Enhanced Version
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.
RVP900 maintains an internal array of up to 1024 dierent 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) is 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° 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 is 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, if:
• Slot-0 defines a default 360° filter-versus-range table.
• Slot-1 defines special filtering within 0 ... 90° that is further modified by ° filters in a
40 ... 50° span.
Then a 45° ray would then be filtered according to Slot-2, a 60° ray would use Slot-1,
and a 100° 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) is invalidated. The result is that no clutter filter is applied to any
of the processed data, regardless of range, AZ, or EL. Moreover, loading a given slot with a
table consisting of 0 bins of filter data invalidates just that 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.
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
---------------------------------------------------------------
| | | | | | | | | | | | | | | | |
| 1 |CLR| 0 1 0 0 0 | Command
|----------------------------------------|---|-------------------| (Enhanced)
Chapter 8 – Host Computer Commands
269
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