Scanning

Sometimes it is desirable to scan the elevation / azimuth angle while taking measurement samples. The details for this scanning are defined in the Scanning tab sheet. When scanning is disabled, a constant elevation angle is used for observations. IWV and LWP retrieval products can be selected in combination with scanning when multi-angular retrievals are provided.
Thus only IWV, LWP and brightness temperature data (BRT, IRT) products may be used with scanning. The elevation mirror moves are subdivided into elementary scans from a start angle to a stop angle with a certain incremental angle and a given number of samples measured at each position. These scans are numbered as Scan#1, Scan#2,....
The radiometer does not execute single scans but only frames of scans. Each frame has a start scan and a stop scan (can be identical) which form a ‘loop’ of scans that can be repeated arbitrarily. The concept of having two levels of movement definitions allows for designing complex scan procedures. The elevation mirror speed is always constant.
A frame is defined by clicking on one of the scans in the start scan list and then clicking on one in the stop scan list. After entering the repetition number, the frame is added (or inserted) to the frame list (Add or Insert). In addition the frame start can be triggered to a certain time increment (e.g. 5 minutes). Three examples illustrate how a frame is executed:
1)	Start: Scan#4, stop: Scan#6, repetitions: 3  
Scan#4,Scan#5,Scan#6,Scan#4,Scan#5,Scan#6,Scan#4,Scan#5,Scan#6
2)	Start: Scan#4, stop: Scan#2, repetitions: 2 
Scan#4,Scan#3,Scan#2,Scan#4,Scan#3,Scan#2
3)	Start: Scan#2, stop: Scan#2, repetitions: 1 
Scan#2

When an optional azimuth positioner (see section 8.2 of the operating manual) is installed, a full 2D sky scan can be programmed by checking the 2D Scan check box. The radiometer then uses the given start and stop angles for elevation and azimuth as edge angles and performs a 2D scan within these limits. E.g. a full sky scan is defined by setting the elevation start angle to 9.0°, the elevation stop angle to 90.0° (a 9° increment angle produces 10 elevation steps at const. azimuth angle), the azimuth start angle to 0.0° and the azimuth stop angle to 350.0° (step increment 10°). This generates a measurement of 378 scan points with an angular resolution of 10°. The radiometer selects an integration time of only 0.4 seconds / sample to reduce the time needed for a full scan to a minimum. The ‘Samples / Pos.:’ entry is ignored in this mode. The full scan in this example takes only about 7 minutes! High speed scanning is feasible due to the parallel architecture of the RPG radiometer’s direct detection filter bank receivers described in section 3.2 of the operating manual. 
In the example above, the 90° elevation angle would be scanned 36 times if the Multiple 90° Samples check box is checked. This leads to redundant zenith information that can be used to derive a time series of other products during full sky scanning. But it increases the time for a full scan due to redundant samples. If the user wants to optimize the scanning time he should uncheck the Multiple 90° Samples check box. In this case the zenith scanning angle is only encountered once during a full sky scan. 
 When products with different integration times are selected for scanning, 2CH.EXE adjusts all integration times to the maximum integration time of the products list. 
Example:
A RPG-HATPRO radiometer is setup for scanning with the brightness temperature product BRT (integration time: 10 sec.) and the IWV product (integration time: 30 sec.) selected. Then the integration time during the scan will be 30 seconds (also for the BRT product).
After the definition of the start angle, stop angle and increment angle, the settings are stored as a scan by clicking Add. Every scan in the scan list can be marked and overwritten with the current settings by using the Edit command. When clicking on a scan number in the scan list, the actual settings of that scan are displayed.
By checking the Enable IRR Elevation Coupling check box, the Infrared radiometer(s) is moved synchroneously together with the internal microwave elevation mirror, if the IRR motor driver option is installed.