User's Manual Part 2

Host Computer Commands
RVP8 User’s Manual
May 2003
6–37
360-degrees of rotation. This gives considerable flexibility in the choice of angles. For
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. Note that on power-up the angle
table is initialized to 360 values corresponding to integer-valued degrees from zero to 359.
The synchronization algorithm works automatically with either clockwise or counterclockwise
antenna rotation, and can tolerate any sequence of changes in direction, e.g., 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 RVP8
will automatically produce an output ray after one second of waiting, even if no trigger angles
have been crossed. This is to avoid timeouts 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 RVP8. 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 GPARM) 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 RVP8 finds the pair of sync angles that enclose the previous trigger
angle. The current ray then runs until the antenna passes outside of either limit, at which point
processing for that ray is terminated. Once this happens, a new trigger angle is assigned based
on which limit was crossed.
The maximum number of pulses that will be present in each ray during angle syncing is still
given the by the Sample Size field of the SOPRM command. The actual number of pulses will
be less only if a trigger angle is crossed before the full pulse count is reached. In general, you
should set the Sample Size somewhat larger than the expected pulse count so that the trigger
angle crossings make the best use of every available pulse when the antenna is scanning at the
expected rate.