Seasat Scatterometer Global Gridded Dealiased
Wind Components (JPL-UCLA-AES)

Summary:

The SEASAT Scatterometer Global Gridded Dealiased Wind Component data set consists of zonal and meridional wind components in meters per second. Each data file contains a 1 degree by 1 degree gridded synoptic wind field which extends from 70° South to 70° North and from 1° East to 360° East. Data are available at 6-hour intervals for the period between 6 September 1978 and 20 September 1978. This data set, also referred to as PO.DAAC product 013, underwent restoration in 1997.

Table of Contents:

1. Data Set Overview:

Data Set Identification:

Seasat Scatterometer Global Gridded Dealiased Wind Components (JPL-UCLA-AES)

Data Set Introduction:

Meridional and zonal wind components were derived from the SEASAT-A Satellite Scatterometer (SASS). The synoptic wind fields are presented on a 1 degree by 1 degree grid at 6-hour intervals for the period between 6 September 1978 and 20 September 1978. Dealiasing of the SASS data was achieved "by hand" using ship observations in a joint effort between JPL, UCLA and AES. This data set underwent restoration in 1997. Updated read software and documentation are now included with this product.

Objective/Purpose:

To calculate global, dealiased, synoptic wind fields from SEASAT-A Satellite Scatterometer measurements of sigma-0.

Summary of Parameters:

The data files contain the following parameters:
  • zonal wind component in meters per second
  • meridional wind component in meters per second
  • data quality flag

Discussion:

This data set consists of zonal and meridional wind components derived from SEASAT-A Satellite Scatterometer (SASS) measurements of normalized radar cross section (NRCS). These data were calculated in a joint effort between the Jet Propulsion Laboratory, the University of California Los Angeles (UCLA), and the Atmospheric Environmental Service (AES) of Canada.

Related Data Sets:

Seasat Scatterometer Global Dealiased Surface Wind Vectors-Atlas (PO.DAAC product 002)

Seasat Scatterometer Global 50km sigma-0 data-Wentz (PO.DAAC product 031)

Seasat scatterometer global dealiased wind vectors-Wentz (PO.DAAC product 029)

2. Investigator(s):

Peter Woiceshyn
Jet Propulsion Laboratory
California Institute of Technology
email: wd21pw@rogue.jpl.nasa.gov

3. Theory of Measurements:

"The Seasat-A Satellite Scatterometer (SASS) was an active microwave sensor (frequency = 14.6 GHz; wavelength = 2.1 cm) which was used to remotely sense wind vectors over the oceans. The physical basis for this technique is the Bragg scattering of microwaves from centimeter length capillary ocean waves. The strength of sigma-0, the normalized radar cross-section (NRCS) backscatter coefficient measured by the SASS, is a function of the capillary wave amplitude that is itself proportional to the wind speed at the sea surface. Moreover, the backscatter response is anisotropic; wind direction can, therefore, be derived using scatterometer measurements at different azimuths." [Boggs, 1982]

4. Equipment:

Sensor/Instrument Description:

Collection Environment:

Seasat satellite

Source/Platform:

"The SEASAT spacecraft consisted of a three-axis-stabilized Lockheed Agena bus carrying a sensor module on which five remote-sensing instruments were mounted."

"The satellite orbit was near-circular, with an inclination of 108 deg, a period of about 101 min, and an altitude of approximately 800km. For the wide-swath instruments (i.e., SASS, SMMR, and VIRR), 95 percent of global coverage was accomplished every 36 h. The ground-track speed was about 6.6 km/s" [Boggs, 1982].

SEASAT Configuration

Source/Platform Mission Objectives:

"The objective of the [Seasat-A Satellite Scatterometer] was to provide a closely spaced grid measurement (~50 km) of ocean-surface wind speed and direction in the range of 4 to >= 26 m/s, accurate to ± 2 m/s or 10 percent (whichever is greater) in magnitude and ± 20 deg in direction" [Boggs, 1982].

Key Variables:

"The physical "observable" measured by the scatterometer is the NRCS backscatter coefficient sigma-0" [Boggs, 1982].

