BYU Daily Browse Images of QuikSCAT Sigma-0 Measurements (D. Long)
Click on image for higher resolution
The QuikSCAT on SeaWinds scatterometer provides normalized radar cross section (sigma-0) measurements of the Earth's surface. While originally designed for wind observation, scatterometers have proven useful in a variety of land and ice studies. To aid in the selection of regions and time periods for study, Brigham Young University has produced this global sigma-0 browse product. This product consists of spatial and temporal averages of QuikSCAT Level 1B sigma-0 values over a one-day period.
As a follow-up/replacement for the NSCAT mission, the QuikScat/SeaWinds scatterometer (QuikSCAT) measures the near-surface ocean wind field with unprecedented coverage and resolution. QuikSCAT makes dual polarization measurements of the normalized radar cross section (sigma-0) at both vertical and horizontal polarization.
This dataset consists of sigma-0 browse images for three regions:
The L1B files report measurements in two forms, eggs and slices, which differ in their spatial sizes and shapes. The nominal instantaneous QuikSCAT antenna footprint is an ellipse. However, the on-board range-doppler processing incorporated in the instruments improves the resolution. Using the on-board processor, 12 individual measurements are obtained for each footprint, though only 8 are reported in the L1B data product. These individual measurements are slices. The slices are typically 6 km long (depending on the instrument mode and antenna beam) by 25 km wide. The summed measurements of the 8 center slices are egg measurements and are reported in the standard L1B product. The effective resolution of the egg measurement nearly matches the elliptical 3 dB antenna footprint (approximately 15 km by 25 km depending on the antenna beam and instrument mode). Although lower in resolution, the egg measurements have smaller measurement variance (Kp²) than slice measurements and are also less sensitive to calibration errors. For this reason the browse product defined here is based on the egg measurements.
To produce the browse images, the center of each L1B egg measurement is located. The measurement is averaged into the image grid element (pixel) in which the center of the measurement falls, i.e. the measurements are gridded into earth-located pixels based on their location. The effective resolution of the image products is less than the pixel resolution, or approximately 55-60 km at the equator. The resulting images are temporal averages over a one-day period of all the sigma-0 measurements whose centers fall within each image pixel area. We note that using slice sigma-0 measurements rather than egg sigma-0 would improve the effective resolution somewhat, though not significantly.
While NSCAT made sigma-0 measurements over a broad range of incidence angles, QuikSCAT makes measurements at each of two nominal incidence angles, 46° and 54.1°, corresponding to the inner and outer beams. The inner beam measurement is horizontal-polarization while the outer beam is vertical. Since it is undesirable to combine measurements from different polarizations, measurements from each beam are kept separate. We note that the variation in incidence angle over an orbit is small, typically less than a few tenths of a degree. For this reason, the incidence angle variation is not considered in this browse product.
The browse image products are stored in the BYU Microwave Earth Remote Sensing (MERS) SIR file format. The file format includes a header which contains location transformation information.
The QuikSCAT L1B data set is available only to selected researchers. However, the following related data sets are available at JPL's PO.DAAC:
Other QuikSCAT-based data sets from the MERS Lab will soon be available at PO.DAAC:
Several Level 3 products (such as this one) have been produced by members of the SeaWinds on QuikSCAT Science Working Team. A list of the publicly available QuikSCAT Level 3 products produced by SeaWinds on QuikSCAT Science Working Team members can be found on the PO.DAAC SeaWinds on QuikSCAT Web Site Links Page.
| Investigator: | Dr. David G. Long |
| Title: | Director,
BYU Center for Remote Sensing Professor, Department of Electrical & Computer Engineering |
| Address: | 459 Clyde Building Brigham Young University Provo, UT 84602 |
| Email: | long@ee.byu.edu |
| Phone: | (801)378-4383 |
| Fax: | (801)378-6586 |
In addition to its primary wind observation mission, NSCAT has also had an impact on a variety of polar ice and tropical vegetation studies [Long and Drinkwater, 1999]. It is clear that scatterometers will continue to play an increasingly important role in monitoring tropical vegetation and polar ice in the future.
