3.6 Tip Curve Calibration

The tip curve calibration measures the atmospheric attenuation at different elevation angles. While lowering the elevation, the atmospheric attenuation rises. From this tip curve an absolute offset in the radiometer reading may be calibrated, the offset gets applied as feed-horn loss if the calibration is accepted by the operator. There should be constant "clear sky" conditions during this test to get reasonable results. If the elevation axis is motorised the RMC move the antenna automatically. If there is no motorised axis the user have to move the antenna manual.

The tip curve calibration is started by clicking to 'Start tip curve calibration' at the Calibration page. The antenna immediately starts to move the maximum elevation angle set for the tip curve calibration. From there the radiometer performs the following steps for each elevation angle to measure. If there is no motorisation the user have to move the antenna manually to the elevation angles, which the RMC shows in the Web-Interface.

1. Wait the settling time / measurement delay
2. Take the number of samples to average
3. Store the averaged atmospheric attenuation for this elevation angle
4. Move the antenna down to the next elevation angle.

The radiometer always measures / calibrates all available channels in parallel. When the tip curve calibration has finished, the program shows a table with the measurement results instead of the normal readings page. The table shows for each channel the following parameters:

• Slope --- The slope of the linear regression function calculated from the measured points.
• Offset --- The y-axis offset of the linear regression function calculated from the measured points. (should be zero for a perfect calibrated radiometer)
• Correlation --- The correlation parameter gives measure of the quality of the linear regression function. A value near 1.0 should be reached.
• Old Feed Loss --- The feed loss used up to now
• New Feed Loss --- The feed loss value calculated from the tip curve.
• Accept --- YES/NO labels are shown for each channel in the last table row. Clicking to a label toggles it from YES to NO and vice versa. Only the calibration of channels labelled with YES gets updated when the calibration process is finished with OK.

To quit the measurement without updating any calibration values, click to "Cancel". Clicking to "OK" accepts the new feed-horn loss calibration for those channels where the results are marked with "YES".

The complete measurement results are stored as a file TIPCURVE.TXT at the radiometer's flash memory. You may download the file by entering "http://aaa.bbb.ccc.ddd/TIPCURVE.TXT" at the address bar of your favourite web browser.

After a manual Tip Curve Calibration, the user have to move the radiometer manual to the desired elevation angle.