Principles of Operation:

"Four dual-polarized (vertical (V) and horizontal (H) polarizations) fan-beam antennas were aligned so that they pointed 45 and 135 deg relative to the spacecraft flight direction (in the orbit plane) to produce an X-shaped illumination pattern on the Earth. In this way a given surface location was first viewed by a forward antenna, and then viewed, somewhere between a few seconds and about three minutes later -- depending on the location's cross-track distance from the subsatellite track -- near-orthogonally by an aft antenna. Thus, sigma-0 measurements of the same region were provided at two azimuthal angles separated by approximately 90 deg."

"The illumination pattern for each antenna was active for 1.89-s measurement periods. The 1.89-s measurement interval was repeated continually and contiguously, but a different antenna or polarization was activated for each consecutive sampling period. Each of eight possible SASS science operational modes was associated with a different prescribed antenna/polarization sequence ordered .... All modes were characterized by an antenna switching-cycle period of 7.56 s, during which four antenna-beam/polarization combinations were cycled through. This timing was to provide sigma-0 measurements spaced approximately 50km apart (footprint area center-to-center distance) in the along-track direction"

"Fifteen Doppler filters were used to electronically subdivide each full antenna footprint into 15 measurement resolution "Doppler" cells of approximate dimension 20 km (cross-beam) by 50 km (along-beam). The intersection of the antenna-beam pattern and Doppler lines determined the resolution cell size, orientation, and location on the Earth.... The instantaneous-field-of-view (IFOV) cell boundaries were determined by the Doppler filter noise bandwidth and the antenna 3-dB beamwidth (.5 deg) in the narrow-beam dimension. [The integrated cell is the area swept out by a sequence of 61 overlapping IFOV cells generated over the course of a 1.89-s measurement period.] The surface area of this final integrated Doppler resolution cell is greater than the instantaneous illuminated region because the satellite moved (about 12.5-km ground-track distance) during the measurement period. Each of these integrated Doppler cells is a SASS footprint, and has one sigma-0 backscatter measurement value associated with it" [Boggs, 1982].

Scatterometer Ground Pattern and Swath

Sensor/Instrument Measurement Geometry:

"Twelve of the Doppler filters ...generated the two primary sigma-0 measurement swath strips that lie on either side of the subtrack (for two-sided modes). These two swaths typically extend from about 200 to 950km in cross-track distance from nadir (for two-sided modes) with incidence angles ranging from 22 to 67 deg. The remaining three Doppler filters yielded resolution cells with incidence angles near nadir -- at about 0, 4, and 8 deg, respectively -- that generated two (for two sided modes) 90-km-wide overlapping nadir-region strips" [Boggs, 1982].

Manufacturer of Sensor/Instrument:

NASA Langley Research Center
Hampton, VA 23681

Calibration:

Specifications:

"A minor frame of SASS data from the SDR is the time-tagged collection of engineering unit data, footprint locations, and supporting parameters associated with the readings derived from one antenna beam illumination pattern; i.e., a 15-Doppler cell data set. SASS data frames therefore occur nominally every 1.89 s. A SASS instrument science-frame cycle nominally occurs during each 242.05-s interval and consists of 4 consecutive data calibration frames followed by 124 consectuive science frames, occuring in the order dictated by 31 repetitions of the mode sequence [shown in the below table]. Thus, after every group of 124 frames -- about 4 min of data -- science data is interrupted by the presence of 4 calibration frames in the data stream."

"This calibration data is in the form of known calibration signal levels applied to the scatterometer receiver. This data was used to determine the current time-varying system gain, which was then used to process the subsequent 124 science data frames. Since measurements leading to sigma-0 backscatter data were not generated during the calibration frames, the SASS sensor data has an X-shaped gap in the (two-sided) swath. The along-track dimension of the gap is evidently given by one or two antenna beam patterns, depending on whether the instrument mode was double- or single-sided, respectively. The subsatellite point moved nearly 1600 km between nominal occurences of calibration frame sets." [Boggs, 1982]

