The Defense Meteorological Satellite Program (DMSP) is the Department of Defense program responsible for designing, building, launching and operating polar orbiting meteorological satellites. The satellites can broadcast visual, infrared and microwave imagery directly to transportable tactical sites around the world. The data are also stored for transmission to the Navy's Fleet Numerical Meteorology and Oceanography Center (FNMOC) and to the Air Force Global Weather Central (AFGWC).

In December 1972, DMSP data were declassified allowing access by the civil/scientific community. As a result, both AFGWC and FNMOC relay the Special Sensor Microwave/Imager (SSM/I), the Special Sensor Microwave Temperature Sounder (SSM/T-1) and the Special Sensor Microwave Water Vapor Profiler (SSM/T-2) data to the National Environmental Satellite, Data, and Information System (NESDIS).

In May 1994, the President directed the Departments of Commerce (DoC) and Defense to converge their separate polar orbiting environmental satellite programs. DMSP is now operated by the two departments and NASA. In June 1998, DoC took over the primary responsibility for flying both satellite systems until the converged systems are ready for launch in the 2007-2010 timeframe.

Each of the DMSP satellites flies in a sun-synchronous, low altitude, near-polar orbit. For a satellite in sun synchronous orbit, the ascending equatorial crossing time remains relatively constant with respect to the local time throughout the lifetime of the satellite. The orbital period is 101 minutes and the nominal altitude is 833 km.

The Comprehensive Large Array-data Stewardship System (CLASS) distributes data from three DMSP instruments: 1) the SSM/T-1; 2) the SSM/T-2; and 3) the SSM/I from which antenna temperatures (Temperature Data Records - TDR), brightness temperatures (Sensor Data Records - SDR) and derived geophysical parameters (Environmental Data Records -EDR) are derived.

Each instrument has different characteristics, resolutions, scan properties, etc. which are described below. CLASS archives data beginning with satellite F10.

Data are transmitted in real time to tactical terminals worldwide. Data are also stored using on-board recorders for transmission to and processing by the Air Force Global Weather Central (AFGWC), Offutt AFB, Nebraska and the Fleet Numerical Meteorology and Oceanography Center (FNMOC), Monterey, California. Both AFGWC and FNMOC relay the SSM/I, SSM/T-1 and SSM/T-2 data to the National Environmental Satellite, Data, and Information System (NESDIS). AFGWC also sends the entire data stream to the National Geophysical Data Center (NGDC).

Note that the orbit file names are generated by NESDIS using the information found in the file header. The file header is created by FNMOC and does not always accurately reflect the start and end times of the data in the file.

SSM/I Introduction

The Special Sensor Microwave/Imager (SSM/I) is part of the instrument suite flown onboard the DMSP series of satellites. The SSM/I is a seven-channel, four-frequency, linearly-polarized, passive microwave radiometric system. The SSM/I measures atmospheric, ocean, and terrestrial microwave brightness temperatures at 19.35, 22.235, 37.0, and 85.5 Ghz.

The SSM/I continuously rotates about an axis parallel to the local spacecraft vertical at 31.6 rpm. The SSM/I measures upwelling scene brightness temperatures over an angular section of 102.4 degrees about the sub-satellite track. When looking in the forward direction of the spacecraft, the scan is directed from left to right with active scene measurements lying 51.2 degree about the forward direction. A conical scan with a swath width of 1,400 km results. Global coverage is obtained in 24 hours. The spacecraft sub-satellite point travels 12.5 km during the 1.9 second period.

For each scan, 128 uniformly spaced 85.5 Ghz scene measurements are taken over a 102.4 degree scan region. The sampling interval is 4.22 msec and equals the time for the beam to travel 12.5 km in the cross track direction. Radiometric data at the remaining frequencies are sampled every other scan with 64 uniformly spaced samples being taken. The sampling interval for these remaining frequencies is 8.44 msec. The start and stop times of the integrate and dump filters at 19.35, 22.235, and 37.0 GHz are selected to maximize the radiometer integration time to achieve concentric beams for all sampled data. The effect of the radiometer integration times is to increase the effective along scan beam diameter to make the beams at 37 and 85 GHz nearly circular.

