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Methods and apparatus are described and illustrated for producing GPS corrections, comprising: collecting measurements from a plurality of network reference stations; determining network corrections from the measurements; determining residual errors at one or more vernier-cell reference stations; an

Methods and apparatus are described and illustrated for producing GPS corrections, comprising: collecting measurements from a plurality of network reference stations; determining network corrections from the measurements; determining residual errors at one or more vernier-cell reference stations; and preparing vernier-cell corrections to compensate the residual errors within a vernier-cell region. Network correction streams are described and illustrated which contain network corrections derived from a plurality of network reference stations and residual error corrections derived from one or more vernier-cell reference stations. Methods and apparatus are described for employing such network correction streams in a virtual reference station to produce corrections and/or virtual measurements for use in a GPS receiver.

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1. System for generating GPS corrections comprising:a. at least one data collection hub for collecting measurements from a plurality of reference stations;b. at least one network processor for processing the measurements to produce corrections by determining network corrections from the measurements

1. System for generating GPS corrections comprising:a. at least one data collection hub for collecting measurements from a plurality of reference stations;b. at least one network processor for processing the measurements to produce corrections by determining network corrections from the measurements, determining residual errors at one or more of the reference stations comprising a vernier cell, and preparing vernier-cell corrections to compensate the residual errors within a vernier-cell region; andc. a distribution system for distributing the corrections for use by mobile units wherein the reference stations are organized in a plurality of regional subnets, the reference stations of each regional subnet having a set of common-visibility GPS satellites, and the regional subnets overlapping such that each adjacent pair of regional subnets includes at least one reference station in common. 2. The system of claim 1, wherein each regional subnet has a respective data collection hub. 3. The system of claim 1, wherein each regional subnet has a respective network processor. 4. The system of claim 3, wherein said at least one network processor comprises a wide-area network processor, and wherein the regional network processors receive correction data from the wide-area network processor. 5. System for generating GPS corrections comprising:a. at least one data collection hub for collecting measurements from a plurality of reference stations;b. at least one network processor for processing the measurements to produce corrections by determining network corrections from the measurements, determining residual errors at one or more of the reference stations comprising a vernier cell, and preparing vernier-cell corrections to compensate the residual errors within a vernier-cell region; andc. a distribution system for distributing the corrections for use by mobile units wherein a plurality of reference stations is located within the vernier-cell region and wherein geographical density of reference stations located within the vernier-cell region is greater than geographical density of reference stations located outside the vernier-cell region. 6. A method for producing GPS corrections, comprising:a. collecting measurements from a plurality of GPS reference stations;b. determining network corrections from the measurements;c. determining residual errors at one or more of the reference stations located within a vernier-cell region; andd. preparing vernier-cell corrections to compensate residual errors within the vernier-cell region,wherein the reference stations located within the vernier-cell region are of geographically greater density than the reference stations located outside the vernier-cell region. 7. The method of claim 6, wherein the vernier-cell corrections comprise satellite error field corrections applicable within a defined range. 8. The method of claim 6, wherein the vernier-cell corrections comprise satellite error field corrections applicable within a defined range and at least one of: an identification of the vernier cell; a geographic center of the vernier cell; and range around the geographic center of the vernier cell. 9. The method of claim 8, wherein the vernier-cell corrections comprise, for each of a plurality of satellites visible in the vernier cell: a satellite identifier; an indication of satellite-signal quality; a time tag; and a set of error-field parameters. 10. The method of claim 8, wherein the defined range is defined by a centroid component, a first slope, and a second slope orthogonal to the first slope. 11. The method of claim 6, wherein the range is defined by a centroid component, an east-west slope, and a north-south slope. 12. The method of claim 6, wherein determining residual errors at one or more of the reference stations comprises applying the network corrections to the measurements to determine an error field for a satellite visible in the vernier-cell region. 13. The method of claim 6, wherein the vernier-cell region has a centroid and wherein preparing vernier-cell corrections comprises: determining a centroid component which defines an error field for a satellite visible in the vernier-cell region. 14. The method of claim 6, wherein the vernier cell has a centroid and wherein preparing vernier-cell corrections comprises: determining a centroid component, determining an east-west slope, and determining a north-south slope, such that the centroid component, the east-west slope, and the north-south slope define an error field for a satellite visible in the vernier-cell region. 