Using Code Based GPS Double Differences for UAV Guidance to Moving Platforms

Kurzfassung

We report on using Global Navigation Satellite System (GNSS) pseudorange based double differences for relative navigation to guide a remotely piloted vehicle onto a ship board landing platform. The German Federal Police for Maritime Tasks (Bundespolizei See) is responsible for border control in North and Baltic Sea, surveillance of maritime traffic, and investigation of violations against environmental or fishing regulations. Maritime police operations can be enhanced by unmanned aerial vehicles (UAV) operated from ships. Highly automated unmanned systems can for example provide real-time surveillance also in geographically or climatically challenging or safety critical situations. The UAV could be based on the coast guard ship, fly surveillance missions and then return to the landing deck, usually located on the rear of the ship. In this case, the place of landing is not fixed in space and may even be moving with a specific velocity and direction. In this case, landing aids that are normally used in aviation will not work as they require a location fixed in space. In surveying, real time kinematics (RTK) is an established technique that uses GNSS carrier phase measurements for highly precise position estimation. Part of the RTK process is to form the double difference observable between master and rover (receiver) and between pairs of satellites. With this double difference and knowledge of the satellite geometry, it is possible to calculate a relative position between two GNSS receivers. When using carrier phase measurements, the rover needs to be stationary for some time in order to initialize the resolution of the carrier phase ambiguities. The same technique can be used on pseudorange measurements without the limit of stationarity for initialization, albeit at the cost of a decreased accuracy. Almost all correlated errors are removed from the solution in the process of differencing [1] and the resulting double difference observable has an accuracy that is comparable to the ground based augmentation system. For the double difference code measurements it is possible to calculate protection levels and thus - guaranteeing the integrity of the solution [2]. Ground tests show that the scalarized accuracy of the baseline vector is at 1m RMS (root mean square). We apply the double difference code technique to a system of aircraft receiver and ground marker in order to calculate the relative vector between the two GNSS antennas. We used ad Bölkow Bo105 as large scale UAV and equipped it with a Septentrio SB Multifrequency GNSS receiver. On the patrol ship, we installed a Topcon NetG3 GNSS receiver. In a first post processing we evaluate the performance of the double difference algorithm using a post processed precise point position trajectory as a reference. Furthermore, we evaluate whether using the correct covariance matrix instead of an assumption of independent and equal measurement errors yields any operational benefit for UAV landings. The system could also be used in situations when it is not possible to perform extensive surveying of the landing site, for example at military forward operating bases.