A technique for estimating the boresight direction of a Global Positioning System (GPS) receiver antenna involves utilization of the relationship between the strengths of received signals and the direction-dependent antenna gain pattern. The technique is fundamentally different from, and much less precise than, other attitude-determination techniques based on interferometry with multiple antennas. The major advantage of this technique is that it quickly gives a coarse estimate, using data from only one antenna. The coarse estimate is not suitable for fine-attitude applications like aiming a telescope or a laser beam, but it can be used, for example, to guide the orientation of a broad-beam communication antenna, to aim a solar panel, or to initialize a fine attitude-determination algorithm or instrument.
The technique is most easily practiced in the case of an antenna with a broad radiation pattern in which the gain decreases monotonically with increasing angle off boresight. The GPS receiver used in this technique must be one that generates data on the signal-to-noise ratio (SNR) of the signal received from each GPS satellite that it tracks. Once the GPS receiver has computed its position from the received GPS signals, the direction to each tracked GPS satellite is known as a byproduct.
The SNR of the signal received from each tracked GPS satellite is taken as a crude measure of the relative strength of the signal and, as such, is used as a weighting value to obtain a vector sum: The unit vector in the known direction to each tracked satellite is multiplied by the SNR for that satellite. The sum of such scalar·vector products for all the tracked satellites is a vector, the direction of which is taken to be the estimated antenna boresight direction. The length of the vector also constitutes ancillary information about the geometric properties of the constellation of tracked GPS satellites.
If only one GPS satellite is being tracked, then the estimated boresight points directly at that satellite; such an estimate is usually erroneous, but it could be helpful in finding other satellites to track and thus obtain a better estimate. When six to eight GPS satellites are being tracked, the estimated boresight differs from the actual boresight by no more than about 15°.
This work was done by Charles Dunn and Courtney Duncan of Caltech for NASA's Jet Propulsion Laboratory. NPO-20323
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Estimating attitude from GPS measurements on one antenna
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Overview
The document discusses the development and implementation of a novel technique for estimating the attitude of a GPS receiver antenna using a single antenna, specifically in the context of the GPS-MET instrument aboard the Microlab-1 satellite launched in April 1995. Traditionally, attitude determination required multiple antennas due to the reliance on interferometric techniques. However, this innovation allows for effective attitude estimation with just one antenna, which is a significant advancement in the field.
The primary challenge addressed in the document is the incorrect assumption made by the GPS receiver software regarding the antenna's pointing direction. Initially, the software assumed the antenna was pointing backward, leading to considerable data loss during off-pointing maneuvers when the antenna was actually directed sideways or forward. This limitation resulted in the tracking of only 3-4 satellites during such periods.
To overcome this issue, the development team, led by Dr. Dunn and Mr. Duncan, implemented an extended algorithm that utilized tracking data to estimate the antenna's pointing direction more accurately. This algorithm was uploaded to the GPS-MET receiver, resulting in a dramatic increase in the number of satellites tracked during off-pointing periods, from an average of 3-4 to 7-8 satellites.
The technique relies on satellite tracking data, which includes signal-to-noise ratio (SNR) information, a crude measure of the received signal level. The GPS-MET's simple patch antenna has a hemispherical gain pattern, with maximum gain near the boresite and declining gain as the angle from boresite increases. The algorithm calculates the position of each tracked GPS satellite, allowing the receiver to estimate the antenna's boresite direction based on the weighted sum of the SNRs from multiple satellites.
The current implementation achieves an accuracy of about 15 degrees, suitable for coarse attitude determination but not for fine applications like aiming narrow beam telescopes. Future improvements are anticipated, including better characterization of the antenna's gain pattern and using joint pointing solutions from antennas directed in different orientations to enhance three-dimensional platform orientation.
Overall, this document highlights a significant advancement in GPS technology, enabling more effective satellite tracking and attitude estimation with a single antenna, paving the way for various applications in space and communication technologies.
