An improved method of calibrating a large directional radio antenna of the type used in deep-space communication and radio astronomy has been developed. This method involves a raster-scanning-and-measurement technique denoted on-the-fly (OTF) mapping, applied in consideration of the results of a systematic analysis of the entire measurement procedure. Phenomena to which particular attention was paid in the analysis include (1) the noise characteristics of a total-power radiometer (TPR) that is used in the measurements and (2) tropospherically induced radiometer fluctuations. The method also involves the use of recently developed techniques for acquisition and reduction of data. In comparison with prior methods used to calibrate such antennas, this method yields an order-of-magnitude improvement in the precision of determinations of antenna aperture efficiency, and improvement by a factor of five or more in the precision of determination of pointing error and beam width. Prerequisite to a meaningful description of the present method is some background information concerning three aspects of the problem of calibrating an antenna of the type in question:
- In OTF mapping measurements in which a TPR is used, the desired data are the peak temperature corresponding to a radio source, the pointing offset when the antenna is commanded to point toward the source, and the shape of the main lobe of the antenna beam, all as functions of the antenna beam elevation and azimuth angles. These data enable one to calculate the (1) antenna aperture efficiency by comparing the measured peak temperature with that expected for a 100-percent-efficient antenna, (2) the mechanical pointing error resulting from small misalignments of various parts of the antenna structure, and (3) misalignments of the antenna subreflector and other mirrors.
- For practical reasons having to do with obtaining adequate angular resolution and all-sky coverage, it is necessary to perform azimuth and elevation scans fairly rapidly.
- Many natural radio sources used in calibrating antennas are only approximately pointlike: some sources subtend angles greater than the beam width of a given antenna. In such a case, the antenna partially resolves the source structure and does not collect all of the radiation emitted by the source.
This makes it necessary to estimate how much of the total known radiation from the source would actually be collected by the antenna if it were 100-percent efficient. The resulting estimate, leading to a source-size correction factor, introduces another degree of uncertainty to the measurements. OTF mapping can remove this uncertainty. The key to using OTF mapping to solve all three aspects of the calibration problem is to maintain a constant, known angular velocity when scanning the antenna along a given direction. To ensure alignment of the individual subscans within the full raster, the angular position of the first data point of each subscan is determined from readings of azimuth-and elevation-angle encoders, while the angular positions of the rest of the subscan data points are determined by timing at the constant angular velocity.
Hence, if the TPR reading is sampled at a constant known rate, then the relative angular position at which each datum is taken is known with high accuracy, and antenna-settling time is no longer an issue. The data-acquisition algorithms used in OTF mapping provide for computation of the angular positions of radio sources, such that at any given time, the position of the antenna relative to a source is known. The acquisition of data in the OTF mode necessarily entails attenuation of high-frequency information as a consequence of the integration that occurs during the sampling intervals. The high-frequency information can be recovered in an inverse-filtering computation. Even though the antenna beam does not sample all of the radiation from an ex- tended radio source at a given instant, the completed raster scan does cover the entire solid angle subtended by the source and, hence contains a sampling of all the radiation from that source. Consequently, no source-size correction is necessary in OTF mapping. The resulting set of data registered on a two-dimensional field of sampling points (see figure) can be used to determine a least-squares-best-fit main beam pattern. The calibration parameters can then be determined from the main beam pattern.
This work was done by David Rochblatt, Paul Richter, and Philip Withington of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-30648
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On-the-Fly Mapping for Calibrating Directional Antennas
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