| NOTE: | This section was obtained entirely from the QuikSCAT Science Data Product User's Manual [Dunbar et al, 2000]. Please refer to the User's Manual for more information. |
| Nominal Orbital Parameters | |
|---|---|
| Recurrent Period | 4 days (57 orbits) |
| Orbital Period | 101 minutes (14.25 orbits/day) |
| Local Sun Time at Ascending Node | 6:00 A.M. ± 30 minutes |
| Altitude above Equator | 803 km |
| Inclination | 98.616° |
| Quantity | Requirement | Applicable Range |
|---|---|---|
| Wind Speed | 2 m/s (rms) | 3-20 m/s |
| 10% | 20-30 m/s | |
| Wind Direction | 20° (rms) selected ambiguity | 3-30 m/s |
| Spatial Resolution | 25 km | sigma-0 cells |
| 25 km | Wind Vector Cells | |
| Location Accuracy | 25 km (rms) | Absolute |
| 10 km (rms) | Relative | |
| Coverage | 90% of ice-free ocean every day | |
| Mission Duration | 36 months |
QuikSCAT/SeaWinds is a conically scanning pencil-beam scatterometer. The SeaWinds scatterometer design used for QuikSCAT is a significant departure from the fan-beam scatterometers flown on previous missions (Seasat SASS and NSCAT). QuikSCAT employs a single 1-meter parabolic antenna dish with twin offset feeds for vertical and horizontal polarization. The antenna spins at a rate of 18 rpm, scanning two pencil-beam footprint paths at incidence angles of 46° (H-pol) and 54° (V-pol). The transmitted radar pulse is modulated, or "chirped", and the received pulse (after Doppler compensation) is passed through an FFT stage to provide sub-footprint range resolution. The range resolution is commandable between 2 km and 10 km, with the nominal value set at about 6 km. The nominal pulse repetition frequency is 187.5 Hz (also commandable). Each telemetry frame contains data for 100 pulses. Signal and noise measurements are returned in the telemetry for each of the 12 sub-footprint "slices." Ground processing locates the pulse "egg" and "slice" centroids on the Earth's surface. The sigma-0 value is then computed for both the "egg" and the best 8 of the 12 "slices" (based on location within the antenna gain pattern).
QuikSCAT "programmability" includes commanding of major mode selection and range resolution, antenna spin rate and PRF, and the ability to uplink new Doppler compensation and range tracking tables as changes in the orbit occur, or to conduct special engineering tests. Mode changes will be made periodically to obtain additional calibration data.
| NOTE: | This section, except the description of the MERS Lab, was obtained from the QuikSCAT Science Data Product User's Manual [Dunbar et al, 2000]. Please refer to the User's Manual for more information. |
| Region | Image Type | Horizontal
Polarization | Vertical Polarization |
| Global: all lats, all lons | average sigma-0 |
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| standard deviation |
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Southern Hemisphere: lats < -52.0°, all lons | average sigma-0 |
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| count |
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| standard deviation |
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Northern Hemisphere: lats > 52.0°, all lons, | average sigma-0 |
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| standard deviation |
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Polar images (north and south): 388 by 388 pixels, or 22.5 km/pixel using a 70° reference latitude.
The naming scheme for the auxiliary files is:
QS_XbTR3C.date
| T | image type | 'x' = longitude 'y' = latitude |
| R | region | 'G' = Global
'N' = Northern Hemisphere 'S' = Southern Hemisphere |
| date | 11-digit date of file production (yyyydddhhmm) | |
| Image Type | Minimum | Maximum | No Data Indicator |
|---|---|---|---|
| average sigma-0 | -32.0 | 0.0 | -33.0 |
| count | 0¹ | 50 | -100 |
| standard deviation | 0.0 | 1.0 | -1.0 |
Each file also has header information. The following sample output was printed by program sir2gif as it created the image at the beginning of this document.
