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Staggered PRT for NEXRAD

Staggered PRT for NEXRAD. Dr. Sebastian Torres CIMMS/NSSL. A presentation to the Data Quality Team June 15, 2007. Range and Velocity Ambiguities. Doppler Dilemma: r a v a = c l /8 NEXRAD specifications l ≈ 10 cm, r a = 230 km Overlaid echoes are more likely with shorter PRTs

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Staggered PRT for NEXRAD

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  1. Staggered PRT for NEXRAD Dr. Sebastian Torres CIMMS/NSSL A presentation to the Data Quality TeamJune 15, 2007

  2. Range and Velocity Ambiguities • Doppler Dilemma: rava = cl/8 • NEXRAD specificationsl≈ 10 cm, ra = 230 km • Overlaid echoes are more likely with shorter PRTs • Range unfolding algorithm is not 100% effective resulting in “purple haze” • Goal: Reduce velocity aliasing and obscuration from overlaid echoes Doppler Velocity from KTLXBatch Mode Doppler Velocity from KOUNStaggered PRT va ≈ 16 m s-1

  3. Alternating batches of long-PRT and short-PRT pulses. Long-PRT reflectivities are used to unfold short-PRT velocities (at most, strongest overlaid trip can be recovered) Uniform PRT 1 scan at each elevation angle Batch PRT 1 scan at each elevation angle Split Cuts2 scans at each elevation angle Long-PRT scan followed by short-PRT scan.Long-PRT reflectivities are used to unfold short-PRT velocities (at most, strongest overlaid trip can be recovered) Legacy Mitigation Strategy 19.5° 7.0° 1.5° 0.5°

  4. Alternating long-PRT and short-PRT pulses. Range-unfolded reflectivities and velocities with good maximum unambiguous velocity (ORDA Build 10+) Uniform PRT (Legacy) 1 scan at each elevation angle Staggered PRT 1 scan at each elevation angle Phase coding (SZ-2)2 scans at each elevation angle Long-PRT scan followed byphase-coded short-PRT scan.Long-PRT reflectivities are used to unfold short-PRT velocities (two strongest overlaid trips can be recovered) (ORDA Build 9) Evolutionary Mitigation Strategy 19.5° ? 7.0° ? 1.5° 0.5°

  5. The Staggered PRT Technique • Transmitter alternates two PRTs • T1 < T2, PRT ratio: k = T1/T2 = m/n • Maximum unambiguous range • Reflectivity: ra = ra2 • Doppler velocity and spectrum width: ra = ra1 • Maximum unambiguous velocity • SPRT Velocity Dealiasing Algorithm • va = m va1 = n va2 2/3 → Preferred ratio … T1 T2 T1 T2 time va = 2va1 = 3va2

  6. T1 T2 T1 T2 … time SPRT Velocity Dealiasing Algorithm • Velocities can be estimated for each PRT • v1 can be estimated from the short-PRT pairs • v2 can be estimated from the long-PRT pairs • Maximum unambiguous velocities for the short and long PRT pairs are different • Velocities v1 and v2 alias in different ways • The true (de-aliased) velocity can be obtained by “analyzing” how v1 and v2 alias R1 R2 R2 R1

  7. ±25 m/s ±16.7 m/s v1 v2 + + 3 simple rules 0 m/s 0 m/s SPRT Velocity Dealiasing Algorithm (II) • Algorithm relies on how the short- and long-PRT velocity estimates alias • Example (l = 10 cm) • T1 = 1 ms → va1 = 25 m/s • T2 = 1.5 ms → va2 = 16.7 m/s

  8. v1 - v2 True velocity v2 ^ add 2va1 to v1 Rule 3 va2 closest level ^ ^ v1 – v2 Velocity dealiasing rules based on velocity difference transfer function SPRT Velocity Dealiasing Algorithm (III) ^ ^ • v1 and v2 are computed from short and long PRT pairs • A velocity difference transfer function determines the intervals for the different dealiasing rules • Beware: Estimates have errors Aliased v Rule 1 Rule 2 Rule 3 v1 va1 True v

  9. Advantages of Staggered PRT • Staggered PRT has the potential of • … producing “clean” fields of reflectivity, velocity, and spectrum width • Likelihood of overlaid echoes can be minimized by using longer PRTs • At least double the current inherent maximum unambiguous range for Doppler • … increasing the maximum unambiguous velocity • … producing reflectivity values with improved accuracy

