Calibration of a length-modulated radiometer

MOPITT is a satellite instrument which will be launched in 1998 on the EOS-AM1 platform of the Earth Observing System. The primary objective of the MOPITT instrument is to enhance our knowledge of the lower atmosphere by measuring atmospheric profiles of carbon monoxide (CO) and methane. Operationally MOPITT will employ a new form of correlation radiometer known as the length modulated radiometer (LMR). To date, the LMR has been successfully implemented in a ground-based remote sounding instrument measuring CO, and is currently being implemented on two airplane-based instruments known as MATR and MOPITT-A. The operating principle of the LMR is the modulation of a static gas cell path length by means of an optically inert filler material. This paper will describe aspects of the operation of an LMR. Topics that will be covered include a discussion of the sources of optical imbalance in the LMR and the radiometric calibration of the LMR with CO. An analysis of the sources of error in the radiometric calibration of an LMR will also be presented.