Patent Application
20110156956
Earth station to satellite communication systems require a tracking system that maintains a precision Earth station antenna orientation with the target satellite. For large antennas to track satellites with non-trivial astrodynamics, for example operating in the Ka band, fine movement control is required. Tracking systems that orient the entire antenna assembly with a high level of precision, including main reflectors that may be of significant dimensions, may be cost prohibitive.
Commonly owned U.S. Pat. No. 6,943,750, “Self-Pointing Antenna Scanning” issued Sep. 13, 2005 to Brooker et al, hereby incorporated in its entirety by reference, discloses a motorized subreflector with orthogonal adjustment capability via x and y axis drive screws to move the subreflector with respect to the main reflector to achieve a limited range of antenna beam orientation, separate from manipulation of the primary antenna mount supporting the entire antenna assembly. Feedback loops incorporating the received signal characteristics may be used to enable precision tracking. However, the tracking accuracy is limited by the time requirements for the drive screws to move forward and back, driving the subreflector past an optimal orientation to obtain a signal peak indication.
Therefore, it is an object of the invention to provide an apparatus that overcomes deficiencies in the prior art.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the general and detailed descriptions of the invention appearing herein, serve to explain the principles of the invention.
The inventor has recognized that a subreflector tracking system incorporating sensor feedback of the precise subreflector position may be obtained by combining conical-scan subreflector tracking with predictive or adaptive main reflector tracking to allow a Ka-band antenna system to track a non-geostationary satellite without a true monopulse receiver tracking system and corresponding high-accuracy main reflector positioner hardware.
One skilled in the art will recognize that the subreflector tracking system and method(s) disclosed herein, in addition to tracking variances from geosynchronous satellite orbits with high precision may also be used to track a wide range of satellite orbits, for example inclined orbit geosynchronous satellites and/or lower altitude orbits.
A typical satellite communications earth station antenna system for use with the invention includes:
These elements are well known in the art, and as such are not described with greater detail herein.
A nutating movement capability is added to the subreflector to generate a conical scan, for example by a spinning turntable or shaft upon which the subreflector is mounted, slightly off center. Thus, as the subreflector rotates, a nutation/conical scan is generated with a magnitude proportional to the offset distance between the center of the subreflector and the axis of rotation.
A sensor array such as a resolver, synchro, Hall effect or the like operative as position sensor(s) are provided to sense the current angular position of the subreflector with a high level of precision and a sample frequency corresponding at least to the subreflector speed of rotation. An angle and velocity estimator module receives the position sensor inputs and outputs an estimated current angle and sync pulse, such as a position sensor reporting top dead center of the subreflector. The angle and velocity estimator may also receive rotation control commands from a supervisory module and also output speed control feedback to a motor speed control driving the motor rotating the subreflector. The angle and velocity module estimator and motor speed control may be remote mounted proximate the subreflector, with a data network connection, for example via Ethernet or optical fiber to the supervisory module.
A satellite signal receiver system capable of determining the instantaneous strength of a reference signal such as a continuous wave RF beacon is coupled to the reflector antenna, detecting signal strength variances as the subreflector rotates through the conical scan.
An antenna control system capable of measuring or estimating the instantaneous angle of rotation of the nutating subreflector and digitally processing the received signal strength from the receiver system over, for example, each rotation of the nutating subreflector to produce an error vector for driving the antenna to peak on the reference signal.
A tracking algorithm designates the angle of rotation from each revolution where the peak signal is detected as the error vector and converts this error vector, for example, to x and y axis drive commands for the positioner motors, for example drive screws and/or gear or belt driven slides or the like to move the sub reflector carriage towards the peak signal location. Further, where the range of signal beam movement obtainable by subreflector tracking is approached, the tracking algorithm may output lower resolution drive instructions to the antenna main mount and drive the subreflector into the other end of its drive range, in anticipation of the main mount displacement.
The tracking algorithm may be selected from a range of different tracking algorithms according to the drive resolution, processing power available and the expected type of target satellite orbit and/or orbital distortions.
