K.M. St Germain

The WindSat spaceborne polarimetric microwave radiometer: sensor description and early orbit performance

The global ocean surface wind vector is a key parameter for short-term weather forecasting, the issuing of timely weather warnings, and the gathering of general climatological data. In addition, it affects a broad range of naval missions, including strategic ship movement and positioning, aircraft carrier operations, aircraft deployment, effective weapons use, underway replenishment, and littoral operations. WindSat is a satellite-based multifrequency polarimetric microwave radiometer developed by the Naval Research Laboratory for the U.S. Navy and the National Polar-orbiting Operational Environmental Satellite System Integrated Program Office. It is designed to demonstrate the capability of polarimetric microwave radiometry to measure the ocean surface wind vector from space. The sensor provides risk reduction for the development of the Conical Microwave Imager Sounder, which is planned to provide wind vector data operationally starting in 2010. WindSat is the primary payload on the Department of Defense Coriolis satellite, which was launched on January 6, 2003. It is in an 840-km circular sun-synchronous orbit. The WindSat payload is performing well and is currently undergoing rigorous calibration and validation to verify mission success.

A preliminary survey of radio-frequency interference over the U.S. in Aqua AMSR-E data

A spectral difference method is used to quantify the magnitude and extent of radio-frequency interference (RFI) observed over the United States in the Aqua AMSR-E radiometer channels. A survey using data from the AMSR-E instrument launched in May 2002 shows the interference to be widespread in the C-band (6.9 GHz) channels. The RFI is located mostly, but not always, near large highly populated urban areas. The locations of interference are persistent in time, but the magnitudes show temporal and directional variability. Strong and moderate RFI can be identified relatively easily using an RFI index derived from the spectral difference between the 6.9- and 10.7-GHz channels. Weak RFI is difficult to distinguish, however, from natural geophysical variability. These findings have implications for future microwave sensing at C-band, particularly over land areas. An innovative concept for radiometer system design is also discussed as a possible mitigation approach.

Radiometric measurements of the microwave emissivity of foam

Radiometric measurements of the microwave emissivity of foam were conducted during May 2000 at the Naval Research Laboratorys Chesapeake Bay Detachment using radiometers operating at 10.8 and 36.5 GHz. Horizontal and vertical polarization measurements were performed at 36.5 GHz; horizontal, vertical, +45/spl deg/, -45/spl deg/, left-circular, and right-circular polarization measurements were obtained at 10.8 GHz. These measurements were carried out over a range of incidence angles from 30/spl deg/ to 60/spl deg/. Surface foam was generated by blowing compressed air through a matrix of gas-permeable tubing supported by an aluminum frame and floats. Video micrographs of the foam were used to measure bubble size distribution and foam layer thickness. A video camera was boresighted with the radiometers to determine the beam-fill fraction of the foam generator. Results show emissivities that were greater than 0.9 and approximately constant in value over the range of incidence angles for vertically polarized radiation at both 10.8 and 36.5 GHz, while emissivities of horizontally polarized radiation showed a gradual decrease in value as incidence angle increased. Emissivities at +45/spl deg/, -45/spl deg/, left-circular, and right-circular polarizations were all very nearly equal to each other and were in turn approximately equal to the average values of the horizontal and vertical emissivities in each case.

Deep-space calibration of the WindSat radiometer

The WindSat microwave polarimetric radiometer consists of 22 channels of polarized brightness temperatures operating at five frequencies: 6.8, 10.7, 18.7, 23.8, and 37.0 GHz. The 10.7-, 18.7-, and 37.0-GHz channels are fully polarimetric (vertical/horizontal, /spl plusmn/45/spl deg/ and left-hand and right-hand circularly polarized) to measure the four Stokes radiometric parameters. The principal objective of this Naval Research Laboratory experiment, which flys on the USAF Coriolis satellite, is to provide the proof of concept of the first passive measurement of ocean surface wind vector from space. This paper presents details of the on-orbit absolute radiometric calibration procedure, which was performed during of a series of satellite pitch maneuvers. During these special tests, the satellite pitch was slowly ramped to +45/spl deg/ (and -45/spl deg/), which caused the WindSat conical spinning antenna to view deep space during the forward (or aft portion) of the azimuth scan. When viewing the homogeneous and isotropic brightness of space (2.73 K) through both the main reflector and the cold-load calibration reflector, it is possible to determine the absolute calibration of the individual channels and the relative calibration bias between polarimetric channels. Results demonstrate consistent and stable channel calibrations (with very small brightness biases) that exceed the mission radiometric calibration requirements.

Spaceborne polarimetric microwave radiometry and the Coriolis WindSat system

WindSat is a satellite-based multi-frequency polarimetric microwave radiometer being developed for the U.S. Navy and the National Polar-orbiting Operational Environmental Satellite system (NPOESS) Integrated Program Office. WindSat is designed to test the viability of using polarimetric microwave radiometry to measure the ocean surface wind vector from space, and is the primary payload on the Air Force Coriolis satellite, which is scheduled to launch in December 2001. The WindSat radiometer operates in discrete bands at 6.8, 10.7, 18.7, 23.8, and 37.0 GHz. The 10.7, 18.7 and 37.0 GHz channels are fully polarimetric, while the 6.8 and 23.8 GHz channels an dual polarized only (vertical and horizontal). The spaceborne segment of the WindSat program has recently completed the Critical Design Review.

