Department of Electrical & Computer Engineering
Brigham Young University
I. IntroductionSatellite observation systems are crucial to the monitoring of the motion of the atmosphere and oceans. In particular, oceanic winds play a key role in driving the oceans and in modulating fluxes between the air and sea. Past observational efforts, using conventionally obtained data, have been severely hampered by the lack of accurate wind measurements with high resolution, global coverage, and frequent sampling. The Seasat Scatterometer, flown for 3 months in 1978, first demonstrated the capability of scatterometers to obtain the needed measurements of near-surface vector winds from space. Since scatterometers are widely recognized as the only all-weather instruments capable of measuring vector winds from space, they play a crucial role in current and future Earth remote sensing systems.
II. Principles of ScatterometryA good review of wind scatterometry can be found in Naderi et al. (1991). The wind scatterometer does not directly measure the wind. It measures the radar backscatter of the ocean's surface which is related to the wind via a geophysical model function. To measure the radar backscatter, the instrument transmits a pulse of RF energy and measures the backscattered power. From knowledge of the parameters of the radar equation, the normalized radar cross-section (denoted so) of the ocean's surface can be computed during ground processing. The nearsurface wind vector is then estimated from the so measurements using a geophysical model function. Due to the nature of the geophysical model function (which exhibits a bi-harmonic dependence on the wind direction), multiple co-located measurements of so from different azimuth angles are required to determine the wind vector at the ocean's surface. When retrieving (estimating) the wind from the so measurements, several possible wind directions may occur. A second step, known as "ambiguity removal,'' is used to determine a unique wind vector.
III. Scatterometer SystemsScatterometer data has been available since 1992 from the European Space Agency (ESA) Earth Remote Sensing System (ERS-1). With the successful launch of ERS-2, ESA will provide scatterometer data through 1997. The NASA Scatterometer (NSCAT) will fly in 1996 aboard the Japanese Advanced Earth Observing System (ADEOS-I). NSCAT will provide data through 1998. NASA is currently developing an advanced scatterometer known as SeaWinds for a three year mission aboard ADEOS-II to be launched in 1999.
To address the crucial need for continuous multiple decade scatterometer datasets and to provide for more frequent coverage, additional scatterometer systems are needed. Unfortunately, traditional fan- beam scatterometer designs such as SASS, ERS-1/2, and NSCAT require significant power and mass and have antenna systems which are difficult to accommodate aboard spacecraft. Further, the current fiscal environment has placed severe restrictions on the remote sensing missions. New designs for smaller, lighter, and less-costly scatterometers are required. One example is a single-beam scanning scatterometer flown on a dedicated small satellite such as the system proposed by Long (1993). Another example would be a scaled down version of SeaWinds. Probably the most cost-effective missions will integrate the scatterometer with the spacecraft on small satellite missions.
IV. Technology IssuesThe relative maturity of scatterometry is a crucial advantage in the effort to develop new design concepts: the issues associated with wind retrieval from the scatterometer measurements and the techniques for predicting system performance are quite well understood. Nevertheless, to achieve a small, low-cost design, technical innovations in the area of antennas and radar electronics may be needed. NASA's Jet Propulsion Laboratory has developed a number of antenna concepts including various types of phased arrays and a Lunberg lens-based antenna. Other concepts leading to smaller and cheaper missions are possible.
As an active radar sensor, a key component of the scatterometer is the final RF amplifier. Typically, this has been based on a traveling wave tube. Solid state amplifiers could improve the system reliability while reducing the weight of the transmitter. RF power requirements vary depending on the antenna design but a minimum 20 to 100 W will be required. The ERS-1/2 scatterometers share an amplifier with a synthetic aperture radar (SAR) mode and use a 5 kW peak transmitter tube. U.S. scatterometers have traditionally operated at Ku-band (14 GHz) while ESA scatterometers have been at C- band (5.3 GHz).
The goal in developing new scatterometer designs must be to make them more flyable. A "flyable" scatterometer is one which minimizes costs to the point they can be afforded by NOAA and other funding agencies; be relatively easily accommodated on (or integral to) a spacecraft; be built, integrated, and tested on a schedule commensurate with potential flight opportunities; and be sufficiently capable so that its data are scientifically and operationally useful. The latter can be specified in terms of coverage and wind measurement accuracy. Coverage of 90% of the Earth's oceans in two days or less is required, while wind measurements accuracy must be +/- 2 m/s and 20 deg rms. Timeliness of the data transmission to the ground (within 6 hours for future systems) is also a requirement.
REFERENCESF. Naderi, M. H. Freilich, and D. G. Long, "Spaceborne Radar Measurement of Wind Velocity Over the Ocean--An Overview of the NSCAT Scatterometer System", invited paper, Proceedings of the IEEE, pp. 850-866, Vol. 79, No. 6, June 1991.
D. G. Long, "ASCAT: A Light-Weight, Low-Cost Scatterometer," in Microwave Instrumentation for Remote Sensing of the Earth, James. C. Shiue, ed., Proc. SPIE 1935, pp. 28-38, Orlando, Florida, April 13-14, 1993.