An increased incidence of AIRS Level 1B CalFlag bit 1 ("telemetry out of limit condition") was observed across the majority of granules sampled since October 4, 2026. This behavior is expected (as explained below) and does not indicate a present degradation in AIRS radiometric data quality.
The telemetry limit thresholds that trigger this flag were established to monitor the instrument's operating environment under the nominal EOS Aqua 1:30 PM equator-crossing (ascending node) orbit. These static bounds were not designed to account for the orbital drift that Aqua has accumulated over its extended mission life. As the spacecraft's local solar crossing time continues to drift from the original 1:30 PM design reference, the resulting shift in the instrument's thermal and solar illumination environment can cause specific telemetry parameters to fall outside their original limit bounds, even though the instrument continues to operate nominally.
Critically, this condition is isolated to the telemetry limit-check bit itself. The independent gain-calculation and offset-calculation anomaly bits — which would indicate an actual breakdown in the radiometric calibration — remain unset throughout the affected granules. This confirms that the underlying calibration and the resulting calibrated radiances are presently unaffected.
Accordingly, the AIRS Level 1B data quality continues to meet the needs of meteorological applications, including the assimilation into operational numerical weather prediction systems. Users are advised that CalFlag bit 1 ("telemetry out of limit condition") can be presently disregarded for data quality screening.