Communications: directive radio wave systems and devices (e.g. – Directive – Including antenna orientation
Reexamination Certificate
1999-06-29
2001-02-06
Tarcza, Thomas H. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including antenna orientation
C342S077000, C342S357490
Reexamination Certificate
active
06184825
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to satellite radio frequency (RF) beam pointing. In particular, the present invention relates to integrating mechanical and electronic beam pointing in a feedback controlled beam pointing method and apparatus.
Satellites use RF beam pointing techniques to point an antenna at terrestrial and space based targets. The targets may be of interest for space/ground communication, space/space intersatellite links, and for radar beam directed imaging, as examples. Two beam pointing techniques are commonly used: mechanical beam pointing and electronic beam pointing.
Mechanical beam pointing involves mechanically moving or slewing a satellite, or individual antennas on the satellite, to direct the beam generated by the antenna to a particular target. Mechanical pointing can be cost effective for certain applications, but often body and antenna dynamics can result in low to moderate slew rates.
Moreover, because satellites are not perfect rigid bodies, the satellite may take a significant amount of time to dynamically settle, during which beam pointing is relatively inaccurate. Therefore, during the time it takes the satellite and its components to settle, the system is generally non-operational or suffers significant performance degradation. As a general rule for radar satellite imaging systems, imaging is suspended until the pointing error due to dynamic settling of the satellite reaches {fraction (1/10)} or {fraction (1/20)} of a beamwidth or less.
Referring now to
FIG. 1
, the target access regions
102
,
104
for a typical synthetic aperture radar (“SAR”) imaging satellite are shown. A SAR system relies on relative motion to increase its effective imaging aperture and therefore has difficulties imaging directly below, directly in front, or directly behind the direction of flight. Attenuation and power constraints limit imaging at long distances, near the Earth limb. The result is a “butterfly” instantaneous imaging field-of-regard (“FOR”). In
FIG. 1
, the FOR is assumed constrained by a 70 degree ground elevation angle (GEA)
106
and a 20 degree GEA
108
.
The satellite direction of travel
110
and apparent target motion
112
are also shown.
The target must remain inside the FOR for the duration of the image. Orbits with relatively low altitudes are often desired to reduce radar power, but these orbits also result in rapid (approximately 7 km/sec) relative satellite motion with respect to the ground targets, such that targets remain inside the FOR for relatively short durations (for example, less that one minute). Because multiple targets are often of interest inside the FOR, there is a strong motivation to image each target as quickly as possible.
As will be explained in more detail below with regard to
FIGS. 2 and 3
, however, mechanical slew induced settling errors prevent the satellite from accurately imaging the target for significant amounts of time. The resolution of each target, the total number of targets that may be imaged in a FOR, and the overall effectiveness of the radar imaging system are correspondingly reduced.
FIG. 2
shows a position error profile
200
for a computer simulation of a mechanical RF beam pointing system used on board a low earth orbit (“LEO”) satellite. The position error profile
200
results from the mechanical slew angle profile
300
shown in FIG.
3
. The simulation represented in
FIG. 3
assumes a RF beamwidth of approximately 0.2 degrees and a 12 second simulated mechanical satellite body slew (beginning at t=0) of 90 degrees to adjust the attitude of the satellite and its rigidly mounted antenna. The settling time required before the pointing accuracy required for nominal operation (approximately 0.01 degree-0.02 degree as indicated by reference numeral
202
) was reached was approximately sixteen seconds (from t=12 to approximately t=28).
Thus a significant fraction of the overall available satellite time must be spent waiting for the satellite to slew and settle before capturing images. Unfortunately, precise mechanical pointing with rapid settling is extremely expensive and extremely difficult to implement.
The long slew times and long settling times associated with mechanical pointing systems are not present in electronic pointing systems. Moreover, electronic pointing systems are often more accurate than mechanical pointing systems because jitter and body dynamics associated with mechanical pointing and control hardware are not experienced. However, eliminating all mechanical pointing through the implementation of a broad angle two dimensional (e.g, steerable in azimuth and elevation) phased array is extremely costly and complex.
Primarily, broad angle two dimensional electronic beam pointing is prohibitively expensive because it requires a great number of variable time delay transmit/receive (“TR”) modules and RF radiating and receive elements closely spaced together. Furthermore, physical constraints on TR module separation may also limit angular coverage. Another significant drawback of a broad angle two dimensional electronic beam pointing system is the increased backend signal generation and signal processing complexity (as well as increased system power and weight) required to properly operate the two dimension phase array.
Radar is only one example of an application adversely effected by settling errors. As another example, communications applications also suffer from mechanical slew induced settling errors. Because reliable communication requires accurate alignment of transmit and receive antennas, antenna mispointing resulting from settling errors may compromise, as examples, the length of time two entities may communicate, the reliability of the communication, or the rate of communication.
A need has long existed in the industry for a method and apparatus for RF beam pointing with the low cost, broad area coverage features of mechanical pointing and the high accuracy, rapid pointing capability of electronic beam steering.
BRIEF SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved RF beam pointing apparatus and method.
It is an additional object of the invention to provide a method and apparatus for beam pointing with the low cost and broad area coverage features of mechanical pointing and the high accuracy and rapid pointing capability of electronic RF beam pointing.
It is a further object of the invention to provide a feedback controlled apparatus and method for RF beam pointing.
It is a still further object of the invention to provide a RF beam pointing apparatus and method for use with transmit only, receive only, or transmit and receive radar and communications applications.
One or more of the foregoing objects are met in whole or in part by the present invention which provides a method and apparatus for compensating for the effects of mechanical slew induced dynamic settling errors on antenna pointing. A mechanical slew first occurs on a satellite carrying an antenna electronically steerable in at least one dimension. The antenna may be a phased array antenna, for example, and the mechanical slew may be a mechanical pointing maneuver of the satellite itself (e.g., a body slew using thrusters) or the antenna itself (e.g., by actuating antenna mounted gimbals). In response to dynamic settling antenna pointing errors resulting from the mechanical slew, the method performs electronic attitude correction. The mechanical slew thus provides coarse broad area pointing while the electronic attitude correction provides precise, narrow angle, rapid pointing.
The electronic attitude correction includes determining antenna attitude based on a current satellite attitude provided by a satellite attitude reference system, comparing the current antenna attitude to a desired antenna attitude, and electronically steering the antenna toward the desired antenna attitude. The dynamic settling induced antenna pointing errors are thereby reduced to within a predetermined pointing accuracy for nominal operation almost immediately aft
Sherwood Richard B.
Simmons, Jr. Edward J.
Wehner James W.
Phan Dao L.
Tarcza Thomas H.
TRW Inc.
Yatsko Michael S.
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