Mode Antenna/Polarization Sequence Side
1 4V, 1V, 3V, 2V; repeat Both
2 4H, 1H, 3H, 2H; repeat Both
3 4V, 4H, 3V, 3H; repeat Left
4 1V, 1H, 2V, 2H; repeat Right
5 4V, 4V, 3V, 3V; repeat Left
6 1V, 1V, 2V, 2V; repeat Right
7 4H, 4H, 3H, 3H; repeat Left
8 1H, 1H, 2H, 2H; repeat Right
9 Continuous Calibrate
10 Standby

Frequency of Calibration:

"After every group of 124 frames -- about 4 min of data -- science data is interrupted by the presence of 4 calibration frames in the data stream." [Boggs, 1982]

Other Calibration Information:

"The nominal sequence of 4 calibration frames followed by 124 science frames was interrupted only by (1) the appearance of an uncorrected time regression ..., (2) missing data frames (i.e., a time gap in the data stream), and (3) a scatterometer mode change" [Boggs, 1982].

5. Data Acquisition Methods:

"Spacecraft data was received and recorded by tracking stations of the Spaceflight Tracking Data Network (STDN), and transmitted to the Goddard Space Flight Center (GSFC). There, data was sorted, merged, time-tagged, and recorded on magnetic tape, which was then shipped to the Instrument Data Processing System (IDPS) at JPL."

"The data package received from GSFC consisted of the (non-SAR) sensor and engineering data as well as attitude and orbit determination data. This data was decommutated, organized by major frame, and converted from data numbers to engineering units. Footprint locations were calculated, and data was then formatted into archival-quality MSDR (Master Sensor Data Record) tape files..." [Boggs, 1982].

Sensor Data Record (SDR) files were then created from the MSDR tape files. "A Sensor Data Record tape file is a complete record of all data processed by the IDPS for a given data type for a given period of time. Thus, a SASS SDR contains all calculated scatterometer data taken from MSDRs for the desired range in time" [Boggs, 1982].

F.J. Wentz reprocessed the SASS SDR data to obtain sigma-0 values with 50-km by 50-km resolution cells (see PO.DAAC product 031). In a joint effort between the Jet Propulsion Laboratory (JPL), the University of California Los Angeles (UCLA), and the Atmospheric Environmental Service (AES) of Canada, meridional and zonal components of the wind were then calculated from Wentz's sigma-0 values. The results of this effort are presented in this data set.

6. Observations:

Data Notes:

No additional notes.

Field Notes:

No additional notes.

7. Data Description:

Spatial Characteristics:

Spatial Coverage:

Each file contains a 1 degree by 1 degree grid which extends from 70° South to 70° North and from 1° East to 360° East.

Spatial Coverage Map:

Not available.

Spatial Resolution:

Each file contains a 1 degree by 1 degree grid which extends from 70° South to 70° North and from 1° East to 360° East.

Projection:

Not applicable.

Grid Description:

Data are available on a 1 degree by 1 degree grid which extends from 70° South to 70° North and from 1° East to 360° East.

Temporal Characteristics:

Temporal Coverage:

Data exist between 6 September 1978 and 20 September 1978.

Temporal Coverage Map:

Not available.

Temporal Resolution:

Data are given at 6-hour intervals.

Data Characteristics:

Parameter/Variable:

Zonal Wind Component
Meridional Wind Component

Variable Description/Definition:

Zonal Wind Component: magnitude of the wind velocity in the east-west direction at 19.5 meters above the ocean surface.

Meridional Wind Component: magnitude of the wind velocity in the north-south direction at 19.5 meters above the ocean surface.

Unit of Measurement:

Wind components are provided in meters per second.

Data Source:

SEASAT-A Satellite Scatterometer

Data Range:

Wind components range from approximately -30 m/s to 30 m/s.

Sample Data Record:

The following are the latitude, longitude, u component, v component and data flag for latitude 65° South and longitudes 245° East through 255° East in data file 'syn19780914.00z'.