Multi-frequency polarimetric microwave ocean wind direction retrievals

The Naval Research Laboratory (NRL) and the Jet Propulsion Laboratory (JPL) have conducted several aircraft campaigns from 1994-1996 using polarimetric microwave radiometers to measure the ocean surface wind direction. The first flights in 1994 used the JPL 19 and 37 GHz polarimetric radiometers, which measure the first three Stokes parameters (I, Q, U). In 1995, NRL added a 10.8 GHz polarimetric radiometer and a dual polarization 22 GHz radiometer. In 1996, NRL modified the 10.8 GHz radiometer to measure all four Stokes parameters simultaneously. The experiments collected data at a variety of incidence angles during circle flights over National Data Buoy Center (NDBC) buoys, which were used for in situ data. The NDBC buoys reported the wind speed and direction with accuracies of /spl plusmn/1 m/s and +10/spl deg/. In 1995, the buoys used were limited to those reporting every 10 minutes; the 1994 data were collected at buoys reporting hourly. Because there were no buoys near Hurricane Juliette, dropsondes were used for the necessary ground truth. Lastly, four flights were flown in November, 1996, out of NASA/Wallops Flight Facility (WFF) on the NASA/WFF P-3. These polarimetric brightness temperatures, together with in situ buoy wind data, verified the presence of a strong wind direction signal.

Multifrequency Passive Microwave Observation Of Saline Ice Grown In A Tank

The present set of observations suggests several phenomena of interest for interpreting microwave signatures of natural sea ice. During initial ice growth, interference fringe effects can occur both from changing ice thickness and snow or frost which may prove useful in interpreting layer thicknesses. A simulation of multiyear ice was attempted by allowing the ice to desalinate, but the resulting emissivity spectrum was more characteristic of lake ice. A multiyearlike spectrum was obtained when rubble was deposited on the ice surface.

APMIR: airborne polarimetric microwave imaging radiometer

APMIR is a new instrument that is currently under construction at the U.S. Naval Research Laboratory in Washington, D.C. The system will include channels from C- to Ka-band (specifically 6.6, 6.8, 7.2, 10.7 or 10.65, 18.7 or 19.35, 22.23 or 23.8, and 37.0 or 36.5 GHz). Each channel will provide verticallyand horizontally-polarized brightness temperatures, and several (10.7, 18.7, and 37.0 GHz) will be fully polarimetric. This instrument is being built to provide calibration data for two satellite programs, as well as for supporting algorithm development. The first satellite is the Defense Meteorological Satellite Programs SSMIS. The second instrument is the Coriolis WindSat, a joint Navy, NPOESS, and Air Force mission. Additionally, APMIR has many of the same frequency bands as AMSR-E. Though the system has been developed primarily for the calibration effort, a great deal of flexibility has been incorporated, such that other missions can be accommodated. These satellite instruments have extremely tight error budgets, which in turn lead to correspondingly formidable error budgets for the instruments involved in calibration. A new aircraft system, planned and built from the ground up with the design philosophy of controlling the errors that are critical to the calibration of these particular instruments, was thought to be the best way to meet the error budget. An extra degree of care has been exercised in all areas, but particularly antenna pointing determination and control, system environmental control, EMI suppression and power supply stability.

The WindSat space borne polarimetric microwave radiometer: sensor description and mission overview

The wind vector affects a broad range of naval missions, including strategic ship movement and positioning, aircraft carrier operations, aircraft deployment, effective weapons use, underway replenishment, and littoral operations. Furthermore, accurate wind vector data aids in short-term weather forecasting, the issuing of timely weather warnings, and the gathering of general climatological data. WindSat is a satellite-based multifrequency polarimetric microwave radiometer developed by the Naval Research Laboratory for the US Navy and the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Integrated Program Office (IPO). It is designed to demonstrate the capability of polarimetric microwave radiometry to measure the ocean surface wind vector from space. The sensor provides risk reduction for the development of the Conical Microwave Imager Sounder (CMIS), which is planned to provide wind vector data operationally starting in 2010

A microwave technique for mapping ice temperature in the Arctic seasonal sea ice zone

A technique for deriving ice temperature in the Arctic seasonal sea ice zone from passive microwave radiances has been developed. The algorithm operates on brightness temperatures derived from the Special Sensor Microwave/Imager (SSM/I) and uses ice concentration and type from a previously developed thin ice algorithm to estimate the surface emissivity. Comparisons of the microwave derived temperatures with estimates derived from infrared imagery of the Bering Strait yield a correlation coefficient of 0.93 and an RMS difference of 2.1 K when coastal and cloud contaminated pixels are removed. SSM/I temperatures were also compared with a time series of air temperature observation from Gambell on St. Lawrence Island and from Point Barrow, AK weather stations. These comparisons indicate that the relationship between the air temperature and the ice temperature depends on ice type.