Vaisala RVP900 - DPRT-1 Mode

7.7.1 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"
sux 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
eective PRF (up to 20 KHz) 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 is generally quite large (up to 20
KHz); and the reciprocal of this
"eective instantaneous PRF" determines the trigger's short
PRT interval. In this way, all subsequent physical calculations are scaled correctly, for
example, unambiguous velocity, maximum
first trip range, and so on, are all
supposed to be based on the short PRT interval. The host computer must be configured so
that it can request these very high trigger rates.
The duration of the long PRT interval is not specified directly by the host computer. Rather,
the RVP900 "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
applies only in DPRT mode; all other modes that have uniform triggers continue to interpret
the RVP900 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. RVP900 returns the GPARM "Invalid Processor
Configuration" bit if "R2" is requested in DPRT mode. The error bit is also returned if the
number of pulses requested (sample size) is not even. All other error conditions are the
same as FFT mode.
Since the RVP900 "Maximum number of Pulses/Sec" is usedto enforce the
duty cycle limit, it is essential that it not be overwritten by the host computer's upper
PRF limit, which typically is much higher.
You must make sure that the PWINFO command is disabled in the RVP900 Mcsetup
menu. There is no duty cycle protection if you do not do this.
CAUTION!
You can run Dual-PRF velocity unfolding within the DPRT-1mode. The short PRT varies in
the selected 3:2, 4:3, or 5:4 ratio, but the overall duty cycle remains constant. The
combination of Dual-PRF and DPRT-1 is very eective in extending the radar's
unambiguous velocity interval.
Chapter 7 – Processing Algorithms
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