Mf — Clutter filters

When clutter correction is applied to the reflectivity data, you must increase the LOG noise threshold slightly to continue to provide reliable qualification of the corrected values. The reason for this is that the uncertainty in the corrected reflectivity becomes greater after the clutter is subtracted. For example, if we observe 20 dB of total power above receiver noise, and then apply a clutter correction of 19 dB, we are left with an apparent weather signal power of +1 dB above noise. However, the uncertainty of this +1 dB residual signal is much greater than that of a pure weather target at the same +1 dB signal level.

Default residual clutter LOG noise margins:
 Baseline :  0.15 dB/dB for Clutter/Noise above 10dB 
 HiSignal  : 1.00 dB/dB for Clutter/Noise above 50dB

The Residual Clutter LOG Noise Margin allows you to increase the LOG noise threshold in response to increasing clutter power. In the previous example, and with the default setting of 0.15 dB/dB, the LOG threshold increases by 19 x 0.15 = 2.85 dB. This helps eliminate noisy speckles from the corrected reflectivity data.

When RVP10 computes power spectra, the time series data are multiplied by a (real) window before computing the Fourier Transform (DFT). 

Default Spectral Window
 0:User , 1:Rect, 2:Hamming
 3:Black, 4:ExBlack, 5:VonHann, 6:Adaptive:0

You can select the window through SOPRM word #10 0:User, or force a particular window:

  • In the IRIS application, the setting 0:User refers to the window defined in the Setup utility block RVP signal processing options.

  • In the Ascope application, the setting 0:User enables the Spect Win button.

At RVP10 startup, 0:User refers to the settings saved in rvp10.conf block opprm.filter bits 9,10,11.

The Spectral Clutter Filters question defines the clutter filters that operate on power spectra during the DFT-type major modes (PPP, FFT, and RPH).

Filter #0 is reserved as all pass, and cannot be redefined here. For filters #1 to #7, enter a digit to choose the filter type, followed by the parameters that the type requires. See Clutter filtering approaches.

Spectral Clutter Filters 
------------------------  
  Window –1:Default   0:Rectangular  1:Hamming
  Code    2:Blackman  3:ExBlackman   4:VonHann  5:Adaptive 
Filter #1  Type:0(Fixed)              Win:1   WidthPts:1 EdgePts:2 
Filter #2  Type:0(Fixed)              Win:2   WidthPts:2 EdgePts:2 
Filter #3  Type:0(Fixed)              Win:2   WidthPts:3 EdgePts:3
Filter #4  Type:1(Variable)           Win:2   WidthPts:3 EdgePts:2 HuntPts:3 
Filter #5  Type:3(Gaussian Adaptive)  Win:-1  Spectrum width: 0.200 m/sec
Filter #6  Type:3(Gaussian Adaptive)  Win:-1  Spectrum width: 0.300 m/sec 
Filter #7  Type:3(Gaussian Adaptive)  Win:-1  Spectrum width: 0.500 m/sec

Fixed width filters (Type 0)

These are defined by the following additional parameters:

Width
Sets the number of spectral points that are removed around the zero velocity term. A Width of 1 removes the DC term. A Width of 2 removes the DC term plus 1 point on either side. A Width of 3 removes DC plus 2 points on either side, and so on. Spectral points are removed by replacing them with a linear interpolating line.
EdgeMinPts
The endpoints of the line are determined by taking the minimum of EdgeMinPts past the removed interval on each side.

Variable width, single slope (Type 1)

RVP10 supports variable-width, frequency-domain clutter filters. These filters perform the same spectral interpolation as the fixed–width filters, except that their notch width automatically adapts to the clutter.

The filters are characterized by the Width and EdgeMinPts parameters in the Mf menu, except that the Width is now interpreted as a minimum width.

The Hunt parameter allows you to choose how far to extend the notch beyond Width to capture the clutter power. Setting Hunt=0 converts a variable–width filter to a fixed–width filter.

The algorithm for extending the notch width is based on the slope of adjacent spectral points.

  • The beginning (Width-1) points away from 0, the filter is extended in each direction as long as the power continues to decrease in that direction, up to adding a maximum of Hunt additional points.
  • If you run with a fixed Width=3 filter, try experimenting with a variable Width=2 and Hunt=1 filter.
  • If the original fixed width occasionally fails, but you are reluctant to increase it to cover those rare cases, try selecting a variable Width=2 and Hunt=2 filter.
tip In general, make your variable filters "wider" by increasing Hunt rather than by increasing Width. This preserves more flexibility in how they can adapt to whatever clutter is present.

Gaussian model adaptive processing (GMAP) (Type 2)

This is the most advanced form of clutter filtering and moment estimation. See Clutter filtering approaches.

For GMAP processing, you must specify the spectrum width of clutter. The algorithm is not very sensitive to the exact value. Configure several widths to cover the antenna rotation rates that are commonly used. It is useful to turn off clutter filtering (select the all pass filter #0) and then look at measurements of the clutter width while the antenna is rotating using, for example, the Ascope utility or application software such as IRIS.

Whitening Parameters for  Tx :Random

The values in the next line define a secondary SQI threshold that is traditionally used to qualify LOG data in the random phase processing mode. The secondary SQI threshold is applied uniformly in all processing modes when reflectivity data are specified as being thresholded by SQI. 

Secondary SQI Threshold Slope :0.50  Offset:-0.05

The secondary SQI level is computed by multiplying the primary user-supplied SQI threshold by the SLOPE, and adding the OFFSET. See Tuning for optimal performance.

Limits: SLOPE: 0.0 ... 2.0, OFFSET -2.0 ... 1.0

These parameters tune the phase whitening process for SZ(8/64) transmissions.

Whitening Parameters for Tx :SZ (8/64)
––––––––––––––––––––––––––––––––––––
Max power mismatch across octants: 4.0 dB
 High power rejection threshold: 8.0 dB 
Maximum KEY phase error: 12.0 deg