User's Manual

RVP8 User’s Manual
October 2005
TTY Nonvolatile Setups
3–19
Range mask spacing: 125.00 meters
The range resolution of the RVP8 is determined by the decimation factor of the
digital matched FIR filter that computes “I” and “Q”. This decimation factor is the
ratio of the filter’s input and output data rates, i.e., the output rate is some integer
divisor of the IFD Acquisition Clock (See Mc Section). For the legacy RVP7 IFD
operating at its standard frequency of 35.9751MHz, the available range resolutions
(in meters) are: 25.0, 28.3, 36.7, 50.0, 58.3, 66.7, 75.0, 83.3, 91.7, 100.0, 108.3,
116.7, 125.0, and 133.3. The RVP8 IFD operating in the 72MHz range provides
twice the resolution between steps, and four times the number of steps.
The ranges that are selected by the bit mask in the LRMSK command are spaced
according to the range resolution that is chosen here. Also, the upper limit on the
impulse response length of the matched FIR filter (see below) is constrained by the
range resolution. If you choose a range resolution that can not be computed at the
present filter length, then a message of the form: “Warning: Impulse response
shortened from 72 to 42 taps” will appear.
Limits: 25 to 1000 meters.
FIR-Filter impulse response length: 1.33 usec
The RVP8 computes “I” and “Q” using a digital FIR (Finite Impulse Response)
matched filter. The length of that filter (in microseconds) is chosen here.
The filter length should be based on several considerations:
S It should be at least as long as the transmitted pulsewidth. If it were shorter, then
some of the returned energy would be thrown away when “I” and “Q” are
computed at each bin. The SNR would be reduced as a result.
S It should be at least as long as the range bin spacing. The goal here is to choose
the longest filter that retains statistical independence among successive bins. If
the filter length is less than the bin spacing, then no IF samples would be shared
among successive bins, and those bins would certainly not be correlated.
S It should be “slightly longer” than either of the above bounds would imply, so
that the filter can do a better job of rejecting out-of-band noise and spurious
signals. The SNR of weak signals will be improved by doing this.
In practice, a small degree of bin-to-bin correlation is acceptable in exchange for the
filter improvements that become possible with a longer impulse response. The FIR
coefficients taper off to zero on each end; hence, the power contributed by
overlapping edge samples is minimal. SIGMET recommends beginning with an
impulse response length of 1.2–1.5 times the pulsewidth or bin spacing, whichever is
greater.
The maximum possible filter length is bounded according to the range resolution that
has been chosen; a finer bin spacing leaves less time for computing a long filter. For
the RVP8 Rev.A processor, the filter length must be less than 2.92 msec at 125-meter
resolution; for Rev.B and higher this limit increases to 6.67 msec.
NOTE: Cascade filter software is being contemplated that will extend the maximum
impulse response length to at least 50 msec. This is of interest when very long
(uncoded CW) transmitted pulses are used.