NOAA Assessment of the Oceansat-2 Scatterometer

Seubson (Golf) Soisuvarn, Zorana Jelenak and Paul S. Chang

NOAA/NESDIS/StAR

OceanSat-2 Scatterometer- OSCAT

• OSCAT is a Ku-band rotating pencil beam scatterometer with similarities to QuikSCAT

– Developed by ISRO– Launched Oct 2009 on board of OceanSat-2 satellite

Parameter QuikSCAT(NASA/JPL)

OSCAT(ISRO)

Operational Frequency 13.402 GHz 13.515 GHzPolarization (Inner/Outer) HH/VV HH/VV

Antenna Diameter 1 m 1 mAltitude at Equator 803 km 720 km

Orbit Near Repeat Cycle 4 days 2 daysLocal time at asc/desc node 6:00 a.m. at asc node noon at desc nodeHH 3dB footprint (Az x El) 24 x 31 km 26.8 x 45.1 kmVV 3 dB footprint (Az x El) 26 x 36 km 29.7 x 68.5 km

Incidence Angle (Inner) 46 deg 49 degIncidence Angle (Outer) 54 deg 57 degSwath Diameter (Inner) 1400 km 1400 kmSwath Diameter (Outer) 1800 km 1836 km

Pointing Accuracy +/- 0.05 deg +/- 0.15 degElevation Pointing (w/ attitude error) +/- 0.05 deg +/- 0.25 deg

NRT OSCAT Data via ISRO and EumetSat

3

OSCAT Data Availability @ NOAA

• NOAA has been receiving delayed (~24 hours) OSCAT data via ISRO FTP server since September 2010

• Timely OSCAT data flow from ISRO to EUMETSAT commenced in February 2011.– Since then EUMETSAT has been receiving

approximately 10-14 orbits per day in a timely fashion

– In March 2011 timely OSCAT data flow began at NOAA via EUMETSAT dedicated FTP server

– ISRO currently working on providing all orbits in a timely fashion and working out procedures to operationally support the data flow

• NOAA is currently receiving all three levels of OSCAT data: L1B, L2A and L2B

• L2A and L2B (gridded @ 50 km WVC)

180min150min

Courtesy of T. Heinemann EumetSat

NRT L2B data coverage for all received orbits in May 2011. High coverage indicated by red and yellow colors, low by blue and violet colors

NRT OSCAT North Atlantic Data Gaps

• Data gap in North Atlantic observed in ascending nodes

• This data gap problem is a result of the definition of data-orbits from North Pole to North Pole.

• The reception stations in Svalbard and Tromsø are not at the North Pole and due to technical constraints at the Oceansat-2 satellite, the satellite dump needs to be cut-off before the transmission starts. Therefore, the measurements between the reception stations and the North Pole are missing when the satellite reaches the reception station from the Atlantic side of the globe.

• ISRO and EUMETSAT try to find a solution for this problem.

Courtesy of T. Heinemann EumetSat

Ongoing Efforts(In collaboration with JPL, KNMI and ISRO)

• Understand and characterize the L1B performance

• Develop and implement sigma0 corrections as appropriate

• Develop and implement NOAA unique L2 products to meet NOAA user requirements

Data Analysis Overview

• The two latest L1B data were used in this analysis– February 2011 processing (OLD)

• Implemented in current NRT processor

– July 2011 processing (NEW)• There is currently no overlap period between the two data sets

– Two different days were used in analysis, but approximately equal amount of data• Day 094, 2011 (OLD) – 14 orbits• Day 030, 2010 (NEW) – 15 orbits

– All L1B QC flags were used except bit:4 (in order to show all slices) and bit:9 negative sigma0 flag

• L1B latest known Discrepancy– L1B was matched up with NCEP model winds and NSCAT2 GMF was used to compute

modeled sigma0– Noise Subtraction Algorithm

• Calculate Signal power and Noise power

– Scan Angle Dependence– Ascending Descending Dependence

Noise Subtraction

PS 0X PSN PN

PN 0XSNR

Noise subtraction inconsistent among slices for pre-July ISRO OSCAT processing version

