System for simultaneously deriving position information from a plurality of satellite transmissions

• US 5,194,871 A
• Filed: 01/22/1991
• Issued: 03/16/1993
• Est. Priority Date: 03/01/1982
• Status: Expired due to Term

• Current Assignee: Western Atlas International Incorporated (Northrop Grumman Corporation)
• Original Assignee: Western Atlas International Incorporated (Northrop Grumman Corporation)
• Inventors: Counselman, III, Charles C.
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US07/643,771
• Time in Patent Office: 784 Days
• Field of Search: 342/352, 342/357, 342/450, 342/451, 342/465, 342/417, 342/444, 342/463, 342/464, 342/453, 342/442, 375/1, 370/104, 370/121
• US Class Current: 342/357.26
• CPC Class Codes: G01C 15/00 Surveying instruments or accessories not provided for in groups G01C1/00 - G01C13/00
• CPC Class Codes: G01S 19/35 Constructional details or hardware or software details of the signal processing chain
• CPC Class Codes: G01S 19/44 Carrier phase ambiguity resolution; Floating ambiguity; LAMBDA [Least-squares AMBiguity Decorrelation Adjustment] method

Abstract

A method and a system are disclosed for measuring the baseline vector b between a pair of survey marks on the ground by radio interferometry using radio signals broadcast from the earth orbiting satellites of the NAVSTAR Global Positioning System (GPS), the radio signals broadcast by the satellites being double-sideband modulated with their carriers suppressed. An antenna is positioned at each survey mark. The signals received by one antenna during a predetermined time span are separated into upper and lower sideband components. These separate components are filtered, converted to digital form, and then multiplied together. Their product is analyzed digitally by means of correlation with quadrature outputs of a local oscillator to determine the power, and the phase relative to that local oscillator, of the carrier wave that is implicit in the double-sideband signal being received from each satellite. Differences in Doppler shift are utilized to distinguish the carriers of different satellites. The signals received at the same time by the other antenna are processed in the same manner. Thus, the powers and carrier phases of the signals from a plurality of satellites are measured simultaneously and numerical data representing the measurement results are obtained at each survey mark. The measurements are performed in real time at each mark without reference to signals that are received at any other place and without knowledge of any of the coded signals that modulate the GPS carriers. The data from the measurements performed simultaneously but independently at the two survey marks, once per second for a predetermined time span, are then processed together to determine the baseline vector that extends from one mark to the other. Two methods of processing are disclosed in either method, an "ambiguity function" is computed which is a function of the measurement data and of a trivial value of the baseline vector. The vector space of b is systematically searched to find the unique value of b that maximizes the computed function. This value of b is taken to be the desired determination of b. By using signals from a plurality of five satellites and a time span of about 5000 seconds, a baseline vector determination can be obtained by the method of the present invention with an accuracy of about 5 millimeters in each coordinate for a baseline length of about 100 meters.

Claims

1. The method of determining position information from suppressed-carrier radio signals modulated by an unknown pseudo-random code, transmitted by earth-orbiting satellites, comprising:

(a) receiving the signals from a plurality of satellites simultaneously with an omni-directional antenna;

(b) generating predictions of the Doppler shifts of the signals received by said antenna;

(c) correlating the signals received by said antenna with the predictions to cancel the Doppler shifts and correlating the signals received by said antenna with themselves to square the unknown pseudo-random code modulation so that the resulting correlation products have slowly varying phase angles related to the phases of carrier waves implicit in the received signals;

(d) time-averaging said correlation products to obtain data indicating the phase of a carrier wave implicit in the signal received from each satellite, for a plurality of satellites simultaneously; and

(e) processing said data indicating the phase of a carrier implicit in the signal received from each satellite to obtain position information.

3. The method of claim 1 wherein the step of correlating the signals received by said antenna with themselves further comprises;

separating the signals received by said antenna into a first portion representing a first band of frequencies received by said antenna and a second portion representing a second band of frequencies received by said antenna, and cross correlating said first and second portions of the signals received by said antenna.

4. The method of claim 1 wherein the step of correlating the signals received by said antenna with themselves further comprises:

filtering the signals received by said antenna with a filter matched to the spectrum of a selected unknown function of time included in the modulation of said signals, and correlating the filtered signals.

5. The method of claim 1 wherein the step of correlating the signals received by said antenna with themselves further comprises;

(f) separating the signals received by said antenna into a first portion representing a first band of frequencies received by said antenna and a second portion representing a second band of frequencies received by said antenna;

(g) filtering each of said portions with a filter matched to the spectrum of a selected unknown function of time included in the modulation of said signals; and

(h) cross correlating said separated and filtered portions of the signals received by said antenna.

2. The method of determining the position of a first point with respect to the position of a second point using suppressed-carrier radio signals modulated by an unknown pseudo-random code, transmitted by earth-orbiting satellites and received at said first and second points, comprising:

(a) receiving the signals from a plurality of satellites simultaneously with a first omni-directional antenna at said first point and a second omni-directional antenna at said second point;

(b) generating predictions of the Doppler shifts of the signals received by each antenna;

(c) correlating the signals received by each antenna with the predictions to cancel the Doppler shifts and correlating the signals received by each antenna with themselves to square the unknown pseudo-random code modulation so that the resulting correlation products have slowly varying phase angles related to the phases of carrier waves implicit in the received signals;

(d) time-averaging said correlation products to obtain data indicating the phase of a carrier wave implicit in the signal received from each satellite, at each antenna, for a plurality of satellites simultaneously; and

(e) combining said phase indication data from both antennas to determine the position of the first point with respect to the second point.

