Data processing: measuring – calibrating – or testing – Measurement system – Orientation or position
Reexamination Certificate
2001-02-01
2003-07-22
Bui, Bryan (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Orientation or position
C702S094000, C702S150000, C701S200000, C701S213000, C701S215000, C342S357490, C342S357490, C342S423000
Reexamination Certificate
active
06598009
ABSTRACT:
DOCUMENT DISCLOSURE REFERENCE
This application claims priority of Disclosure Document No. 467,710, filed Jan. 18, 2000.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention generally relates to radio navigation techniques using Global Positioning System (GPS) signals. More particularly, the present invention relates to the augmentation of conventional GPS receivers with attitude determination capability that can operate in an interference-rich environment by using adaptive nulling and constrained beamforming monopulse techniques.
2. Description of the Prior Art
GPS is one of the most advanced satellite radio navigation systems, maintained by the government of the United States of America. GPS radio navigation relies upon a constellation of twenty-four active satellites in six different orbits around the globe. GPS position fixes are obtained by measuring the propagation delay times of GPS radio signals broadcast by the orbiting GPS satellites. Normally, a user must receive the signals from at least four GPS satellites in order to solve the variables of longitude, latitude, and altitude, as well as timing error that are needed to precisely determine location and time. As its name implies, GPS is a positioning device. In order to see at least four satellites simultaneously on or near the earth, an antenna with a hemispheric reception gain pattern is typically used. See B. W. Parkinson and J. J. Spilker Jr. (eds.),
Global Positioning System: Theory and Applications
, Published by the American Institute of Aeronautics and Astronautics, Inc., 1996 and E. D. Kaplan (ed.),
Understanding GPS: Principles and Applications
, Artech House Publishers, 1996 for a detailed description of GPS and its operations and applications.
GPS phase measurements turn out to be of very good quality, and this has prompted a widespread use of interferometric techniques. More successful use is in surveying applications with double-difference kinematic GPS. GPS interferometric attitude determination has also seen some applications. GPS interferometric attitude determination requires at least three antennas at the tips of two non-parallel baselines separated by several wavelengths. For a given level of phase measurement quality, the attitude estimation accuracy of the technique is proportional to the baseline length. However, continuous outputting of attitude information hinges on reliable resolution of phase integer ambiguity and repair of cycle slips after their detection. Both techniques are computationally very demanding in real time. Furthermore, most interferometric configurations for GPS attitude determination do not include any provision against the presence of interference such as jamming and/or multipath.
Recently, array antennas have been introduced mostly in military applications for jam-suppression and signal to noise ratio (SNR) enhancement. It is therefore an object of this invention to utilize array antennas for GPS attitude determination in addition to conventional positioning and timing with an improved capability of operating under interference.
However, a straightforward combination of GPS interferometry and multiple antenna arrays for attitude determination, assuming it would work, is not cost-effective. Each antenna array with an anti-jam spatial filter may serve as a jam-free antenna. However, the spatial filters associated with these antenna arrays have to affect the signal in a very similar manner in order to preserve the gain and phase relationships. This is too costly and cumbersome. More critically, anti-jamming manipulation of GPS signals would introduce adverse effects on kinematic carrier phase attitude determination that are difficult to constrain or minimize. That is, the very same carrier phase measurements needed for attitude estimation have been adjusted in gain and/or phase by an anti-jam spatial filter in antenna electronics to attenuate jammers. Even if a jamming signal is totally suppressed from the output of an anti-jam spatial filter that inputs from an antenna array, the jamming signal still corrupts individual array antenna elements. As a result, the spatially filtered antenna array signal, as a whole, can be used for positioning but not for attitude determination. This is because of unpredictable phase distortion. Similarly, individual antenna elements cannot be used for attitude determination, as in a conventionally interferometric manner, because these antenna elements still contain jamming signals. It is therefore an object of this invention to seek other rather interferometric techniques for GPS attitude determination under interference.
Quinn and Crassidis (see D. A. Quinn and J. C. Crassidis, “GPS ‘Compound Eye’ Attitude Sensor,”
NASA Tech Briefs
, May 1999) outlines an approach, different from interferometry, to attitude determination using GPS signals. In their design, multiple directional antennas are mounted on a convex hemispherical surface with polyhedral arrangement. Each antenna is thus aimed to receive GPS signals from a certain field of view, called a visualization cone. Their idea is to use the special GPS antennas arranged in a “compound eye” as star trackers and the GPS satellites as well-known, well-behaved stars. As GPS satellites pass through the various fields of view, attitude can be determined in a manner identical to what has been employed by standard star trackers for many years. Compared to interferometric GPS, the “compound eye” is easy to get a first fix to attitude and its accuracy is not limited by structure size. Because it does not require carrier phase or Doppler shift estimation, it is much less sensitive to Doppler and multipath effects as well as line biases. The antenna geometry provides maximum sky coverage without the need for self-survey calibrations. However, the attitude estimation accuracy of the “compound eye” depends upon the number of antennas and, more critically, upon the precision at which individual antenna patterns can be physically shaped, oriented, and mounted. The latter is imaginably associated with substantial cost of manufacturing and installation particularly when it is small.
Alternative to the interferometric GPS and different from the special antenna system of a “compound eye” attitude sensor, the present invention solves the directions of arrival of GPS incident signals for attitude determination using GPS monopulse. Monopulse is a mature technique widely used in surveillance and tracking radar. See, for example, D. K. Barton,
Modern Radar Systems Analysis
, Artech House, 1988, M. Sherman,
Monopulse Principles and Techniques
, Artech House, 1988, and I. Leanov and K. I. Fomichev,
Monopulse Radar
, Artech House, 1986. In simple terms, monopulse angular measurement is based upon distinct angular responses of directional antennas (i.e., with finite beamwidth) to signals incident from different directions. For each angular dimension (azimuth or elevation), two squint antenna patterns are formed with an angular displacement typical half of the beamwidth, from which a pair of sum and difference beams are generated. The ratio of the sum over difference beam responses provides the measurement of an angular offset of the incident wavefront off the array boresight. The squint beams can be generated either physically with directional antennas or with omnidirectional antenna arrays electronically or digitally in a digital beamforming process.
In general, angular accuracy, in terms of estimation error, is inversely proportional to aperture size. For an array antenna with a small number of elements, the GPS monopulse technique may provide an angular solution less accurate than that of the GPS interferometry technique. However, the latter can only do so if good phase measurements are available as previously analyzed. Besides, it may suffer from the problem of carrier cycle integer ambiguity, further complicated by cycle slip. Its solution cannot be made available until code and carrier tracking have been achieved and maintained in the receiver. In comparison, the GPS monopulse technique is fast
Bui Bryan
LaMorte & Associates
Vo Hien
LandOfFree
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