RTK GPS demonstration

This is a visual demonstration of  RTK GPS.  The demo was made with Navio board and Raspberry Pi running RTKLIB. Plots display how RTK GPS solution compares to a single GPS solution from the same receiver.

RTK is a type of differential GPS where two receivers are used – one is stationary and acts as reference station by calculating corrections and sending them to another receiver – rover, that can apply those corrections and increase the accuracy of coordinates.

Corrections in RTK are calculated based on carrier phase measurements and resulting coordinates can be accurate to millimeters. But unlike code cycles, phase cycles are indistinguishable and that creates a problem of integer ambiguity.

In RTK terminology there are different types of solutions:

  • Fix (green on plots) – Solution with fixed integer ambiguity (centimeter accurate).

  • Float (yellow) – Integer ambiguity is not fixed (accuracy floats, may be decimeters).

  • Single (red) – No differential corrections applied (meters accuracy).

RTK is a well-known technology in the field of geodetics, but commercial systems could cost a fortune.

Navio is equipped with a special version of u-blox NEO6 receiver – NEO6-T, which outputs raw GPS measurements. And as RTK is differential GPS – two receivers should be used, but one of them could be replaced by corrections from the internet.

We have conducted several tests with NAVIO boards. The antenna was placed on the roof of a vehicle and kept static.

After that it was driven in a circular path.

RTK is an advanced feature and multiple factors influence quality of the solution – antenna, RF interference, satellite visibility. To keep track of the latter, we usually check ourselves with SatPredictor tool before going to the field. It creates a nice plot with quantity of visible satellites based on your area, date and elevation mask (common rule is to set it to 15deg).

We have different antennas, but usually use Tallysman dual-feed patch as it has very low noise figure and high gain. A rule of a thumb for good RTK performance is to have several satellites with SNR (signal/noise ratio) around 50. A good idea is to have the base permanently installed on the roof of your home or on the roof of club building at the field and broadcast corrections over 3G. This way you can share the base station with your friends and colleagues. Permanent installation would result in a more reliable and predictable satellite visibility. In some areas RTK corrections are already available from universities or other organizations, search for NTRIP servers nearby.

P.S. By the way, float solution does not mean bad solution.