Dr. Penina Axelrad

Colorado Center for Astrodynamics Research

University of Colorado


Characterization of GPS Multipath for Aircraft Landing Systems


Systems that rely on measurements from GPS are subject to errors caused by indirect signal paths from the satellite to the receiving antenna, known as multipath.  These indirect paths also affect communications systems, but in those applications, the primary concern is for signal loss due to destructive interference.  For navigation systems, multipath is a more significant issue because accuracy is limited by precise knowledge of the signal path.  For differential GPS implementations including both civil and military landing systems, multipath is currently a dominant error source in the use of both code and carrier phase observations.  For code-based differential measurements, carrier smoothing is often used to reduce multipath effects; however, this has implications on system robustness and availability.  In carrier-phase based solutions such as those planned for Navy shipboard landing systems, multipath effects on code and carrier can compromise the ability to perform reliable integer ambiguity resolution.   I will discuss research being conducted to characterize and effectively bound the effects of multipath in various GPS-based landing system scenarios using a combination of analytical, simulation, and experimental approaches.