The coldest possible brightness temperatures observed by a downward-looking
microwave radiometer from space are often produced by calm oceans under cl
oud-free skies and very low humidity. This set of conditions tends to occur
with sufficient regularity that an orbiting radiometer will accumulate a u
seful number of observations within a period of a few days to weeks. Histog
rams of the radiometer's coldest measurements provide an anchor point again
st which very small drifts in absolute calibration can be detected. This te
chnique is applied to the TOPEX microwave radiometer (TMR), and a statistic
ally significant drift of several tenths of a Kelvin per year is clearly de
tected in one of the channels. TMR housekeeping calibration data indicates
a likely cause for the drift, as small changes in the isolation of latching
ferrite circulators that are used in the onboard calibration-switch assemb
ly. This method cars easily be adapted to other microwave radiometers, espe
cially imagers operating at frequencies in the atmospheric windows, In addi
tion to detecting long-term instrument drifts with high precision, the meth
od also provides a means for cross-calibrating different instruments, The c
old reference provides a common tie point, even between sensors operating a
t different polarizations and/or incidence angles.