Sebastian Torres NEXRAD Range-Velocity Ambiguity Mitigation Fall 2004 – Technical Interchange...

Sebastian Torres

NEXRAD Range-Velocity Ambiguity Mitigation

Fall 2004 – Technical Interchange Meeting

http://www.nssl.noaa.gov/wrd/internal/presentations/logos/cimms.gif

Part One

Recap of last TIM


Surprise! GMAP

• GMAP adopted as the ORDA GCF in FY04

• Ice et al. (2004) performed initial assessment– Blackman window required for 50 dB+

suppression– Input noise required for small number of

samples

• Devoted good part of FY04 to study GMAP– Implemented in MATLAB from source code


Yet Another Windows® Update

• Window choice– Sachidananda et al. (1998)

recommended using von Hann window

– Blackman window is more aggressive

• Larger standard error of estimates (needs to be quantified)

• Recommendation: Consider an adaptive scheme that uses the presence/strength of clutter for window selection


Let’s Make Some Noise

• Noise estimation– A rank order noise estimation algorithm is

invoked if noise is not provided to GMAP– Filter notch width depends on noise level

• A good noise estimate is critical for the filter’s performance

– SZ(8/64) phase coded out-of-trip echoes appear as noise

– Recommendation: Provide reliable noise estimate to GMAP


Filling the Void

• Spectral reconstruction– GMAP fills notched spectrum using Gaussian

interpolation• Biases are minimized• Process assumes coherent

signal spectrum

– Recommendation: • Clutter with strong signal: use spectral reconstruction• Clutter not with strong signal: bypass spectral

reconstruction (notch filter)– GMAP modification: enable/disable spectral

reconstruction


C:\Program Files\Matlab\bin\win32\matlab.exe

It’s just a phase…

• Time-series reconstruction– GMAP operates in the power spectrum domain

• Phase information is lost

– SZ-2 requires time-series for cohering process

– Recommendation: Save unfiltered phase spectrum and use zeroes in the gap

• GMAP modification: return number of spectral components with clutter


SZ-2 Clutter Filtering (I)

• Conditions for filtering– Determined by maps

• Bypass map• Clutter censor zones

– Determined by clutter strength• Clutter power is not available in maps• GMAP removed power is a good estimate of clutter

power only for CSR > 0 dB

– Recommendation: • Use maps as in legacy WSR-88D • Use total power if clutter is not with the two

strongest trips


SZ-2 Clutter Filtering (II)

• Sequence of operations – Clutter must be removed first

• Cohere to trip with clutter• Apply GMAP

• Censoring– Recommendation: Do not recover weak

signal if clutter is not with the strong signal


Part Two

Status of the SZ-2 Algorithm


SZ-2 Evolution

• Initial recommendation on Aug 15, 2003• Interim recommendation on May, 2004

– Incorporation of GMAP (with a couple of modifications)– Ability to handle clutter in any trip– PNF optimization– Spectrum width computation

• New recommendation on June 1, 2004– GCF bypassing using CSR (in addition to maps)– New censoring rules and refined thresholds

• Thresholds may require further refinement after operational tests

• Errata on June 22, 2004


Scan Strategy

• Long PRT (non phase coded) used to retrieve– Filtered powers– GMAP removed powers– Spectrum widths

• Short PRT (phase coded) used to retrieve– Strong and weak trip velocities– Strong trip spectrum width

• Weak trip spectrum width from the long PRT scan


The SZ-2 Algorithm (I)

• Determine overlaid trips

• Determine clutter location(s): 3 cases• Window time series• If needed, filter clutter

– Cohere to trip with clutter– Apply GMAP

range1st trip 2nd trip 3rd trip 4th trip

PL

Pth

P1

P2

P3 P4

Long PRT powers are clutter filtered


The SZ-2 Algorithm (II)

• Determine strong and weak trips– Cohere to trips with recoverable signals– Compute autocorrelations

• Cohere to strong trip

• Compute strong trip velocity

• Apply PNFStrong trip

coheredWeak trip modulated

PNF

vvs


The SZ-2 Algorithm (III)