Latitude Longitude U Component V Component Data Quality Flag
Degrees North Degrees East m/s m/s
-65 245 -0.49 3.62 4
-65 246 -1.43 4.03 4
-65 247 -1.53 2.57 4
-65 248 -1.59 0.46 4
-65 249 -1.64 -1.59 4
-65 250 -1.61 -3.66 4
-65 251 -2.01 -6.10 4
-65 252 -7.03 -7.77 4
-65 253 -12.93 -1.65 4
-65 254 -13.70 -5.30 4
-65 255 -8.43 -7.98 4

File Naming Convention:

The naming convention for these files is as follows:

synyyyymmdd.hhz

where:
syn denotes synoptic wind fields
yyyy denotes the four digit year
mm denotes the two digit month
dd denotes the two digit day
hh denotes the two digit hour
z denotes hours zulu

8. Data Organization:

Data Granularity:

A general description of data granularity as it applies to the Earth Observing System Data Gateway appears in the EOSDIS Glossary.

The basic granule is one data file.

Data Format:

The data are available in ASCII.

9. Data Manipulations:

Formulae:

Derivation Techniques and Algorithms:

Refer to Data Processing Sequence.

Data Processing Sequence:

Processing Steps:

In a joint effort between the Jet Propulsion Laboratory (JPL), the University of California Los Angeles (UCLA), and the Atmospheric Environmental Service (AES) of Canada, meridional and zonal components of the wind were calculated from Wentz's sigma-0 values. The 50-km values of sigma-0 were used to determine the wind speed for 100-km cells. Because the SASS is a two-azimuth-look scatterometer, as many as four wind directions, called aliases, were possible for each wind value. Dealiasing of the ambiguities was accomplished by hand using in-situ measurements. The dealiased winds were then input into a flat plate triangulation scheme to obtain 1 degree by 1 degree global wind fields. Quality flags were assigned based on the results of this interpolation. For example, if the triangle was considered to be too large or if one of the angles in the triangle was considered to be too small (resulting in a long, skinny triangle), the data were marked as poor quality. The latitude, longitude and time of observation of the 1 degree by 1 degree wind fields were then calculated at 6-hour intervals using a cubic spline interpolation in time. [Personal Communication with Glenn Cunningham, 1997]

Processing Changes:

None.

Calculations:

Special Corrections/Adjustments:

None.

Calculated Variables:

None. For a description of the variables in this product, refer to Data Characteristics.

Graphs and Plots:

Not applicable.

10. Errors:

Sources of Error:

Because these data were dealiased "by hand" using in-situ measurements, the amount of error depends partially on the "skill and experience of the analyst" [Wurtele et al., 1982] as well as on the accuracy of the in-situ data.

"The measurement accuracy of sigma-0 was affected primarily by communication noise, attitude pointing uncertainty, instrument processing (e.g., quantization errors and gain uncertainty), and various bias errors. Bias errors are in general deterministic and, depending upon the existence of adequate comparison data, are removable. The remaining errors which are random in nature and not removable, limit the ultimate accuracy of scattering coefficient measurements" [Boggs, 1982].

Quality Assessment:

Data Validation by Source:

Information not available.

Confidence Level/Accuracy Judgement:

A data quality flag is given for every 1 degree by 1 degree cell. Flag values of 4 or higher are given to "good" quality data. Values of 3 or lower represent "poor" quality data. These flags were assigned based on the results of the flat plate triangulation scheme. For example, if the triangle was considered to be too large or if one of the angles in the triangle was considered to be too small (resulting in a long, skinny triangle), the data were marked as poor quality.

Measurement Error for Parameters:

Information not available.

Additional Quality Assessments:

None.

Data Verification by Data Center:

This data set underwent restoration in 1997. The data set integrity was validated, and new read software and documentation were also created.

11. Notes:

Limitations of the Data:

Sigma-0 measurements used to calculate the wind vectors may be affected by the presence of clouds and light rain.

Radar returns from land and ice correspond to different scattering processes than those over open ocean, and can contaminate wind vector estimates.

Known Problems with the Data:

None.

Usage Guidance:

If a longer time series is required, please refer to NSCAT (NASA scatterometer) data. NSCAT data currently exist from September 1997 to present. ERS-1 and ERS-2 satellites also carry scatterometers. ERS-1 data currently exist from 1991 through May 1996, and ERS-2 data currently exist from May 1996 to present. Wind speed data (without direction) from 1987 to present can also be obtained from the DMSP SSM/I.

Reference height for surface winds
The reference height for SASS wind vectors is 19.5 meters.

