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Factors responsible for GPS signal errors

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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.

Factors responsible for GPS signal errors

     

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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Posted on: March 11, 2008

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Comments:1
Korfakos
September 28, 2012
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that it would take about 10 minutes to get a liaotcon/signal when turning it on after travelling great distances and found this to be true. I did find the same problem as other people- some of the newer roads were not on the map. But I still never got lost and that was great. You can save your hotel as a favorite address and never lose your way home. It was also great for finding gas stations, amusement parks, restaurants, etc. My 9 year old son loves to watch it and tell me what streets we have to turn on. I always know where I am, how far I have to go to the next turn, how far I have to go in my trip and what time I am expected to be there. Barring any traffic problems, I have found it to be very accurate. I shopped around at various stores testing out similar Garmin and Magellen models and kept coming back to this one. My husband is a skeptic and does not read directions. I showed him how to use this in 10 minutes. Now he uses it for his own trips (when I am not using it of course.) The portability makes it easy to take from one car to another. I bought a medium sized camera bag to store the unit in. I just detach the entire unit from the windshield and put it in the camera bag (still attached to the windshield mount.) This makes it easy to carry in and out of my house (I was told to avoid extreme temperatures to prevent damage to the LCD screen.) It also gives me the ability to put the bag in my trunk when leaving my car while in the city. I have been using this unit daily since I got it for Christmas (4 months ago). I have found the windshield mount to be quite easy, convenient and stable. On only 2 occassions, the unit could not locate an address, one of which was a 10 year old house, and had to use MapQuest as back up. Overall, I think this unit is great and have recommended it to many people. I can't imagine how I did this job without this unit! This is mandatory equipment for anyone who spends a lot of time on the road.
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