Errors Sources

 The final position of the survey station is influenced by : 

(1) The error in the range measurement 

(2) The satellite receiver geometry 

(3) The accuracy of the satellite ephemerides 

(4) The effect of atmospheric refraction 

(5) The multipath environment at the receivers 

(6) The quantify and quality of satellite data collected 

(7) The connections between the observed GPS network and the existing control 

(8) The processing software used 

It is necessary therefore to consider the various errors involved some of which have already been mentioned 

The majority of the error sources are eliminated or substantially reduced if relative positioning is used rather than single point positioning 

This fact is common to many aspects of surveying 

For instance in simple leveling it is generally the difference in elevation between points that is required 

Therefore if we consider two points A and B whose heights Ha and Hb were obtained by measurements from the same point which had an error H in its assumed height then :

with the result that H is differenced out and difference in height is much more accurate than the individual heights Thus if the absolute position of point 

A fixed by GPS was 10 m in error the same would apply to point B but their relative position would be comparatively error free 

Then knowing the actual coordinates of A and applying the computed difference in position between A and B would bring B to its correct relative position 

This should be borne in mind when examing the error sources in GPS 

Receiver clock error 

This error is a result of the receiver clock not being compatible and in the same time system as the satellite clock 

Range measurement ( pseudo range ) is thus contaminated 

As the speed of light is approximately 300 000 km s then an error of 0.001 s results in a range error of about 3000 km 

As already shown this error can be evaluated using four satellite or cancelled using differencing software 

Satellite clock error 

Excessive temperature variations in the satellite may result in variation of the satellite clock from GPS time 

Careful monitoring allows the amount of drift to be assessed and included in the broadcast message and therefore substantially eliminated if the user is using the data 

Differentail procedures eliminate this error 

Satellite ephemeris error 

Orbital data has already been discussed in detail with reference to Broadcast and Precise Ephemeris 

Errors are still present and influence baseline measurement in the ratio :

The specification for GPS is that orbital errors should not exceed 3.7 m but this is always possible 

Error in the range of 10-20 m may occur using the Broadcast Ephemeris Thus for an orbital error of 10 m on a 10 km baseline would be 5 mm 

This error is substantially eliminated over moderate length baselines using differential techniques