GPS (Global Positioning System)- Definition, Requirement, Principle, Working, Errors Explained

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  • GPS is a satellite navigational system, funded and controlled by the US Department of Defence.
  • GPS is a ground based satellite navigational system that provides location and time information in all weather condition and anywhere on the Earth.
  • It was made by the US Government for their military use and later on it was provided to civilians also. It is a free service for everyone with a GPS Receiver.
  • GPS is backbone for modernising the global air and marine traffic system

Requirement

  • Chp V/ Reg 19.2.1.6 of SOLAS Code, requires all ships irrespective of size to have a receiver for a global navigation satellite system or a terrestrial radio navigation system, or other means, suitable for use at all times throughout the intended voyage to establish and update the ship’s position by automatic means.
  • Global Positioning Satellite System (GPSS) equipments installed on ships to fulfil above SOLAS regulation includes following categories of equipments:
    • Global positioning system (GPS) equipment.
    • Global navigational satellite system (GLONASS) equipment.
    • Differential global position system (DGPS) equipment.
    • Differential global navigation satellite system (DGLONASS) equipment and
    • Combined global position system and global navigation satellite system (GPS/GLONASS) receiver equipment.

Segments of GPS

  • Space Segment;
  • Control Segment or Ground Segment;
  • User Segment

Space Segment

  • It consists of total 24 satellites fixed evenly in SIX orbits. Which means each orbit has FOUR Satellites.
  • Each of these satellites are placed at 60° apart from each other in each orbit.
  • Each orbit is inclined at 55° to the equinoctial. Each Satellite moves at a speed of 3.9 km/sec at a height of 22000 km above Earth Surface.
  • Each satellite makes TWO complete orbits each day, repeating the same ground track each day.
  • At least FIVE Satellites are available for the user at any given time.

Control Segment (Ground Segment/ Control Station)

  • The Control Segment of GPS consists of a global network of ground facilities that track the GPS Satellite, monitor their transmission, performance analysis and send commands and data to the constellation.
  • Control Segment of the GPS consists of:
    • A master control station;
    • An alternative master control station;
    • 16 dedicated Ground based Monitoring stations (6 from Air Force and 10 from NGA);
    • 12 Ground Antennas as Upload Station
GPS-Ground-Segment
GPS Ground Segment Flow Diagran
How Control or Ground Segment works
  • The monitoring stations track the satellite.
  • Obtain data from the satellite and pass that information to Master Control Station.
  • The Master Control Station predict the future path and position of all the satellites.
  • The updated data is fed to the Upload Station which then transmit the same data to each satellite three times a day.
  • The Master Control Station can selectively degrade the satellite data. This degradation is known as Selective Availability, and it can cause position error of up-to 100 meters.

User Segment

  • User Segments are end users like Ships, Air crafts, our mobile phones, cars
  • User Segment consists of:
    • A Receiving Antenna;
    • A Receiver with built-in Computer;
    • A Display Unit
How User Segment works
  • The Receiver locks on the satellite, and from this satellite is obtained the almanac (the time) of all the other satellites and therefor selects the FOUR most suitable satellite for position fixing.
  • The final fix obtained is displayed on the display unit along with the other information like, SOG, COG, UTC, etc.
  • Each Satellite transmit a Navigational message of 30 secs duration using 50 bits/ sec data frame.

Navigational Message

  • Each Satellite transmit a Navigational message of 30 secs duration using 50 bits/ sec data frame.
  • This data which is different for each satellite and is previously supplied to the satellite by Master Control Station and is divided into FIVE Sub-frames
  • The First Sub-frame: contains the data related to satellite clock correction
  • The Second and Third Sub-frame: contains the satellite ephemeris data, which defines the position of the satellite. Ephermeris is a table or data file giving the calculated position of a celestial object at regular interval throughout a period.
  • The Fourth Sub-frame: It is an open gate and passes alphanumeric data to the user and one will be used of Upload Station has a need to pass some specific message or important message.
  • The Fifth Sub-frame: It gives the almanac of all the other satellites which include data on satellite health and identify codes thus allowing the user for optimum choice or the best choice of the satellite for the position fixing.

Position Fixing OR How GPS works?

