Vaisala RVP900

Chapter 7 __________________________________________________ Host Computer Commands
VAISALA______________________________________________________________________ 315
example, if local obstructions cause shadows in the radar image, then those
regions can be skipped merely by omitting table entries in their vicinity.
Likewise, as the antenna rotates data can be acquired within one or more
sectors by simply specifying the appropriate sets of contiguous bearings at
whatever angular resolution is desired. On power-up the angle table is
initialized to 360 values corresponding to half-integer-valued degrees from
0.5 to 359.5.
The synchronization algorithm works automatically with either clockwise
or counterclockwise antenna rotation, and can tolerate any sequence of
changes in direction, for example, if the antenna itself is scanning a sector,
or if it is turning erratically. Moreover, the trigger angles do not have to be
hit exactly in order to start each new ray – the antenna need only move
across them. This minimizes the possibility of losing data due to missing
codes in the angle encoders. The RVP900 will automatically produce an
output ray after one second of waiting, even if no trigger angles have been
crossed. This is to avoid time-outs with the host computer when the
antenna is not moving at all.
To use the synchronization mode, the trigger angle table is first loaded
using the LSYNC command. The user chooses the number of table entries
and then writes the required number of words to the RVP900. The angles
must be supplied in a clockwise strictly increasing order, and they must
neither reach nor pass zero degrees by the table's end. The first value,
however, may be zero. Binary angle representation is used wherein Bit 15
represents 180 degrees, Bit 14 represents 90 degrees, etc. The Ld bit must
be set in the command word to indicate that a new table size and set of
angles are being loaded. A flag bit is to be set (see Section 7.9 Get
Processor Parameters (GPARM) on page 290) if errors are detected when
loading the table of angles.
To actually enable synchronized operation the Ena command bit must
eventually be set, and EL and BCD should be either set or cleared
according to the user's needs. These bits may be used independent of
reloading the actual table values. Thus, antenna synchronization may be
turned on and off without having to reload the table each time. However,
if there were errors when the table was last loaded, the processor ignores
the Ena bit and synchronization is forced off. Once enabled, PROC
commands are then issued in the usual manner to acquire and process the
radar data. Either the single-cycle or free-run PROC mode may be used.
Data collection proceeds as usual, except that the rays are now
automatically aligned with the trigger angles.
The angle sync algorithm is dynamic and works as follows. Each ray
begins immediately upon the user's request, or upon completion of the
previous ray when in continuous processing mode. At the start of the ray,
the RVP900 finds the pair of sync angles that enclose the previous trigger
angle. The current ray then runs until the antenna passes outside of either