twistedenginestarter,
GPS has a number of errors, civil aviation units are known to have the following errors
- Ephemeris error 4m ( GPS satellite not where it should be)
- Clock errors 2m
- Receiver noise 1m
- Ionosphere 8m
- Troposphere 3m
- Selected availability 32m
The RMS average of the above is about 33m, along with the PDOP will give you a position of close to 100 m only
95% of the time.
The atmospheric effects are predicted and corrected to the GPS position, as the atmosphere is anywhere between 130-190 km the amount of slow down will vary. Military GPS gets rid of this error by using two frequencies from each satellite.
You may have more than 12 sats in view, but the GPS receiver will drop out any sats that are less than 7.5 degrees above the horizon (a TSO C129 standard).
The selected availability which has been turned off now used to make the sats transmit a false position so the military of non US alliance members could not use the GPS against the US. The still change the clock frequency which alters the time output in the GPS signal which give a false position.
PDOP is position dilution of precision, ie if you have sats low on the horizon PDOP is high, if the sats are high in the sky PDOP is low.
BTW with baro aiding only 3 sats are required to get a 3d fix, the baro aiding gives the gps unit a psudo sat and solves one of the variables in the four x/y/z/time equations.
With RAIM the GPS unit compares the positions of multiple sat sets and gets the best position and discards the problem ones.
Jamming is unlikely, the US military thought this through very well, the GPS signals are transmitted at a very low power, and actually use the earths background noise for amplification. This makes it near impossible to jam. High power signals are easy to jam i.e radar, low power very hard.
As 2Donkeys said problems occur when the data you put in is crap, you get crap out. The crap in this case is the waypoint positions. GPS receivers normally use the WGS84 datum, if your waypoints are in another datum, the GPS will take you to the wrong spot, ie the position you entered but on the WGS85 datum.
This is still a problem today, I understand that Jepp have just issued a notam in the current database cycle that they have removed a number of airports, and waypoints in Mexico as the Mexican aviation authority are in the process of changing datum’s but is not consistent throughout an approach or departure.
GPS tracks are normally more accurate than that displayed on charts or conventional aids. Ground based aids need to be surveyed all the time to get the magnetic variation to apply to the true tracks. Most countries do this only every 2-5 years which can lead to errors particularly with enroute vors. The GPS calculates the earths magnetic field for the latitude, longitude, and time and comes up with a variation which is statistically within 200nT of the real value.
Because of all these errors, the GPS units will not become a sole means navigational equipment because it cannot meet the accuracy, integrity, availability and continuity of service without some form of WAAS.
[This message has been edited by Zeke (edited 06 July 2001).]