Description of SSM/I & SSMIS Data Products

Version Notes

Introduction

SSM/I & SSMIS Data

Graphic Image Maps

Map Dates and Times

Binary Data Files

Acknowledgement

Version-7 SSMIS F16, F17 Ocean Products

DMSP F16 and F17 satellites carry the SSMIS instrument, an instrument similar to the SSM/I, but with sounding capabilities. Remote Sensing Systems now provides the same set of ocean data products for F16 and F17 SSMIS that we have provided for F8, F10, F11, F13, F14 and F15 SSM/I. However, these new SSMIS data products are created using an updated algorithm that we refer to as Version-7 (V7). The previous SSM/I products will be reproduced with V7 in the near future. The SSMIS data have been carefully intercalibrated on the brightness temperature level with the previous SSM/I and therefore extend this important time series of ocean winds, vapor, cloud and rain values.

There are 3 major V7 algorithm changes:
(1)	the water vapor continuum absorption model was re-derived
(2)	the clear-sky bias in cloud water was removed (cloud data format has changed)
(3)	the beamfilling correction in the rain algorithm was modified
The effects of V7 changes relative to V6:
vapor	increased vapor values in the range of 50-60 mm by 1% increased vapor values above 60 mm by 2-3%
cloud	changed the range of cloud water values to: -0.05 to 2.45 mm (cloud data format has changed)
rain	increased the global mean rain rates by about 16% (mostly due to changes in the extratropical values)

More detail of these changes are described in the following AMS poster:

Description of Remote Sensing Systems Version-7 Geophysical Retrievals
by Hilburn et al., 2010.

Introduction

These Special Sensor Microwave/Imager (SSM/I) and Special Sensor Microwave Imager Sounder (SSMIS) data products are produced as part of NASA's Pathfinder Program. Remote Sensing Systems generates SSM/I & SSMIS data products using a unified, physically based algorithm to simultaneously retrieve ocean wind speed (at 10 meters), water vapor, cloud water, and rain rate. This algorithm is a product of 20 years of refinements, improvements, and verifications. While the algorithms have evolved over time, a substantial background to the radiative transfer function used to derive the geophysical parameters is described in the following two papers:

"A Well Calibrated Ocean Algorithm for SSM/I"	PDF format
"SSM/I Rain Retrievals Within a Unified All-Weather Ocean Algorithm"	PDF format

The above two articles are in the postscript and PDF formats. You will need a postscript viewer such as GSview, or PDF viewer such as Adobe Acrobat Reader in order to view the files online. These articles have been published in the following journals:

Wentz F. J. 1997, "A well-calibrated ocean algorithm for SSM/I", J. Geophys. Res., Vol. 102, No. C4, pg. 8703-8718.

Wentz, Frank J. and Roy W. Spencer, May 1, 1998, "SSM/I Rain Retrievals within a Unified All-Weather Ocean Algorithm", Journal of the Atmospheric Sciences, Vol. 55, pg. 1613-1627.

SSM/I & SSMIS Data

The RSS SSMI geophysical dataset consists of data derived from observations collected by SSM/I and SSMIS instruments carried onboard the DMSP series of polar orbiting satellites. These satellite are numbered:

There are gaps within these data. If you select a date for which no data is available, either a list of acceptable dates will appear, or a blank map with text stating "Data not available" will be posted.

Remote Sensing Systems performs a detailed processing of SSM/I & SSMIS instrument data in two stages.

The first stage produces an interim product (identified by "rt" within the file name) which we make available as soon as possible, generally within hours of when the data are recorded. This product can contain geolocation errors and erroneous brightness temperatures inherent in the data supplied to us.

These errors are removed and better ice flagging is performed by the final processing stage, usually completed within 15 days of the intial product. The final product replaces the interim product automatically, with the "rt" designation in the file name changing to "v6" or "v7".

