Vaisala RVP900

Chapter 2 _______________________________________________ Introduction and Specifications
VAISALA_______________________________________________________________________ 47
false alarms (speckles). Both a 1D (single azimuth ray) and 2D (three
azimuth rays by three range bins) are supported.
2.9.1.8 Velocity Unfolding
A special feature of the RVP900 processor is the ability to "unfold" mean
velocity measurements based on a dual PRF algorithm. In this technique,
two different radar PRFs are used for alternate N-pulse processing
intervals. The internal trigger generator automatically produces the correct
dual-PRF trigger, but an external trigger can also be applied. In the later
case, the ENDRAY_ output line provides the indication of when to switch
rates. The RVP900 measures the PRF to determine which rate (high or
low) was present on a given processing interval. It then unfolds based on
either a 2:3, 3:4, or 4:5 frequency ratio. Table 1 on page 45 provides typical
unambiguous velocity intervals for a variety of radar wavelengths and
PRFs.
2.9.2 RVP900 Pulse Pair Time Domain
Processing
Pulse pair processing is done by direct calculation of the autocorrelation.
Prior to pulse pair processing, the input “I” and “Q” values are filtered for
clutter using a time domain notch filter, frequency domain fixed, or
variable width filters. IIR filters of various selectable widths are available
for either 40 dB or 50 dB stop band attenuation. The filtered I/Q values are
processed to obtain the autocorrelation lags R0, R1, and R2. The unfiltered
Table 1 Examples of Dual PRF Velocity Unfolding
Unambiguous Velocity (m/s)
for Various Radar Wavelengths
PRF1 PRF2
Unambiguous
Range (km) 3 cm 5 cm 10 cm
500 * 300 3.75 6.25 12.50 No Unfolding
1000 * 150 7.50 12.50 25.00
2000 * 75 15.00 25.00 50.00
500 333 300 7.50 12.50 25.00 Two Times
Unfolding
1000 667 150 15.00 25.00 50.00
2000 1333 75 30.00 50.00 100.00
500 375 300 11.25 18.75 37.50 Three Times
Unfolding
1000 750 150 22.50 37.50 75.00
2000 1500 75 45.00 75.00 150.00
500 400 300 15.00 25.00 50.00 Four Times
Unfolding
1000 800 150 30.00 15.00 100.00
2000 1600 75 60.00 100.00 200.00