User's Manual Part 2

Processing Algorithms (draft)
RVP8 User’s Manual
April 2003
5–30
5.5 Dual PRT Processing Mode
The RVP8 supports two major modes for Dual PRT processing, i.e., algorithms using triggers
that consist of alternate short and long periods. Most of the Doppler parameters are available in
each of these modes. You may also request time series data in both cases; the samples will be
organized so that the first pulse of a short PRT pair always comes first.
5.5.1 DPRT-1 Mode
The DPRT-1 trigger consists of a very short PRT from which Doppler data are obtained,
followed by a much longer PRT whose purpose is to limit the average duty cycle of the
transmitter. No information is extracted from the long PRT pair, but Dual-PRF techniques can
still be used by varying the short period from ray to ray. The “-1” suffix in the name for this
mode is a reminder that Doppler parameters are computed from the short PRT only. The
DPRT-1 mode is intended for millimeter wavelength radars that must run at a very high effective
PRF (up to 20KHz) to get an acceptable unambiguous velocity, but which also have a much
lower duty cycle constraint on the average number of pulses transmitted each second.
In DPRT-1 mode the requested PRF from the host computer will generally be quite large (up to
20KHz); and the reciprocal of this “effective instantaneous PRF” will determine the trigger’s
short PRT interval. In this way, all subsequent physical calculations will be scaled correctly,
e.g., unambiguous velocity, maximum first trip range, etc., are all supposed to be based on the
short PRT interval. The host computer must therefore be configured so that it can ask for these
very high trigger rates.
The duration of the long PRT interval is not specified directly by the host computer. Rather, the
RVP8’s “Maximum number of Pulses/Second” setup parameter is used to compute how much
delay to insert in order to insure that the transmitter’s duty cycle is not exceeded. This special
treatment applies only in DPRT mode; all other modes that have uniform triggers continue to
interpret the RVP8’s trigger bound as a simple “Maximum PRF”.
Since DPRT-1 mode uses only the short pairs of pulses, it is not possible to run the “R2” moment
estimation algorithms. The RVP8 will return the GPARM “Invalid Processor Configuration” bit
if “R2” is requested in DPRT mode. The error bit will also be returned if the number of pulses
requested (sample size) is not even. All other error conditions are the same as FFT mode.
Warning: Since the RVP8’s “Maximum number of Pulses/Sec” is used to enforce
the duty cycle limit, it is essential that it not be overwritten by the host
computer’s upper PRF limit, which typically will be much higher. To insure
this, you must make sure that the PWINFO command is disabled in the RVP8
“Mc” setup menu. You will have no duty cycle protection if you do not do this.
Note: You may still choose to run Dual-PRF velocity unfolding within the
DPRT-1 mode. What will happen is that the short PRT will vary in the selected
3:2, 4:3, or 5:4 ratio, but the overall duty cycle will remain constant. The
combination of Dual–PRF and DPRT-1 is tremendously effective in extending
the radar’s unambiguous velocity interval.