A first tracking algorithm is general predictive pointing with empirical optimization. This is accomplished as follows:
The main reflector may be driven continuously typically using predictions based on furnished Keplerian orbital elements and well-known astrodynamic calculations to produce a local look angle (either az/el or hour-angle/declination). This will allow the system to approximately track the satellite; however, path distortions such as refraction and scintillation, mechanical distortions in the antenna, and small errors in the Keplerian elements may produce a significant error in the tracking that is difficult to correct without feedback.
To correct these errors, the subreflector may be independently allowed to “float” to a continuously determined peak by using the error vector from the nutating subreflector (or using other techniques using only az and er displacement and parabolic curve fitting, for example as described in U.S. Pat. No. 6,657,588, “Satellite Tracking System Using Orbital Tracking Techniques” issued Dec. 2, 2003 to Strickland et al, hereby incorporated by reference in its entirety) processing system to actuate the X-Y carriage. When the signal is peaked, the conical scan produced by the nutating subreflector will center on the peak signal. As long as the tracking error of the main reflector is less than the range of the subreflector tracking systems, the system can track with only the additional cost of the small loss caused by the nutating subreflector's offset.
As the nutating subreflector floats, if an offset from center is persistent, the main reflector can be offset as well to center the subreflector, or to advance or retard the timing of the orbital track.
A second possible tracking algorithm is completely empirical pointing, accomplished as follows:
The main reflector is not initially driven continuously using predictions but rather to react to measured movement detected by the subreflector. As the nutating subreflector floats, the angular velocity of the target can be measured, and the control system directed to drive the main reflector at a continuous rate that matches this angle, and the floating nutating subreflector again optimizes the look angle. If a persistent bias is determined in the look angle, again, the main reflector look angle can be offset, or the rates changed, to adjust the timing of the orbital track. It is also possible to use a path planner to split movements between the main reflector and the subreflector movements.
In any case, as long as the sub-reflector is kept on the peak and within its limits of travel the system can track the satellite even if the main reflector's control system introduces errors larger than the half-power beamwidth of the antenna. The result is a significantly less expensive main beam mount (due to the complete elimination of the need for, for example, sub-10-arc-minute tolerances in positioning) and the elimination of the need for a complex receiver subsystem.
An exemplary subreflector 2 with a nutation mechanical arrangement 1 may be implemented, for example as shown in
The position sensor(s) 22 may be mounted to sense passage of a target or the like as best shown on
The data network message(s) 33 may be de-jittered using a, for example, software implemented phase-locked-loop in a nutator model 40, which estimates the current nutation angle 42 output to an accumulator control 44 that feeds into an array of signal accumulator(s) 46 (X+, X−, Y+, Y−) which integrate the nutation angle 42 with a corresponding signal strength 48 from the tracking receiver 34.
The tracking receiver 34 is sampled a number of times per revolution, for example 32 times per revolution, and the samples are integrated selectively based on the quadrant that the present nutation angle 42 is in to create an error vector. Assuming that b[0] through b[32] are the samples, Gx and Gy are the feedback gains for a simple control loop, and that b[0] is taken at top dead center, the nutated samples give direct errors errorx and errory as:
With the resulting error (x,y) then output as drive instructions 52 from a latch 50 synced by the sync pulse 26 for the respective x and y axis positioner controller(s) 54 of the subreflector 2 carriage and/or main drive.
One skilled in the art will recognize that the present invention represents a significant improvement to prior satellite earth station antenna tracking apparatus, systems and methods. Further, the solution(s) provided are lightweight, compact and low power. Thereby, improved cost, manufacturing, operation and/or maintenance efficiencies may be realized.
Where in the foregoing description reference has been made to ratios, integers, components or modules having known equivalents then such equivalents are herein incorporated as if individually set forth.
While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus, methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of applicant's general inventive concept. Further, it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope or spirit of the present invention as defined by the following claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US09/68589 | 12/17/2009 | WO | 00 | 2/25/2011 |
| Number | Date | Country | |
|---|---|---|---|
| 61138148 | Dec 2008 | US |
