Networked differential GPS system

A differential GPS (DGPS) system for providing GPS correction information to a plurality of users according to each user's position, comprising:

• a regional network of a plurality of reference stations for tracking a plurality of satellites (SV's) in the GPS constellation of satellites wherein at least five reference stations track each SV with unauthorized GPS receivers;
• a central processing facility (CPF) in communication with the regional network of reference stations and having means for computing satellite orbit and clock errors and self-synchronizing clock means for deriving a DGPS system clock from a plurality of said unauthorized GPS receivers;
• and
• communication means connected to the CPF for obtaining reference station measurements and for distributing DGPS corrections to users, wherein the use of complex ionospheric and tropospheric delay models are avoided and authorized GPS receivers are unnecessary.

A dual-frequency differential GPS network (DGPS), comprising:

• a plurality of reference stations not exceeding thirty-three in number substantially equally distributed around the earth at locations on land for continuous tracking of every one of the satellites in the GPS constellation by at least one of the reference stations at any one time;
• a central processing facility (CPF) in communication with all of the reference stations;
• a plurality of dual-frequency non-authorized GPS receivers wherein at least one each is located at each of the reference stations and provide measurements of signals from various GPS satellites for the computation of ionospheric delays in the local area of the respective reference station;
• and
• computing means in communication with the plurality of reference stations for estimating global ionospheric delays for a plurality of user locations and for computing satellite orbit and clock corrections without authorized precision code access.

A method of reducing data communications volume and bandwidth in a networked differential GPS system comprising a plurality of reference stations, a central processing facility (CPF) and group control centers, the method comprising the steps of:

• estimating GPS satellite orbit ephemerides in four-dimensions and in real-time at a CPF or a group control center;
• communicating GPS satellite clock corrections derived from the step of estimating at a first update rate from the CPF or group control centers to a plurality of users;
• and
• communicating GPS satellite orbit corrections derived from the step of estimating at a second update rate from the CPF or group control centers to a plurality of users, said second update rate being substantially slower than said first update rate.

A method of producing real-time GPS satellite corrections in a networked differential GPS system, the method comprising the steps of:

• receiving a plurality of GPS position signals with a plurality of codeless GPS receivers distributed amongst a plurality of reference stations;
• processing said GPS position signals with a GPS navigation processor to determine orbit position and orbit ephemerides for at least one orbiting GPS satellite;
• estimating GPS satellite orbit position errors and their orbit ephemerides from said GPS positions signals with a least squares estimator;
• and
• estimating GPS satellite clock errors from said GPS position signals with a least-squares estimator.

A method of estimating GPS satellite orbit parameters to produce stable estimates that stay fresh for at least ten minutes, the method comprising the steps of:

• receiving a plurality of GPS position signals with a GPS receiver;
• processing said GPS position signals with a GPS navigation processor to determine orbit position and orbit ephemerides for at least one orbiting GPS satellite;
• initializing an epoch counter;
• estimating the XYZ orbit position of a plurality of GPS satellites received by said GPS receiver at one epoch;
• repeating the estimating of the XYZ orbit position for at least ten minutes;
• and
• feeding said orbit position estimates gathered over an epoch to an orbit relaxation estimator which estimates at least fifteen orbit parameter corrections each thirty to sixty minutes.

A rapid GPS network space vehicle (SV) orbit ephemeris parameter estimator that receives both SV position and velocity such that an immediate Kepler is available, comprising:

• means for obtaining measurements from a plurality of reference stations comprising carrier-smoothed pseudoranges that have been corrected for both ionospheric and tropospheric delays and that produces an ionospheric delay measurement;
• a central processing facility for computing SV positions from said reference station measurements;
• carrier correction means for correcting an SV carrier for both ionospheric and tropospheric delays at each reference station thereby providing a precise measurement of corresponding SV velocity;
• estimating means for generating a set of SV orbit ephemeris parameters which process all of said reference station measurements through one of a least-squares estimator or Kalman filter;
• and
• means for broadcasting said set of SV orbit ephemeris parameters to a plurality of users such that user navigation is substantially improved thereby.

