Optimizing Channel Selection for the JPALS´ Land-based Integrity Monitor

JPALS (Joint Precision Approach and Landing System) is a precision landing system that provides LAAS (Local Area Augmentation System) like capabilities in a military operating environment. This paper discusses the landbased JPALS application and the enhancement of a selection algorithm called EXM (Executive Monitor). The EXM examines the pass/fail outputs of other monitors to define a common set of data based on exclusion of detected satellite or receiver faults. Prior versions of the EXM used heuristic rather than optimal criteria. Using data collected from Stanford s LAAS IMT prototype, we evaluated the EXM algorithms with an analysis tool called the JTEP (JLIM Test Platform), where JLIM stands for the JPALS Land-based Integrity Monitor. The VPL (Vertical Protection Limit) is a high-integrity estimation of the landing aircraft’s vertical positioning error. It is a function of satellite geometry, the number of receivers, and the quality of each satellite’s ranging signal. When the VPL is below the VAL (Vertical Alarm Limit) the landing system is available. Our paper shows that an EXM decision method based on minimizing the VPL provides greater availability than previously achieved. The traditional method of defining a common set by maximizing the number of receivers is less complex to code and less computationally demanding than the VPL based method. However, our results show the VPL method provides better availability performance with sufficient computational efficiency to enable real-time operation. To establish the performance benefits of the VPL-based method, this paper presents two analyses. The first details a comparison of EXM decision logic for the all-in-view satellite case. The second shows the robustness of VPLbased algorithms to a degraded one-satellite out case. The degraded constellation reflects a user aircraft which is unable to track all the satellites, an even which may occur commonly in JPALS due to jamming, interference, or aerial maneuvers (obstruction). 1.0 INTRODUCTION JPALS (Joint Precision Approach and Landing System) is a GPS landing system which provides LAAS (Local Area Augmentation System)-like precision approach and landing capabilities in a military environment. This paper discusses the development of a specific integrity monitor called the Executive Monitor (EXM) for land-based JPALS. This integrity monitoring capability is generally referred to as the JLIM (JPALS Land-based Integrity Monitor). One of the subsystems of the JLIM is a function called the EXM (for Executive Monitoring logic). The responsibility of this function is to take as inputs the pass/fail outputs of other monitors and to determine what action should be taken. Primarily, the EXM will look at which receiver/satellite channels have been passed and decide what the best common set to proceed with is. A channel represents an individual receiver-satellite pairing, e.g., satellite 2 on receiver 1 constitutes one channel. The common set is the group of satellites viewed on more than one receiver that the EXM approves for navigation. In the past, little attention has been given to EXM logic, because for nominal operations, the details of the EXM implementation have little impact on availability. The case is different under stress conditions. With jamming, or with marginal satellite visibility, which may be the norm for JPALS deployment, the EXM algorithm can make a significant difference in availability. If the intention is to maximize availability, then it is feasible to define a common set algorithm based on an availability metric. Since VPL is compared to VAL (Vertical Alarm Limit) in the user aircraft to determine availability, we have devised a VPL based algorithm. Prior versions of the EXM would choose a common set based on using the maximum number of GPS receivers that have current and valid observations (pseudorange and carrierphase), along with valid ephemeris. Since this system’s availability is determined by the VPL and not by that of some heuristic value such as the number of receivers, it is logical to incorporate the VPL into the decision making monitors of the system, i.e. the EXM. We will be comparing the performance results of different methods to see which method is optimal. Thus, we expand the domain of algorithms which will determine the common set as an output of the EXM. These algorithms include maximizing the number of receivers, maximizing the number of satellites, minimizing the VPL, and minimizing the VPL when there will be a discrepancy between the tracked satellites of the ground station and user aircraft. The focus of this paper is to devise a selection method for the EXM which will maximize the availability of the JLIM system. This paper is comprised of four main sections as follows: 1) understanding the JLIM and the EXM, 2) specifying the system’s objective, 3) devising algorithms to achieve those objections, and 4) analyzing the performance of those algorithms. 2.0 JLIM AND EXM BACKGROUND The JLIM is a comprehensive collection of subsystems which process incoming measurements to determine code-phase corrections and simultaneously provide sufficient integrity to the user. The JLIM Test Platform, which is being developed at Stanford University, is referred to as the JTEP, and provides a means to test the relative advantages of various monitoring algorithms and architectures. This testbed has evolved from the LAAS Integrity Monitoring Testbed (IMT) [1,2], which is a similar development tool for LAAS which was also developed at Stanford. The JTEP has been developed to improve upon and extend the capabilities of its predecessor. Additions include the ability to handle an arbitrary (but plural) number of receivers, various input data formats, and more than one frequency (L1 & L2). Also, the JTEP has been coded in Matlab instead of Ccode to allow for easier development across platforms and to incorporate Matlab’s embedded utility libraries.