SSM/I's output voltages are converted into antenna temperatures (Temperature Data Records - TDR), brightness temperatures (Sensor Data Records - SDR) and derived geophysical parameters (Environmental Data Records -EDR). The EDRs contain geophysical parameters derived from the TDRs and SDRs. CLASS archives TDR, SDR and EDR data beginning with satellite F10.

SSM/I Applications

SSM/I data are available as antenna temperatures (TDRs), brightness temperatures (SDRs) and derived geophysical parameters (EDRs). EDRs measure various parameters over the ocean, ice and land surfaces. There are five oceanic parameters: surface wind speed, cloud water content, water vapor content, rainfall intensity and liquid water content. There are four ice parameters: ice concentration, ice age, ice edge and cloud water content over ice. There are eight land parameters: rain intensity, liquid water content, surface moisture, cloud water content, snow water content, surface character, surface temperature and cloud amount. Not all parameters are simultaneously possible.

SSM/I Data Acquisition

SSM/I's output voltages are transmitted to the Fleet Numerical Meteorology and Oceanography Center (FNMOC) in Monterey, California, where they are converted to sensor counts. FNMOC then converts the sensor counts into antenna temperatures (Temperature Data Records - TDR), brightness temperatures (Sensor Data Records - SDR) and derived geophysical parameters (Environmental Data Records -EDR). The TDRs, SDRs, and EDRs are sent to NESDIS for archival.

SSM/I Data Description

SSM/I data from CLASS consist of 12-bit precision antenna temperatures for TDRs, brightness temperatures for SDRs, or derived geophysical parameters for EDRs, along with satellite ephemeris, earth surface positions for each pixel, and instrument calibration.

No subsetting of data is performed on any DMSP data at CLASS.

SSM/I Spatial Coverage

The 1,400 km wide conical scan of the SSM/I obtains global coverage every 24 hours. The channel footprint varies with channel energy, position in the scan, along scan or along track direction and altitude of the satellite. The 85 GHz footprint is the smallest at 13 x 15 km and the 19 Ghz footprint is the largest at 43 x 69 km.

SSM/I Temporal Coverage

The scanning period for the SSM/I is 1.9 seconds during which the spacecraft sub-satellite point travels 12.5 km. For 540 msec of that period, 128 uniformly spaced 85.5 Ghz scene measurements are taken over the 102.4 degree scan region. The sampling interval is 4.22 msec and equals the time for the beam to travel 12.5 km in the cross track direction. Radiometric data at the remaining frequencies are sampled every other scan with 64 uniformly spaced samples being taken. The sampling interval for these remaining frequencies is 8.44 msec. The remaining portion of the sampling period allows the SSM/I to rotate through the remaining 257.6 degrees to once again be positioned to start acquiring scene measurements.

The overall coverage of the SSM/I data archived at CLASS is shown in the following tables. However, associated with equipment malfunctions, there may be short gaps in the time ranges.

TDR

SDR

EDR

SSM/I Calibration

A small mirror and a hot reference absorber are positioned off axis such that they pass between the feed horn and the parabolic reflector, occulting the feed once each scan. The mirror reflects cold sky radiation into the feed, thus serving, along with the hot reference absorber, as calibration references for the SSM/I. This scheme provides an overall absolute calibration which includes the feed horn every 1.9 seconds. Corrections for spillover and antenna pattern effects from the parabolic reflector are incorporated in the data processing algorithms.

SSM/I TDR Data Processing Software

To obtain DMSP SSM/I TDR data processing software files, click on the appropriate links below. You will be presented with the source code. To save the source code, go to your file menu and select "Save As."

C language program ssmitdrta.c reads the SSM/I TDR data set file and writes the antenna temperatures, satellite ID, time, revolution number, latitudes, and longitudes to an output file. This output file is read by FORTRAN program ssmitdrtb.f which converts the antenna temperatures to brightness temperatures.

C language program ssmitdrlatlon.c reads the TDR data set and creates a new TDR file containing scans within specified latitude-longitude boundaries.

DMSP/README.tdr
DMSP/ssmitdrta.c
DMSP/ssmitdrtb.f
DMSP/ssmitdrlatlon.c
DMSP/read_ssmi_netcdf.pro <new>

SSM/I Comprehensive Information

Details of the SSM/I TDR data sets can be found at: http://www.osdpd.noaa.gov/PSB/SHARED_PROCESSING/TDR.HTML.
F-16 - TDR data formats.

