What's Moving Base RTK For?

GPS receiver output correct course over ground (COG) information when in motion. It represent movement direction. Without movement, the output COG information may be incorrect.

Some applications, such as marine vessel autopilot, require knowledge of accurate heading direction regardless of movement direction.  Two NS-HP or NS-HP-GL configured in moving base RTK mode gives precise heading in the direction from base antenna to rover antenna. When placed in stern/bow direction, it gives heading of the vessel. Such RTK based method is preferred over magnetometer based method due to latter accuracy is affect by surrounding metallic environment.

Accuracy of the moving base RTK heading is related to the base/rover antenna separation (baseline length), larger the baseline distance higher the accuracy. Below gives some (1-sigma) heading error we measured with respect to baseline length using survey grade antenna HX-CSX601A:

14.8 meter baseline: 0.009 degree

2 meter baseline: 0.067 degree

1 meter baseline: 0.135 degree

50 cm baseline: 0.27 degree

33 cm baseline: 0.32 degree

14.8 meter baseline 1 hour test result (can zoom from your web browser to view the numbers more clearly)

33 cm baseline 30 minutes test result

Recently we are seeing some customers testing our low-cost RTK receivers in moving base mode for applications similar to 3Z RF Aligner, DJI RTK-G, and Furuno SC-30 Satellite Compass. Such applications normally would have fixed baseline length; i.e. base/rover antenna separation. When configuring the rover for moving base mode, providing a baseline length within 5cm accuracy will speed up time to reaching RTK Fix state and output correct heading information via PSTI,032 message "Baseline Course" field. 

Below example shows providing baseline length constraint, RTK Fix time improved from 34sec average to 11sec average for a short baseline test case.

With Baseline Length Constraint

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Without Baseline Length Constraint

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For DJI RTK-G type of application, there is Advanced Moving Base mode that enables rover to have both precise heading and RTK position by using three NS-HP-GL modules; it is described in section 10 page-29 of the User Guide.