Livio Gratton

Orbit ephemeris monitors for local area differential GPS

Methodologies sufficient to monitor GPS satellite orbit ephemeris for category I aircraft precision approach navigation are described. In the absence of a satellite maneuver, it is shown that a monitor based on the projection of previously validated ephemeris parameters is adequate to meet navigation integrity and availability requirements. After scheduled stationkeeping maneuvers, no previously validated ephemerides are available, so a measurement-based method is required. The feasibility of such a monitor is also established.

Ephemeris type a fault analysis and mitigation for LAAS

The Local Area Augmentation System (LAAS) has been developed by the FAA to enable precision approach and landing operations using the Global Positioning System (GPS). Each LAAS installation provides services through a LAAS Ground Facility (LGF) which is located at the airport it serves. By monitoring the GPS signals, measurements, and navigation messages, the LGF is able to exclude unhealthy satellites and broadcast real-time range-correction messages for healthy satellites to users via a VHF data link. Airborne users apply these corrections to remove errors that are common between the LGF and the aircraft. The LGF is also responsible for warning the aircraft of any potential integrity threats that cannot easily be resolved by excluding unhealthy satellites. One source of potential errors is the satellite broadcast ephemeris message, which users decode and use to compute GPS satellite positions. In LAAS, potential GPS ephemeris faults are categorized into two types, A and B, based upon whether or not the fault is associated with a satellite maneuver. This work focuses on aviation navigation threats caused by Type A faults. To detect and mitigate these threats, we investigate two LGF monitors based on comparing expected ranges and range rates (based on broadcast ephemeris) with those measured by the LGF. The effectiveness of these monitors is analyzed and verified in this paper.

Carrier Phase Relative RAIM Algorithms and Protection Level Derivation

The concept of Relative Receiver Autonomous Integrity Monitoring (RRAIM) using time differential carrier phase measurements is investigated in this paper. The precision of carrier phase measurements allows for mitigation of integrity hazards by implementing RRAIM monitors with tight thresholds without significantly affecting continuity. In order to avoid the need for cycle ambiguity resolution, time differences in carrier phase measurements are used as the basis for detection. In this work, we examine RRAIM within the context of the GNSS Evolutionary Architecture Study (GEAS), which explores potential architectures for aircraft navigation utilizing the satellite signals available in the mid-term future with GPS III. The objectives of the GEAS are focused on system implementations providing worldwide coverage to satisfy LPV-200 operations, and potentially beyond. In this work, we study two different GEAS implementations of RRAIM. General formulas are derived for positioning, fault detection, and protection level generation to meet a given set of integrity and continuity requirements.

Orbit ephemeris monitors for category I LAAS

This paper describes sufficient methodologies to monitor GPS satellite orbit ephemeris for Category I aircraft precision approach navigation. In the absence of a satellite maneuver, it is shown that a monitor based on the projection of previously validated ephemeris parameters is adequate to meet navigation integrity and availability requirements. After scheduled stationkeeping maneuvers, no previously validated ephemerides are available, so a measurement-based method is required. The feasibility of such a monitor is also established.

Monitoring Global Positioning System Satellite Orbit Errors for Aircraft Landing Systems

Ground-based augmentations of the global positioning system (GPS) demand the greatest safety and reliability to support aircraft precision approach and landing navigation. One troublesome failure mode for these systems is the possibility of large orbit errors; discrepancies between the locations of GPS satellites in space and the locations derived by the ephemeris data that they broadcast. To counter this possibility, several ephemeris monitor algorithms detecting orbit errors are described. The method is based on a comparison between satellite positions given by the current satellite ephemeris [today’s ephemeris (TE)] and the ephemeris broadcast by the same satellite on its preceding pass [yesterday’s ephemeris (YE)]. Variants of this YE‐TE test are shown to provide protection against ephemeris errors and also to support minimum detectable errors as low as 1145 m, which will minimize the resulting impact on ground-based augmentation system user availability. In addition, to initialize these monitors when no earlier validated ephemerides are available, means of using raw measurements are proposed. The results show that the YE‐TE and the measurement-based methods together are adequate to meet navigation integrity and availability requirements for category 1 precision approaches.