Details of the SSM/I SDR data sets can be found at: http://www.osdpd.noaa.gov/PSB/SHARED_PROCESSING/SDR.HTML.
F-16 - SDR data formats.

Details of the SSM/I EDR data sets can be found at: http://www.osdpd.noaa.gov/PSB/SHARED_PROCESSING/EDR.HTML.
F-16 - EDR data formats.

SSM/T-1 Introduction

The Special Sensor Microwave Temperature (SSM/T-1) sounder is part of the instrument suite flown onboard the DMSP series of satellites. The SSM/T-1 is a seven channel microwave sounder, designed to provide global, synoptic scale soundings of temperature throughout the troposphere and lower stratosphere. All seven channels are within the 50 - 60 Ghz oxygen band, with one channel acting as a surface window channel. A single cross-track reflecting antenna is used to direct the upwelling atmospheric radiation through a fixed circular horn which is coupled to the Dicke radiometers. The incoming broad band signal is first split into two bands having orthogonal polarization, and then filtered into the seven discrete channels whose center frequencies and bandwidths are listed below.

Temperature soundings are obtained operationally using a minimum variance approach whose covariance matrices are constructed from a fixed set of simulated SSM/T data and corresponding temperature profiles.

In processing SSM/T-1 data, special attention is placed on the utilization of the two lowest frequency channels. The 50.5 GHz channel receives nearly 70 percent of its energy from the surface and is considered a "window" channel. Since the measurements are strongly dependent on precipitation and surface emissivity variations, the channel is not used as a temperature predictor. However, based on a minimum threshold brightness temperature of about 245 K, the window channel is used to edit the data for precipitation over the oceans. Furthermore, the 50.5 GHz channel also provides surface emissivity corrections for the lowest sounding channel.

The lowest sounding channel (53.20 GHz) responds to changes in atmospheric temperature around 700 mb where the weighting functions peaks. However, it receives about 20 percent of its energy from the surface and therefore requires corrections for the effects of surface emissivity and high elevation (greater than 1 km) on the brightness temperature. The window channel provides an emissivity correction while elevation adjustments are based on terrain height information. The next highest peaking channel (54.35 GHz) lies within the more opaque region of the oxygen band where the surface contribution is only 2 percent. As a result, this channel and the remaining channels are generally found to have negligible surface effects.

The SSM/T-1 completes one scan of seven Earth view measurements and two calibration measurements every 32 seconds. Each scan of the instrument covers a swath of roughly 1,500 km perpendicular to the orbital track. Each orbit covers a different area of the earth, except poleward of 57 degrees latitude, where successive orbits begin to overlap. For each of the seven channels, there are 18,900 earth view measurements collected per day.

The SSM/T-1 instrument data, also known as the SSM/T-1 raw data, are received and processed to the 1b level by the NOAA/NESDIS/IPD.

SSM/T-1 Applications

The SSM/T-1 is a seven channel microwave sounder, designed to provide global, synoptic scale soundings of temperature throughout the troposphere and lower stratosphere. The measurements are within 1K (rms) of the brightness temperatures computed from radiosonde data. The retrievals are generally within 2.5K (rms) of radiosonde temperatures for pressures less than 850 mb. These results are independent of cloud cover and more accurate than the retrievals obtained from the TIROS Operational Vertical Sounder (TOVS), particularly for cloudy atmospheres.

SSM/T-1 Data Description

DMSP SSM/T-1 files are stored at CLASS under the following naming convention:

NSS.SSMT.Sn.Dyyjjj.Shhmm.Ehhmm.Axxxxxxx.NS

where the upper case characters remain fixed and the lower case characters vary from spacecraft to spacecraft and from orbit to orbit.

