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 system 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 system 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 the 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 actual 3-dimensional situation, velocity, and time offset via receiver clock.

The main objectives behind the development of the GPS system are following:-

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

Basically, there are three segments included in the working GPS

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

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.

GPS 2

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-code at 10.23 MHz frequency.

The L2 carrier signals have only one code modulated upon it at 10.23 Mhz.

GPS 3
Figure: General representation of different modulated codes used by GPS at different wave frequencies.

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, 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.

GPS 4

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. Defence application (Missile guidance)
  4. Natural resource management
  5. Agriculture
  6. Transportation
  7. Mapping
GPS 5

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; it is very important that the user knows 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 in relation to our own position, we can calculate our own position in relation to those three points. The navigation technique is known as “Trilateration”, which is based on the measurements of difference in distance to 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).

Principle of GPS

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 own locations and clock timings.

Using the signal transit time to satellites, two circles with radius 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.

Principle of GPS 1

Comparison between different Satellite-Based Navigation Systems

System NameCountryNo of SatellitesOrbitAltitudeType
NAVSTARU.S24620200kmglobal
GLONASSRussia24319100kmglobal
GalileoEurope27336000kmglobal
BeidouChina355 in Geostationary orbit 3 in geosynchronous orbit 28 36000km   36000km   21500m Regional
QZSSJapan3340000km to 32000km in different orbital planesregional
IRNSSIndia73 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

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