Method and Apparatus for Generating and Using a Regional-Terrain Model

Abstract

A method and apparatus for computing position using a regional-terrain model is provided. The method includes obtaining from at least three satellites pseudorange measurements, computing a transitional position by using a default altitude with a large uncertainty, using this transitional position to obtain from a terrain model altitude information associated with a region, and computing an accurate three-dimensional position as a function of the pseudorange measurements and the altitude information. The region defines a boundary, and the boundary includes the transitional position.

What is claimed is: 
     
         1 . A method for improving the quality of a position, comprising:
 computing first pseudorange measurements at a location using a first set of satellites;   computing the position based on the first pseudorange measurements, the position including computed latitude, longitude, and altitude;   using the computed latitude and longitude, retrieving a modeled altitude from a regional-terrain model; and   comparing a difference between the computed altitude and the modeled altitude to a prescribed range to determine a validity of the position.   
     
     
         2 . The method of  claim 1 , further comprising:
 determining the position valid if the difference is within the prescribed range and invalid if the difference is outside the prescribed range.   
     
     
         3 . The method of  claim 2 , further comprising:
 if the difference is outside the prescribed range, computing second pseudorange measurements at the location using a second set of satellites and re-computing the position based on the second pseudorange measurements.   
     
     
         4 . The method of  claim 1 , wherein the regional-terrain model comprises a first modeled region, the first modeled region having a boundary defined by a first coordinate-system point and a first radius. 
     
     
         5 . The method of  claim 4 , wherein the first radius is based on a first altitude and a first altitude uncertainty associated with the first modeled region. 
     
     
         6 . The method of  claim 5 , wherein the first radius is such that the first modeled region includes all coordinate-system points having altitudes that fall within a range of altitudes defined by the first altitude and the first altitude uncertainty associated with the first modeled region. 
     
     
         7 . The method of  claim 4 , wherein the computed latitude and longitude fall within the first modeled region, and wherein retrieving the modeled altitude comprises retrieving the first altitude associated with the first modeled region 
     
     
         8 . A method for computing a position, comprising:
 computing pseudorange measurements at a location using a set of satellites;   computing a transitional position of the location using the pseudorange measurements and a fixed altitude;   using the transitional position, retrieving a modeled altitude of the location from a regional-terrain model, wherein the regional-terrain model comprises a first modeled region, the first modeled region having a boundary defined by a first coordinate-system point and a first radius; and   computing the position using the modeled altitude and the pseudorange measurements.   
     
     
         9 . The method of  claim 8 , wherein the fixed altitude is a default value or a last-known altitude value. 
     
     
         10 . The method of  claim 8 , wherein the first radius is based on a first altitude and a first altitude uncertainty associated with the first modeled region 
     
     
         11 . The method of  claim 10 , wherein the first radius is such that the first modeled region includes all coordinate-system points having altitudes that fall within a range of altitudes defined by the first altitude and the first altitude uncertainty associated with the first modeled region. 
     
     
         12 . The method of  claim 8 , wherein the transitional position falls within the first modeled region, and wherein retrieving the modeled altitude comprises retrieving the first altitude associated with the first modeled region. 
     
     
         13 . A receiver, comprising:
 a positioning module configured to compute first pseudorange measurements at a location using a first set of satellites;   a memory configured to store a regional-terrain model, wherein the regional-terrain model comprises a first modeled region, the first modeled region having a boundary defined by a first coordinate-system point and a first radius, the first radius based on a first altitude and a first altitude uncertainty associated with the first modeled region; and   a processor configured to retrieve, using a transitional position of the location, a modeled altitude of the location from a regional-terrain model and to compute a position of the location using the modeled altitude and the first pseudorange measurements.   
     
     
         14 . The receiver of  claim 13 , wherein the processor is further configured to compute the transitional position of the location using the first pseudorange measurements and a fixed altitude associated with the receiver. 
     
     
         15 . The receiver of  claim 14 , wherein the fixed altitude includes a default altitude value or a last-known altitude value. 
     
     
         16 . The receiver of  claim 13 , wherein the first radius is such that the first modeled region includes all coordinate-system points having altitudes that fall within a range of altitudes defined by the first altitude and the first altitude uncertainty associated with the first modeled region. 
     
     
         17 . The receiver of  claim 13 , wherein the transitional position falls within the first modeled region, and wherein the processor is configured to retrieve the first altitude associated with the first modeled region. 
     
     
         18 . The receiver of  claim 13 , wherein the processor is further configured to compare a difference between an altitude of the transitional solution and the modeled altitude to a prescribed range to determine a validity of the transitional solution. 
     
     
         19 . The receiver of  claim 18 , wherein the processor is further configured to determine the transitional position valid if the difference is within the prescribed range and invalid if the difference is outside the prescribed range. 
     
     
         20 . The receiver of  claim 19 , wherein the processor is further configured, if the difference is outside the prescribed range, to compute second pseudorange measurements at the location using a second set of satellites and to re-compute the transitional position based on the second pseudorange measurements.