Power after noise subtraction

Noise power

zero line zero line

NEW = July 2011 deliveryOLD – Pre July 2011

INNER BEAM OUTER BEAM

OLD OLD

NEW NEW

Signal Power

Noise Subtraction – 2

• Signal-to-noise ratio are improved at low wind speeds• In both versions signal power appear to saturate at ~ 3-4

m/s

NEW

Ps

Pn

OLD

Ps

Pn

Negative Sigma0

• Low value of sigma0 anomalies with wind speed is now gone.

OLD NEW

INNER BEAM OUTER BEAM

OLD – Pre July 2011

NEW

Sigma0 bias vs Scan Angle

Large biases between ascending & descending are still observed: ~ 1 dB for inner beam (H-pol) and ~ 0.5 dB for outer beam (V-pol)

Biases with respect to NSCAT-2 model function operating on GDAS winds.

INNER BEAM OUTER BEAM

Xfactor vs Scan Angle

OLD – Pre July 2011

INNER BEAM OUTER BEAM

Noise Power vs Scan Angle

OLD – Pre July 2011

ISRO L2B Data Analysis(Pre July 2011 Data Set )

ISRO OSCAT L2B Selected Wind VectorsWind Speeds Wind Directions

4≤ws<24m/sECMWF rms 2.08m/sGDAS rms 2.64m/s

4≤ws<24m/sECMWF rms 18.24ºGDAS rms m/s 20.42º

6<ws<24m/sECMWF rms 1.76m/sGDAS rms 1.89m/s

6<ws<24m/sECMWF rms 18.04ºGDAS rms m/s 19.84º

Average zonal winds bias vs ECMWF for March-July 2011 ISRO OceanSAT-II Near Real Time Winds. Pattern likely due to cross track biases.

Courtesy of Svetla Hristova-Veleva JPL

NOAA OSCAT Wind Data Processor

• Process OSCAT data from Level 1B

• Grid Sigma0 @ 25 km WVC (L2A)

• MLE based wind retrieval

• Median filter based ambiguity removal

ISRO derived WVC index (i,j) from satellite position and velocity vectors(not currently available in routine L1B processing )

Derived WVC index (i,j) by approximation from orbital elements given in L1B attribute parameters, i.e.-Inclination = 98.28 deg-Semi-major axis = 7098.14 km-Eccentricity = 0.00113-Equator crossing longitude (descending node) = varied orbit-by-orbit--New version of OSCAT L1B files contain state vectors

NOAA orbit (25 km)ISRO orbit (50 km)

NOAA 25km OSCAT L2A Product

SS

SSS

compX

X 0

0

σ0

1

σ0

2

σ0

3

We calculate standard deviation of each Sigma0 from the following formulation

2

00

0 1

Scomp

SS

SS

XX

NSTD

TC Bret07/20/2011

TC Cindy07/21/2011

Hurricane Dora07/20/2011

Summary and Conclusions

• OSCAT data has been flowing to NOAA in near real-time via EUMETSAT since March 2011 (not all orbits yet)

• OSCAT L1B/L2A investigation shows:– High wind speed retrievals from OSCAT should definitely be

possible– Sigma0 residual biases are significantly high at low wind speeds– Sigma0 are dependent on antenna scan angle– Sigma0 are also exhibit an Ascending & Descending pass

dependence• Development of enhanced L2A product from ISRO’s L1B

– 25 km WVC grid– L2A product will contain standard deviation of composite Sigma0

• Is proving to be useful parameter in definition of objective function normalization during retrieval process

• OSCAT has the potential to provide a high utility product to largely mitigate the impacts due to the loss of QuikSCAT in NWS weather forecasting and warning products and services 24