The lower case characters correspond to the following variables:

n = Spacecraft identification (e.g. 6 for satellite F-12)
yy = Year of the century
jjj = day of the year
hhmm = Time in hours and minutes
xxxxxxx = Orbit Number

The upper case characters represent the following:

NSS.SSMT = NESDIS SSM/T-1
D = Day
S = Start Time
E = End Time

SSM/T-1 Header Record Format

Each SSM/T-1 level 1b file contains a header record in the following format:

SSM/T-1 Data Record Format

Each subsequent data record contains one scan of SSM/T-1 data in the following format:

SSM/T-1 Spatial Coverage

The radiometer has a field of view (FOV) of 14.4 degrees. At the nominal 833 km altitude, the spatial resolution at nadir is an approximate circle with a 174 km diameter. At the far end of each scan, the footprint degrades to an ellipse 213 x 304 km in size. The seven cross-track scan positions are separated by 12 degrees with a maximum cross-track scan angle of 36 degrees. The SSM/T-1 swath width is 1,528 km. Each orbit covers a different area of the earth, except poleward of 57 degrees latitude, where successive orbits begin to overlap. Elsewhere, there is a data coverage gap between successive orbits.

SSM/T-1 Temporal Coverage

The overall coverage of the SSM/T-1 data archived at CLASS is shown in the following tables. However, associated with equipment malfunctions, there may be short gaps in the time ranges.

SSM/T-1 Calibration

The calibration of the SSM/T-1 instrument is provided by two additional scan steps. One position views cold space (~2.7 K) and the second an ambient temperature target (~300K) attached to the scan structure. Ground-based data processing results in individually calibrated brightness temperatures for the seven channels by linearly relating the output channel response to the monitored target temperature and 2.7K cold space temperature. The total scan period is 32 seconds with an integration time of 2.7 seconds for each of the Earth viewing and calibration positions.

The in-flight calibration system is a well-matched, closed-path configuration with very low dissipative wall losses. A shroud on the reflector allows direct coupling to both the cold path and warm load. The cold path is an oversized circular transmission line that is used to restrict the radiometer field of view so that extraneous input signals due to both the surrounding spacecraft and the earth's atmosphere are minimized. Due to the location of the sensor on the spacecraft it is not possible for the sensor antenna to view the sky directly. Therefore, it is necessary to utilize a reflecting miter bend in the cold path to direct the antenna pattern in the proper direction. The warm load is an extended microwave radiator made up of a large number of tapered absorbing sections and is designed to provide a stable blackbody temperature source at approximately 300 K. An accurate measurement of the surface temperature of the load is provided as a result of the warm load thermal design. A shroud allows direct coupling to the antenna and a sun shield located on the antenna reflector prevents the warm load from viewing the sun, thereby enhancing the thermal stability of this load.

The electrical performance requirements of the SSM/T-1 sensor system are a maximum calibration uncertainty of 1 K and maximum NETD for the various channels of 0.4 to 0.6 K.

SSM/T-1 Data Processing Software

To obtain DMSP SSM/T-1 data processing software files, click on the appropriate link below. You will be presented with the source code. To save the source code, go to your file menu and select `Save As'.

The C language program, ssmt1rdc.c, reads the SSM/T-1 data set file and generates an intermediate file with all the necessary data from the data set. FORTRAN program ssmt1rdf.f reads this intermediate file into arrays.

DMSP/README.ssmt1
DMSP/ssmt1rdc.c
DMSP/ssmt1rdf.f

SSM/T-2 Introduction

The Special Sensor Microwave Water Vapor Profiler (SSM/T-2) is part of the instrument suite flown onboard the DMSP series of satellites. The SSM/T-2 is a cross-track scanning, five channel, passive, total power, microwave radiometer. Of the five channels, three are water vapor channels centered around the 183.31 GHz water vapor line. The other two are window channels at 91.655 GHz and 150.0 GHz. The SSM/T-2 is designed to provide global monitoring of the concentration of water vapor in the atmosphere under all sky conditions by taking advantage of the reduced sensitivity of the microwave region to cloud attenuation.

The SSM/T-2 observation rate is 7.5 scans per minute. The instrument utilizes a step-scan motion in the cross-track direction of +/- 40.5 degrees. The SSM/T-2 scan mechanism is synchronized with the SSM/T-1 so that the beam cell patterns of the two sensors coincide. There are 28 observations (beam positions) per scan for each of the five channels. All five channels have coincident centers. The total swath width for the SSM/T-2 is approximately 1,500 km.