SIR file header: 'QS_XbvaG3C2001001.20010032220' Title: 'Grid image of Globe Revs 08000-08014' Sensor: 'QSCAT L1B' Type: 'A image (QS_XbvaG3C2001001.20010032220)' Tag: '(c) 1999 BYU MERS Laboratory' Creator: 'BYU MERS QS_SigBrw V1.0' Created: ' 3Jan2001' Size: 1800 x 900 Total:1620000 Offset: -33 Scale: 500 Year: 2001 JD range: 1-1 Region Number: 500 Type: 1 Form: 0 Polarization: 2 Frequency: 13.600000 MHz Datatype: 2 Headers: 1 Ver:20 Nodata: -33.000000 Vmin: -32.000000 Vmax: 0.000000 Rectangular Lat/Long projection: Size (deg): 360.000000 , 180.000000 Lon, Lat scale: 5.000000 , 5.000000 (pix/deg) Offsets: -180.000000 , -90.000000 Image Min, Max: -32.000000 , 0.000000
QS_XbpTR3Cyyyyddd.date
| p | polarization | 'h' = horizontal 'v' = vertical |
| T | image type | 'a' = average sigma-0 'C' = count 'V' = standard deviation |
| R | region | 'G' = Global
'N' = Northern Hemisphere 'S' = Southern Hemisphere |
| yyyy | four-digit year | |
| ddd | three-digit day of year, from 001 to 366 | |
| date | 11-digit date of file production (yyyydddhhmm) | |
The naming scheme for the auxiliary files, as well as their meanings, is described above.
A general description of data granularity as it applies to the IMS appears in the EOSDIS Glossary.
A SIR format file consists of one or more 512-byte headers followed by the image data and additional zero padding to insure that the file is a multiple of 512 bytes long. The file header record contains all of the information required to read the remainder of the file and the map projection information required to map pixels to lat/lon on the Earth surface. The image pixel values generally represent floating point values and may be stored in one of three ways. The primary way is as 2 byte integers (with the high order byte first), though the pixels may be stored as single bytes or IEEE floating point values. Scale factors are stored in the header to convert the integer or byte pixel values to native floating point units. The image is stored in row-scanned (left to right) order from the lower left corner (the origin of the image) up through the upper right corner. By default, the location of a pixel is identified with its lower-left corner. The origin pixel (1,1) is the lower left corner of the image. The array index n of the (i,j)th pixel where i is horizontal and j is vertical is given by
n = (j - 1) × Nx + iwhere Nx is the horizontal dimension of the image. The last pixel stored in the file is at (Nx, Ny).
The sir file header contains various numerical values and strings which describe the image contents. For example, the value for a no-data flag is set in the header as well as a nominal display range and the minimum and maximum representable value. Optional secondary header records (512 bytes) can be used to store additional, non-standard information. The standard SIR file format supports a variety of image projections including:
Any of the programs described in Software below decodes SIR headers.
The L1B files are part of a series of level-conversion stages, starting from raw data from the SeaWinds on QuikSCAT instrument and producing more refined products at each stage. See Related Data Sets. Those products are created in the following order:
Rain also attenuates the signal over land and ice. Generally, however, the land backscatter is larger over land/ice than over the ocean so rain is less of a problem. No correction for rain is applied in generating this or other sigma-0 products.
The normalized standard deviation of sigma-0, known as Kp, is computed to give an estimate of the measurement uncertainty of the backscatter. Major sources of Kp in the scatterometer system are:
Instrument processing is another source of error, as are uncertainty in attitude pointing and various bias errors. The latter two can reduce the effective resolution of the highest resolution images (in effect smearing them), but they are not considered to be important sources of error for most sigma-0 image products including this one.
Since the images are averages over time, sampling variability over the averaging period affects the reported backscatter values.
