A Self-Calibrating Pseudolite Array (SCPA) is a self-deployable GPS pseudolite-based local-area navigation system applicable to future-generation Mars rovers. By utilizing bi-directional GPS transceivers (incorporating separate GPS pseudolites and GPS receivers) deployed in a ground-based array, the SCPA can provide all the benefits of satellite-based Carrier-phase Differential GPS (CDGPS), such as drift-free, centimeter-level, and three-dimensional positioning, without requiring a satellite constellation on Mars. Relative geometry change by moving the rover enables the SCPA to self-calibrate both the array locations and the rover trajectory to centimeter-level accuracy. This self-calibration capability of the SCPA overcomes the difficulty of autonomous robotic deployment of the pseudolite-based navigation system on Mars, eliminating the need for accurate a priori position information or precise placement of the array.
K9 SCPA Field Trial at the Marscape 02/11/04
This page contains new results from the latest field trial of the SCPA conducted in February 2004 using the K9 Mars rover platform operated in the Marscape at NASA Ames Research Center. The Marscape is a 3/4 acre lot of the Ames outdoor test site with Mars-analog terrains. The terrains were designed to incorporate typical aspects of the Martian environment and geology of greatest scientific interest. The SCPA was deployed on a dry lakebed area of the Marscape. This field trial demonstrates the entire scenario of the SCPA operation, starting with autonomous array deployment, followed by array self-calibration and centimeter-level navigation for a long traverse. The results show that a 0.2% drift rate is achieved by the SCPA navigation (0.4 meter of the final positioning error after a 174.7 meter traverse).
The operation of the SCPA starts with deploying stationary GPS transceiver units autonomously in a fixed array. One possible deployment method is that the K9 rover carries the stationary transceivers to be deployed and places them on the field one by one. After placing one of the stationary units, the distance between the K9 rover and the first unit can be calculated from the delta range between the two based on the ICP Doppler measurements. Although the ICP-based delta range is given in centimeter-level accuracy by the bi-directional transceiver ranging, the measurement of the absolute distance between the two is corrupted by a meter-level initial bias caused by the uncertainty of robotic deployment. The initial bias of the placement uncertainty can be assumed to be roughly less than a half meter, provided that the stationary transceiver is placed somewhere within a half meter from the K9. This placement uncertainty is calibrated later to centimeter-level accuracy in the self-calibration process; therefore no precise deployment capability on the K9 rover is necessary.
SCPA Array Deployment
Once the second stationary unit is deployed, the K9 rover can instantly start meter-level 2-D positioning via a triangulation of the two delta ranges. Again, this inaccuracy is caused by the placement uncertainty of the two stationary units. This meter-level performance is sufficient for many rover tasks such as general navigation between waypoints and collision avoidance. Placing the third stationary unit completes the SCPA deployment with the minimum configuration required for full operation. This third transceiver provides better geometry for the triangulation and also provides the capability of array calibration to centimeter-level accuracy, where three delta ranges, or three measurement constraints, are required to resolve all the uncertainties to be calibrated. Once all the initial biases in the range measurements are calibrated, the K9 rover is ready to operate with centimeter-level navigation anywhere within the array.
Long Rover Traverse with Self-Calibrated Array
K9-SCPA at the Marscape 04/02/11 [MPG VCD 360x240 22MB]
Self-Calibrated K9 Trajectory
Last modified Mon, 1 Nov, 2010 at 14:40