The channel characteristics for the SSM/T-2 are listed below:

The SSM/T-2 employs a single offset parabolic reflector with a 2.6 inch diameter projected aperture. The reflector is shrouded to eliminate the possibility of rays from the sun striking either of the calibration paths and causing unwanted thermal gradients. The feedhorn is a corrugated pyramidal horn with a flare designed to minimize phase center separation over the bandwidth (91 to 183.3 GHz), while providing a spherical wave illumination of the reflector. A 3.3 degree beamwidth is achieved for the 183.3 GHz channels and larger beamwidths of approximately 3.7 degrees and 6.0 degrees for 150 and 91.665 GHz, respectively. These correspond to the Field-of-View (FOV) parameters given in the table above.

To achieve the cross-track scanning, the reflector alone rotates. The rotation of the reflector produces a rotation of the plane of polarization of the upwelling scene Brightness Temperatures which is permitted provided that the polarization remains identical for the two window channels and the 183.3 +/- 7 Ghz channel. These channels must have the same polarization characteristics because they measure contributions from both the atmosphere and the surface. Note that all SSM/T-2 channels possess the same polarization.

The SSM/T-2 observed raw data are processed into the SSM/T-2 Level 1b data set by NOAA/NESDIS Information Processing Division (IPD) and are made available by CLASS. The 1b data set contains earth located and calibrated SSM/T-2 data. Each data set contains one orbit's worth of data and is allowed to accumulate up to 120 minutes of data. Approximately 14 level 1b orbital data sets are generated per day.

SSM/T-2 Applications

The SSM/T-2 is designed to provide global monitoring of the concentration of water vapor in the atmosphere under all sky conditions by taking advantage of the reduced sensitivity of the microwave region to cloud attenuation.

Major mid-latitude weather phenomena such as fronts and extratropical cyclones have excellent signatures in SSM/T-2 data, including three-dimensional structure. Other phenomena such as tropical cyclones, tropical plumes, subtropical anticyclones and surface states such as sea ice and snow cover may be identified.

Applications other than profiling are also possible with the SSM/T-2. The retrieval of vertically integrated water vapor is possible due to the strong sensitivity of the 183.31 GHz water vapor absorption line.

SSM/T-2 Data Description

DMSP SSM/T-2 files are stored at CLASS under the following naming convention:

NSS.SSMT.Sn.Dyyjjj.Shhmm.Ehhmm.Axxxxxxx.NS

where the upper case characters remain fixed and the lower case characters vary from spacecraft to spacecraft and from orbit to orbit.

The lower case characters correspond to the following variables:

n = Spacecraft identification (e.g. 6 for satellite F-12)
yy = Year of the century
jjj = day of the year
hhmm = Time in hours and minutes
xxxxxxx = Orbit Number

The upper case characters represent the following:

NSS.SMT2 = NESDIS SSM/T-2
D = Day
S = Start Time
E = End Time

SSM/T-2 Header Record Format

Each SSM/T-2 level 1b data set contains a header record in the following format:

The header record can be broken down by the following data groups which are described below.

Identification Block

The identification block is contained in Bytes 1 - 48 in the header record. This data group contains the data set name, the number of scans and the number of gaps in the data. The data set name provides the spacecraft identification, the orbit start day, orbit start and end times, and the processing block identification.

The spacecraft identification is a numerical ID assigned to a spacecraft (e.g. ID=6 corresponds to spacecraft F-12). The start and end times of the level 1b orbit are rounded off to the nearest five minutes. Therefore, the first scan may not necessarily be at the start time and the last scan may not necessarily be at the end time. The processing block identification contains a letter code followed by a five digit starting rev number and a two digit ending orbit number. The ending orbit number is obtained by incrementing the last two digits of the starting orbit number by one. The total number of scan records in the data set is given by the number of scans parameter. The number of data gaps parameter corresponds to missing data. If the scene data block is empty or missing, then it is considered as a data gap. If a data gap covers one or more consecutive scans, then it is counted as one data gap. The data gap parameter can be used to determine the completeness of the data.

Preflight Calibration Data Group

The preflight calibration data group is contained in bytes 49 - 156 in the header record. This data group includes the coefficients used to convert warm load calibration counts to the corresponding temperatures, as well as the correction terms used to compute the slope and intercepts. To compute the temperatures, a set of eleven counts and the corresponding temperatures are provided for each thermistor. Included in the correction terms are a cold path temperature correction term and a warm path temperature correction term for each channel. The preflight calibration data are retrieved from the SSM/T-2 constants file.

Note: Preflight calibration data have no utility to the user. These data are intended for troubleshooting calibration related problems.

