GPS Observing Methods

The use of GPS for positioning to varying degrees of accuracy in situations ranging from dynamic ( navigation ) to static ( control networks ) has resulted in a wide variety of different field procedures using one or other of the basic observables 

Generally pseudo range measurements are used for navigation whilst the higher precision necessary in engineering surveys requires carrier frequency phase measurements 

The basic point positioning method used in navigation gives the X , Y ,  Z position to an accuracy of better than 20 m by observation to four satellites 

However the introduction of Selective Availability ( SA ) see below degraded this accuracy to 100 m or more and so led to the development of the more accurate differential technique 

In this technique the vector between two receivers ( baseline ) is obtained i.e the difference in coordinates ( X , Y , Z ) 

If one of the receivers is set up over a fixed station whose coordinates are known then comparison with the observed coordinates enables the differences to be transmitted as corrections to the second receiver ( rover )

In this way all the various GPS errors are lumped together in a single correction 

At its simplest the corrections transmitted could be in a simple coordinate format i.e X , Y , Z which are easy to apply 

Alternatively the difference in coordinate position of the fixed station may be used to derive corrections to its own observations before computing its position 

The fundamental assumption in Differential GPS ( DGPS ) is that the errors within the area of survey would be identical 

This assumption is acceptable for most engineering surveying where the areas involved are small compared with the distance to the satellites 

Where the area of survey becomes extensive this argument may not hold and a slightly different approach is used called Wide Area Differential GPS 

It can now be seen that using DGPS the position of a roving receiver can be found relative to a fixed master or base station without significant errors from satellite and receiver clocks ionospheric and tropospheric reference and even ephemeris errors

This idea has been expanded to the concept of having permanent base stations established throughout a wide area or even a whole country 

As GPS is essentially a military product the US Department of Defense has retained the factiliy to reduce the accuracy of the system by interfering with the satellite clocks and the ephemeris of the satellite 

This is known as Selective Availability ( SA ) of the Standard Positioning Service ( SPS ) 

This from of degradation has been switched off since May 2000 and it is unlikely though possible that it will be reintroduced as there are other ways that access to the system can denied be to a hostile power 

The P can also be altered to a Y code to prevent imitation of the PPS by hostile forces and made unavailable to civilian users 

This is known as Anti Spoofing ( AS ) 

However the carrier wave is not affected and differential methods should correct for most SA effects 

Using the carrier phase observable in the differential mode produces accuracies of 1 ppm of the baseline length Post processing is needed to resolve for the integer ambiguity if the highest quality results are to be achieved 

Whilst this depending on the software can result in even greater accuracies than 1 ppm ( up to 0.01 ppm ) it precludes real time positioning 

However the development of Kinematic GPS and on the fly ambiguity resolution makes real time positioning possible and greatly reduces the observing times 

The following methods are based on the use of carrier phase measurement for relative positioning using two receivers