A GPS network space vehicle (SV) orbit ephemeris estimator, comprising:

• means for obtaining both code and carrier measurements from a dual-frequency GPS receiver of ionospheric delays over a period not longer than the period an individual SV is in view from a single reference station in a sparse network of reference station receivers tracking SV's in the GPS constellation, wherein each reference station comprises means for ionospheric processing with a codeless L2 receiver to use night-time measurements that represent stable initial estimates of ionosphere and for using ionospheric rates from L2 to get estimates of ionospheric delays at zenith;
• means for preprocessing measurements to remove tropospheric and measured ionospheric signal delays;
• means for estimating SV orbit ephemeris parameters using one of a least-squares estimator and Kalman filter;
• and
• means for broadcasting SV orbit ephemeris information to a plurality of users.
• means for estimating SV orbit ephemeris parameters using one of a least-squares estimator and Kalman filter;
• and
• means for broadcasting SV orbit ephemeris information to a plurality of users.

A differential GPS (DGPS) system that provides GPS correction information to a plurality of users according to each user's position, comprising:

• a global network of a plurality of reference stations for tracking a plurality of satellites (SV's) in the GPS constellation of satellites wherein at least five reference stations track each SV;
• a central processing facility (CPF) having means for computing satellite orbit and clock errors, comprising:
• means for using carrier-smoothed pseudoranges that have been tropospherically and ionospherically corrected as inputs to construct a measurement vector Y.sub.k that contains all of a plurality of pseudoranges at one epoch from a plurality of reference stations;
• means for selecting a linear transformation matrix, [D.sub.OR ], in which the product of the matrix and a measurement matrix [B] of partials of pseudorange errors with respect to reference receiver clock errors, is zero:[D.sub.OR ][B]=0, said linear transformation being used to eliminate unwanted reference receiver clock errors in Y.sub.k which leads to a corrected measurement vector y.sub.k ;
• and
• a least-squares estimator for computing SV position errors x.sub.k for all SV's at one epoch from y.sub.k ;
• and
• communication means connected to the CPF for obtaining reference station measurements and for distributing DGPS corrections to users.

A differential GPS (DGPS) system that provides GPS correction information to a plurality of users according to each user's position, comprising:

• a global network of a plurality of reference stations for tracking a plurality of satellites (SV's) in the GPS constellation of satellites wherein at least five reference stations track each SV;
• a central processing facility (CPF) having means for computing satellite orbit and clock errors;
• and
• communication means connected to the CPF for obtaining reference station measurements and for distributing DGPS corrections to users;
• wherein each of the plurality of reference stations include means for using measurements from the SV's to estimate a zenith component of ionospheric delay at each reference station;
• and
• the CPF comprises means to combine said zenith components to calibrate a model of ionospheric delay as a function of time and comprises means to distribute said model to a plurality of single-frequency users.

A differential GPS (DGPS) system that provides GPS correction information to a plurality of users according to each user's position, comprising:

• a global network of a plurality of reference stations for tracking a plurality of satellites (SV's) in the GPS constellation of satellites wherein at least five reference stations track each SV;
• a central processing facility (CPF) having means for computing satellite orbit and clock errors;
• and
• communication means connected to the CPF for obtaining reference station measurements and for distributing DGPS corrections to users;
• wherein each reference station comprises means for the measurement of slant ranges that are mapped to reference station coordinates.

A differential GPS (DGPS) system that provides GPS correction information to a plurality of users according to each user's position comprising:

• a global network of a plurality of reference stations for tracking a plurality of satellites (SV's) in the GPS constellation of satellites wherein at least five reference stations track each SV;
• a central processing facility (CPF) having means for computing satellite orbit and clock errors;
• and
• communication means connected to the CPF for obtaining reference station measurements and for distributing DGPS corrections to users;
• wherein the CPF comprises means for generating quality control information by using over-specified reference station information and applying a root-sum-of-squares algorithm.

A differential GPS (DGPS) system that provides GPS correction information to a plurality of users according to each user's position, comprising:

• a global network of a plurality of reference stations for tracking a plurality of satellites (SV's) in the GPS constellation of satellites wherein at least five reference stations track each SV;
• a central processing facility (CPF) having means for computing satellite orbit and clock errors;
• and
• communication means connected to the CPF for obtaining reference station measurements and for distributing DGPS corrections to users;
• wherein the CPF comprises means for building SV orbits generating real-time ephemerides in a two step least-squares estimator which estimates the XYZ position of SV's and then estimates their orbital ephemeris terms.