staggered prt and phase coding algorithms
DESCRIPTION
NEXRAD Range-Velocity Ambiguity Mitigation. Staggered PRT and Phase Coding Algorithms. Sebastian Torres. Part One. Staggered PRT Current Status. Block 1. Block 2. Block 3. Pattern. Pattern. Pattern. RRDA Capabilities Staggered PRT. Expanded VCP definitions - PowerPoint PPT PresentationTRANSCRIPT
Sebastian Torres
NEXRAD Range-Velocity Ambiguity Mitigation
Staggered PRT andPhase Coding
Algorithms
Part One
Staggered PRTCurrent Status
RRDA CapabilitiesStaggered PRT
• Expanded VCP definitions▪ Staggered PRT modes are specified using patterns
T1 T1 T2 T2 T2 T3 T1 T1 T2 T2 T2 T3 T1 T1 T2 T2 T2 T3 …
• Expanded set of PRTs▪ Exact PRT ratios▪ Resolution given by 9.6 MHz clock
• Real-time staggered PRT algorithm▪ Hardware and software modifications
• Level I and II recorder▪ Uninterrupted data collection for up to 8 hours
Block 1 Block 2 Block 3
Pattern Pattern Pattern
The Staggered PRT Technique
• Transmitter alternates two PRTs▪ T1 < T2
▪ PRT ratio: K = T1/T2 = m/n (m,n integers)▪ ra1 = cT1/2, ra2 = cT2/2▪ va1 = /4T1, va2 = /4T2
• Maximum unambiguous range▪ ra = ra2 (one-overlay resolution)
• Maximum unambiguous velocity▪ va = m va1 = n va2 (velocity dealiasing)
T1 T2
time
T1 T2…
The Staggered PRT Technique
va = 25.36 m s-1 va = 45.17 m s-1
148 km184 km
KTLXVCP 11 – Batch Mode
KOUNStaggered 184/276EL = 2.5 deg
04/06/03 4:42 GMT
The Staggered PRT Algorithm
• Computation of autocovariances▪ P1, R1 for short range sweeps▪ P2, R2 for long range sweeps
▪ P1, R1, and R2 computed up to ra1
▪ P2 computed up to ra2
T1 T2 T1
o oo oP1 P1
R1 R1
P2 P2
R2 R2
The Staggered PRT Algorithm
• Ground clutter filtering▪ Magnitude squared of DC component is
removed from autocovariances▪ Bypass map is used▪ Filter is simple but suppression is limited to
about 10 dB▪ Future work: Test other filtering schemes
• Sachidananda’s GCF (Rep. 3 & 4)– Frequency domain filter
• Regressive filters• Others
Clutter Filter Performance
KOUNStaggered PRT
KOUNUniform PRTEL = 0.5 deg
03/17/03 23:06 GMT
The Staggered PRT Algorithm
• Velocity dealiasing algorithm▪ v1 and v2 are computed from R1 and R2
vv2va2
v1
va1
v1 – v2^ ^
v1 - v2
closest level
True velocity
add 2va1 to v1^
^ ^
Velocity Dealiasing Algorithm Performance
va = 45.1 m s-1 va = 34.6 m s-1
KOUNStaggered 184/276
KOUNStaggered 240/360EL = 2.5 deg
04/06/03 4:50 GMT
184 km
240 km
Velocity Dealiasing Algorithm Performance
• What happens if SD(v1) and SD(v2) are large?
v1 – v2^ ^
v1 - v2
v
v1
va1
closest level
True velocity
closest level
Wrong velocity
Catastrophic error!!
Velocity Dealiasing Algorithm PerformanceVelocity
Staggered 240/360Spectrum WidthStaggered 240/360EL = 2.5 deg
04/06/03 4:48 GMT
Can be used for censoring
The Staggered PRT Algorithm
• Reflectivity computation▪ Use clean powers
▪ Computed to ra2
▪ Future work: Extend Z to 2ra1
• Censoring▪ Overlaid echoes do not bias v, but act as noise▪ Future work: Test Sachidananda’s one-overlaid resolution scheme
(Rpt. 4)T1 T2
I II I II III
Censoring
ReflectivityStaggered 184/276
VelocityStaggered 184/276EL = 1.5 deg
03/18/03 3:28 GMT
276 km
184 km
Statistical errors
Summary
• Range coverage▪ Z to ra2 and v to ra1, where ra1/ra2 = m/n = K▪ Natural “match” for NEXRAD requirements
• Extension of maximum unambiguous velocity▪ va = m va1 = n va2
• Range-velocity ambiguities▪ Uniform PRT
• rava = c/8 → Inadequate for = 10 cm
▪ Staggered PRT • ra1va = m(c/8)• ra1 vs. va trade-off controlled by PRTs
PRT Trade-Off
Long PRTsStaggered 336/466
va = 26.7 m s-1Medium PRTs
Staggered 240/360va = 34.6 m s-1
Short PRTsStaggered 184/276
va = 45.1 m s-1
336 km
240 km
184 km
K = 2/3
K = 2/3
Conclusions
• Algorithm works with any PRT ratio▪ No need to add new PRTs to the system
(initially)▪ Only need exact ratios for Sachidananda’s
ground clutter filter and one-overlaid recovery
• Need good velocity estimates to avoid catastrophic errors▪ Future work: Determine maximum allowable
errors for a given set of PRTs
Conclusions
• Recommended for intermediate elevations to replace legacy Batch Mode▪ Need better ground clutter filters to be useful
at lower elevations▪ Future work: Derive optimum choice of PRTs
to match current performance
• Achieves “clean” separation of echoes
• Results in very simple algorithm
Part Two
Phase CodingSZ-2 AlgorithmCurrent Status
RRDA CapabilitiesPhase Coding
• Expanded VCP definitions▪ Can specify phase coding sequence number for each scan
• Standard (or predefined)• Downloadable
– Proposed new RPG-RDA Message
• Real-time 1st-trip decoding of phase-coded signals▪ Hardware and software modifications▪ Use WSR-88D phase shifter (7 bits)
• Level I and II recorder▪ Uninterrupted data collection for up to 8 hours
SZ-2 Algorithm
• Transmitted pulses are phase-modulated with SZ(8/64) switching code
• Phase-coded scan is preceded by long-PRT surveillance scan▪ Surveillance scan is not phase coded▪ Powers from the surveillance scan are used to
determine overlaid trips in the phase-coded scan▪ Spectrum widths from the surveillance scan can
be used for censoring• Future work: Study limitations of spectrum width
estimates obtained from long PRTs
SZ-2 Algorithm
• Censoring and overlaid trip determination
▪ Significant return? ▪ Above noise plus sum of out-of-trip powers?▪ Within recovery region?
