Vaisala RVP900 - Output Data

3.3.9.3 Random Phase Processing for Second Trip Echo
Second trip echoes can be a serious problem for applications that operate at a high PRF.
Second trip echoes can appear separately, or can be overlaid on first trip echoes (second
trip obscuration).
The random phase technique
1)
separates the first and second trip echoes so that:
• In most cases, the second trip echo can be removed from the first trip, even in the case
of overlapped
first and second trip echoes. The benefit is a clean first trip display
• The second trip echoes can be recovered and placed at their proper range at first trip/
second trip signal ratios of up to 40 dB dierence for overlapped echoes. Because of
the wide dynamic range of weather echoes, this power limit is sometimes exceeded.
The technique requires that the phase of each pulse be random. Digital phase correction is
applied in the processor for the
first and second trips. The adaptive filter removes the echo
of the other trip to increase the SNR.
Magnetron radars have a naturally random phase. For Klystron radars, a digitally controlled
precision IF phase shifter is required. RVP900 provides an 8-bit RS422 output for the phase
shifter.
3.3.9.4 Polarization Mode Processing
Polarization processing uses a time domain autocorrelation approach to calculate the
parameters of the polarization co-variance matrix, that is, ZDR,LDR, PhiDP, RhoHV, PhiDP
(Kdp), and so on.
The standard moments T, V, Z, and W are also calculated.
The available parameters and the algorithms used to calculate them, depends on the type of
polarization radar, for example, single channel switching, STAR, or dual channel switching.
Vaisala is licensed by US National Severe Storms Laboratory (NSSL) to use the STAR
hardware and processing techniques and algorithms.
Polarization measurements require calibration of the ZDR and LDR
osets. The use of a
clutter filter for the polarization variables can sometimes bias the derived parameters.
Because of this, you must decide whether to use
filtered or unfiltered time series.
3.3.10
Output Data
RVP900 output data for standard moment calculations include mean radial velocity (V),
spectrum width (W), corrected reflectivity (Z or dBZ), and uncorrected reflectivity (T or
dBT).
Other data outputs include I/Q time series, DFT/FFT power spectrum points, and
polarization parameters.
The output can be made in either 8-bit or 16-bit format. 16-bit format is preferred for most
applications. Time series and DFT are always 16-bit formats.
1) Joe, P., and A. Siggia, 1995: Second Trip Unfolding by Phase Diversity Techniques. Preprints of the 27th Conference on Radar
Meteorology, American Meteorological Society, 770-772.
Chapter 3 – Functional Description
47