6. The method of claim 2 wherein the step of correlating the signals received by each antenna with themselves further comprises:

separating the signals received by each antenna into a first portion representing a first band of frequencies received by each said antenna and a second portion representing a second band of frequencies received by each said antenna, and cross correlating said first and second portions of the signals received by each said antenna.

7. The method of claim 2 wherein the step of correlating the signals received by each antenna with themselves further comprises;

filtering the signals received by each antenna with a filter matched to the spectrum of a selected unknown function of time included in the modulation of said signals, and correlating the filtered signals.

8. The method of claim 2 wherein the step of correlating the signals received by each antenna with themselves further comprises:

(f) separating the signals received by each antenna into a first portion representing a first band of frequencies received by each said antenna and a second portion representing a second band of frequencies received by each said antenna;

(g) filtering each of said portions with a filter matched to the spectrum of a selected unknown function of time included in the modulation of said signals; and

(h) cross correlating said separated and filtered portions of the signals received by each said antenna.

9. Apparatus for determining position information from suppressed-carrier radio signals modulated by an unknown pseudo-random code, transmitted by earth-orbiting satellites, comprising:

(a) an omni-directional antenna for simultaneously receiving signals from a plurality of satellites;

(b) means for generating predictions of the Doppler shifts of the signals received by said antenna;

(c) means for correlating the signals received by said antenna with the predictions to cancel the Doppler shifts and means for correlating the signals received by said antenna with themselves to square the unknown pseudo-random code modulation so that the resulting correlation products have slowly varying phase angles related to the phases of carrier waves implicit in the received signals;

(d) means for time-averaging said resulting correlation products to obtain data indicating the phase of a carrier wave implicit in the signal received from each satellite, for a plurality of satellites simultaneously; and

(e) means for processing said data indicating the phase of a carrier wave implicit in the signal received from each satellite to obtain position information.

10. The apparatus of claim 9 wherein the means for correlating the signals received by said antenna with themselves further comprises:

means for separating the signals received by said antenna into a first portion representing a first band of frequencies received by said antenna and a second portion representing a second band of frequencies received by said antenna, and means for cross correlating said first and second portions received by said antenna.

11. The apparatus of claim 9 wherein the means for correlating the signals received by said antenna with themselves further comprises:

means for filtering the signals received by said antenna with a filter matched to the spectrum of a selected unknown function of time included in the modulation of said signals, and means for correlating the filtered signals.

12. The apparatus of claim 9 wherein the means for correlating the signals received by said antenna with themselves further comprises;

(f) means for separating the signals received by said antenna into a first portion representing a first band of frequencies received by said antenna and a second portion representing a second band of frequencies received by said antenna;

(g) means for filtering each of said portions with a filter matched to the spectrum of a selected unknown function of time included in the modulation of said signals; and

(h) means for cross correlating said separated and filtered first and second portions of the signals received by said antenna.

13. Apparatus for determining the position of a first point with respect to the position of a second point using suppressed-carrier radio signals modulated by an unknown pseudo-random code, transmitted by earth-orbiting satellites and received at said first and second points, comprising;

a first omni-directional antenna at said first point and a second omni-directional antenna at said second point for simultaneously receiving signals from a plurality of satellites;

(b) means for generating predictions of the Doppler shifts of the signals received by each antenna;

(c) means for correlating the signals received by each antenna with the predictions to cancel the Doppler shifts and means for correlating the signals received by each antenna with themselves to square the unknown pseudo-random code modulation so that the resulting correlation products have slowly varying phase angles related to the phases of carrier waves implicit in the received signals;

(d) means for time-averaging said resulting correlation products to obtain data indicating the phase of a carrier wave implicit in the signal received from each satellite, at each antenna, for a plurality of satellites simultaneously; and

(e) means for combining said phase indication data from both antennas to determine the position of the first point with respect to the second point.

14. The apparatus of claim 13 wherein the means for correlating the signals received by each antenna with themselves further comprises:

means for separating the signals received by each antenna into a first portion representing a first band of frequencies received by each said antenna and a second portion representing a second band of frequencies received by each said antenna, and means for cross correlating said first and second portions received by each said antenna.

15. The apparatus of claim 13 wherein the means for correlating the signals received by each antenna with themselves further comprises:

means for filtering the signals received by each antenna with a filter matched to the spectrum of a selected unknown function of time included in the modulation of said signals, and means for correlating the filtered signals.

16. The apparatus of claim 13 wherein the means for correlating the signals received by each antenna with themselves further comprises;

(f) means for separating the signals received by each antenna into a first portion representing a first band of frequencies received by each said antenna and a second portion representing a second band of frequencies received by each said antenna;

(g) means for filtering each of said portions with a filter matched to the spectrum of a selected unknown function of time included in the modulation of said signals; and

(h) means for cross correlating said separated and filtered first and second portions of the signals received by each said antenna.