Data processing: vehicles – navigation – and relative location – Navigation – Employing position determining equipment
Reexamination Certificate
1999-02-26
2001-11-27
Cuchlinski, Jr., William A. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Navigation
Employing position determining equipment
C701S207000, C701S213000, C701S214000, C701S226000, C342S357490, C342S357490, C342S357490
Reexamination Certificate
active
06324474
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an improved method for establishing the coverage area of a Wide-Area Differential Global Positioning System (WADGPS), and specifically to predict the level of accuracy when navigating within a coverage area to transmit information and determine a user's position.
2. Description of the PriorArt
WADGPS have been developed for a multiple of applications, including aircraft navigation. Such systems use a combination of the U.S. Department of Defense Global Positioning System (GPS) Space Vehicles (SV), a network of Ground Reference Stations (GRS), a Central Control Station (CCS), and differential corrections continuously broadcast from either a ground or a space based reference point. A ground transmitter may be placed at a fixed geographic location, while a space based transponder may be placed aboard a geosynchronous earth orbiting SV (GEO SV). The CCS generates the differential corrections using the GRS observable measurements and the differential correction transmitter continuously broadcasts the WADGPS information to users within the wide area. One such system being developed ior the U.S. Federal Aviation Administration is the Wide-Area Augmentation System (WAAS), as described in specification FAA-E-2892.
Various methods have been proposed in the prior art to assess the effectiveness of such GPS augmentation systems. The standard GPS coverage models quantify accuracy and availability in terms of Dilution of Precision. While generally effective, these prior art methods do not capture the spatial and temporal dynamics of a WADGPS system. The additional GRSs within a WADGPS enables the space vehicle Signal-in-Space (SIS) errors to be decomposed into their ephemeris and clock components. The present invention utilizes such a decomposition to more closely approximate the SV SIS accuracy and project these error sources onto a lattice of user locations to predict WADGPS navigation accuracy. The inventive method allows prediction of the effective user position accuracy over a wide geographical area. This will determine the operational characteristics of an existing WADGPS system or optimize a proposed WADGPS system configuration.
Two alternatives exist to evaluate user position determination accuracy in a WADGPS. The first alternative involves the use of a discrete simulation of the WADGPS network and the evaluation of the various performance metrics by enumeration. However, such a discrete simulation requires a precise definition of the network and numerous Monte Carlo scenarios to generate meaningful statistics. A second alternative is the use of an analytical model that captures the underlying physics and dynamics of the WADGPS configuration. The present invention uses such analytical models to predict the WADGPS navigation accuracy and its effective coverage area.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method to predict the effectiveness over a wide-area of a differentially corrected space vehicle based navigation service such as the U.S. FAA WAAS. This will incorporate the spatial and temporal variations of the constellation of space vehicles when observed by the network of WADGPS ground reference stations.
A second object of the present invention is to provide a method for predicting the accuracy of the users navigation position determination.
Briefly, this invention describes a method for measuring the effectiveness of a WADGPS by projecting the least-squares error covariance of the SVs and GRSs through the central control station. Through generation of the differential corrections at the CCS, the SV-GRS error covariance is then projected onto the WADGPS user locations. A covariance analysis approach is then used to capture the dependencies of the SV and the observability of the SV to the GRSs and to the user locations within the WADGPS coverage area.
The SIS least-squares error covariance captures the SV ephemeris short-term and long-term errors, the SV clock short-term errors, the GRS clock and receiver errors, and the Selective Availability short-term clock dither prediction error. A key component of the error covariance approach is the ability to capture the spatial and temporal variations of the GPS constellation relative to the GRS locations. The GRS is a critical factor in the WADGPS SIS accuracy. Their placement depends on amount of GRSs, GRS construction cost, GRS to CCS communication availability, and GRS maintenance convenience. This invention quantifies the impact of the GRS quantity and their geographic placement on overall WADGPS SIS accuracy.
The user positional accuracy is predicted following a multi-step process that projects the SIS error covariance onto a lattice of potential user locations. During this process, additional error sources such as user receiver and residual ionospheric errors are induced. At each potential user location, a subset of the best SVs are selected. Best in this sense is defined in the art as those SVs for which the users position determination will be minimally impacted. This is commonly known in the art as minimum position dilution of precision (PDOP). A typical avionics user receiver has limited SV tracking capacity and would select only the best-of-six SVs for the purpose of determining a navigation fix. In all cases though, the WADGPS user must receive the differential corrections continuously broadcast by the WADGPS. Thus the user must always have in view either a GEO SV or a ground based transmitter. In WAAS, a geostationary (GEO) satellite continuously transmits the differential corrections. An important factor in the user avionics receiver SV selection process is the minimum antenna elevation mask angle. This elevation angle acts as a constraint on the number of SVs visible to the user. The invention considers the user and the GRS mask angles to be different. A lower user mask angle may significantly increase the WADGPS coverage area, however, the airframe will likely induce larger multipath effects at low elevation angles. A lower GRS angle significantly increases the duration of the GRS-SV observables, but the slant range will likely induce larger delay effects at low elevation angles.
The SIS error covariance of the selected SVs is projected along the line-of-sight vector to the user location. In summary, our method is comprised of the following steps:
Generation of a lattice of user location points that converts from geodetic latitude and longitude coordinates to earth-centered, earth-fixed coordinates.
Initialization of a common epoch time and duration based on the available trajectory data.
At each time step:
Propagation of the GPS and GEO SV position.
Generation of the SIS least-squares error covariance using the available GRSs and the SVs visible to those GRSs.
Generation of the user navigation positional accuracy for each lattice point using the projected SIS error covariance.
Evaluation of WADGPS-specified performance metrics for each lattice point.
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Beisner Henry M.
Dorsey Arthur J.
Parker Timothy C.
Aitken Andrew C.
Arthur Gertrude
Cuchlinski Jr. William A.
Lockhead Martin Corporation
Venable Baetjer Howard & Civiletti LLP
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