GES DISC DAAC Data Guide:
Special Sensor Microwave Water Vapor Profiler (SSM/T-2) Sensor

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Summary:

Table of Contents:

• 1 Sensor/Instrument Overview
• 2 Sensor/Instrument Layout, Design, and Measurement Geometry
• 3 Manufacturer of Sensor/Instrument
• 4 Calibration
• 5 References
• 6 Glossary of Terms
• 7 List of Acronyms
• 8 Document Information

1. Sensor/Instrument Overview:

Sensor/Instrument Long Name, Sensor/Instrument Acronym:

Special Sensor Microwave Water Vapor Profiler, SSM/T-2

Sensor/Instrument Introduction:

The SSM/T-2 is a five channel, total power, microwave radiometer with three channels situated symmetrically about the 183.31 GHz water vapor resonance line and two window channels. This instrument was flown on all DMSP Block 5D-2 satellites starting with F11 launched in 1991.

Sensor/Instrument Mission Objectives:

SSM/T-2, a spaceborne instrument, was 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.

Key Variables:

Microwave radiances

Scanning or Data Collection Concept/Principles of Operation:

The SSM/T-2 is a cross-track scanning, five channel, passive total power microwave radiometer system which measures microwave radiance at 183.3 +/- 1, +/- 3, +/- 7 and at 150.0 and 91.655 GHz.

A microwave radiometer is a highly sensitive receiver capable of measuring low levels of microwave radiation. When a scene is observed by a microwave radiometer, the radiation received by the antenna is partly due to the self-emission by the scene and partly due to the reflective radiation originating from the surroundings.

Through proper choice of the radiometer parameters (wavelength, polarization and viewing angle) and weighting functions, a relationship is established between the magnitude of the energy received and a profile of the atmospheric water vapor.

2. Sensor/Instrument Layout, Design, and Measurement Geometry:

List of Sensors:

Special Sensor Water Vapor Profiler, SSM/T-2

Sensor Description:

The SSM/T-2 is a cross-track scanning, five channel, passive total power microwave radiometer system which consists of a single, self-contained module with 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 sensor coincide. The SSM/T-2 observation rate is 7.5 scans per minute. There are 28 observations (beam positions) per scan for each of the five channels, with each observation having a spatial resolution of approximately 48 km. All five channels have coincident centers. The total swath width for the SSM/T-2 is approximately 1500 km.

The SSM/T-2 employs a single offset parabolic reflector with a 2.6 inch diameter projected aperature. The reflector is shrouded to eliminate the possiblity 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 seperation over the bandwith (91 to 183.3 GHz), while providing a spherical wave illumination of the reflector.

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 TBs which is permitted provided that the polarization remains identical for the two window channels and 183.3 +/- 7 GHz. 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.

3. Manufacturer of Sensor/Instrument:

SSM/T-2 was built by Aerojet Electronic Systems Division under the direction of SMC/CI.

4. Calibration:

Specifications:

During each scan period, and for all five channels at twenty-eight discrete earth viewing positions, four discrete calibration measurements of a hot-load target (~300K), and cosmic background radiation (~3K) are monitored.

The SSM/T-2 inflight warm-load calibration target is a derivative of the SSM/T-2 warm load calibration target. The warm load (~300 K) 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 (~3 K) 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 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.

Tolerance:

The minimal detectable temperature difference is 0.45 K.

Frequency of Calibration:

The SSM/T-2 employs a calibration period of 8 seconds in which four samples are taken of a warm-load calibration target along with four samples of the cosmic background.

Other Calibration Information:

The radiometer sensitivity, i.e. the minimum detectable change in the radiometric antenna temperature deltaT of the observed system is:

By design the contributions to the deltaT from the video and quantization noise are much smaller than the wide-band thermal noise term. The contribution of the deltaG/G are minimized by employing stable components and frequent radiometer calibrations at the input to the antenna aperature.

Ground based calibration estimates of the deltaT are obtained by operating the SSM/T-2 in a thermal/vacuum chamber in a configuration that resembles actual orbital conditions. Two primary standard calibration targets operating in the range of 300 K and 80 K are sequentially viewed by the SSM/T-2 every 8 seconds for a duration of four samples. The warm primary standard is subsequently viewed for 28 samples which corresponds to the number of earth viewed scenes on orbit. This procedure is repeated until eight scans of data are collected. At this point the warm and cold calibration data are fitted by a linear transfer funcion to characterize the state of the radiometer over the eight scans. The sampled variance of the 228 samples (8 * 28) of raw output counts is computed next and referenced to the input of the antenna aperture using the slope of the transfer function. This procedure is repeated many times, averaged and the square root taken to obtain an estimate of the deltaT.

 

5. References:

• F. Ulaby, R. Moore, A. Fung (1981), Microwave Remote Sensing, Active and Passive, ISBN 0-201-10759 (v.1), Addison-Wesley Publishing Company, Inc.
• Falcone, V.J., M.K. Griffin, R.G. Isaacs, J.D. Pickle, J.F. Morrissey, A.J. Jackson, A. Bussey, R. Kakar, J. Wang, P. Racette, D.J. Boucher, B.H. Thomas, A.M. Kishi: DMSP F11 SSM/T-2 Calibration and Validation Data Analysis, Phillips Laboratory, Hanscom Air Force Base, MA, 29 October 1992.
• Tadepalli, K.: SSM/T-2 Level 1b Interface Control Document, NOAA/NESDIS, Suitland, MD, 11 July, 1994.

6. Glossary of Terms:

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

7. List of Acronyms:

• AFB - Air Force Base

• DMSP - Defense Meteorological Satellite Program

• DoD - Department of Defense

• FNMOC - United States Navy Fleet Numerical and Oceanography Center

• NESDIS - National Environmental Satellite, Data, and Information Service

• OLS - Operational Linescan System

• RF - Radio Frequency

• SMC/CI - TBD

• SSM/I - Special Sensor Microwave Imager

• SSM/T-1 - Special Sensor Microwave Temperature Sounder

• SSM/T-2 - Special Sensor Microwave Water Vapor Profiler

• USAF - United States Air Force

• URL - Uniform Resource Locator

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

8. Document Information:

Document Revision Date:Thu Apr 25 09:23:02 EDT 2002

March 05, 1996

Document Review Date:

March 05, 1996

Document ID:

SSM/T2 SENSOR

Change History

Version 1.0

Version baselined on addition to the GES Controlled Documents List, March 5, 1996.