The GPS receiver requires the current time, the orbital position of the satellite and a measurement of signal delay. Calculating accurate co-ordinates of an object primarily depends on three data. To measure the signal delay, the receiver prepares an identical sequence of the Coarse-acquisition (C/A) code and compares it with the sequence received from the satellite. The changes in bit transition can be calculated an approx. of 10 nanoseconds for the code. As the signal travels at a speed equal to the light, the net error can be taken as 3 meters.
However, there are several factors that affect directly to the accuracy of the GPS signal and some of them are given bellow.
Atmospheric effect – Any change in atmospheric condition can affect GPS radio signals as signals travel through the thick layer of the atmosphere. Both ionosphere and troposphere can leave little or more impact on the speed of the radio signals. Dual frequency measurement or comparing the two frequencies L1 and L2 can reduce the error and also define a precise location of the receiver.
Multipath effects – This occurs when signals are get reflected or bounced by coming in contact with surrounding hills, buildings or any kind of hard core object before it reaches the receiver. Thus, increase in travel time of signals cause error. However, with the help of the technique named narrow collector spacing the receiver recognizes the long delay signals and does not take into account while calculating the precise time and location. Also there are some specialized antennas are used to measure the exact data from the signals that are slightly deviated or delayed. But it’s a bit difficult as the fluctuation is very little in atmospheric delay.
Receiver clock error – The receiver enabled with a clock that is not as accurate as the atomic clock in satellite and so produce a little timing error.
Orbital error – This occurs when the satellite’s orbital location is calculated wrongly. As the right location of the satellites position in the orbit is essential, even a small error can leave a large difference as far as accuracy is concerned.
Satellite geometry - It is the exact position of the satellite at a given period of time. Ideally when satellites are located in wide angles from each other, it becomes easy to measure distance. However, poor satellite geometry occurs when they are in a line or in a tight grouping.
Artificial degradation of the satellite signal – The Department of Defense of US recently restricted certain highly accurate GPS signals (military related) from public use as these can be misused and so intentionally degraded. This is called Selective Availability (SA) in which kinds of intentional errors like creating noise in the satellite clock or transmitting some false orbital data to the satellite that can come back to the receiver at the ground. However, receivers of Defense Department have the capability of rectifying those errors.
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