Abstract
Abstract. We analyzed intrusions of the Moon in the deep space view of the Advanced Microwave Sounding Unit-B on the NOAA-16 satellite and found no significant discrepancies in the signals from the different sounding channels between 2001 and 2008. However, earlier investigations had detected biases of up to 10 K, by using simultaneous nadir overpasses of NOAA-16 with other satellites. These discrepancies in the observations of Earth scenes cannot be due to non-linearity of the receiver or contamination of the deep space view without affecting the signal from the Moon as well. As neither major anomalies of the on-board calibration target nor the local oscillator were present, we consider radio frequency interference in combination with a strongly decreasing gain the most obvious reason for the degrading photometric stability. By means of the chosen example we demonstrate the usefulness of the Moon for investigations of the performance of microwave sounders in flight.
Highlights
Photometric stability of the measurement devices is an indispensable prerequisite for a reliable characterization of global change in atmospheric properties
In this work we investigate why the flux calibration of the different channels seemed to diverge with time by using the radiation from the Moon when entering the deep space view (DSV) as a third reference flux, in addition to the cosmic microwave background (CMB) and the on-board calibration target (OBCT)
We find a difference of −0.113◦ ± 0.019◦, i.e. the DSV direction determined with the Gauss fit leads the one calculated with AAPP by about 0.1◦ in the scan direction
Summary
Photometric stability of the measurement devices is an indispensable prerequisite for a reliable characterization of global change in atmospheric properties. This basic rule is valid for space-based instruments, because they cannot be checked in the laboratory again once the operational phase has begun. In order to first characterize and reduce these errors in the case of AMSU-A (Advanced Microwave Sounding Unit-A), Zou and Wang (2011) determined time-dependent calibration offsets and nonlinear coefficients from simultaneous nadir overpass (SNO) regressions, which resulted in more consistent multi-satellite radiance observations for all respective channels. SNOs and other inter-calibration methods that rely exclusively on the comparison of two space instruments without a third source of information about the Earth scenes, can, as a matter of principle, never remove all spurious trends in the data, because they cannot identify the relative contributions of either instrument to offsets and drifts
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