Catalog and Description of GPS and WAAS L1 C/A Signal Deformation Events

Abstract

The possibility of signal deformations on GPS signals has been a long standing threat requiring detection and mitigation with safety of life augmentation systems such as WAAS and GBAS. Numerous papers have been written on deformation monitoring and the threat model used to represent it. The propensity of signal deformation events to occur, however, can sometimes get lost since they are very rare. Over the passage of time, deformations have been observed on GPS and WAAS satellite signals and one of the purposes of this paper is to provide a list of known events and their characterization. The deformation events that will be discussed are actual signal generation failures that occurred on GPS satellites, smaller deformations from satellite operations such as satellite power management and switching use of components, SVN 49 since it is a rather unique case of deformation, and lastly, a signal deformation event that was associated with WAAS GEO satellites. The WAAS GEO event occurred when two GEO signals were emanating from essentially the same point in space as one satellite traversed from/to its intended orbital location. The paper will attempt, where possible, to describe the actual mechanism that caused these deformations. The paper will also analyze one of the events in the context of signal deformation threat model parameters to show the differential error possible from this satellite for a WAAS corrected User for the entire class of allowed GPS equipment. Since aviation equipment allowed with WAAS and GBAS has a fairly large design space in terms of receiver correlator spacing and processing bandwidth, the relatively small errors observed with the WAAS and GBAS ground systems can be manifested as larger errors for a differentially corrected User.