The SSM/I & SSMIS Pathfinder data products include the daily geophysical data and time-averaged data as follows:

daily	orbital data mapped to 0.25 degree grid, data overwritten by later data
3-day	average of 3 days ending on file date
weekly	average of 7 days ending on the Saturday file date
monthly	average of all data within month

Geophysical graphic images can be viewed on our web page and binary data files can be downloaded from our ftp server: ftp.ssmi.com/ssmi

Graphic Image Maps

Each daily, 3-day, weekly and monthly graphic image map displays one geophysical parameter, 10 meter Surface Wind Speed (W), Columnar Water Vapor (V), Cloud Liquid Water (L), or Rain Rate (R). The daily maps display the local morning or evening satellite passes separately. The date of the data displayed is the date at Greenwich Mean Time when each orbit occured and the data were collected (See the following section on Map dates and Times). The scale for each map is located next to the map for reference. The scale bar extends between zero and the defined maximum listed below:

White areas on the daily map represent regions of ice (greater than 0% sea ice) as determined by the SSM/I or SSMIS instrument. In the time-averaged maps, ice is shown when ice is present over 50% of the time, or when the number of times a cell is identified as containing ice is greater than the number of times the cell contains data. Land regions are colored gray. Those areas where data are not available are black. For the daily maps, the black color includes areas where the satellite did not pass over and no data were collected, areas where data were collected but were determined to be bad, coastal areas, areas affected by sun glint, areas of rain on the surface wind speed map, and areas of heavy rain on the water vapor map.

Map Dates and Times

It is currently 4 pm in New York City on August 18th and you see on our web page the daily map of data shown below.

August 18th Evening Passes F14 Data

The F14 satellite passes overhead at approximately 8:30 pm each evening, local time. How can there be data shown in the map for a time that hasn't occurred yet??? Did we make an error???

Each map shown on this web page is for morning or evening passes of a satellite on that date at Greenwich, England. For example, the data you see in the above map were collected near 8:30 pm local time. 8:30 pm in New York City is 1:30 am in Greenwich, England (8:30 + 5 hour time difference). Since it must be August 18th in Greenwich (the date of the map), then the data you see were collected on Aug 17th, local time.

Similar time "problems" occur for each of the satellites. The table below demonstrates another example, this time for data collected by the F13 satellite. F13 crosses the equator at approximately 5:30 am.

Here, the F13 morning pass time of 5:30 (local time) is used to determine the GMT (Greenwich Mean Time) of the map date. The corresponding GMT time and date are then used to determine the date at the given location. When the September 19th F13 morning pass was collected, it was actually September 20th in New Guinea, a location that is 8 hours ahead of Greenwich.

The best way to determine the time for scientific comparisons is to use the time data set in the daily binary data files. Each pixel location of an ocean parameter map has a specific GMT minute of day listed in the time array. Use of time in this manner will keep your data processing in the correct order.

Binary Data Files

We produce Daily binary data files and Time-Averaged (3-day, weekly and monthly) data files. The daily files consist of SSM/I or SSMIS geophysical products mapped to a regular grid complete with data gaps between orbits. Two maps exist for each parameter, one of ascending orbit segments and the other of descending orbit segments. Data on each of the segment maps are overwritten at both the high latitudes where successive orbits cross and at the "seam" or region where the last orbit of the day overlaps the first orbit of the day. Daily data files contain a time data set consisting of the time (in minutes UTC) of the data within that cell. Time-Averaged data files do not contain any time information.

Each binary data file available from our ftp site consists of ten (daily) or four (averaged) 0.25 x 0.25 degree grid (1440,720) byte maps. For daily files, five local morning (descending, except F08) maps in the following order, Time (T), 10 meter Surface Wind Speed (W), Atmospheric Water Vapor (V), Cloud Liquid Water (L), and Rain Rate (R), are followed by five local evening (ascending, except F08) maps in the same order. Time-Averaged files contain just the geophysical layers in the same order [W,V,L,R]. The daily, 3-day and monthly maps are stored by instrument in appropriate year and month subdirectories. The weekly data files are stored by instrument in the /weeks directory.

The file names have the following naming conventions:

Where ss, yyyy, mm, dd, and vv stand for:

The center of the first cell of the 1440 column and 720 row map is at 0.125 E longitude and -89.875 latitude. The center of the second cell is 0.375 E longitude, -89.875 latitude.

The data values fall between 0 and 255. Specific values have been reserved:

The data values between 0 and 250 need to be scaled to obtain meaningful geophysical data. To scale the data, multiply by the scale factors listed below (and add offset to cloud in Version-7):

Version-6

Version-7

Read Routines:

Binary file read routines and verification files are available on our ftp server in the ssmi/ssmi_support directory.

SSM/I data are produced by Remote Sensing Systems and sponsored by the NASA Earth Science MEaSUREs DISCOVER Project. Data are available at www.remss.com.

Have You Used These Data?

Last updated: October 07, 2011