Any Other Relevant Information about the Study:

None.

12. Application of the Data Set:

Global monitoring of atmospheric dynamics, air-sea interaction studies, ocean modelling

13. Future Modifications and Plans:

None.

14. Software:

Software Description:

A Fortran program is provided below as well as on the PO.DAAC FTP site (podaac.jpl.nasa.gov) in the pub directory. 
           program read_seasat_013
C
C     READ_SEASAT_013: A Fortran program to read the SEASAT 
C                      Scatterometer global gridded dealiased
C                      wind vectors produced in a joint effort
C                      between JPL, UCLA and AES (referred to
C                      as PO.DAAC product 013).
C
C     This program was adapted from program READTAPE.FOR.  Comments
C     from READTAPE.FOR are included below.
C
C     4/97 K.L. Perry
C
C
c*****************************  READTAPE  **************************
c
c   This program reads the JPL-UCLA-AES Dealiased Gridded Surface
c   Wind Vectors data set described in the NODS Data Archives 
c   Contents Document.
c
c   This program reads in global 1x1 degree gridded synoptic 
c   wind fields of Seasat scatterometer data dealiased and produced 
c   as described in Peteherych et al., 1984, Proceedings of the URSI
c   Commision F Symposium and Workshop, Shoresh, Israel, May 14-23,
c   NASA Conf. Publ. CP-2303, 575-585, and Wurtele et al., 1982,
c   "Wind direction alias removal studies of Seasat scatterometer
c   derived wind fields", Journal of Geophysical Research, 87, 3365-77.
c
c   The field parameters are:
c
c       u(ni,nj)  - zonal wind component, in m/s
c       v(ni,nj)  - meridional wind component, in m/s
c       ia(ni,nj) - data quality flag 
c                   (a quality flag which is greater than 3 implies good data)
c                   (a quality flag which is less than 4 implies good data)
c
c   The arrays cover 1x1 degree gridded fields from 70S -> 70N, 1E -> 360E
c   at 6-hour intervals starting from 00z:6-Sep-78 to 18z:20-Sep-78.
c
c   The tape is written at 6250 bpi with recl=2160, block=32400. Each 
c   global field is contained in one file with an EOF mark at the end
c   of each file.
c
c*******************************************************************
C
C     Declare Variables

      integer ni,nj
      parameter(ni=360)
C                 =gridded longitude (1 East to 360 East)
      parameter(nj=141)
C                 =gridded latitude (70 South to 70 North)

      
      character*6 indata(ni*nj*3)

      real utmp(ni*nj),vtmp(ni*nj)
      integer iatmp(ni*nj)

      real u(ni,nj),v(ni,nj)
      integer ia(ni,nj)

      integer lon(ni),lat(nj)

      integer i,j,n


C     Open and read the input file
C
C     Please note that the data are not stored as 2D arrays.
C     They are stored in one giant 1D array (ie, 1 row which
C     contains 360x241x3 values)

      open(1,file='syn19780907.18z')

      read(1,91) (indata(i),i=1,ni*nj*3)
 91   format(1552280a6)

      close(1)

C     Convert the character array 
      
      do 100 i=1,ni*nj
         read(indata(i),'(f6.2)') utmp(i)
         read(indata(i+ni*nj),'(f6.2)') vtmp(i)
         read(indata(i+ni*nj*2),'(i6)') iatmp(i)
 100  continue

C     Convert the one dimensional arrays to 2D arrays
C     Calculate the latitude and longitude

      n=1
      do 300 j=1,nj
         do 200 i=1,ni
            u(i,j)=utmp(n)
            v(i,j)=vtmp(n)
            ia(i,j)=iatmp(n)
            lon(i)=i
            n=n+1
 200     continue
         lat(j)=j-71
 300  continue

C     Write the output

      do 500 j=1,nj
         do 400 i=1,ni
            if (ia(i,j).gt.0) then
               write(*,*) lat(j),lon(i),u(i,j),v(i,j),ia(i,j)
            endif
 400     continue
 500  continue

      stop
      end

Software Access:

Software is available in the Software Description section or on the PO.DAAC FTP site (podaac.jpl.nasa.gov) in the pub directory.