Antenna Pattern Correction Data Group

The antenna pattern correction data group is contained in bytes 157 - 436 in the header record. This data group provides the coefficients required to perform antenna pattern correction. No correction to the antenna pattern is performed by the Level 1b software. Therefore, the coefficients to perform this correction as a post-processing step are supplied in this data group. The correction coefficients are stored in the following order:

Quality Control Summary Group

The quality control (QC) summary data group is contained in bytes 437 - 450 in the header record. This data group provides a summary of the quality of the earth locations, scene data and calibration data at the orbital level. The overall quality of the level 1b data can be determined from the QC summary. The QC summary is reported as a percentage of the total number of samples upon which the quality control is performed. The criteria used to assess the quality of a scan or channel are as follows.

While computing earth locations, the two ephemeris minute vectors are verified for valid data. The scan time is verified to determine if it lies between the ephemeris times. The earth locations are not computed if the ephemeris data are deemed invalid, or if the scan time is not bounded by the ephemeris times and a filler value of 0 is used for the earth locations.

If a scene data block is empty or missing, the corresponding scan is treated as bad as far as the scene data are concerned. No filler values are provided as substitutes for missing data.

The calibration algorithm uses the averages of calibration data taken from 8 scans (four preceding scans, the scan being calibrated, and three succeeding scans). A scan or channel is treated as bad, as far as the calibration data are concerned, when one of the following conditions is satisfied:

1) Warm load counts failed the limit check, or the difference of warm load temperatures computed from the two thermistors exceeded the pre-defined limit, or less than four good scans were obtained to perform the averages.
2) All cold view or warm view counts from a given channel failed the limit check.
3) The averages of cold view and warm view counts were identical in a given channel.
4) Slope or intercept failed the limit check in a given channel.

When the first condition is satisfied, the entire scan is not calibrated and the individual channels are treated as bad. When conditions 2, 3 or 4 are satisfied, the corresponding channel is treated as bad.

In all other instances, the quality of earth location, scene data and calibration counts is validated to be good and identified as such. The level 1b software compiles the data accumulated over the entire orbit, computes statistics and stores the results in the header record in terms of percentages.

SSM/T-2 Data Record Format

Each SSM/T-2 level 1b data set contains up to 900 data records. Each data record contains one scan of SSM/T-2 data in the following format:

Each data record can be broken down by the following data groups which are described below.

Scan Information Group

The scan information data group is contained in bytes 1 - 20 in the data record. This data group includes the orbit number, scan number, scan index and scan time. The orbit number corresponds to the rev number provided in the readout header of the raw data. The scan number is a sequential number assigned to a scan. It also coincides with the data record number, i.e. the scan number in the first data record is 1, the scan number in the second data record is 2, and so on. The scan index is used for collocation with SSM/T-1 data. The index is a composite of the scan group and scan sequence number. The scan group is a sequential number assigned to a set of 4 SSM/T-2 scans and the scan sequence number is a number which identifies individual scans within that group. The scan index is stored as the scan group times 10 plus the scan sequence number. For example, if the scan number is 75, the corresponding index is 193 (scan numbers 73-76 becomes group 19; scan number 75 is the third scan in that group, i.e. 19*10+3).

The scan time contains the year of the century and the day, OLS time in seconds, and TS in milliseconds. The OLS time is the time extracted from the first subframe (data ident = 0, 8, 16 or 24) of 8 subframes which make up the SSM/T-2 scan. TS is the time between beam position 1 and the following readout enable. TS is used as an additive term in the earth location algorithm and has no utility to the user. The scan time is stored in the following format:

Note: The day parameter is not reset for orbits which are on a day boundary.

Earth Location Data Group

The earth location data group is contained in bytes 21-132 of the data record. This data group includes the earth locations for each of the 28 beam positions. The earth locations (latitude-longitude pairs) appear in the following order:

and so on.

The earth locations are specified in degrees and the following convention is used for latitude-longitudes:

Latitudes: North > 0 and South < 0; (-90 <= lat <= 90)
Longitudes: East > 0 and West < 0; (-180 <= long <= 180).

Note: The earth locations provided in the data record must be descaled to determine the appropriate latitudes and longitudes. If the earth locations of all beam positions are identically equal to zero, it indicates missing earth locations and these locations must not be used.

