What is a Scatterometer?
A scatterometer is a microwave radar sensor used to measure the reflection or scattering effect produced while scanning the surface of the earth from an aircraft or a satellite.
Description of the SeaWinds Scatterometer and How It Works
The SeaWinds scatterometer is a microwave radar designed specifically to measure ocean near-surface wind speed and direction.
The SeaWinds scatterometer consists of three major parts called subsystems. They are the Electronics Subsystem (SES), the Antenna Subsystem (SAS), and the Command and Data Subsystem (CDS).
The Electronics Subsystem is the heart of the scatterometer and it contains a transmitter, receiver and digital signal processor. It generates and sends high radio frequency (RF) waves to the antenna. The antenna transmits the signal to the Earth's surface as energy pulses. When the pulses hit the surface of the ocean it causes a scattering affect referred to as backscatter. A rough ocean surface returns a stronger signal because the waves reflect more of the radar energy back toward the scatterometer antenna. A smooth ocean surface returns a weaker signal because less of the energy is reflected. The echo or backscatter is routed by the antenna to the SES through waveguides (rectangular metal pipes that guide RF energy waves from one point to another). The SES then converts the signals into digital form for data processing.
The CDS is essentially a computer housing the software that allows the instrument to operate. It provides the link between the command center on the ground, the spacecraft and the scatterometer. It controls the overall operation of the instrument, including the timing of each transmitted pulse and collects all the information necessary to transform the received echoes into wind measurements at a specific location on Earth. To locate the precise position on Earth at which the echo was taken, the CDS collects (for each pulse) the antenna rotational position, spacecraft time, and an estimate of the spacecraft position. The CDS also collects instrument temperature, operating voltages and currents, so that the overall health of the instrument can be monitored. It is through the CDS that the other two subsystems receive the commands that control all of their functions.
The SAS consists of a one-meter parabolic reflector antenna mounted to a spin activator assembly, which causes the reflector to rotate at 18 Rpm's (revolutions per minute). The activator assembly provides very accurate spin control and precise position or pointing information to the CDS. Optical encoders, glass disks with small patterns printed on the surface, tell the CDS exactly where the antenna is pointing to about 10/1000 of a degree. The antenna spins at a very precise rate, and emits two beams about 6 degrees apart, each consisting of a continuous stream of pulses. The two beams are necessary to achieve accurate wind direction measurements. The pointing of these beams is precisely calibrated before launch so that the echoes may be accurately located on the ground from space.
Why is Scatterometry Important?
Data derived from ocean scatterometers is vital to scientists in the their studies of air-sea interaction and ocean circulation, and their effects on weather patterns and global climate. These data are also useful in the study of unusual weather phenomena such as El Niño, the long-term effects of deforestation on our rain forests, and changes in the sea-ice masses around the polar regions. These all play a central role in regulating global climate.
Computer modeling of global atmospheric dynamics for the purpose of weather forecasting has become an increasingly important tool to meteorologists. Scatterometer data, with wide swath coverage, have been shown to significantly improve the forecast accuracy of these models. By combining scatterometer data of ocean-surface wind speed and direction with measurements from other scientific instruments, scientists gather information to help us better understand the mechanisms of global climate change and weather patterns.
A History of Scatterometry
In the past, weather data could be acquired over land, but our knowledge of surface winds over oceans came from infrequent, and sometimes inaccurate, reports from ships and buoys.
Scatterometery has its origin in early radar used in World War II. Early radar measurements over oceans were corrupted by sea clutter (noise) and it was not known at that time that the clutter was the radar response to the winds over the oceans. Radar response was first related to wind in the late 1960's. The first scatterometer flew as part of the Skylab missions in 1973 and 1974, demonstrating that spaceborne scatterometers were indeed feasible. The Seasat-A Satellite Scatterometer (SASS) operated from June to October 1978 and proved that accurate wind velocity measurements could be made from space. A single-swath scatterometer flew on the European Space Agency's Remote Sensing Satellite-1 (ERS-1) mission.
The NASA Scatterometer (NSCAT), which launched aboard Japan's ADEOS-Midori Satellite in August, 1996, was the first dual-swath, Ku-band scatterometer to fly since Seasat. From September 1996 when the instrument was first turned on, until premature termination of the mission due to satellite power loss in June 1997, NSCAT performed flawlessly and returned a continuous stream of global sea surface wind vector measurements. Unprecedented for coverage, resolution, and accuracy in the determination of ocean wind speed and direction, NSCAT data has already been applied to a wide variety of scientific and operational problems. These applications include such diverse areas as weather forecasting and the estimation of tropical rain forest reduction. Because of the success of the short-lived NSCAT mission, future Ku-band scatterometer instruments are now greatly anticipated by the ocean winds user community. The NSCAT mission proved so successful, that plans for a follow-on mission were accelerated to minimize the gap in the scatterometer wind database. The QuikSCAT mission launched SeaWinds in June 1999.