15. The method of claim 6, wherein preparing vernier-cell corrections comprises determining error-field parameters for a satellite visible in the vernier-cell region from a set of clock-adjusted instantaneous range error values for at least three reference stations located within the vernier-cell region. 16. The method of claim 6, wherein preparing vernier-cell corrections comprises: computing a set of error-field parameters for a satellite visible in the vernier-cell region from a set of data comprising a clock error for the satellite and measurements for the satellite from each of at least three reference stations located within the vernier-cell region. 17. The method of claim 6, wherein the vernier-cell corrections comprise a set of error-field parameters for a plurality of satellites visible in the vernier-cell region, and wherein the method further comprises checking the vernier-cell corrections by: calculating a position fix for each of a plurality of reference stations located within the vernier-cell region using the network corrections and the error-field parameters; and comparing the calculated position fix with the known position of the reference station to determine a residual error after application of the vernier-cell corrections. 18. The method of claim 17, further comprising determining validity of the error-field parameters by comparing the residual error after application of the vernier-cell corrections of a plurality of reference stations located within the vernier-cell region. 19. Apparatus for producing GPS corrections, comprising:a. means for collecting measurements from a plurality of GPS reference stations;b. means for determining network corrections from the measurements;c. means for determining residual errors at one or more of the reference stations located within a vernier-cell region; andd. means for preparing vernier-cell corrections to compensate residual errors within the vernier-cell region,wherein the reference stations located within the vernier-cell regions are of geographically greater density than the reference stations located outside the vernier-cell regions. 20. The apparatus of claim 19, wherein the vernier-cell correction comprise satellite error field corrections applicable within a defined range. 21. The apparatus of claim 19, wherein the vernier-cell corrections comprise satellite error field corrections applicable within a defined range and at least one of: an identification of the vernier cell; a geographic center of the vernier cell; and range around the geographic center of the vernier cell. 22. The apparatus of claim 21, wherein the vernier-cell corrections comprise, for each of a plurality of satellites visible in the vernier cell: a satellite identifier; the quality of the satellite signal; a time tag; and a set of error-field parameters. 23. The apparatus of claim 19, wherein the range is defined by a centroid component, a first slope, and a second slope orthogonal to the first slope. 24. The apparatus of claim 19, wherein the range is defined by a centroid component, an east-west slope, and a north-south slope. 25. The apparatus of claim 19, wherein the means for determining residual errors at one or more of the reference stations applies the network corrections to the measurements to determine an error field for a satellite visible in the vernier-cell region. 26. Apparatus for producing GPS co rrections, comprising:a. means for collecting measurements from a plurality of GPS reference stations;b. means for determining network corrections from the measurements;c. means for determining residual errors at one or more of the reference stations located within a vernier-cell region; andd. means for preparing vernier-cell corrections to compensate residual errors within the vernier-cell region,wherein the vernier-cell region has a centroid and wherein the means for preparing vernier-cell corrections determines a centroid component which defines an error field for a satellite visible in the vernier-cell region. 27. Apparatus for producing GPS corrections, comprising:a. means for collecting measurements from a plurality of GPS reference stations;b. means for determining network corrections from the measurements;c. means for determining residual errors at one or more of the reference stations located within a vernier-cell region; andd. means for preparing vernier-cell corrections to compensate residual errors within the vernier-cell region,wherein the vernier cell has a centroid and wherein the means for preparing vernier-cell corrections determines a centroid component, an east-west slope, and a north-south slope, such that the centroid component the east-west slope, and the north-south slope define an error field for a satellite visible in the vernier-cell region. 28. Apparatus for producing GPS corrections, comprising:a. means for collecting measurements from a plurality of GPS reference stations;b. means for determining network corrections from the measurements;c. means for determining residual error at one or more of the reference stations located within a vernier-cell region; andd. means for preparing vernier-cell corrections to compensate residual errors within the vernier-cell region,wherein the means for preparing vernier-cell corrections determines error-field parameters for a satellite visible in the vernier-cell region from a set of clock-adjusted instantaneous range error values for at least three reference stations located within the vernier-cell region. 29. Apparatus for producing GPS corrections, comprising:a. means for collecting measurements from a plurality of GPS reference stations;b. means for determining network corrections from the measurements;c. means for determining residual errors at one or more of the reference stations located within a vernier-cell region; andd. means for preparing vernier-cell corrections to compensate residual errors within the vernier-cell region,wherein the means for preparing vernier-cell corrections computes a set of error-field parameters for a satellite visible in the vernier-cell region from a set of data comprising a clock error for the satellite and measurements for the satellite from each of at least three reference stations located within the vernier-cell region. 