Tight coupling of GPS and navigation estimates with reducer or eliminated inertial measurement unit data
A system and method for generating a navigation solution in high interference and dynamic environments using Global Positioning System (GPS) and navigation such as dead reckoning is described. The system configuration is a multi-satellite tracking loop structure obtained by closing each satellite's tracking loop through a multi-state navigation filter. This generates a robust navigation solution that can track GPS signals in a lower signal to noise ratio than can the standard GPS tightly coupled tracking loops. The system contains an Advanced Tightly Coupled (ATC) tracking processor which accepts early, late, and on-time I and Q data from the GPS signal tracker and outputs vehicle to satellite range, range rate and range acceleration residual measurements to a navigation Kalman filter. The ATC includes nonlinear discriminators which transform I and Q data into residual measurements corrupted by unbiased, additive, and white noise. It also includes an amplitude estimator configured to operate in rapidly changing, high power noise; a measurement noise variance estimator; and a linear residual smoothing filter for input to the navigation filter.
1. A method of eliminating the use of an inertial measurement unit (IMU) for a global positioning system (GPS) receiver while still retaining anti-jamming benefits of tightly coupling with navigation estimates, the method comprising:
determining a position associated with the GPS receiver via GPS signals received by the GPS receiver such that the position is known, the GPS receiver having code and carrier tracking loops;
directly advancing, using course, speed, acceleration, and time obtained from GPS, the known position obtained via GPS to generate a propagated position associated with the GPS receiver for a next navigation filter cycle of the GPS receiver without the use of the IMU;
generating range and range rate estimates to satellites, wherein each range and range rate estimate for a particular satellite is generated by projecting (a) vehicle position, velocity, and attitude estimates and (b) satellite position, velocity, and acceleration (PVA) data, wherein the satellite PVA data is based on ephemeris data that had been retrieved from a navigation message from the particular satellite;
driving the code and carrier tracking loops of the GPS receiver based on the generated range and range rate estimates, which are based on the propagated position of the GPS receiver and on positions of satellites that are being tracked, such that the tracking loops are tightly coupled with the propagated position;
wherein determining, advancing, and driving are performed by the GPS receiver during tracking of previously acquired GPS signals such that the GPS signals remain tracked.
2. The method of claim 1 , wherein advancing the known position comprises multiplying a velocity vector by a measured time interval.
3. The method of claim 1 , wherein advancing the known position is performed only as a backup when the IMU fails or there is insufficient power for the IMU, and when an operating IMU is available and sufficient power is available, the known position is advanced using data from the IMU.
4. The method of claim 1 , further comprising projecting the propagated position along a line of sight computation for a satellite.
5. The method of claim 1 , wherein the method is performed in real time.
6. An apparatus for eliminating the use of an inertial measurement unit (IMU) while still retaining anti jamming benefits of tightly coupling with navigation estimates, the apparatus comprising:
a navigation processor configured to generate a propagated position for a next navigation filter cycle of a GPS receiver directly from course, speed, acceleration, and time, all of which are obtained via GPS, and a known position obtained via GPS without data from the IMU, wherein the propagated position and the known position are associated with the GPS receiver;
a navigation message decoder configured to calculate a satellite's position, velocity, and acceleration (PVA) based on ephemeris data that had been retrieved from a navigation message for the satellite;
code and carrier tracking loops that are configured to track GPS signals; and
a line of sight projection processor configured to update line of sight calculations based on the propagated position and positions of satellites being tracked;
wherein the code and carrier tracking loops are aided during tracking for previously acquired GPS signals for the next navigation filter cycle based on the updated line of sight calculations, such that the tracking loops are tightly coupled with the propagated position.
7. The apparatus of claim 6 , wherein the navigation processor is further configured to multiply a velocity vector by a measured time interval to advance the known position.
8. The apparatus of claim 6 , wherein the apparatus further comprises an IMU, wherein the navigation processor is further configured to generate the propagated position without data from the IMU only as a backup when the IMU fails or there is insufficient power for the IMU, and wherein the navigation processor is further configured to advance the known position based on IMU data when an operating IMU is available and sufficient power is available.
9. The apparatus of claim 6 , wherein the apparatus comprises a GPS receiver.