• Based on plots of SD(vw) on the Ps/Pw vs. ww plane
range1st trip 2nd trip 3rd trip 4th trip
PL
Pth
P1
P2
P3 P4
SZ-2 Algorithm
• 1st trip cohering▪ Use measured switching code
• Ground clutter filtering▪ Use bypass map▪ Frequency domain filter▪ Future work: Study other filtering schemes
2nd trip modulated1st trip cohered
Ground clutter
3rd trip modulated4th trip modulated
v
SZ-2 Algorithm
• Lag-one autocorrelation computation▪ From cohered data for two strongest trips
• Final strong/weak trip determination▪ Use |R(Ts)| for the two strongest trips
• Strong-trip cohering
• Strong-trip velocity computation (vs)
v
Strong trip cohered
Weak trip modulated
Weak trip cohered
Strong trip modulated
vs
SZ-2 Algorithm
• Processing notch filter (PNF)▪ Location determined by vs and presence of
clutter
▪ Notch Width determined by strong and weak trip numbers
• 8 replicas → NW = 3M/4• 4 replicas → NW = M/2
1st trip cohered
2nd trip modulatedPNFPNF
vvsvs/2
SZ-2 Algorithm
• Weak-trip cohering
• Weak-trip velocity computation (vw)
▪ From lag-one autocorrelation of notched and cohered weak signal
v
Weak trip cohered
Sidebands
Strong trip residue
vw
SZ-2 Censoring
• Power adjustments▪ Windowing▪ PNF▪ Weak-trip
• Assignment of correct range▪ Trip numbers are used to assign correct range
location to strong- and weak-trip moments
• Censoring and thresholding▪ Tag trips with significant powers that are
unrecoverable
SZ-2 Algorithm Performance
ReflectivityLong PRT
VelocitySZ-2 with short PRTEL = 0.5 deg
04/06/03 4:26 GMT
117 km
234 km
SZ-2 Algorithm Performance
VelocityNon PC “Split cut”
VelocitySZ-2 with medium PRTEL = 0.5 deg
04/06/03 4:28 GMT
175 km175 km
va = 23.7 m s-1
SZ-2 vs. Staggered PRT
VelocityStaggered 240/360
VelocitySZ-2 with medium PRTEL = 0.5 deg
04/06/03 4:30 GMT
240 km 175 km
va = 34.6 m s-1 va = 23.7 m s-1
Conclusions
• SZ-2 uses a non-phase-coded, long-PRT, surveillance scan to determine overlaid trips▪ Substitute for “split cuts” in the legacy WSR-88D
• SZ-2 handles up to 2 trips out of 4 possible▪ Two strongest trips are selected▪ Future work: Fine-tune thresholds
• Can use overlapping radials if M ≠ 64▪ Future work: Test this technique with real data
Conclusions
• Phase coding may require ground clutter filters with zero phase response▪ Future work: Study alternatives to recursive filters▪ Future work: Study ways to compensate for phase
distortions
• Censoring in SZ-2 is simpler than in the stand-alone version (SZ-1)▪ Use P and v from surveillance or both scans
▪ Future work: Fine-tune/add(?) parameters
Conclusions
• SZ-2 is very sensitive to clutter residue▪ From the long-PRT surveillance scan
• Recovery region test does not pass• Ring of censored data at the beginning of 2nd, 3rd,
and 4th trips
▪ From the phase-coded scan• Noisy data at the beginning of 2nd, 3rd, and 4th trips• Could add CSR as a censoring parameter
• SZ-2 is very sensitive to out-of-trip leakage▪ Fixed by fine-tuned censoring parameters
SZ-1 vs. SZ-2
VelocitySZ-1
No cens., No GCF, 1st and 2nd trips only
VelocitySZ-2EL = 0.5 deg
04/06/03 4:30 GMT