A method of determining the attitude of a vehicle equipped with a Global Positioning System (GPS) receiver with multiple nonaligned antennas has been invented. As used here, “nonaligned antennas” does not signify antennas that lack alignment; rather, it signifies antennas that generally point in different directions (in contradistinction to antennas oriented identically as in prior methods). The method is applicable to a land vehicle, aircraft, spacecraft, ship, or almost any other vehicle.
For the purpose of this or any other method of determining attitude from GPS measurements, the positions and orientations of the antennas with respect to the vehicle and to each other must be known. In general, the attitude of the vehicle, referenced to lines of sight between the vehicle and a set of GPS satellites, is determined from differences among the phases of the GPS carrier signals received by the various antennas.
Prior methods of determining attitude from GPS measurements are based partly on the assumption that the antennas are oriented identically. This assumption simplifies attitude determination in the following way: GPS signals are right-hand circularly polarized (RHCP). In the case of identically oriented antennas, the effects of the circular polarization on the phases of the signals received by all the antennas are equal; therefore, the phase-difference measurements and the attitude determination are unaffected by the circular polarization.
When the antennas are nonaligned, the phase effects of the circular polarization differ among antennas, so that the prior methods cannot be used. In the present method, these effects are taken into account, making it possible to use nonaligned antennas for attitude determination. The major advantage gained through this method is that by using a sufficient number of antennas pointed in different directions, one can maximize the probability that a sufficient number of GPS satellites will be in view at any given time, making it possible to determine the current attitude of the vehicle, regardless of its position, attitude, or state of motion (the vehicle could even be tumbling).
The figure schematically depicts selected aspects of a typical GPS receiver system that implements the present method. Preferably, there are eight antennas and the radio-frequency (RF) section of the receiver can accommodate as many as six GPS channels per antenna. The signals in the various channels are digitized, then sent to a processor that includes a navigation module and an attitude module. The navigation computations and most of the attitude computations are performed according to conventional GPS methods. The distinct aspect of the present method lies in the software of the attitude module. This software provides phase-difference corrections based on the calculable effects of (1) the known relative positions and orientations of the antennas and (2) the phases of the RHCP GPS signals that they receive.
The primary obstacle to incorporating the RHCP phase-difference corrections into the attitude computations is the fact that the attitude of the vehicle is embedded in expressions of the reference frames of the various antennas. A circular-reasoning condition arises in that to calculate the attitude, it is necessary to know the RHCP corrections, but to calculate the RHCP corrections, the attitude must be known. If the attitude of the vehicle is already approximately known, perhaps from a prior estimate, the RHCP phase corrections can be estimated and then iterated, along with a new attitude solution, to convergence toward a final attitude solution. If no prior estimate of attitude is available, then an initial estimate can be constructed, for example, by use of the line of sight from each antenna to the source of the strongest GPS signal received on that antenna.
This work was done by Edgar Glenn Lightsey of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronic Components and Systems category.
This invention has been patented by NASA (U.S. Patent No. 6,005,514). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Goddard Space Flight Center; (301) 286-7351. Refer to GSC-13907.