VCPs for Staggered PRT

Sebastián Torres and David Warde

CIMMS/NSSL

Data Quality MOU – Technical Interchange Meeting

Fall 2008

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Outline

• Designing VCPs for Staggered PRT (recap)

– Performance indicators

• Test VCPs 14 and 15 (recap)– Design criteria

• Benefits of using shorter PRTs– Trade-offs

• A new test VCP– Performance analysis

• Recommendation andFuture Work

Staggered PRT

ra = 184 km, va = 45.1 m/s

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VCP Performance Indicators

• Acquisition time– Dwell times

• Maximum unambiguous range– Surveillance (Z)– Doppler (V & W)

• Maximum unambiguous velocity• Spectrum width saturation• Errors of estimates– Statistical errors– Dealiasing errors

• Clutter suppression

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PRT Choice

• Shorter PRTs– Larger Nyquist– Larger maximum

spectrum width– More samples for the

same dwell times – Lower errors of

estimates– Lower rate of

catastrophic errors– Better clutter

suppression

• Longer PRTs– Longer unambiguous

range

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VCP Design Criterion

• Make T1 as short as possible– No-overlay condition

• ra,1 ≥ rmax

– T1 ≥ 2rmax / c

– Algorithm limitation• ra,2 ≥ rmax

– T1 ≥ 4rmax / 3c

• ra,D = ra,1 and ra,S = ra,2

– The no-overlay condition is more restrictive, but not a requirement

– The current SPRT algorithm does not handle range unfolding

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Test VCPs for Staggered PRT

• 2007: Staggered PRT subcommittee– Two test VCPs based on VCP 12

• Intermediate and high elevations running SPRT ( = 2/3)

• Design criterion

– VCP 14: Same dwell times as VCP 12– VCP 15: Double dwell times as VCP 12 for SPRT

EL < 4 degEL > 4 deg

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VCPs 14 and 15

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• If we allow some overlay– ra,1 ≤ rmax ≤ ra,2

• No overlay: ra,1 = rmax

• Maximum overlay: ra,2 = rmax

– Shorter PRTs– Must have technique to recover overlaid

echoes

Overlay Trade-off

ra,2

ra,1

rmax

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New VCP Design Criterion

• We can trade shorter PRT for increased likelihood of overlaid echoes

– Benefits are realized for elevations above 2.4 deg

Solid: oldDashed: new

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Choosing the Dwell Times

• Dwell times must be chosen to…– Meet error requirements for reflectivity,

velocity, and spectrum width– Meet clutter filtering requirements– Satisfy operational needs

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Dwell Times for SACHI (I)

Maximum clutter suppression based on Reflectivity requirements

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Dwell Times for SACHI (II)

Maximum clutter suppression based on Spectrum Width requirements

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Dwell Times for SACHI (III)

Maximum clutter suppression based on Velocity requirements

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Performance of SACHI

• Excessively long dwell times seem to be needed to meet clutter filtering velocity requirements

– Feasible, but operationally not acceptable

• Recommend using dwell times to meet error requirements

– Longer dwell times than with VCP 12, but operationally acceptable

• We have anecdotal evidence of operators switching from VCP 12 to VCP 21 to get what they want

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Performance of SACHI

• Only Doppler velocities are affected corresponding to gates …

– … with strong clutter contamination and … – … with velocities around 0, ±va/5, ±2va/5 and …– … where SACHI doesn’t pick the right wx “replica” and …

– … where the ORPG velocity dealiasing algorithm fails to fix spatial discontinuity

• This has not been quantified!

• Censoring of “bad velocities” is possible– Reflectivities are still good for rainfall rate estimates

• Standard errors are more than 3x better than with the Batch mode

– Spectrum widths are still good for turbulence

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Proposed VCP

• T1 for maximum allowable overlay

• Dwell times to meet error requirements

– VCP time ~6 min

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Proposed VCP Performance• Acquisition time– Longer dwell times lead to VCP time ~6 min

• Maximum unambiguous range and velocity– Great improvement w.r.t legacy VCPs

• Spectrum width saturation– Acceptable

• Errors of estimates– Statistical errors meet requirements– Catastrophic errors effectively handled by ORPG’s

VDA

• Clutter suppression– Meets requirements for Z and W– V dealiasing errors may be handled by ORPG’s VDA

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Summary

• Staggered PRT provides significant benefits …

– Longer unambiguous range– Larger Nyquist velocity– More accurate reflectivity estimates

• … that must be traded for …– Longer acquisition times (~VCP 21)

• Operationally acceptable

– Increased rate of velocity dealiasing errors• Handled by ORPG’s VDA

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Future Work

• Recommendation– Implement proposed test VCP– Collect more data cases – Process data using recommended SPRT

algorithm including SACHI and modified ORPG velocity dealiasing algorithm

– Assess data quality

• Future work– Complete evaluation of technique

to recover overlaid echoes for SPRT– Research ways to improve SACHI

Back up slides

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VCP 12