEO) at an altitude of approximately 20,200 km (12,550 miles).
Each satellite circles the Earth twice a day.
The satellites in the GPS constellation are arranged into six equally-spaced orbital planes surrounding the
Earth. Each plane contains four "slots" occupied by baseline satellites. This 24-slot arrangement ensures
users can view at least four satellites from virtually any point on the planet.

5. The control segment
• The control segment of the GPS system consists of a worldwide network of
tracking stations.
• The master control station (MCS) located in the United States at Colorado
Springs, Colorado.
• The primary task of the operational control segment is tracking the GPS
satellites in order to determine and predict satellite locations, system integrity,
behavior of the satellite atomic clocks, atmospheric data, the satellite almanac,
and other considerations.
The User segment
• The user segment includes all military and civilian users. With a GPS receiver
connected to a GPS antenna, a user can receive the GPS signals, which can be
used to determine his or her position anywhere in the world. GPS is currently
available to all users worldwide at no direct charge.

6. • How it work?
When a GPS receiver is first turned on, it downloads orbit information from all the satellites
called an almanac.
Once this information is downloaded, it is stored in the receiver’s memory for future use.
The GPS receiver calculates the distance from each satellite to the receiver by using the
distance formula: distance = velocity x time.
The receiver determines position by using triangulation.
When it receives signals from at least three satellites the receiver should be able to calculate its
approximate position (a 2D position).
The receiver needs at least four or more satellites to calculate a more accurate 3D position. The
position can be reported in latitude/longitude.

7. GPS signal structure
The two GPS codes are;-
1. Coarse acquisition (or C/A-code)
2. Precision (or P-code).
3. The C/A-code is modulated onto the L1 carrier only, while the P-code is
modulated onto both the L1 and the L2 carriers. This modulation is called
biphase modulation, because the carrier phase is shifted by 180° when the code
value changes from zero to one or from one to zero

8. Source of GPS error
• Satellite clock errors: Caused by slight discrepancies in each satellite’s four atomic
clocks. Errors are monitored and corrected by the Master Control Station.
• Orbit errors:Satellite orbits vary due to gravitational pull and solar pressure
fluctuations. Orbit errors are also monitored and corrected by the Master Control
Station.
• Ionospheric interference: The ionosphere is the layer of the atmosphere from 50 to
500 km altitude that consists primarily of ionized air. Ionospheric interference
causes the GPS satellite radio signals to be refracted as they pass through the
earth’s atmosphere – causing the signals to slow down or speed up.

9. Application
• GPS instruments have been deployed to measure plate motions, volcanoes, and earthquakes.
• GPS aids us in better understanding our planet by allowing us to measure how the surface of the Earth
moves. Much of this motion causes earthquakes, builds mountains, and, indirectly, creates volcanoes.
• InSAR—Satellite-based technique captures overall deformation "picture“InSAR (Interferometric Synthetic
Aperture Radar) is a technique for mapping ground deformation using radar images of the Earth's surface
that are collected from orbiting satellites.
• Snow depth is estimated through a calculation of the relative change of the effective multipath
reflector height with respect to a snow free surface.
• topographic analysis using Geographic Information System (GIS) techniques has helped determine suitable
locations to install monitoring instrument stations.
• Light Detection and Ranging technology uses laser scanners to measure the elevation of the ground surface

10. References
• Gps spotlite https://spotlight.unavco.org/how-gps-works/how-gps-works.html
• https://spotlight.unavco.org/how-gps-works/gps-basics/gps-basics.html
• https://spotlight.unavco.org/how-gps-works/gps-and-tectonics/gps-and-tectonics.html
• https://spotlight.unavco.org/how-gps-works/gps-and-the-water-cycle/gps-and-the-water-cycle.html
• https://spotlight.unavco.org/how-gps-works/gps-and-the-water-cycle/gps-snow-depth.html
• https://spotlight.unavco.org/how-gps-works/gps-and-the-water-cycle/gps-soil-moisture.html
• GLOBAL POSITIONING SYSTEM AND IT'S WIDE APPLICATIONS (PDF) GLOBAL POSITIONING SYSTEM AND IT'S WIDE
APPLICATIONS (researchgate.net), DOI:10.5707/cjit.2015.9.1.22.32
• Introduction to Global Positioning System
• Global Positioning System (GPS) Technology for Community Supervision: Lessons Learned, Tracy M. L. Brown ; Steven A. McCabe ;
Charles Wellford
• GPS: The Global Positioning System A global public service brought to you by the U.S. government https://www.gps.gov/
• MOHINDER S. GREWAL LAWRENCE R. WEILL ANGUS P. ANDREWS GLOBAL POSITIONING SYSTEMS, INERTIAL
NAVIGATION, AND INTEGRATION pg no. 10-65
• Application of Global Positioning System (GPS) in Earth Sciences teaching https://www.researchgate.net/institution/Southern-Taiwan-
University-of-Science-and-Technology
• Introduction to GPS The Global Positioning System Ahmed El-Rabbany pg no 4-30

11. Thank you