Expanding the Coverage of Local Area Differential Correction

Abstract

Japan has been developing QZSS (quasi-zenith satellite system) as a regional satellite navigation system operating together with the US GPS. The QZSS satellites broadcast GPS-like ranging signals on L1, L2, and L5 frequencies and additional augmentation signals for submeter and centimeter level positioning services. QZSS satellites are launched into a 24-hour elliptic orbit inclined 43 degrees in order to broadcast signals from high elevation angles into urban canyons. With the full constellation of QZSS consisting of three or more satellites on the same groundtrack, at least one of QZSS satellites always exists at near zenith so that the system always provides augmentation information to users. The submeter service of the QZSS, separated from the SBAS service, has been designed and they are planning to broadcast the intended corrections based on the local area differential correction technique. The sets of differential corrections, each set is generated by the associate reference station, are broadcast simultaneously on the single augmentation signal. According to this implementation, Japan is covered by some reference stations settled at some biggest cities. A possible problem of this implementation is selection or interpolation of corrections; Which station a user between reference stations should apply although the service coverages of reference stations have some overlaps each other. In order to ensure the performance for the whole of Japanese territory, it is necessary to define an interpolation methodology implemented inside user receivers. By employing an appropriate interpolation methodology, the service coverage of each reference station could be expanded. The authors have considered some methods to expand the service coverage of the local area differential correction. Two methods are tested: (i) Virtual Station DGPS: Constructing the virtual station inside a user receiver based on weighted-averaging of corrections for surrounding DGPS stations; and (ii) Delta WADGPS Correction: Adjusting the DGPS corrections based on the difference of WADGPS correction between user location and DGPS station. As a result of experimental trial, it is observed that Virtual Station DGPS method improves position accuracy inside reference station network while the accuracy of Delta WADGPS Correction seems similar with local DGPS. Note that this trial is conducted just for the nominal ionosphere condition.