A radiometer system is calibrated with a tip-curve. This is a standard procedure by the on-site installation and the calibration can be checked periodically. For this procedure the elevation angle have to be well know (better than 0,5°).
A tip-curve should only be conducted under clear sky conditions, because the atmosphere is reliable homogeneous. The antenna moves from zenith (theta=90°) in direction to the earth (theta=0°). Every few degrees the atmospheric attenuation is measured. In the zenith the attenuation a_z is the smallest, because the way through the atmosphere is the shortest.
The attenuation follows a secant law as show in the following equation.
For an easier analysis of the results, the elevation angles are linearised with equation 5.21.
(5.21)
The conclusion is a straight as described in equation 5.22.
(5.22)
A tip-curve should only be conducted under clear sky conditions, because the atmosphere is reliable homogeneous. The antenna moves from zenith (theta=90°) in direction to the earth (theta=0°). Every few degrees the atmospheric attenuation is measured. In the zenith the attenuation is the smallest, because the way through the atmosphere is the shortest.
A best fit line is calculated from the measuring points. The standard deviation and of the points and the intersection with the y-axis is also calculated.
If the radiometer system is calibrated, the line intersects the y-axis at the 0dB point. If this is not the reason, the loss of the horn has to be corrected with the factor c from equation 5.27.
(5.27)