55 Global Position System: Different Segments of GPS, its working Principle, Popular Substitute of GPS

Introduction

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:-

1. To provide the location of the user with proper coordinates and elevation.
2. To provide 3-dimensional positioning capability operating in all weather conditions.
3. To offer the potential for different civilian general applications.
4. PPS: This service is used by the US government agencies as well as by the aligned country of us:
   1. The estimated accuracy of the PPS is 22 m & 27.7 m Horizontal & Vertical accuracy respectively.
   2. UTC accuracy of 200 ns time.
5. SPS: this signal service is used the general civilian purposes without any restriction.
   1. The estimated accuracy of the SPS is 100 m & 156 m Horizontal & Vertical accuracy respectively.
   2. 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

1. Space-based segment
2. Control segment
3. User segment

Space-Based 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.

Control Segment

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.

User Segment

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:

1. Surveying
   1. Geodetic control survey
   2. Cadastral survey
2. Aerial/Marine/Land navigation
3. Defense application (Missile guidance)
4. Natural resource management
5. Agriculture
6. Transportation
7. Mapping

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
NAVSTAR	U.S	24	6	20200km	global
GLONASS	Russia	24	3	19100km	global
Galileo	Europe	27	3	36000km	global
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

FPS FTP GBAS GDOP GEO GIC GIM GIS GLONASS GNSS GOTEX GPS GRS HDOP HIRAN HOW lAG IAT IAU ICRF IERS IF IGEB IGEX IGS ILS INMARSAT INS IOC ION IRM IRP ISU ITRF ITS ITU IUGG IVHS IWV JD JPL JPO LAAS LEO LEP LORAN MEDLL MEO MIT MITES MJD MLS MRSE NAD NAGU NANU NASA NAVSTAR NGS NIMA NIS NMEA NNSS NSWC OCS OEM OTF OTR PCMCIA PDD PDOP PLL PPS PRC PRN RAIM RDS RF RINEX RRC RTCM RTK SA SBAS SD SEP SERIES SINEX SLR SNR SPOT SPS SV TACAN TCAR TDOP TEC TLM TOPEX TOW TRF TT TVEC UERE UHF URL USCG USGS USNO UT UTC UTM VDOP VHF VLBI VOR VRS WAAS WADGPS WGS WRC WWW

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) Hand-Over Word International Association of Geodesy International Atomic Time International Astronomical Union IERS (or International) Celestial Reference Frame International Earth Rotation Service Intermediate Frequency 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 Julian Date 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 On-the-Fly On-the-Run PC Memory Card International Association Presidential Decision Directive Position Dilution of Precision Phase Lock Loop Precise Positioning Service Pseudorange Correction Pseudorandom Noise Receiver Autonomous Integrity Monitoring Radio Data System Radio Frequency Receiver Independent Exchange (format) Range Rate Correction Radio Technical Commission for Maritime (services) Real-Time Kinematic Selective Availability Satellite-Based Augmentation System Selective Denial Spherical Error Probable Satellite Emission Range Inferred Earth Surveying Software Independent Exchange (format) Satellite Laser Ranging Signal-to-Noise Ratio Satellite Probatoire d’Observation de la Terre Standard Positioning Service Space Vehicle Tactical Air Navigation Three-Carrier Ambiguity Resolution Time Dilution of Precision Total Electron Content Telemetry (word) (Ocean) Topography Experiment Time-of-Week (count) Terrestrial Reference Frame Terrestrial Time Total Vertical Electron Content User Equivalent Range Error Ultra-High Frequency Uniform Resource Locator U.S. Coast Guard U.S. Geological Survey U.S. Naval Observatory Universal Time 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

Recommended Books for the Detail Study of Global Position System