The sun as a test source for boresight calibration of microwave antennas

Although the diameter of the sun at microwavelengths is a little over half a degree, the sun can be used for establishing, say between 3 and 30 cm, the pointing corrections (boresight calibration) of microwave antennas, with high precision. There is, however, a discrepancy between the position of the microwave sun on any given day and the position of the center of the solar disc as tabulated in [16]. Over the period 1966-1969 this discrepancy had an rms value of 1.6' east-west and 1.2' north-south and a range of 11.4' east-west and 9.2' north-south. These conclusions are based on sun maps, made at 9.1 cm with an antenna having a beamwidth of approximately 3', which are published regularly. From such a map the microwave centroid can be determined with an error less than 1'. In the absence of such a map, the centroid can be estimated to within 2' or slightly better by taking account of optical data such as sunspots or plages. The correction on any one day is strongly correlated with that of the day before, but the correlation is small after about 5 days. The correlation returns in part after 27 days.