What is GPS, its errors and details?

GPS FEATURED
Written by Amit Sharma

GPS is the abbreviation of the full form of GLOBAL POSITIONING SYSTEM

The Global Positioning System (GPS) is a space-based  navigation  system  which  consists of 24 navigational satellites in 6 different orbits that provides location and time information in all weather conditions, anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites.

As its name implies

  • It is Global.
  • It gives real time Position.

And it is a System comprising of space segment with its 24 satellites, a control segment based on land and the receiver segment consisting cheap and simple to install and operate sub-systems.

WHAT ARE THE COMPONENTS OF  GPS?

The system consists of three segments

  1. Space Segment
  2. Control Segment
  3. User Segment

SPACE SEGMENT

This segment consists of 24 orbiting satellites in 6 different orbit paths at about 20000 km altitude.Each Satellite has an orbital period of about 12 hrs.At any time at least four satellites are above the user’s horizon at minimum elevation of 9.5 deg.

Following data is transmitted by each satellite over two frequencies

  • Its Identity
  • Its Position.
  • Extremely accurate Time of sending the message which is provided by the atomic clocks fitted on each of the satellites.

CONTROL SEGMENT

The control segment is composed of:

  1. A master control station (MCS),
  2. An alternate master control station,
  3. Four dedicated ground antennas, and
  4. Six dedicated monitor stations.

gps Control SegmentThe satellites are controlled by keeping them on their correct orbital paths and updating them regularly with information so that data transmitted by them are accurate.

The Control Segment is the sole responsibility of the Department of Defense (USA).

USER SEGMENT

gps user Segment

  • The user segment consists of the antenna and the receiver which is fitted on board a ship.
  • There are multiplier supplier of these equipment, who pay license fees to USA for use of positional information broadcast by the GPS satellites.
How position fixing is done by  GPS?

GPS uses the ranging principle to fix the ship.

The GPS concept is based on time and the known position of specialized satellites. The satellites carry very stable atomic clocks that are synchronized to each other and to ground clocks. Any drift from true time maintained on the ground is corrected daily. Likewise, the satellite locations are known with great precision. GPS receivers have clocks as well; however, they are not synchronized with true time, and are less stable. GPS satellites continuously transmit their current time and position. A GPS receiver monitors multiple satellites and solves equations to determine the precise position of the receiver and its deviation from true time. The time difference between data transmitted from the satellite and its reception at the receiver is measured (which is in nano-seconds) and integrated with speed of light (3 X 10 meters per second). i.e. d= s x ∆t

At a minimum, four satellites must be in view of the receiver for it to compute four unknown quantities (three position coordinates and clock deviation from satellite time).

Features of GPS Signals: 

Carriers

The GPS satellites transmit signals on two carrier frequencies.

  1. The L1 carrier is 1575.42 MHz and carries both the status message and a pseudo-random code for timing.
  2. The L2 carrier is 1227.60 MHz and is used for the more precise military pseudo-random code.
What is Pseudo Random Code?

The Pseudo Random Code (PRC) is a fundamental part of GPS. Physically it’s just a very complicated digital code, or in other words, a complicated sequence of “on” and “off” pulses.The signal is so complicated that it almost looks like random electrical noise. Hence the name “Pseudo-Random

There are two types of pseudo-random code

The first pseudo-random code is called the C/A (Coarse Acquisition) code. It modulates the L1 carrier. It repeats every 1023 bits and modulates at a 1 MHz rate. Each satellite has a unique pseudo-random code. The C/A code is the basis for civilian GPS use.

The second pseudo-random code is called the P (Precise) code. It repeats on a seven day cycle and modulates both the L1 and L2 carriers at a 10 MHz rate. This code is intended for military users and can be encrypted. When it’s encrypted it’s called “Y” code.

Since P code is more complicated than C/A it’s more difficult for receivers to acquire. That’s why many military receivers start by acquiring the C/A code first and then move on to P code.

What are Errors  Of  GPS System?
GDOP(Geometric Dilution of Precision)

The idea of Geometric DOP is to state how errors in the measurement will affect the final state estimation.  Geometric Dilution of Precision  Estimate of satellite conditions for a given location & time, Given in distance units (meters or feet), When visible navigation satellites are close together in the sky, the geometry is said to be weak and the DOP value is high; when far apart, the geometry is strong and the DOP value is low. Consider two overlapping ring  of different centres . If they overlap at right angles, the greatest extent of the overlap is much smaller than if they overlap in near parallel. Thus a low DOP value represents a better positional precision due to the wider angular separation between the satellites used to calculate a unit’s position.

Ideal GDOP: One Satellite directly overhead w/an abundance of additional satellites spaced evenly around the sky

Inospheric and  Tropospheric delays of signals.

The ionosphere is that part of the upper atmosphere where free electrons occur in sufficient density to have an appreciable influence on the propagation of radio frequency electromagnetic waves.

Less than 1% of constituents ionized Lower boundary at ~ 60 km Upper boundary at ~ 1000 km,

The ionosphere introduces a variable time delay in the propagation of signals from the satellite to the receiver, which affects positioning.

Satellite clock error:

All satellites contain atomic clocks that control all onboard timing operations, including broadcast signal generation. Although, these clocks are highly stable still they lack of perfect synchronization between the timing of the satellite broadcast signals and GPS system time. Satellite clock correction terms  and time of clock  , account for this lack of synchronization. These will lead to inaccuracy in time measurement

User clock error:

Same as above these are eliminated by using an extra satellite equation.

Deviation  of satellite from their predicted orbit.

Receiver errors:

Errors in the receiver’s measurement of range caused by receiver clock error, thermal noise, software accuracy. These are small and are caused due to internal noise computational error etc

Multi path error:

GPS multipath is caused by the reception of signals arrived not only directly from satellites, but also reflected or diffracted from the local objects. Multipath results in an error in  pseudo range measurements and thus affects the positioning accuracy, since the multipath signal takes a longer path than the direct signal.

OR

Multipath results when the direct path to your receiver is blocked (by  your body,  your house,  roof,  trees,  mountains,  buildings,  etc) and the signal from the satellite is REFLECTED by some object.  The reflecting surface may be:  buildings,  mountains,  the ground,  or any object that happens to be a radio reflector at 1.6Ghz. 

So we can conclude that multipath error is Caused due to poor sitting on the receiver antenna.

Summary of GPS errors

gps error

Factors affecting the accuracy of GPS data
  • Satellite Clock Errors
  • Ephemeris Errors
  • Atmospheric Effects
  • Receiver Errors
  • Operator knowledge and awareness
  • Number of visible satellites
  • Satellite Geometry
  • Multipath

 

Related Searches:

  1. Why two GPS on ship show slight different positions?
  2. Why DR navigation is preferred over GPS navigation in performing search operation at sea?
  3. What is DGPS?
  4. What are features of DGPS (Differential Global Positioning System)?

 

 

About the author

Amit Sharma

Graduated from M.E.R.I. Mumbai (Mumbai University), After a brief sailing founded this website with the idea to bring the maritime education online which must be free and available for all at all times and to find basic solutions that are of extreme importance to a seafarer by our innovative ideas.