on. 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 wavelength channels given in Table 4.
Figure 13: IFOV of TOMS projected onto Earth's surface (image credit: NASA)
Legend to Figure 13: The right portion (samples 1-19) of two consecutive scans are shown, and a portion of a scan from the previous orbit is also shown to illustrate the inter-orbit coverage at the equator. In descending mode (North to South) ADEOS/TOMS scans West to East.
Table 4: ADEOS/TOMS albedo calibration constants and gain range ratios
ADEOS-TOMS operations, data processing and data distribution are conducted by the Science Operation Center (SOC) at Goddard Space Flight Center. The SOC processes all near-real-time data obtained at US receiving stations and JAXA's EOC (Earth Observation Center). The preliminary Level 3' data set is available to science or operational users via Internet.
Figure 14: Photo of the TOMS instrument (image credit: NASA)
Figure 15: Sample ozone image of the TOMS instrument on ADEOS (image credit: NASA) 29)
POLDER (Polarization and Directionality of the Earth's Reflectances):
POLDER is a passive optical imaging radiometer of CNES (as passenger instrument on ADEOS). Objectives: Observation of bidirectionality and polarization of the solar radiation reflected by the atmosphere: tropospheric aerosols (inversion of the physical properties); ocean color (accurate determination of sea surface reflectances); land surfaces (determination of surface BRDF and improvement in the correction of the surface bidirectional and atmospheric effects on vegetation indices); Earth radiation budget (determination of cloud BRDF and classification of clouds according to their bidirectional properties). POLDER is capable to observe an area from various directions along with the spectral characteristics of the reflected solar light. 30) 31) 32) 33) 34) 35) 36)
a) Measurement of polarized reflectance in VIS/NIR
Six of POLDER's eight frequencies are optimized for observing atmospheric aerosols, clouds, ocean color, and land surfaces. The other two frequencies are centered on the H2O and O2 absorption bands 