Global positioning system (GPS) is also known as Navigation System with Time And Ranging Global Positioning System (NAVSTAR) GPS. It is a GNSS (global satellite-based navigation system) system that covers the entire earth. GPS was originally developed for military purposes, but after some time it is being used by civilians for various purposes such as Marnie navigation, surveying, and car navigation.
The GPS is operated by the US DOD (Department of Defence). There are different signals used by the GPS for different purpose applications, for the military perspective it uses precise positioning services (PPS), and for civilian uses, it uses the standard positioning services (SPS). SPS services can be freely accessed by the general public but on the other hand, the PPS services can be used by authorized government agencies. With the help of GPS satellite-coded signals, the user can make it possible to compute the actual 3-dimensional situation, velocity, and time offset via the receiver clock.
The main objectives behind the development of the GPS are the following:-
- To provide the location of the user with proper coordinates and elevation.
- To provide 3-dimensional positioning capability operating in all weather conditions.
- To offer the potential for different civilian general applications.
- PPS: This service is used by the US government agencies as well as by the aligned country of us:
- The estimated accuracy of the PPS is 22 m & 27.7 m Horizontal & Vertical accuracy respectively.
- UTC accuracy of 200 ns time.
- SPS: this signal service is used the general civilian purposes without any restriction.
- The estimated accuracy of the SPS is 100 m & 156 m Horizontal & Vertical accuracy respectively.
- UTC accuracy of 340 ns time
There are also some other available GNSS systems like GLONASS (an acronym for Globalnaya navigatsionnaya sputnikovaya Sistema) operated by Russian Aerospace defense forces, BeiDou Navigation System (BDS) operated by Chinese satellite navigation system which is operational since 2000, and Galilieo built by the European Union and Europe space agency. There are also some regional space-based navigation systems available such as IRNSS (Indian regional navigation satellite system) operated by the Indian space research organization and QZSS (Quasi-Zenith Satellite System) is a Japanese satellite positioning system composed mainly of satellites in quasi-zenith orbit. Details and comparative looks can be discussed later in this chapter.
Different Segments of GPS
There are three segments included in the working GPS
- Space-based segment
- Control segment
- User segment
It comprises 24 satellites orbiting the earth at approximately 20200km every 12 hours. There are six orbital planes with nominally four satellites present in each orbit.
The segment of space is designed for the objective that there will always be a minimum of at least four satellites visible above 15 degrees cut off/mask at any point on the surface of the earth at any time in any weather condition. Each satellite carries a highly precise atomic clock onboard which operates at a fundamental frequency of 10.23 MHz. these clocks play a major role in generating signals which are broadcasted by the satellite.
Satellite normally generates or broadcast two carrier signals (L1 carrier signal at the frequency of 1575.42 MHz and L2 at the frequency of 1227.60MHz) which are in the L band of wavelength.
These carrier waves have codes modulated upon them. The L1 signals carrier has two codes known as C/A (coarse acquisition) and P (precision) codes. C/A codes are modulated at 1.023 MHz but on the other hand P-codes at 10.23 MHz frequency.
The L2 carrier signals have only one code modulated upon it at 10.23 Mhz.
GPS control segment includes a master control station with 5 monitoring stations. The main responsibility of these stations is to track and control the orbital positions of the satellites.
Control sites are situated in Hawaii, Colorado Springs, Ascension islands, and Diego Garcia Kwajalein. It is very important to calculate the actual orbital position of each satellite to predict its path 24*7.
The signals generated by the GPS satellite onboard sensors are detected by the GPS receiver which later enables the exact position of each of the satellites. Signals from the satellites are recorded by the control stations which estimate the measurement errors. These errors are later transmitted to the master station in Colorado springs. Information from the master control station is resent to the monitoring stations which are later then uploaded to these satellites.
The final segment of GPS is the user segment that is used to receive the modulated signals generated by the satellite. These signals can be used for navigation and other application uses. Following are some applications of GPS:
- Geodetic control survey
- Cadastral survey
- Aerial/Marine/Land navigation
- Defense application (Missile guidance)
- Natural resource management
Working principle of GPS
The signals translated by each satellite to the earth are known as GPS navigation messages. The main aim of these signals is to estimate the current position of the satellites as well as to calculate the transit time of the signal; the user must know these encoded signals. Datastream is transmitted at 50 bits per second. GPS uses a variety of ways to estimate position coordinates, the accuracy of which is determined by the user and the type of GPS utilized. The two-point problem in plane table surveying is equivalent to the basic approach employed by GPS receivers for the immediate determination of locational coordinates.
The essential idea is that if we know the distance between three places to our position, we can calculate our position in relation to those three points. The navigation technique is known as “Trilateration”, which is based on the measurements of the difference in distance between two or more stations (located at known coordinates) that transmit signals (at a known time) (In simple words it is defined as a process of the 3 accurate measurements of the distance from 3 satellite location which is known and computes positions by defining a point in 3-dimensional space).
