Navy Need For Wind Information

WindSat strives to answer the battlespace environment question, "What's the wind speed and direction around the carrier strike group?" Winds over the ocean affect nearly every aspect of naval operations, including carrier operations, mission planning for precision guided munitions, surf forecasting for expeditionary forces, and avoidance of nuclear, biological, and chemical clouds. The global ocean surface wind vector (speed and direction) provides essential information for short-term weather forecasts and warnings, nowcasting, and climatology and oceanography studies in both the civilian and military sector. This can lead to improved accuracy in tropical cyclone forecasting and improved ship routing. Carrier post-cruise reports following deployments in the Adriatic Sea stated that the most critical piece of meteorological data was the wind direction. The same reports indicated that wind direction was the least-understood parameter in their environment. Despite this critical need, the Navy has not been able to obtain global wind direction information from space. Space-borne passive microwave sensors, such as the Special Sensor Microwave Imager (SSM/I), operationally provide environmental data such as tropospheric water vapor mass, cloud liquid water mass, sea ice age and concentration, and ocean surface wind speed.1 One parameter that has not been provided by microwave radiometers is wind direction. However, recent work and advance in polarimetric radiometry suggest that it may be possible to measure the complete ocean surface wind vector (speed and direction) from space-borne microwave radiometer (such as SSM/I) if the instrument is modified to include measurement of the full Stokes vector (the current generation of SSM/I sensors measure only the first two components of the four-element vector).2,3

WindSat is a satellite-based multifrequency polarimetric microwave radiometer developed by the Naval Research Laboratory Remote Sensing Division and the Naval Center for Space Technology for the U.S. Navy and the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Integrated Program Office (IPO). WindSat is designed to demonstrate the viability of using polarimetric microwave radiometry to measure the ocean surface wind vector from space. It is the primary payload on the Air Force Coriolis satellite, which is sponsored jointly by the DoD Space Test Program (STP) and the Navy (SPAWAR PMW-155). WindSat shares this mission with the Solar Mass Ejection Imager (SMEI) developed by the Air Force Research Laboratory (AFRL). A spacecraft developed by Spectrum-Astro of Gilbert, Arizona, supports both payloads. The WindSat/Coriolis mission was launched on a Titan II rocket from Vandenberg Air Force Base on 6 January 2003. In addition to potentially providing the Navy with badly needed ocean surface wind vector measurements, WindSat provides risk reduction data that the NPOESS will use in the development of the Conical Microwave Imager Sounder (CMIS), which would provide wind vector measurements operationally beginning in the 2009 time frame, building on the pathfinding work of WindSat.

Polarimetric Radiometry Background

Microwave radiometry is a well-established technology for remote sensing of the environment. Radiometers such as WindSat measure the microwave emission from the field of view (FOV) of its antenna. Figure 1 illustrates that the received energy is a combination of energy emitted from the surface (ocean), radiation from the atmosphere, and energy from the sky reflected off the surface. The measured quantity is known as the brightness temperature. For the surface emission, it is related to the physical temperature by

where T_B,p is the brightness temperature in polarization p, e is the scene emissivity, T_phys is the physical temperature of the scene, and (θ,φ) represent the viewing geometry. By definition, a perfectly absorbing and emitting blackbody has an emissivity of one. Therefore, at thermal equilibrium it has a brightness temperature equal to its physical temperature. All other scenes have an emissivity less than one. The emissivity depends not only on the geometry and polarization, but also on the physical properties of the medium. For sea water, in particular, the emissivity is a function of the water temperature, salinity, and the roughness of the medium's surface.

FIGURE 1
Conceptual description of principles of microwave radiometer measurements.

It has long been known that the microwave emission from the ocean surface depends on the wind speed at the surface. As the winds increase, the seas become rougher and the microwave emission increases. However, the wind-driven waves on the ocean surface are not isotropic; their distribution varies with wind direction. Therefore, the intensity of the emission depends not only on the wave structure, but also on the orientation of the wind-driven waves.

WindSat is the first space-borne polarimetric microwave radiometer. As a polarimetric radiometer, WindSat measures not only the principal polarizations (vertical and horizontal), but also the cross-correlation of the vertical and horizontal polarizations. The cross-correlation terms represent the third and fourth parameters of the modified Stokes vector, defined as

In this definition, T_v, T_h, T₄₅, T_-45, T_lc and T_rc represent brightness temperatures (radiances) at vertical, horizontal, plus 45°, minus 45°, left-hand circular, and right-hand circular polarizations, respectively. The Stokes vector provides a full characterization of the electromagnetic signature of the ocean surface and the independent information needed to uniquely determine the wind direction.

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