Over the ocean all backscatter measurements (from a single polarization) falling within a single pixel are averaged. Thus, the forward- and aft-looking measurements are averaged, and the resulting average is over the various azimuth angles of the measurements. The azimuth angles of the measurements hitting a given location vary with pixel location and time and may be further affected by missing or low-quality data. Discontinuities and artifacts can occur at swath edges and in areas where the surface shows significant azimuth dependence (e.g. over the ocean). The normalized standard deviation images can be useful in evaluating temporal and azimuth variation in the backscatter measurements.
The QA reports are available on the PO.DAAC FTP site.
Some areas may not be covered in a single day; see Spatial Coverage.
Sigma-0 measurements may be contaminated when rain is present.
For the most current information concerning the SeaWinds on ADEOS-II mission, please refer to the JPL SeaWinds on ADEOS-II web site.
| Language | Program Name | Description |
|---|---|---|
| C | sir_ez_example.c | read SIR file, print values of 4 corner pixels |
| sir2byte.c | convert SIR file to raw, unsigned byte file | |
| sir2gif_ez.c | convert SIR file to GIF | |
| sir2gif.c | ||
| sir2bmp.c | convert SIR file to BMP | |
| Fortran | fsirexample_EZ.f | read SIR file, create an unsigned byte file |
| fsirexample.f | ||
| sir2byte.f | ||
| fsir_locmap.f | read SIR file, create latitude and longitude maps like the auxiliary files | |
| fsir_locmap_EZ.f | ||
| IDL | xsir_idl.pro | load SIR file, save to file, display image, do forward/inverse transforms |
| PV-WAVE | xsir.pro, xsir_pvwave.pro | load SIR file, save to file, display image, do forward/inverse transforms |
| MATLAB | loadsir.m, writesir.m, showimage.m, ... | load SIR file, save to file, display image, do forward/inverse transforms |
The IDL and PV-WAVE programs reside in one directory due to the similarity between the languages. xsir_idl.pro, xsir.pro, and xsir_pvwave.pro call the same functions, though the file loadsir.pro must be modified for PV-WAVE.
| email: | podaac@podaac.jpl.nasa.gov |
| url: | http://podaac.jpl.nasa.gov |
| phone: | (626)744-5508 |
| fax: | (626)744-5506 |
| mail: | JPL PO.DAAC User Services Office Jet Propulsion Laboratory MS Raytheon-299 4800 Oak Grove Drive Pasadena, CA 91109, U.S.A.
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Dr. David Long of BYU is the source of this dataset. Please contact him with more detailed questions. See Investigator for contact information.
This product is also available on CD-ROM. Orders may be placed using the PO.DAAC on-line order form, http://podaac.jpl.nasa.gov/order, or the Earth Observing System (EOS) Data Gateway, http://poseidon.jpl.nasa.gov/~imsww/pub/imswelcome/.
Dunbar, R.S. et al, 2000. QuikSCAT Science Data Product User's Manual, Version 2.0, JPL Document D-18053, Jet Propulsion Laboratory, Pasadena, CA.
Long, D.G., 2000. A QuikSCAT/SeaWinds Sigma-0 Browse Product, Version 2.0, Brigham Young University, Provo, UT.
Long, D.G. and M.R. Drinkwater, 1999. Cryosphere Applications of NSCAT Data, IEEE Trans. on Geoscience and Remote Sensing, Vol. 37, No. 3, pp. 1671-1684
Huddleston, J.N. and B.W. Stiles, 2000. Multidimensional Histogram (MUDH) Rain Flag Product Description, Version 2.1, Jet Propulsion Laboratory, Pasadena, CA.
Remund, Q.P. and D.G. Long, 2000. Iterative Estimation of Antarctic Sea Ice Extent Using SeaWinds Data, Proc. Int. Geosci. Rem. Sens. Sym., Honolulu, Hawaii, 6-10 July 2000
Remund, Q.P. and D.G. Long, 1998. Sea Ice Mapping Algorithm for QuikSCAT and Seawinds, Proc. Int. Geosci. Rem. Sens. Sym., Seattle, Washington, 6-10 July, 1998, pp. 1686-1688.
See the EOSDIS Glossary for a more general listing of terms related to the Earth Observing System project.