Boresight pointing calibration for the Chilbolton radar using galactic radio sources and satellite targets

From time to time, it is desirable to assess the pointing accuracy of the large paraboloid antennas which are typically used for research applications in radar meteorology and related fields. Sources of error affecting the difference between apparent and actual antenna main-lobe pointing angles (in both azimuth and elevation) include: displacement of the feed from the prime-focus; distortion of the reflector profile; backlash in the mechanical drives and gearboxes; errors in the servo-system; and offsets in the encoders of the position readout system.

The authors recently undertook an experimental campaign to determine the quality of calibration of the position readout system used on the 25 m-diameter dish of the Chilbolton S-band 'CAMRa' radar in relation to the antenna's true boresight pointing angle, as determined by observations of various galactic radio sources. Having performed this calibration, the performance of the system was verified by observing transits of several satellite targets in low and medium earth orbits, the positions and timings of which were computed to high accuracy from two-line element (TLE) orbital-parameter data in conjunction with software which included corrections for gravitational perturbations and atmospheric drag.

We demonstrate that, using the radar receiver as a radiometer (by employing suitable signal processing techniques and a specially installed, low-noise GaAsFET pre-amplifier), the measurements of galactic radio sources can be used to determine the antenna's true pointing angle to within a reproducibility of considerably better than the -3 dB two-way beamwidth of 0.25 °. This method achieves superior accuracy compared to more traditional approaches, such as the use of tethered or suspended balloon targets, beacon transmitters or transponders.

The subsequent observations of satellite transits serve to prove the calibration of the radar as a complete system in the azimuth, elevation, range and, albeit approximately, in the radar cross section (RCS) domains. In the case of certain satellite targets, these tests have also revealed some interesting polarimetric signatures.

In this paper, we discuss the procedures used to collect both the galactic noise source and satellite observations, the constraints imposed by certain aspects of the CAMRa radar hardware for the optimum processing of the recorded data, and the analysis and implications of the results. It is hoped that the approach described will be of use to other users of large-aperture, steerable radar antennas who may wish to periodically evaluate and monitor pointing-accuracy performance.