30. Apparatus for producing GPS corrections, comprising:a. means for collecting measurements from a plurality of GPS reference stations;b. means for determining network corrections from the measurements;c. means for determining residual errors at one or more of the reference stations located within a vernier-cell region; andd. means for preparing vernier-cell corrections to compensate residual errors within the vernier-cell region,wherein the vernier-cell corrections comprise a set of error-field parameters for a plurality of satellites visible in the vernier-cell region, and wherein the apparatus further comprises means for checking the vernier-cell corrections by: calculating a position fix for each of a plurality of reference stations located within the vernier-cell region using the network corrections and the error-field parameters; and comparing the calculated position fix with the known position of the reference station to determine a residual error after application of the vernier-cell corrections. 31. The method of claim 30, further comprising means for determining validity of the error-field paramete rs by comparing the residual error after application of the vernier-cell corrections of a plurality of reference stations located within the vernier-cell region. 32. Method of using a network correction stream in a navigator to produce corrected position fixes, comprising:a. receiving network correction data derived from a plurality of network reference stations and residual error corrections derived from one or more vernier-cell reference stations; andb. preparing from the network correction data and residual error corrections a set of corrections suitable for a declared location,wherein the residual error corrections are derived from reference stations located within a plurality of vernier-cell regions in which the reference stations are of geographically greater density than the reference stations located outside the vernier-cell regions. 33. The method of claim 32, wherein the vernier-cell corrections comprise satellite error field corrections applicable for each vernier-cell region within a defined range. 34. The method of claim 33, wherein the vernier-cell corrections comprise satellite error field corrections applicable within a defined range and at least one of: an identification of the vernier cell, a geographic center of the vernier cell, and range around the geographic center of the vernier cell. 35. The method of claim 32, wherein the vernier-cell corrections comprise satellite error field corrections applicable for each vernier-cell region, wherein the vernier-cell regions cover respective geographical areas of differing sizes which overlap one another at least in part in an overlap area, wherein the declared location lies within the overlap area, and wherein the residual error corrections from which the set of corrections is prepared is applicable to the smallest of the vernier-cell regions which includes the overlap area. 36. Method of using a network correction stream up a navigator to produce corrected position fixes, comprising:a. receiving network correction data derived from a plurality of network reference stations and residual error corrections derived from one or more vernier-cell reference stations; andb. preparing from the network correction data and residual error corrections a set of corrections suitable for a declared location,wherein the vernier cell has a centroid and the error field of the vernier cell is defined by a centroid component. 37. Method of using a network correction stream in a navigator to produce corrected position fixes, comprising:a. receiving network correction data derived from a plurality of network reference stations and residual error corrections derived from one or more vernier-cell reference stations; andb. preparing from the network correction data and residual error corrections a set of corrections suitable for a declared location,wherein the vernier cell has a centroid, and the error field about the centroid is defined by a centroid component, an east-west slope, and a north-south slope. 38. The method of claim 37, wherein preparing vernier-cell corrections comprises: computing an east-west component as a product of the east-west slope and an east-west distance of the declared location from the centroid; computing a north-south component as a product of the north-south slope and a north-south distance of the declared location from the centroid; and summing the centroid component with the east-west component and the north-south component. 39. Method of using a network correction stream in a navigator to produce corrected position fixes, comprising:a. receiving network correction data derived from a plurality of network reference stations and residual error corrections derived from one or more vernier-cell reference stations; andb. preparing from the network correction data and residual error corrections a set of corrections suitable for a declared location,further comprising: determining a calculated L 1 pseudorange for a GPS satellite; applying the residual error correc tions derived from one or more vernier-cell reference stations to determine a difference between the L 1 pseudorange and a virtual measurement of L 1 pseudorange at the declared location; and applying the difference to the calculated L 1 pseudorange to obtain a virtual L 1 pseudorange. 40. The method of claim 39, further comprising: applying network correction data representing satellite clock and position errors to the calculated L 1 pseudorange. 41. The method of claim 40, further comprising: applying an ionospheric delay adjustment to the virtual L 1 pseudorange to determine a virtual L 2 pseudorange. 42. The method of claim 40, further comprising: applying an ionospheric delay adjustment to the virtual L 1 pseudorange to determine an L 1 phase measurement and an L 2 phase measurement.