TMR : Topex Microwave Radiometer

The TMR acquires measurements via three separate frequency channels to determine the path delay of the altimeter's radar signal due to atmospheric water vapour.

The TMR (50 kg including partial redundancy, 25 W) measures the microwave emissivity (brightness temperatures) of the sea surface at three frequencies (18, 21 and 37 GHz) to provide the total vapour content in the troposphere along the altimeter beam. The 21 GHz channel is the primary channel for water vapour measurement. The 18 and 37 GHz channels are respectively used to remove the effects of wind speed and cloud cover (liquid water contribution) from the water vapour measurement. Measurements are combined to obtain the error in the satellite range measurements caused by pulse delay due to water vapour, and to obtain the sigma naught correction for liquid water absorption.

Function

The TMR measures water vapour content in the atmosphere so that we can determine how it impacts radar signal propagation. Its measurements can also be used directly for studying other atmospheric phenomena, particularly rain.

Principle

The TMR is a passive receiver that collects radiation reflected by the oceans at frequencies of 18.7, 23.8 and 34 GHz.
Radiation measured by the radiometer depends on surface winds, ocean temperature, salinity, foam, absorption by water vapour and clouds, and various other factors. To determine atmospheric water vapour content accurately, we need to eliminate sea surface and cloud contributions from the signal received by the radiometer. This is why the TMR uses different frequencies, each of which is more sensitive than the others to one of these contributions. The main 23.8 GHz frequency is used to measure water vapour, the 34 GHz channel provides the correction for non-rainbearing clouds, and the 18.7 GHz channel is highly sensitive to wind-driven variations in the sea surface. By combining measurements acquired at each of these frequencies, we can extract the water vapour signal.

Further information on Instrument Description: Radiometer(s) (Nasa/JPL website)