Instrument Description and Measurement Modes
The main purpose of the microwave radiometer is the measurement of the tropospheric path delay for the altimeter through the measurement of the atmospheric integrated water vapor content and the estimate of the attenuation of the altimetric signal by the liquid water content of the clouds. This is achieved by measuring the brightness temperature at 23.8 and 36.5 Ghz which are, over ocean, very sensitive to the content of water vapor and liquid water in the troposphere. The water vapor is related to the excess pathlength as the liquid water is to the attenuation. The relation between brightness temperature and geophysical parameters, ie liquid water and water vapor, is of statistical nature and is obtained by linear regression using a radiative transfer mode. Basics for retrieval of ocean surface and atmospheric parameter from satellite microwave radiometry can be found in Ref. 27 (Wilheit and Chang, 1980). This information can be used also in radiation budget studies (e.g. surface energy budget associated with the surface temperature measured by the ATSR infra-red channels and the scatterometer wind, ground humidity, surface emissivity etc.), in ice formations studies (the limits of the ice, in association with the altimeter) and for studying the properties of the continental ice pack.
Each channel, 23.8 and 36.5 Ghz, has a bandwidth of ±200 Mhz wide and operates in Dicke mode by comparing the antenna temperature to an internal reference source with a switching frequency of 1 kHz. The output signal from the sensor is integrated and sampled every 150 ms (synchronised with the Infra-red Radiometer scan rate), and is transmitted to the ground as a numerical count together with the temperatures of the microwave components. Internal calibration is achieved every 38.4s by either connecting the receiver input to a skyhorn observing the cold temperature of space, or to a second internal reference load. The temperatures of the microwave components are monitored every 4.8 s using precise resistance measurements of platinum thermistances. As some drift of the resistance measurement system may occur with temperature variations, four precisely calibrated reference resistances are measured to calibrate the system. The main antenna is an offset antenna, with one feed horn for each frequency. Each channel is then pointing at an angle close to the nadir, with the channel 36.5 in the forward direction and the channel 23.8 in the backward direction. Each channel is linearly polarised in the orbit plane (vertical polarisation). The 3 dB footprint diameter for both channels is about 21 km.
[From "RA/ATSR products - User Manual", Ref.: C2-MUT-A-01-IF V, 2.3]