Microwave holography – Calibrating radio telescopes from space

I recently read an interesting article on Microwave holography ? a method used to do high precision calibration of large (> 30 m) radio telescopes. The maximum frequency of operation is determined by how well the surface can be calibrated. Microwave holography, as applied to reflector antennas, is a technique that utilizes the Fourier transform relation between the complex farfield radiation pattern of an antenna and the complex aperture distribution [1]. The far-field amplitude and phase response of the antenna is measured by a geosynchronous satellite doing a high resolution raster-scan. This method can be compared to optical holography where both the light intensity and depth (or phase information) is recorded which gives the image a life-like perception.

The captured far-field data is used to calculate a surface error map which is used to adjust (or calibrate) the individual panels in an overall reflector. ?Here are some examples of improved performance taken from the reference article: ?At Ka band, by improving the root-mean-squared offset error of the surface relative to that of a ?perfect? dish from 0.67mm to 0.25mm, the gain is improved by 3dB (i.e. you get double the signal!) ?This is independent of the dish size ? and on a 34m diameter dish, improving the accuracy from 0.67 to 0.25mm is such a miniscule change, it is impressive that it has such an effect! These big dishes seem real simple, but are high-precision engineering on a grand scale.

The image below is processed far-field data captured by the Microwave holography technique. The red and blue colors represent regions that are deformed by a constant value of +- 0.2mm, respectively where the green color represents a perfect dish surface.

Microwave holography example of reflector surface error.

Microwave holography example of reflector surface error.

[1] Microwave Antenna Holography by David J Rochblatt, Chapter 8.

Author: Robert Kellerman

Leave a Reply

Your email address will not be published. Required fields are marked *