Curtis A. Shively

A Method of Over Bounding Ground Based Augmentation System (GBAS) Heavy Tail Error Distributions

The purpose of this paper is to describe a statistical method of modeling and accounting for the heavy tail fault-free error distributions that have been encountered in the Local Area Augmentation System (LAAS), the FAA’s version of a ground-based augmentation system (GBAS) for GPS. The method uses the Normal Inverse Gaussian (NIG) family of distributions to describe a heaviest tail distribution, and to select a suitable NIG family member as a model distribution based upon a statistical observability criterion applied to the FAA’s LAAS prototype error data. Since the independent sample size of the data is limited to several thousand and the tail probability of interest is of the order of 10-9, there is a chance of mismodeling. A position domain monitor (PDM) is shown to provide significant mitigation of mismodeling, even for the heaviest tail that could be encountered, if it can meet certain stringent accuracy and threshold requirements. Aside from its application to GBAS, this paper should be of general interest because it describes a different approach to navigation error modeling and introduces the application of the NIG distribution to navigation error analysis.

FAA's DGPS CAT III feasibility program: update and test methodology

The FAA has established a program to evaluate the technical feasibility of using guidance from the satellite-based Global Positioning System (GPS) for Category III (CAT III) precision approaches. A combination of flight tests, analyses and simulations will be used to evaluate differential GPS (DGPS) and associated monitoring techniques for CAT III application. Emphasis will be placed on meeting accuracy and integrity requirements for a complete DGPS-based CAT III autoland capability. Contractors will be required to provide the ground and airborne equipment for computing DGPS-based aircraft position and velocity. Contractors will also provide a demonstration aircraft completely equipped with a DGPS-based capability. Feasibility will be demonstrated by meeting either sensor accuracy requirements for CAT III applications or total system accuracy requirements. It is expected that meeting total system accuracy requirements would place less demand on the accuracy of the DGPS-based sensor than meeting sensor requirements, but more demand on automatic total system performance monitoring in the aircraft. This paper presents an updated schedule of program activities and describes the test methodology in detail.<<ETX>>

Performance study of a new crosslink-aided User Range Accuracy (URA) integrity monitor algorithm for LPV-200

A new algorithm for the GPS satellite User Range Accuracy (URA) integrity monitor that incorporates the satellite ranging measurements taken at ground monitor stations, aided by the crosslink ranging measurements taken between the GPS satellites, is presented. The new algorithm provides improved performance and eliminates the need for a key assumption of a previous algorithm that used ground and crosslink measurements. The performance of the new algorithm is analyzed and measured by the values of the minimum monitorable URA (MMU) for the satellites in the constellation. The availability of LPV-200 operations for an aviation GPS receiver that uses the MMU values as URAs to derive its integrity assured navigation position solution is also analyzed. The LPV- 200 availabilities at representative US and worldwide airport locations are presented. Improved performance of the new algorithm is shown as a reduction of the MMU value and an increase of the LPV-200 availability.

Modeling the effect of pressure altimetry on geostationary satellite surveillance accuracy

In the ranging and processing mobile-satellite (RAPSAT) system, aircraft position is estimated from the aircraft altitude in conjunction with the aircraft range (signal transit time) to two surveillance satellites. Due to large variations in atmospheric conditions, altimeter readings deviate significantly from the true geometric altitudes. As a result, the aircraft position estimation accuracy of satellite surveillance systems, such as RAPSAT, may be degraded. The authors model the spatial and temporal deviations between pressure altitudes and true geometric altitudes, and the results are applied to characterize the position estimation error of the RAPSAT system. The results show that the geometric altitude deviated widely from the pressure altitude at different latitudes and altitudes, but the altitude difference remained constant at the same latitudes and altitudes. As a result, these altitude deviations can be modeled as a constant correction over the same latitudes and altitudes. These altitude deviations were observed to be reasonably stable over several days. With the adjustment of altimeter readings, the errors in the estimation of geometric altitudes, including instrumentation errors, were computed to be less than 310 ft (one standard deviation) across conterminous US. The corresponding root mean square error in aircraft position (2-D) estimated by the RAPSAT system varied from 135 ft to 297 ft for six selected locations representing the cross section of the country.<<ETX>>

Operational consideration for use of GPS integrity information in the National Airspace System

The author discusses the integrity of the GPS (Global Positioning System) as applied to flight operations in the National Airspace System. A generic scenario for failure of navigation signals-in-space is defined. Concern is focused on the state of temporary navigation on notification of a failure, and subsequent recovery to continued radionavigation. Operational considerations are identified to reflect the impact on pilot and air traffic control. A comparison of receiver autonomous and GPS integrity channel monitoring is presented with regard to these considerations. General guidelines are suggested for assuring that GPS navigation equipment properly uses the integrity information to satisfy operational considerations of safety and workload.<<ETX>>