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Staggered PRT Practical Considerations

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Staggered PRTPractical Considerations

Sebastian Torres

CIMMS/NSSL

Technical Interchange Meeting

Spring 2005

BATCH MODE

STAGGERED PRT

- Staggered PRT refresher
- Choosing the PRTs
- Away with the Batch Mode!
- Ground clutter filter performance

Improvement in Doppler velocity retrieval using Staggered PRT

T1

T2

T1

T2

…

time

`

Simple algorithm

- Transmitter alternates two PRTs
- T1 < T2, PRT ratio: k = T1/T2 = m/n

- Maximum unambiguous range
- Reflectivity: ra = ra2
- Doppler velocity: ra = ra1

- Maximum unambiguous velocity
- Velocity Dealiasing Algorithm (VDA)
- va = m va1 = n va2

Max. unambiguous velocity can be extended by means of a simple algorithm

Longer PRTs can be used to avoid overlaid echoes

Velocity fields are “clean”

Spectral processing is complicated due to non-uniform spacing of samples

Ground clutter filtering is a challenge

Longer PRTs may result in larger errors of estimates

Choice of PRTs

What are the magicalvalues of T1 and T2?

- How do we choose T1 and T2 for the operational environment?
- Constraints
- System limits
- Range coverage
- Design considerations
- Errors of estimates

- Maximum RF duty cycle for the WSR-88D transmitter is 0.2%
- Duty cycle = t /Ts
- 1.57 ms/Ts≤ 0.002 => Ts≥ 780 ms

- Signal processor memory allocation in the RVP-8 limits the number of range bins to 3072
- Typical sample spacing: 1.67 ms (250 m)
- 1.67 ms x 3072 = 5.12 ms => Ts≤ 5.12 ms

- Maximum unambiguous range
- ra = cTs/2
- The longer the PRT the better!

- WSR-88D requirements:
- Reflectivity up to 460 km
- ra2≥ 460 km => T2≥ 3.07 ms

- Doppler velocity up to 230 km
- ra1≥ 230 km => T1 ≥ 1.53 ms

- Reflectivity up to 460 km

- Actual requirements for intermediate to higher elevations can be relaxed
- Most storms tops are below 18 km
- For a height of 18 km, the slant range (r) is ~ 300 km at an elevation (qe) of 2.4 deg and ~ 150 km at 6 deg

- Actual requirements for range coverage at intermediate elevations are300 km or less
- For example
- At 2.4 deg: ra1≥ 230 km => T1 ≥ 1.53 ms ra2≥ 300 km => T2≥ 2 ms
- At 6.0 deg: ra1≥ 150 km => T1 ≥ 1 ms ra2≥ 150 km => T2≥ 1 ms

- Algorithm assumes T1 < T2
- Preferred PRT ratio is k = 2/3
- Fewest number of rules for the VDA
- Spectral GCF performs best with this ratio (Sachidananda et al, 1999)

- T1 / T2 = 2/3
- Note: None of the existing PRTs in the WSR-88D satisfy this condition!
- RVP-8 and 88D transmitter not limited to 8 PRTs
- Possible ORPG impact
- RDA/RPG ICD does not include the PRT
- VCP definitions use PRT numbers

- Echo coherency is required for precise Doppler measurements
- The spectrum width should be much smaller than the Nyquist interval
- Median value of sv in severe storms is about 4 m/s
- T1 < 2.2 ms
- However, larger sv would require shorter T1

System limits

Range coverage

Design constraints

Signal coherency

PRT ratio

Acceptable PRTs

T1 = 1.5350 ms

T2 = 2.3025 ms

Errors of Estimates

Can these values of T1 and T2 do the work?

T1

T2

T1

T2

…

R1

R2

R2

R1

time

- Uniform PRT uses contiguous pairs
- Staggered PRT uses spaced pairs
- Usual formulas for the standard error of estimates have to be modified
- Formulas for the spectrum width don’t even apply

Staggered PRT assumes echoes don’t extend beyond ra2

T1

T2

I II I II III

- Reflectivity
- Three zones:
- I: Short PRT pulses only
- II: All pulses
- III: Long PRT pulses only

- Zone II estimates are more accurate for large spectrum widths

- Three zones:

Solid lines: short (or long) PRT pulses only

Dashed lines: all pulses

SNR = 40 dB

DT = 60 msk = 2/3

- Velocity
- v1 from short PRT pairs is dealiased using v2 from long PRT pairs

- Spectrum Width
- Depends on power and lag-1 autocorrelation estimates
- Use short or long PRT lag?
- Long PRTs will exhibit saturation for large spectrum widths

- Depends on power and lag-1 autocorrelation estimates

Solid lines: short PRT pairs

Dashed lines: long PRT pairs

SNR = 40 dB

DT = 60 msk = 2/3

Solid lines: short (or long) PRT pulses for power

Dashed lines: all pulses for power

Lines without markers:short PRT pairs for autocorrelation

Lines with markers:long PRT pairs for autocorrelation

SNR = 40 dB

DT = 60 msk = 2/3

Replacing the Batch Mode

Can Staggered PRT performbetter than the Batch Mode?

- 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

- Acquisition time
- Dictated by antenna rotation rate
- Must keep same or shorter dwell time

- Errors of estimates
- Must meet NEXRAD requirements

Extended using long PRT powers

rad: short PRT

ras: long PRT

T1 = 2rad/c

T2 = 2ras/c

ras = (3/2) rad

Batch Mode

Staggered PRT

- According to NEXRAD requirements, standard errors of estimates are specified at SNR > 8 dB and sv = 4 m/s
- For the Batch Mode, we use standard formulas (contiguous pairs)
- For the Staggered PRT, we use modified formulas (spaced pairs)

Requires average of 4 gates in range

All slightly larger than 1 m/s

All better than with the BM

All less than 1 m/s

Batch Mode

Staggered PRT

- Collect more samples per radial
- This requires reducing the antenna rotation rate
- For example, at 2.5 deg need to collect 21 pairs instead of 15, adding about 8 seconds to the scan

- This requires reducing the antenna rotation rate
- Use Sachi’s one-overlay resolution technique and reduce the PRTs
- Computationally complex

- Use oversampling and whitening
- Even a small oversampling factor would work

- Requirements don’t change!
- Everything depends on the required range coverage and the dwell time
- The shorter the dwell time and/or the larger the required range coverage, the larger the errors
- Larger errors increase the likelihood of catastrophic errors in the Staggered PRT velocity dealiasing algorithm

- The shorter the dwell time and/or the larger the required range coverage, the larger the errors

Ground Clutter Filtering with Staggered PRTs

Can we remove the clutteras well as with uniform PRT?

- Batch mode
- Surveillance pulses are filtered with the two-pulse canceller
- About 30 dB of suppression

- Doppler pulses are filtered with GMAP

- Surveillance pulses are filtered with the two-pulse canceller
- Staggered PRT
- Staggered PRT pulses are filtered with a spectral GCF (Sachidananda)
- Filter can incorporate elements of GMAP

- Staggered PRT pulses are filtered with a spectral GCF (Sachidananda)

Batch Mode - 2 pulse canceller

Batch Mode - GMAP

No GCF

Staggered PRT

- Unambiguous range coverage for velocity estimates is greatly improved with Staggered PRT
- Staggered PRT yields larger Nyquist velocity intervals
- For the same dwell times, errors of estimates are better for reflectivity and slightly worse for velocity
- Ground clutter suppression using a spectral filter is much better for reflectivity and comparable for velocity
- Staggered PRT is a viable candidate to replace the legacy Batch Mode at intermediate elevation angles

BATCH MODE

STAGGERED PRT