• Cohere to weak trip

• Compute weak trip velocity

• Compute strong and weak trip powers

• Compute strong trip spectrum width

• Censor unrecoverable data– Eight censoring rules– Recommendation: Censoring thresholds

should be in adaptation data


SZ-2 Censoring

• Three types of returns– Significant– Overlaid-like (purple haze)– Noise-like (not shown on displays)

• SZ-2 censoring occurs in– Doppler velocities– Spectrum widths


SZ-2 Censoring Rules (I)

• Noise-like cells– (1) Low SNR in the long-PRT scan

• Cells with non-significant powers are not considered as candidates for recovery during the short-PRT scan

• KSNR is specified in the VCP definition

– (2) Low SNR in the short-PRT scan• Takes care of advection between long- and short-

PRT scans

• KSNR is specified in the VCP definition


SZ-2 Censoring Rules (II)

• Overlaid-like cells– (3) Low SNR*

• Out-of-trip signals appear as noise

• Thresholds for strong and weak trips are Ks and Kw

– (4) Weak trip not recoverable• Recovery region from plots of SD(v2) in the S1/S2

vs. n1 plane

• Different regions for narrow and wide weak-trip spectrum widths Recoverable

Unrecoverable


SZ-2 Censoring Rules (III)

• Overlaid-like cells (cont’d)– (5) High CSR

• Strong clutter residue makes recovery of overlaid signals very difficult

• Thresholds for the strong and weak trips are KCSR1 and KCSR2

– (6) Clutter location• Weak trip recovery only feasible is clutter is with

strong trip


SZ-2 Censoring Rules (IV)

• Overlaid-like cells (cont’d)– (7) Large weak trip spectrum widths

• Spectrum widths are derived from long-PRT

• v saturates at ~4.8 m/s with PRT #1

• Threshold is v,max

– (8) Triple or quadruple overlay• SZ-2 can recover at most two overlaid trips• Third or fourth strongest trips are censored

0 5 10 15 20 250

2

4

6

8

10

TRUE SPECTRUM WIDTH (m s-1)

ME

AS

UR

ED

SP

EC

TR

UM

WID

TH

(m

s-1

)

True width

Measured width va[ln(M-1/M)]1/2/

RF=2705 MHz PRF = 321 Hz

M = 17


Future Enhancements

• Proposed SZ-2 algorithm provides significant improvement compared to legacy algorithms

• Identified 4 areas for further improvements– Use of GMAP without spectral reconstruction– AP clutter suppression– Recovery of overlaid echoes with comparable

powers– Weak trip spectrum width computation

• Need more research– Cost-benefit analyses


Part Three

Performance of the SZ-2 AlgorithmCase Examples


Set Up

• Data collected with KOUN radar– RRDA with analog or digital receiver– Experimental VCP, lowest elevation angles, 5

scans at each elevation angle• Non PC, long PRT• Non PC, medium PRT• PC, medium PRT• Non PC, short PRT• PC, short PRT

• Proposed SZ-2 algorithm (June 04) completely implemented in MATLAB


Stratiform Precipitation

va = 35.5 m s-1, ra = 117 km va = 8.9 m s-1, ra = 466 km

ReflectivityLong PRT EL = 0.5 deg

10/08/02 15:11 GMT Legacy VelocityShort PRT




SZ-2 VelocityShort PRT EL = 0.5 deg






10/08/02 15:11 GMT SZ-2 CensoringShort PRT





10/08/02 15:11 GMT SZ-2 Censoring*Short PRT




SZ-2 VelocityMedium PRT EL = 0.5 deg

10/08/02 15:11 GMT Legacy VelocityMedium PRT





10/08/02 15:11 GMT SZ-2 CensoringMedium PRT





10/08/02 15:11 GMT SZ-2 Censoring*Medium PRT


Convective Precipitation





Squall Line





MCS-Squall Line





The End


Sebastian Torres NEXRAD Range-Velocity Ambiguity Mitigation Fall 2004 – Technical Interchange...

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Transcript of Sebastian Torres NEXRAD Range-Velocity Ambiguity Mitigation Fall 2004 – Technical Interchange...