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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Overview
The document titled "Technical Support Package for On-the-Fly Mapping for Calibrating Directional Antennas" focuses on the calibration techniques used for antennas in NASA's Deep Space Network (DSN). The DSN is crucial for maintaining communication links with various spacecraft involved in unmanned space exploration. To optimize the performance of these antennas, precise calibration is essential, which has become increasingly important as operating frequencies have risen from S-band (approximately 2.3 GHz) to Ka-band (approximately 32 GHz) and beyond.
The document discusses the On-the-Fly (OTF) mapping technique, which significantly improves measurement precision in antenna calibration. This method allows for the systematic analysis of measurement procedures, focusing on noise characteristics and the implementation of advanced data acquisition techniques. The OTF technique enhances the determination of antenna aperture efficiency and reduces errors in measuring pointing accuracy and beamwidth. It eliminates the need for independently derived source-size correction factors, thus streamlining the calibration process.
Figures included in the document illustrate the stages of data processing for the OTF system, showcasing the 3-D response of raster scan data collected from strong radio sources, such as Venus. The low noise levels in these measurements indicate the effectiveness of the OTF mapping technique in achieving accurate results.
The document also emphasizes the economic implications of antenna calibration improvements, noting that each decibel (dB) of enhancement in the G/T ratio (gain-to-noise temperature) can significantly impact mission support capabilities, with estimates suggesting a value of approximately $6.8 million per percent improvement.
In summary, the document serves as a comprehensive overview of the advancements in antenna calibration techniques within NASA's DSN, highlighting the importance of OTF mapping for enhancing communication efficiency with spacecraft. It provides insights into the technical aspects of calibration, the benefits of improved measurement precision, and the broader implications for space exploration missions. For further information, the document directs readers to additional resources available through NASA's Scientific and Technical Information Program Office.