Scene Data Group

The scene data group is contained in bytes 133-468 of the data record. This data group provides the time at each beam position followed by raw counts of all channels. Scene data appear in the following order:

The raw counts range from 0 to 4,095. These counts are used to compute channel brightness temperatures using the following relationship:

BTn = Bn * Cn + An

where n is the SSM/T-2 channel number (1-5), and BT, B, C and A are the brightness temperatures, slopes, raw counts and offsets, respectively.

The time given at each beam position is the time measured relative to the OLS time. The beam position time is stored in milliseconds and can be converted to an absolute time by adding these milliseconds to the OLS time (byte 13-16). Since the beam position times are derived as a function of ephemeris time, occasionally, the beam position times could lag behind the OLS time by a few seconds. This is a normal condition.

Calibration Data Group

The calibration data group is contained in bytes 469-660 of the data record. Calibration data consists of the following parameters:

Note: Only the channel slope and offset values have utility to the user. The other parameters provided under the calibration data group are intended for troubleshooting calibration related problems.

Quality Control Data Group

The quality control data group is contained in bytes 661-674 of the data record. This data group indicates the quality of earth locations, scene data and calibration at the scan level. The criteria used for quality control are outlined in the header record Quality Control Summary section above.

The QC information appears in the following order:

If the earth locations QC flag contains a non-zero value, the earth locations from that scan must not be used. If the scene data QC flag contains a non-zero value, it indicates missing scene data and the scene data from that scan should be used with caution. If a calibration QC flag contains a non-zero value, the slopes and intercepts computed for that channel are probably erroneous and are not usable.

Note: The QC data provided in this data group are only applicable to the data record in which they are reported.

SSM/T-2 Spatial Coverage

At the nominal 833 km altitude, SSM/T-2 observations are made at a spatial resolution of approximately 45 km. Each of the approximately 14 orbits per day covers a different area of the earth, except poleward of 57 degrees latitude, where successive orbits begin to overlap. Elsewhere, there is a data coverage gap between successive orbits. Each orbit is comprised of ascending and descending passes. All five channels have coincident centers. The total swath width for the SSM/T-2 is approximately 1,500 km.

A 3.3 degree beamwidth is achieved for the 183.3 GHz channels while larger beamwidths of approximately 3.7 degrees and 6.0 degrees are achieved for the 150.0 and 91.665 Ghz channels, respectively. These correspond to nadir Field-of-Views of approximately 48, 54 and 84 km, respectively.

The SSM/T-2 scan mechanism is synchronized with the SSM/T-1 so that the beam cell patterns of the two sensors coincide.

SSM/T-2 Temporal Coverage

The SSM/T-2 makes observations at a rate of 7.5 scans/minute with each scan producing 28 observations or Beam Positions. Each data set contains one orbit's worth of data and is allowed to accumulate up to 120 minutes of data. Approximately 14 level 1b orbital data sets are generated per day per satellite.

The overall coverage of the SSM/T-2 data archived at CLASS is shown in the following tables. However, associated with equipment malfunctions, there may be short gaps in the time ranges.

SSM/T-2 Calibration

The SSM/T-2 employs a calibration period of 8 seconds in which four samples are taken of a warm-load calibration target (~300K) along with four samples of the cosmic background (~3K). The warm load is shrouded to improve radio frequency (RF) coupling of energy to the reflector/feedhorn antenna. This minimizes potential calibration errors arising from the reception of extraneous energy due to scattering of earth or solar radiation off of the spacecraft. The cold path is a cylindrical oversized waveguide tube which permits a direct view of the cosmic background by the antenna reflector during calibration.

The periodic calibration data are modeled by a linear transfer function to characterize the state of the total power radiometer and to remove time variations of the receiver gain and offset for frequencies less than half the reciprocal of the calibration period. As a consequence, relatively large temperature-related receiver gain drifts are taken into account in the periodic construction of the transfer function.

SSM/T-2 Data Processing Software

To obtain the DMSP SSM/T-2 data processing software file, click on the link below. You will be presented with the source code. To save the source code, go to your file menu and select `Save As'.

C language program ssmt2list.c reads and lists the SSM/T-2 file contents.

DMSP/README.ssmt2
DMSP/ssmt2list.c

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