This produces an endless number of locations that, when plotted, create a hyperbolic curve. To narrow down the specific user position, a second measurement must be performed to a new set of stations, resulting in a second curve, and so on.
Let’s assume an example: a user is lost in the desert and would want to know his exact position using a hand-held GPS receiver. Satellites orbiting high above the earth’s surface and passing over the aforementioned desert constantly relay their locations and clock timings.
Using the signal transit time to satellites, two circles with radii R1 and R2 may be created around the satellites. Each radius indicates the computed distance to the satellite, and all potential distances to the satellites will be located on the circle’s circumference.
The user’s GPS receiver will be located at the exact position where the two circles of interest intersect underneath the satellites. This placement, however, is only applicable on a 2D plane when the goal is to obtain the X and Y coordinates.
In practice, identifying coordinates in 3D space requires the availability of a third satellite. This will simply provide the required user position based on the intersection of all three spheres.
Comparison between different Satellite-Based Navigation Systems
|System Name||Country||No of Satellites||Orbit||Altitude||Type|
|Beidou||China||35||5 in Geostationary orbit 3 in geosynchronous orbit 28||36000km 36000km 21500m||Regional|
|QZSS||Japan||3||3||40000km to 32000km in different orbital planes||regional|
|IRNSS||India||7||3 in geostationary orbit 4 in geosynchronous orbit||36000km 36000km||regional|
Note: – Different Abbreviations used in GPS
|Federal Radionavigation Plan|
File Transfer Protocol
Ground-Based Augmentation System
Geometric Dilution of Precision
Geostationary Orbit (satellite)
GPS Integrity Channel
Global Ionosphere Map
Geographic Information System
Global Navigation Satellite System
Global Navigation Satellite System
Global Orbit Tracking Experiment
Global Positioning System
Geodetic Reference System
Horizontal Dilution of Precision
High Range Navigation (system)
International Association of Geodesy
International Atomic Time
International Astronomical Union
IERS (or International) Celestial Reference Frame
International Earth Rotation Service
Interagency GPS Executive Board
International GLONASS Experiment
International GPS Service (for Geodynamics)
Instrument Landing System
International Maritime Satellite (organization)
Inertial Navigation System
Initial Operational Capability
Institute of Navigation
IERS (or International) Reference Meridian
IERS (or International) Reference Pole
International System of Units
IERS (or International) Terrestrial Reference Frame
Intelligent Transportation System
International Telecommunication Union
International Union for Geodesy and Geophysics
Intelligent Vehicle/Highway System
Integrated Water Vapor
Jet Propulsion Laboratory
Joint Program Office
Local Area Augmentation System
Low Earth Orbit (satellite)
Linear Error Probable
Long-Range Navigation (system)
Multipath Estimating Delay Lock Loop
Mean Earth Orbit (satellite)
Massachusetts Institute of Technology
Miniature Interferometer Terminals for Earth Surveying
Modified Julian Date
Microwave Landing System
Mean Radial Spherical Error
North American Datum
Notice Advisories to GLONASS Users
Notice Advisories to Navstar Users
National Aeronautics and Space Administration
Navigation System with Timing and Ranging
National Geodetic Survey
National Imagery and Mapping Agency
Navigation Information Service
National Marine Electronics Association
Navy Navigation Satellite System (or TRANSIT)
Naval Surface Warfare Center
Operational Control System
Original Equipment Manufacturer
PC Memory Card International Association
Presidential Decision Directive
Position Dilution of Precision
Phase Lock Loop
Precise Positioning Service
Receiver Autonomous Integrity Monitoring
Radio Data System
Receiver Independent Exchange (format)
Range Rate Correction
Radio Technical Commission for Maritime (services)
Satellite-Based Augmentation System
Spherical Error Probable
Satellite Emission Range Inferred Earth Surveying
Software Independent Exchange (format)
Satellite Laser Ranging
Satellite Probatoire d’Observation de la Terre
Standard Positioning Service
Tactical Air Navigation
Three-Carrier Ambiguity Resolution
Time Dilution of Precision
Total Electron Content
(Ocean) Topography Experiment
Terrestrial Reference Frame
Total Vertical Electron Content
User Equivalent Range Error
Uniform Resource Locator
U.S. Coast Guard
U.S. Geological Survey
U.S. Naval Observatory
Universal Time Coordinated
Universal Transverse Mercator (projection)
Vertical Dilution of Precision
Very High Frequency
Very Long Baseline Interferometry
VHF Omnidirectional Range (equipment)
Virtual Reference Station
Wide Area Augmentation System
Wide Area Differential GPS
World Geodetic System
World Radio Conference
World Wide Web