15. Data Access:

Contact Information:

User Services Office
Physical Oceanography Distributed Active Archive Center (PO.DAAC)
Jet Propulsion Laboratory (JPL)

Phone: (626) 744-5508
Fax: (626) 744-5506
Email: podaac@podaac.jpl.nasa.gov
URL: http://podaac.jpl.nasa.gov

Data Center Identification:

Jet Propulsion Laboratory
Physical Oceanography Distributed Active Archive Center (PO.DAAC)

Procedures for Obtaining Data:

Data are available on the PO.DAAC FTP site (podaac.jpl.nasa.gov) in the pub/ocean_wind/seasat/wind/6hour_JPL directory. Data may also be ordered on 8mm tape by using the form provided on the PO.DAAC homepage or by contacting the PO.DAAC User Services Office.

Orders can also be placed through the Earth Observing System Data Gateway (EDG) http://podaac.jpl.nasa.gov/imswelcome.

Data Center Status/Plans:

None.

16. Output Products and Availability:

Data are available via anonymous FTP to podaac.jpl.nasa.gov, or on 8mm tape in UNIX TAR format.

17. References:

Boggs, D. H.,1982. Seasat Scatterometer Geophysical Data Record (GDR) Users Handbook, JPL Document D-129, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA.

Brown, R.A., V.J. Cardone, T. Guymer, J. Hawkins, J.E. Overland, W.J. Pierson, S. Peteherych, J.C. Wilkerson, P.M. Woiceshyn, and M. Wurtele, 1982.Surface Wind Analyses for SEASAT, J. Geophys. Res., 87, 3355-3364.

Jones, W.L., L.C. Schroeder, D.H. Boggs, E.M. Bracalente, R.A. Brown, G.J. Dome, W.J. Pierson, and F.J. Wentz, 1982. The SEASAT-A Satellite Scatterometer: The Geophysical Evaluation of Remotely Sensed Wind Vectors Over the Ocean, J. Geophys. Res., 87, 3297-3317.

Wentz, F.J., S. Peteherych, and L.A. Thomas, 1984. A Model Function for Ocean Radar Cross Sections at 14.6 GHz, J. Geophys. Res., 89, 3689-3704.

Wurtele, M.G., P.M. Woiceshyn, S. Peteherych, M. Borowski, and W.S. Appleby, 1982. Wind Direction Alias Removal Studies of SEASAT Scatterometer-Derived Wind Fields, J. Geophys. Res., 87, 3365-3377.

18. Glossary of Terms:

AMBIGUITY REMOVAL
The process of selecting the best wind vector field using the wind solutions retrieved from the scatterometer measurements.

SIGMA-0 (SIGMA-NAUGHT)
Normalized Radar Cross-Section.

See the EOSDIS Glossary for a more general listing of terms related to the Earth Observing System project.

19. List of Acronyms:

AES

 Atmospheric Environmental Service

DMSP

 Defense Meteorological Satellite Program

FTP

 File Transfer Protocol

ERS

 European Remote Sensing satellites

GSFC

 Goddard Space Flight Center

IDPS

 Instrument Data Processing System

IFOV

 Instantaneous Field of View

JPL

 Jet Propulsion Laboratory

NSCAT

 NASA Scatterometer

MSDR

 Master Sensor Data Record

NASA

 National Aeronautics and Space Administration

NRCS

 Normalized Radar Cross-Section

PO.DAAC

 Physical Oceanography Distributed Active Archive Center

SAR

 Synthetic Aperature Radar

SASS

 Seasat-A Satellite Scatterometer

SDR

 Sensor Data Record

SSM/I

 Special Sensor Microwave/Imager

STDN

 Spaceflight Tracking Data Network

UCLA

 University of California Los Angeles

URL

 Uniform Resource Locator

20. Document Information:

Document Revision Date:

June 11, 1997

Document Review Date:

June 11, 1997

Document ID:

...

Citation:

...

Document Curator:

PO.DAAC User Services Office

Document URL:

http://podaac.jpl.nasa.gov:2031/DATASET_DOCS/seasat_windcomp_jpl.html