Sensor Description

The Special Sensor Microwave Imager/Sounder (SSMIS) is a conically scanning passive microwave radiometer with a 53.1 degree Earth incidence angle sensing upwelling microwave radiation at 24 channels covering a wide range of frequencies from 19 – 183 GHz. Data is collected along an active scan of 144 degrees across track producing a swath width on the ground of 1707 km. The first of five sensors was launched on board DMSP F16 on October 18, 2003. The SSMIS is a joint US Air Force/Navy multi-channel passive microwave sensor that combines and extends the imaging and sounding capabilities of the previous DMSP microwave sensors SSM/T, SSM/T2 and SSM/I. It was built by Northrup-Grumman Electronic Systems.

SSMIS channel characteristics of the channels which are intercalibrated in FCDR:

Center Frequencies (GHz) 19.35 19.35 22.235 37.0 37.0 91.655 91.655
Channel Number 13 12 14 16 15 17 18
Polarization V H V V H V H
Bandwidth (MHz) 400 400 450 1500 1500 1500 1500
NEDT (K) 0.7 0.7 0.7 0.5 0.5 0.9 0.9
EFOV (km x km) * 74 x 45 74 x 45 74 x 45 45 x 28 45 x 28 16 x 13 16 x 13
Sampling Interval (km x km) * 12.5 x 25 12.5 x 25 12.5 x 25 12.5 x 25 12.5 x 25 12.5 x 12.5 12.5 x 12.5

SSMIS channel characteristics of the moisture sounding channels which are NOT intercalibrated in FCDR:

Center Frequencies (GHz) 150.0 183.31±1 183.31±3 183.31±7
Channel Number 8 11 10 9
Polarization H H H H
Bandwidth (MHz) 1500 500 1000 1500
NEDT (K) 0.88 1.25 1.0 1.2
EFOV (km x km)* 16 x 13 16 x 13 16 x 13 16 x 13
Sampling Interval (km x km)* 12.5 x 12.5 12.5 x 12.5 12.5 x 12.5 12.5 x 12.5

SSMIS channel characteristics of the temperature sounding channels which are NOT intercalibrated in FCDR:

Center Frequencies (GHz) 50.3 52.8 53.596 54.40 55.50 57.29 59.4 63.283 60.793 60.793 60.793 60.793 60.793
Channel Number 1 2 3 4 5 6 7 19 20 21 22 23 24
Polarization H H H H H RC RC RC RC RC RC RC RC
Bandwidth (MHz) 400 400 400 400 400 350 250 1.5 1.5 1.5 3.0 8.0 30.0
NEDT (K) 0.4 0.4 0.4 0.4 0.4 0.5 0.6 2.4 2.4 1.8 1.0 0.6 0.7
EFOV (km x km)* 27 x 18 27 x 18 27 x 18 27 x 18 27 x 18 27 x 18 27 x 18 27 x 18 27 x 18 27 x 18 27 x 18 27 x 18 27 x 18
Sampling Interval (km x km)* 12.5 x 37.5 12.5 x 37.5 12.5 x 37.5 12.5 x 37.5 12.5 x 37.5 12.5 x 37.5 12.5 x 37.5 12.5 x 75 12.5 x 75 12.5 x 75 12.5 x 75 12.5 x 75 12.5 x 75

* EFOV and Sampling Interval values are km along track x km across track.

Channel characteristics shown above are from the Algorithm and Data User Manual for SSMIS, Table 1 (Bandwidth), Table 3 (EFOV, Sampling Interval), and Kunkee et al. 2006 (Sensitivity).

Local observing times:

  Geolocation

The original SSMIS pixel geolocation is based on predicted spacecraft ephemeris. For the FCDR data we recompute the spacecraft ephemeris using orbital element information contained in two-line element (TLE) files produced by the North American Aerospace Defense Command (NORAD). Using this updated spacecraft ephemeris along with software to compute the pixel geoleocation based on the geometry of the sensor, we perform an examination of geolocation errors and the subsequent impact on the view angle, or Earth incidence angle (EIA). Given that the observed Tb are highly sensitive to the view angle, properly accounting for changes in EIA across an orbit and over time is an important step that must be addressed before attempting to intercalibrate the sensors. Using a previously developed coastline analysis technique, estimates of changes in the spacecraft attitude including deviations in roll, pitch, and yaw have been computed for the life of each of the SSMIS sensors. Applying these corrections results in an improved pixel geolocation, but more importantly provides accurate estimates of the EIA across the scan and throughout each orbit. An analysis of uncertainties in the calculation of EIA shows mean errors within 0.1 degrees, which translates to errors in the calibration of less than 0.2 K for all channels. Equally important, is the availability of improved geolocation and EIA estimates for algorithm developers using the SSMIS data, which is critical for producing unbiased estimates of geophysical parameters for use in climate applications.

Additional Information on Geolocation

  Intercalibration

As with the SSM/I sensors, for the SSMIS sensors (including F16, F17, F18 and F19) multiple intecalibration approaches were implemented. Ultimately, however, the final intercalibration was obtained based on the results from a double difference approach using coincident overpasses with TRMM TMI and geophysical parameters from the ECMWF Interim reanalysis. Unlike the SSM/I sensors where a simple calibration offset was applied independent of scene temperature, with the SSMIS sensors there are significant variations in the calibration differences relative to F13 as a function of the scene tempeature. As a result, a temperature-dependent calibration was applied. Given the lack of an absolute calibration target for the microwave frequencies employed on SSM/I and SSMIS, using multiple approaches provides confidence in the results and an estimate of the residual uncertainty in the intercalibration. The techniques that have currently been used include the following.

Additional Information on Intercalibration

  Data

Get Data

Data Availability

Format Specifications

BASE Files
FCDR Files

  Software

Code to read FCDR files is available in the following languages:

  Documentation

CSU FCDR Documentation
SSMIS Documentation from other Sources

  Monitoring