3.8.3 Gain-elevation corrections

The apparent sensitivity of an antenna varies with elevation for a number of reasons.

As the air-mass increases at low elevations, atmospheric absorption and radiation into the receiver (hence increasing the effective system temperature) become significant, especially at frequencies of 5 GHz and above. Since the antenna surfaces are set for moderate elevations, the antennas may become less efficient at low elevations. Spillover radiation from the ground will also vary with elevation.

At present all these effects are treated together as a gain-elevation correction which is determined empirically for each frequency band using observations of point-cal sources from rising to setting. It must be applied when determining the flux scale (§5.1.5).

At very low elevations ($<10\dg$) the corrections are substantial, typically factors of two or more, and may only be accurate to within 20% (mainly because they vary with azimuth). These errors are partially corrected by amplitude calibration using the phase-cal source, but since this may differ in elevation by up to $5\dg$ some errors will remain. For bright target sources, these residual errors can be corrected by amplitude self-calibration. Especially at 22 GHz the weather has a strong effect and self-calibration or the use of special techniques is required for low-$\delta$ sources.