Sensitivity Analysis of the JPALS Shipboard Relative GPS Measurement Quality Monitor

The Joint Precision Approach and Landing System (JPALS) is being developed as a single replacement for the navigation, precision approach and landing systems currently used by military aircraft. Shipboard Relative GPS (SRGPS) is a JPALS variant being developed to support flight operation in the shipboard environment. Because of the stringent SRGPS performance requirements, differential carrier phase solutions are being pursued. In the system architecture currently being considered, the JPALS Shipboard Integrity Monitor (JSIM) is a reference station based integrity monitoring concept being explored. To some extent, JSIM is a shipboard analog of the LAAS Ground Facility (LGF) and its function is, in part, to detect, isolate, and alarm signalin-space and reference receiver failures. Unlike the LGF however, JSIM is a mobile system and has to deal with reference receiver antenna motion. Because it operates within a shipboard environment, it must also be robust to the effects of multipath. The effect of antenna motion and multipath is to introduce un-modeled correlated carrierphase measurement errors. This paper presents the results of a sensitivity analysis to assess the performance of a JSIM integrity monitor known as the Measurement Quality Monitor relative to its equivalent IMT monitor in the presence of un-modeled correlated carrier-phase measurement errors. A method for analyzing the effect of un-modeled carrier-phase noise, which involves developing a frequency response for the MQM, is presented. The analysis method used is to develop a relationship between carrier-phase measurement noise and MQM alarm limits. The results of this analysis method are used to show that large, un-modeled correlated noise triggers the JSIM’s fault detection logic unnecessarily, increasing the false alarm rate and adversely affecting system continuity. The increase in false alarm rate as a function of un-modeled carrier-phase measurement error is quantified.