Azimuth-dependent elevation threshold (ADET) masks to reduce multipath errors in ionospheric studies using GNSS

Physical structures found in the vicinity of GNSS receivers can introduce multipath interference effects when a signal arrives at the receiver by different routes. Multipath effects are well recognized as one of the most significant sources of error that degrade the accuracy of GNSS signals for navigation and positioning applications. These effects also reduce the quality of GNSS data used for ionospheric studies. The principal cause of multipath effects is proximity of the antenna to reflecting structures and it is more pronounced when the signal comes from a satellite with low elevation. Typically, conservative fixed-elevation thresholds of 20-40° are used to filter out signals from low elevation angles, but this leads to the exclusion of a significant quantity of useable data. In this paper we present a series of azimuth-dependent elevation thresholds that were developed by characterizing the multipath environment of the GPS Ionospheric Scintillation and TEC (Total Electron Content) Monitor (GISTM) receivers installed by SANSA (South African National Space Agency) at Mauritius (20.14°S and 57°E), Marion Island (46.87°S and 37.86°E) and SANAE IV in Antarctica (71.73°S and 2.2°W). The threshold masks were developed from azimuth-elevation maps of the S4 index, σϕ index, the Code-Carrier Divergence Standard Deviation (CC-STDDEV) and the L1 Carrier-to-Noise Density (L1 CNo) from 1-min scintillation data taken over a period of 10-12 months at each location to identify signals that are distorted by multipath effects. Using the azimuth-dependent elevation threshold (ADET) mask typically gives 22-28% more useful data than using a fixed-elevation threshold at the sites studied in this paper.