Real time kinematic RTK

 The previous methods that have been described all require post processing of the results However RTK provides the relative position to be determined instantaneously as the roving receiver occupies a position. 

The essential difference is in the use of mobile data communication to transmit information from the reference point to the rover 

Indeed it is this procedure that imposes limitation due to range over which the communication system can operate 

The system requires two receivers with only one positioned over a known point 

A static period of initialization will be required before work can commence 

If lock to the minimum number of satellites is lost then a further period of initialization will be required 

Therefore the surveyor should try to avoid working close to major obstructions to line of sight to the satellites 

The base station transmits code and carrier phase data to the rover On board data processing resolves the ambiguities and solves for a change in coordinate differences between roving and reference by remaining static for a short time over a point of known position 

The great advantage of this method for the engineering surveyors is that GPS can be used for setting out on site 

The setting out coordinates can be entered into the roving receiver and a graphical output indicates the direction and distance through which the pole antenna must be moved 

The positions of the point to be set out and the antenna are shown 

When the two coincide the centre of the antenna is over the setting out positions 

Real time kinematic on the fly 

Throughout all the procedures described above it can be seen that initialization or reinitialization can only be done with the receiver static 

This may be impossible in high accuary hydographic surveys or road profiling in a moving vehicle 

Ambiguity Resolution On the Fly ( AROF ) enables ambiguity resolution whilst the receiver is moving 

The techniques require L l and 1.2 observation from at least five satellites with a good geometry between the observer and the satellites 

There are also restrictions on the minmum periods of data collection and the presence of cycle slips 

Both these limitations restrict this method of surveying to GPS friendly environment 

Depending on the level of ionospheric disturbances the maximum range from the reference receiver to the rover for resolving ambiguities whilst the rover is in motion is about 10 km with an achievable accuracy of 10 - 20 mm 

For both RTK and AROF the quality of data link between the reference and roving receiver is important 

Usually this is by radio but it may also be mobile phone 

When using a radio the following  issues should be considered :

- In many countries the maximum power of the radio is legally restricted and/or a radio licence may be required 

This in turn restricts the practical range between the receivers 

- The radio will work best where there is a direct line of sight between the receivers 

This may always be possible to achieve so for best performance the reference receiver should always be sited with the radio antenna as high as possible 

- Cable lengths should be kept as short as possible to reduce signal losses