  • It works on the principle of timing and ranging of satellite
  • The signal transmitted by the satellite travel at a speed of light (3 X 108 m/sec).
  • The receiver locks on to one satellite and, from this satellite it obtain almanac of all other satellites and thereby select the four most suitable satellites for position fixing .
  • By noting the time taken by signal to travel, is measured and distance of satellite from the receiver is calculated.
  • This way each Satellite gives a position sphere which after on intersection on the Earth surface gives position circle. Two of such circles gives 2D, but three, gives a 3D fix.
GPS-2D-3D-Fix-PNG
  • An additional fourth satellite re-assessment is taken by the GPS receiver to account for receiver clock bias and thus correct all satellite “Pseudo Ranges” to True Ranges and make the position free of error.
GPS-Position-Fixing-PNG
Speed DIstance Time Formula

Codes

Each satellite provides two types of codes:

  • P- Code or Precision Code: P-Code is only available for US Military and allies. The accuracy of this code is approx. 10 m.
  • C/A Code or Coarse Acquisition Code: available for all civilian receivers. the accuracy fir this code is approx 100 m

Frequencies

  • L1 Signals = 1575.42 MHz, consists of both C/A and P Codes
  • L2 Signals = 1227.6 MHz, consists of only P-Code

C/A Code

  • C/A Code is different for every satellite;
  • The C/A Code is made up of a sequence called chips;
  • This sequence repeats itself every milliseconds;
  • The C/A Code is for civilians.

P-Code

  • The full code length is 267 days;
  • The P-Code is different for each satellite;
  • Due to the extreme long code length, it is difficult to lock on the P-Code;
  • The P-Code is available only for US and allies.

Pseudo Random Code (PRC)

  • Due to the fact that all the satellite works on same frequencies and each circuit has a unique code so all the satellite can use the same frequency with jamming into each other
  • The PRC is a fundamental part of GPS and has a very complicated digital code in other words complicated sequence of “on” and “off” pulses;
  • The signal is so complicated that it almost looks like a random electrical noise has named Pseudo-random

Function of these codes

  • For Satellite identification, since each satellite has a unique code;
  • For the measurement of propagation time, from satellite to the user.
GPS-Pseudo-Random-Code
Pseudo Random Code (PRC)

Errors of GPS

Clock Errors

a. Satellite Clock Error

  • this is caused due to the error between satellite clock and GPS time. Going to extremely high velocity in space that is 3.9 kilometre per second an error can creep in or move on in automatic clock on satellite.
  • This error is monitored by ground based segment and any error in the satellite clock forms part of the 30 sec navigational message sent by MCS.
  • Clock error can be caused positional area of about 1.5 metres.

b. User Clock Error

  • All the (space vessel) satellites are fitted with automatic clocks. It is not possible to install automatic locks on board the receiver because of high cost and huge size of the clock and if the user clock is not well synchronised with the satellite clock the range measurement will not be accurate and hence called Pseudo Ranges
  • This problem can be solved by using a reassurement satellite that is a fourth satellite.
  • 3 Satellites for 2-D Fix and 4 Satellites for 3-D Fix
  • True Ranges – Pseudo Ranges +/- Clock Error Range

Ionospheric and Tropospheric Delay or Errors

The radio waves transmitted by the satellite passes through different layers of atmosphere. this results into the change into the velocity of waves and hence affect the time and range measurement and fix obtained will not be accurate.

GDOP (Geometric Dilution of Precision)

  • The geometry of the position of satellite determines the angle of cut, which will affect the quality of the position obtained.
  • Wider the range of separation between satellites better the position obtained, and the lower the value of GDOP higher the accuracy of position.
  • The value of GDP is always indicated on GPS display.
GDOP

Multi-path Error

  • This is caused by the satellite signal coming to receiver after being reflected from other obstructions, in addition to reach the receiver directly.
  • Two signals are those received simultaneously causes distortion of signal and in term affect the range measurement.
  • This problem is resolved by suitable siting of Antennas.

 

Deviation of satellite from predicate path

The satellites are monitored by ground segment and their paths are prdicted, but some of satellites may drift from the protective path, results into small position in accuracy.

 

 

Types of Position service in GPS

Precision Positioning Services (PPS)

  • Authorised user has access to PPS
  • Uses the P Codes
  • PPS predictable accuracy is 30 metres

Standard positioning services (SPS )

  • Civil users worldwide uses the SPS
    Uses C/A codes
  • Predicted accuracy of SPS is 35 metres
  • SPS is subject to selective availability, International down graduation of accuracy
  • Now it has been announced by the US Government that the international graduation of accuracy cannot be done.