Apr 10 2010
GPS signal and Errors
A GPS signal comprises of about three different bit of information; pseudorandom code, almanac data and ephemeris data. The pseudorandom code is nothing but a ID that recognizes which satellites transmits data. The ID code or satellite number could be viewed on the satellite page of the GPS unit. The Ephemeris data is very essential to determine the current satellite position.
It is transmitted regularly with significant information like current time, current data and healthy or unhealthy state of the satellite. The almanac data carries orbital information of all the satellites orbiting around.
GPS satellites transmits low power radio signals to two levels L1 and L2. GPS used by the civilians utilizes 1575.42 MHz frequency in L1 of the UHF band.
Errors on GPS Signal
Sometimes our GPS devices guides us to a wrong path. Have you ever thought of the reason behind. The GPS signal from the satellites before reaching the GPS receiver, gets degraded, affecting it accuracy. There are about 4 important factors causing GPS errors:
* Troposphere and Ionosphere Delays
The satellite signal as it passes through the earth layers viz Ionosphere and Troposphere, slows down a bit. This time delay may cause error in calculating the exact time and position. To overcome the problem, the GPS system employs a built in model that estimates an average delay time in order to correct the error partially.
*Orbital Error
The orbital errors are caused due to the inaccuracies of the reporting location of satellite. They can be also known as ephemeris errors.
* Signal Multipath
Tall buildings and large rock surfaces reflects off the GPS signal, increasing the travel time to reach the receiver. Since the time reaching the GPS receiver is delayed, the signal causes error.
*Receiver clock error
The receiver clocks on the GPS receivers are not as accurate as the atomic clocks on the GPS satellites. Hence timing errors may occur slightly.
One of the other factors that may lead to inaccuracy of data is visibility of the satellites. More the GPS receiver can see satellites, more accurate the calculations are. The visibility of the satellites cannot reach underwater, indoors and underground hence the GPS device do not work at these places.
The errors caused are inevitable, the only solution to make our sat nav devices accurate is to estimate the delay time appropriately so the the occurrence of error is minimized.
