Multipath Considerations for Ground Based Ranging Sources

摘要

This paper discusses fielding of an airport pseudolite (APL)rnfor the Local Area Augmented System (LAAS), which is arndifferential GPS (DGPS) based precision approach system.rnThe addition of an airport pseudolite (APL) is beingrninvestigated to increase the availability of the LAAS. Onernof the major error sources for the APL link is the ground-to-groundrnmultipath at the LAAS Ground Facility (LGF).rnThis paper investigates the ground multipath performancernof the APL link using a prototype LGF. Using a MultipathrnLimiting Antenna (MLA) for APL transmission, typicalrndouble difference (DD) error terms are calculated using arnpair of High Zenith Antennas (HZA)s or MLAs for GPSrnand APL signal reception. Using GPS DD Error analysis asrna baseline for comparison, the C/A code biases were in thern2.2-2.4 meter range regardless of whether HZA or MLArnantennas were used. These biases were present withoutrnperforming special antenna siting or radio frequency (RF)rngain control and blanking. For tests where the HZAs were used for APL reception, a modest level of desired/undesiredrn(D/U) ratio of 13 dB (11 dB for the MLA and 2 dB for thernHZA) was obtained for the APL link geometry. Arnmultipath error term on the order of ? meter is evidentrnwhile using the HZA for reception, even with the APLrnWideband (WB) code format. When two MLAs were usedrnfor APL and GPS reception, essentially no bias term wasrnpresent in the APL DD Error. The non-existence of anyrnreal bias in the WB APL DD Error is an indication that thernstanding-wave multipath in the APL ground-to-ground linkrncan be eliminated by the use of MLAs for APL transmissionrnas well as reception, along with proper antenna siting. Thisrncorresponds to a total D/U of 22 dB (11 dB for eachrnantenna) for the APL link geometry.