  10. Limitations of Staggered PRT • Maximum unambiguous velocity is extended with a simple Velocity Dealiasing Algorithm (Torres et al, 2004) • Occurrence of catastrophic errors • Ground clutter filtering is effective but computationally more complex (Sachidananda and Zrnic, 2002) • Filter performance degrades with small number of staggered pairs • Use of longer PRTs reduces the likelihood of overlaid echoes but • … limits the range of measurable spectrum widths • … leads to less accurate velocity estimates (compared to standard VCPs)

  11. Range coverage Different for Surveillance (reflectivity) and Doppler (velocity and spectrum width) Must meet NEXRAD requirements but can be adjusted using maximum height of storms Maximum unambiguous velocity Must meet or exceed current standard VCP performance Acquisition time Dictated by antenna rotation rate Must keep same or shorter dwell time Errors of estimates Must meet NEXRAD requirements VCPs Design for Staggered PRT

  12. Required Range Coverage

  13. Range of acceptable PRTs 2.4° Selecting the Staggered PRTs Trade-off Can satisfy ra and va • Maximum unamb. range • ra,S = 3cT1/4 ≥ rmax • ra,D = cT1/2 ≥ rmax • Maximum unamb. velocity • va = l/2T1≤ va,D • Shorter PRTs lead to • Larger max. unamb. velocity • Larger max. measurable spectrum width • Lower errors of velocity and spectrum width • Lower rate of catastrophic errors • Better ground clutter filtering Match va Match ra

  14. Resulting Performance (VCP 11) VCP 11 Batch Mode Staggered PRT

  15. Case Studies • Time series data collected with NSSL’s KOUN radar • Modified hardware and software • Experimental Volume Coverage Pattern (VCP) • Lower elevation angles (0.5º and 1.5º): split cut (VCP 11), phase coding, and staggered PRT. • Intermediate elevation angles (2.5º): batch mode (VCP 11), phase coding, and staggered PRT. • Signal Processing • Baseline ORDA algorithms • Recommended Staggered PRT algorithm • Clutter filtering is included • ORPG’s velocity dealiasing algorithm is not included

  16. April 22, 2004 – 2.5 deg Staggered PRT(k = 2/3, same DT) Reflectivity Batch ModeVCP 11 ra = 466 km ra = 276 km

  17. April 22, 2004 – 2.5 deg Staggered PRT(k = 2/3, same DT) Doppler Velocity Batch ModeVCP 11 ra = 147 km, va = 28.8 m/s ra = 184 km, va = 45.1 m/s

  18. SPRT Velocity Dealiasing Errors “Catastrophic errors” are more likely to occur for larger spectrum widths

  19. June 30, 2004 – 1.5 deg Staggered PRT(k = 2/3, 10% larger DT) Reflectivity Split CutVCP 11 ra = 466 km ra = 360 km

  20. June 30, 2004 – 1.5 deg Staggered PRT(k = 2/3, same DT) Doppler Velocity Split CutVCP 11 ra = 147 km, va = 28.8 m/s ra = 240 km, va = 34.7 m/s

  21. March 3, 2004 – 2.5 deg Staggered PRT(k = 2/3, same DT) Reflectivity Batch ModeVCP 11 ra = 466 km ra = 276 km

  22. March 3, 2004 – 2.5 deg Staggered PRT(k = 2/3, same DT) Doppler Velocity Batch ModeVCP 11 ra = 147 km, va = 28.8 m/s ra = 184 km, va = 45.1 m/s

  23. Summary • Staggered PRT advantages • Extension of maximum unamb. range and velocity with simple algorithm • Lower errors of reflectivity estimates • Staggered PRT disadvantages • May need to define new PRTs for the system • Larger errors of velocity and spectrum width estimates • ~30% larger than in the legacy Batch Mode • Can be mitigated with range oversampling techniques • Occurrence of catastrophic VDA errors (velocity only) • Can be mitigated with continuity check in the RDA and RPG’s existing VDA • Staggered PRT is an excellent candidate to replace the Batch Mode in standard NEXRAD VCPs • It provides significant reduction of velocity aliasing and obscuration due to overlaid echoes

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