sebastian torres nexrad range-velocity ambiguity mitigation fall 2004 – technical interchange...
TRANSCRIPT
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Sebastian Torres
NEXRAD Range-Velocity Ambiguity Mitigation
Fall 2004 – Technical Interchange Meeting
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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
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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
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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
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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
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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
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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
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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
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Part Two
Status of the SZ-2 Algorithm
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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Part Three
Performance of the SZ-2 AlgorithmCase Examples
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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
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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
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Stratiform Precipitation
va = 35.5 m s-1, ra = 117 km va = 35.5 m s-1, ra = 117 km
SZ-2 VelocityShort PRT EL = 0.5 deg
10/08/02 15:11 GMT Legacy VelocityShort PRT
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Stratiform Precipitation
va = 35.5 m s-1, ra = 117 km va = 35.5 m s-1, ra = 117 km
SZ-2 VelocityShort PRT EL = 0.5 deg
10/08/02 15:11 GMT SZ-2 CensoringShort PRT
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Stratiform Precipitation
va = 35.5 m s-1, ra = 117 km va = 35.5 m s-1, ra = 117 km
SZ-2 VelocityShort PRT EL = 0.5 deg
10/08/02 15:11 GMT SZ-2 Censoring*Short PRT
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Stratiform Precipitation
va = 23.7 m s-1, ra = 175 km va = 23.7 m s-1, ra = 175 km
SZ-2 VelocityMedium PRT EL = 0.5 deg
10/08/02 15:11 GMT Legacy VelocityMedium PRT
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Stratiform Precipitation
va = 23.7 m s-1, ra = 175 km va = 23.7 m s-1, ra = 175 km
SZ-2 VelocityMedium PRT EL = 0.5 deg
10/08/02 15:11 GMT SZ-2 CensoringMedium PRT
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Stratiform Precipitation
va = 23.7 m s-1, ra = 175 km va = 23.7 m s-1, ra = 175 km
SZ-2 VelocityMedium PRT EL = 0.5 deg
10/08/02 15:11 GMT SZ-2 Censoring*Medium PRT
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Convective 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
05/17/03 0:39 GMT Legacy VelocityShort PRT
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Convective Precipitation
va = 35.5 m s-1, ra = 117 km va = 35.5 m s-1, ra = 117 km
SZ-2 VelocityShort PRT EL = 0.5 deg
05/17/03 0:39 GMT Legacy VelocityShort PRT
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Convective Precipitation
va = 35.5 m s-1, ra = 117 km va = 35.5 m s-1, ra = 117 km
SZ-2 VelocityShort PRT EL = 0.5 deg
05/17/03 0:39 GMT SZ-2 CensoringShort PRT
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Convective Precipitation
va = 23.7 m s-1, ra = 175 km va = 23.7 m s-1, ra = 175 km
SZ-2 VelocityMedium PRT EL = 0.5 deg
05/17/03 0:39 GMT Legacy VelocityMedium PRT
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Convective Precipitation
va = 23.7 m s-1, ra = 175 km va = 23.7 m s-1, ra = 175 km
SZ-2 VelocityMedium PRT EL = 0.5 deg
05/17/03 0:39 GMT SZ-2 CensoringMedium PRT
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Squall Line
va = 35.5 m s-1, ra = 117 km va = 8.9 m s-1, ra = 466 km
ReflectivityLong PRT EL = 0.5 deg
06/11/03 6:44 GMT Legacy VelocityShort PRT
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Squall Line
va = 35.5 m s-1, ra = 117 km va = 35.5 m s-1, ra = 117 km
SZ-2 VelocityShort PRT EL = 0.5 deg
06/11/03 6:44 GMT Legacy VelocityShort PRT
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Squall Line
va = 35.5 m s-1, ra = 117 km va = 35.5 m s-1, ra = 117 km
SZ-2 VelocityShort PRT EL = 0.5 deg
06/11/03 6:44 GMT SZ-2 CensoringShort PRT
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Squall Line
va = 23.7 m s-1, ra = 175 km va = 23.7 m s-1, ra = 175 km
SZ-2 VelocityMedium PRT EL = 0.5 deg
06/11/03 6:44 GMT Legacy VelocityMedium PRT
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Squall Line
va = 23.7 m s-1, ra = 175 km va = 23.7 m s-1, ra = 175 km
SZ-2 VelocityMedium PRT EL = 0.5 deg
06/11/03 6:44 GMT SZ-2 CensoringMedium PRT
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MCS-Squall Line
va = 35.5 m s-1, ra = 117 km va = 8.9 m s-1, ra = 466 km
ReflectivityLong PRT EL = 1.5 deg
06/26/03 3:14 GMT Legacy VelocityShort PRT
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MCS-Squall Line
va = 35.5 m s-1, ra = 117 km va = 35.5 m s-1, ra = 117 km
SZ-2 VelocityShort PRT EL = 1.5 deg
06/26/03 3:14 GMT Legacy VelocityShort PRT
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MCS-Squall Line
va = 35.5 m s-1, ra = 117 km va = 35.5 m s-1, ra = 117 km
SZ-2 VelocityShort PRT EL = 1.5 deg
06/26/03 3:14 GMT SZ-2 CensoringShort PRT