Interference Direction Finding for Aviation Applications of GPS

The Local Area Augmentation System (LAAS) is being developed by the US Federal Aviation Administration (FAA) to provide satellite navigation performance compliant with the stringent requirements for aircraft precision approach and landing. A primary design goal for LAAS is to insure that failures occurring in the ground or space segments be eliminated by the ground system before differential corrections are broadcast to users. One such failure is unintentional interference or intentional jamming in the GPS frequency band. To protect availability, ground personnel must also be able to locate and disable the interference source. To serve this purpose, the LAAS Ground Facility may well include an Interference Direction Finder (IDF). The IDF can improve LAAS availability by rapidly and accurately estimating the location of the source. IDF activities are implemented in parallel with reference receiver functions and may share components with the reference receivers and processors in existing LAAS prototypes. This paper focuses on interference direction finding using interferometry. Given an undesired interference signal, measuring the signal propagation delay along the baseline between two antennas is used to estimate the direction of the signal. Measurements from the multiple baselines present in the LAAS Ground Facility are combined to estimate the location of the undesired signal transmitter. The paper describes the Stanford Prototype IDF receiver design and presents experimental results that demonstrate the use of the IDF to detect nominal GPS signals and determine the source of interfering signals.