wave radar instrument (a fan-beam Doppler scatterometer), using an array of six antennas that radiate microwave pulses at a frequency of 13.995 GHz across broad regions of the Earth's surface.
An array of six, 3 m long antennas scan two swaths of 600 km width each, one band to each side of the flight path, separated by a gap of 330 km at nadir. Wind speed accuracy = 2 m/s (rms), direction accuracy = 20º (rms), resolution = 50 km; antenna polarization: 6 V, 2 H; antenna beam width=25º (3 dB broad beam) =0.42º (3 dB narrow beam); instrument mass = 280 kg; power=240 W; data rate = 3.2 kbit/s; peak transmit power=110 W; pulse width=5 ms; PRF=62 Hz. Operation requirements: continuous operation for global observation of the oceans (about 190,000 wind measurements/day). Note: NSCAT is an upgraded version of the Radar Scatterometer (SASS) on SEASAT. 23) 24) 25)
Figure 10: Illustration of the NSCAT antenna radiation pattern geometries (image credit: NASA)
NSCAT transmits microwave pulses and receives a backscattered echo from the ocean surface. Changes in wind velocity cause changes in ocean surface roughness, modifying the radar cross section of the ocean and the magnitude of the backscattered power. Multiple collocated measurements acquired from several directions can thus be used to solve for wind speed and direction simultaneously.
Table 3: NSCAT instrument performance parameters
Figure 11: Schematic of NSCAT instrument (image credit: NASA)
TOMS (Total Ozone Mapping Spectrometer):
TOMS is a NASA/GSFC sensor. Objective: Observation of total ozone changes, evaluation of changes in UV radiation and the observation of sulfur dioxide. Measurement wavelengths: 308.6, 312.5, 317.5, 322.3, 331.2 and 360 nm with 1 nm bandpass. Swath width: 2795 km. IFOV= 50 km at nadir; cross-track scan = 111º (37 3º steps). Operation requirements: global observation of illuminated part. TOMS mass=34 kg, power=24 W, data rate=700 bit/s.
TOMS measures the albedo of the Earth's atmosphere at six narrow spectral bands in the near-ultraviolet region. The albedo is measured by comparing the radiance of the Earth with the radiance of a calibrated on-band diffuser plate. Total ozone is derived from the differential albedo in three pairs of spectral bands, which are selected to function at all latitudes and solar illumination conditions. 26) 27) 28)
Figure 12: Illustration of the TOMS instrument (image credit: NASA)
A new feature on ADEOS/TOMS is the ability to monitor solar diffuser reflectance. A device referred to as the RCA (Reflectance Calibration Assembly) was added to the new series of TOMS. This assembly employs a phosphor light source with peak emission over the TOMS wavelength range. When powered on, the lamp illuminates the exposed diffuser surface which is then viewed using the TOMS scan mirror. The scan mirror also rotates to view the phosphor surface directly. The ratio of signals at the two scan mirror positions is a measure of relative diffuser reflectance.
The ADEOS/TOMS has 11 operating modes during normal operations. The most important of these are:
1) Standby mode
2) Scan mode
3) Solar calibration mode
4) Wavelength monitoring mode
5) Electronic calibration mode
6) Reflectance calibration mode
7) Direct control mode.
The primary operating mode of the TOMS is scan mode. It is in this mode that the scanning mirror samples the 37 scenes corresponding to the scanner view angles, measuring the backscattered Earth radiances used for deriving column ozone. During the nighttime portion of the orbit the instrument is placed in standby mode, at which time the scan mirror points into the instrument at a black surface. During solar calibration mode the scanner moves to view the exposed diffuser surface. The remaining modes are specialized for calibration purposes as the names indicate. The direct control mode is used when overriding the standard instrument modes. On ADEOS/TOMS, the reflectance calibrations were re-defined using direct control.
In the normal Earth scanning mode, ADEOS/TOMS measured the Earth backscatter ultraviolet at the six wavelengt