Chapter 7 __________________________________________________ Host Computer Commands
VAISALA______________________________________________________________________ 291
Enhanced Version
The RVP900 supports an enhanced version of the LFILT command that
provides "Clutter Maps", that is, much greater flexibility by allowing filter
choices to depend on antenna angle as well as range. This lets you specify
a 2D or even 3D table of clutter filter selections that are dynamically
selected during live data processing.
The RVP900 maintains an internal array of up to 1024 different filter-
versus-range tables, each of which is keyed to a particular solid angle
AZ/EL sector. Each enhanced LFILT command fills in one of these slots
with a filter selection table similar to that of the legacy command, except
that the number of range bins is specified explicitly and eight bits are used
for filter selection rather than three. Then, for each live ray being
processed, the RVP900 applies clutter filters according to the filter slot
whose solid sector includes the midpoint AZ/EL of the ray. If the antenna
angle of the ray does not fall within any of the defined filter sectors, then
the all-pass filter (#0) will be used at all ranges.
The low and high angle limits in each filter slot are inclusive; hence, the
pair (0x000, 0xFFFF) would span the full 360-degree circle with no gaps.
Also, the filter array can be sparse (not all slots filled in), and have
overlapping sectors (in which case the highest numbered slot that spans a
ray's AZ/EL midpoint will be used). Choosing the highest numbered
encompassing slot is a subtle but important detail that allows complex
regions to be defined as a layered hierarchy of overlapping sectors. For
example Slot-0 might define a default 360-degree filter-versus-range table,
while Slot-1 defines special filtering within 0-90 degrees that is further
modified by Slot-2 filters in a 40-50 degree span. A 45-degree ray would
then be filtered according to Slot-2, a 60-degree ray would use Slot-1, and
a 100-degree ray would use Slot-0.
If the CLR bit is set in the opcode, then no additional arguments follow and
the entire internal filter array (all slots) will be invalidated. The result is
that no clutter filter will be applied to any of the processed data, regardless
of Range, AZ, or EL. Moreover, loading a given slot with a table consisting
of zero bins of filter data will invalidate just that one slot. This allows some
data to be removed from the table without having to resort to a complete
CLR operation.
The legacy LFILT command is equivalent to calling the enhanced
command first with the CLR bit set, followed by a second call that writes
the legacy filter choices into slot #0 using AZ/EL limits that cover all of
space. Thus, the legacy behavior is obtained as a special case of the
enhanced mechanism.
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