Sky Tipping Calibration

The sky tipping (or tip curve) calibration technique is described in detail in the operating manual, section 4. 
The scanning angles listed in the Scan Angles [DEG] group box are predefined to give equidistant air mass samples in the sky tipping scan (the air mass is proportional to 1 / sin(α), see section 4.5). They can be modified by Add and Delete but it is recommended to only define angles >20°. If the radiometer’s horizontal view is blocked by obstacles, the lowest elevation angle should be adjusted appropriately but should not be >30° to maintain the calibration accuracy. In addition, the azimuth angle for the sky tipping direction can be selected independently from other observation positions, if an azimuth positioner option is installed. The calibration reliability may be improved by checking Bilateral Tipping (Handling group box). With bilateral tipping the radiometer scans the elevation angles in both quadrants (0° and 180° azimuth angle) to detect possible inhomogeneous water vapour distributions which might violate the requirements for a stratified atmosphere. This of course assumes a free view to both observation sides between 20° and 90° elevation. 
Sky Tipping is enabled by checking Enable Sky Tipping. The user can specify how often a calibration shall be performed by selecting a period between 10 Minutes and 24 Hours. Practical periods are 2 to 24 hours because the radiometer gain is normally calibrated every 5-10 minutes by a gain calibration and a full parameter recalibration (including Tsys and Tn) is not required so frequently. In addition, a tip curve interrupts the measurement for more than three minutes which should be kept to a minimum.
Furthermore, it is possible to define the time of the first tip curve calibration in the measurement. By checking period  absol. (0:0:0), the calibration will start relative to midnight time, e.g. with a period of 6 hours and a measurement start at 3:00 pm the first calibration will take place at 18:00 assuming that first cal. @ mea. start is not checked. If rel. to measure. start is checked, the calibration timing is relative to measurement start time.
As explained in section 4.5, tip curve calibrations are only applicable for humidity profiler channels. That is why the Temp. Profiler checkbox is disabled.
So far three quality checks (thresholds) are implemented: 

•	Zenith threshold. In the presence of clouds (which violate the assumption of a stratified atmosphere) the observed zenith brightness temperatures (BTs) are significantly higher than under clear sky conditions. The user can define maximum BTs as a threshold for each channel to reject the calibration. 
•	Linear correlation factor. This measures the correlation of the optical thickness samples (as a function of air mass) with a straight line. Typical linear correlation factor thresholds are >0.9995. The linear correlation factor is not sensitive for the noise of the optical thickness samples caused by clouds etc. 
•	χ2-test. This measures the variance of the optical thickness samples relative to the straight line. Typical threshold values are <0.4 for a good quality calibration.

With Define Zenith Thresh., Define CHI^2 Thresh. and Correlation Thresh. the corresponding thresholds can be entered in the Threshold Definition group box.
The main purpose of a sky tipping calibration is to recalibrate the noise diode secondary standards (see section 4.2). In order to avoid sudden calibration jumps in the Tb data, subsequent successful skydips can be averaged, so that the calibration noise is reduced. The averager depth can be set between 50 and 100. When averaging is used, the preset thresholds for linear correlation and standard deviation should be relaxed to achieve more successful calibrations for a better calibration statistics. Also the period should be adjusted for more calibration trials, e.g. one hour or less.
Default Settings resets all skydip parameters to reasonable standard values, optimized for the specific instrument in use.
The radiometer PC is capable of averaging successful sky tipping calibrations in order to avoid sudden calibration jumps in the calculated brightness temperatures (TBs). If calibration averaging is desired, the checkbox  Enable in the ‘Averaging’ sub-menu should be checked. If enabled, the averager uses the number Av. Depth of successive calibrations in the averaging process. Reasonable values are in the range 40-100.