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This study examines the trustworthiness of the Advanced Along-Track ScanningRadiometer (AATSR) and the Infrared Atmospheric Sounding Interferometer(IASI-A), as on-orbit reference instruments that are useful in re-calibratingthe Advanced Very High Resolution Radiometer (AVHRR) series (Mittaz andHarris, 2011). To do this, a 39-month period (1 January 2008 to31 March 2011) of AATSR and IASI-A inter-comparisons of top-of-atmosphere(TOA) radiance measurements is examined. Our inter-comparison revealsfeatures of the AATSR and IASI-A bias with respect to scan angle, scenetemperature, time and orbital maneuvers, and gives insight into theirtrustworthiness as an in-orbit reference instruments. </br>The first feature that our study reveals is that the AATSR (nadir view) andIASI-A are both stable (have no perceptible trends in the period ofstudy). The second feature is that IASI-A is perhaps more accurate( ∼  0.05 K) than its stated accuracy (0.5 K). In fact the AATSRand IASI-A bias is close to the AATSR pre-launch bias (plus a small offsetof +0.07 K) implying that IASI-A can get close to pre-launch levels ofaccuracy. Third, a very small scan angular dependence of AATSR and IASI-Abias indicates that the IASI-A response vs. scan angle algorithm is robust,while the instrument is in orbit. </br>Inter-comparisons of AATSR with IASI-A further reveal the impact of orbitalmaneuvers of the ENVISAT, the platform carrying AATSR, done in October 2011and not anticipated previously. Our study reveals that this maneuverintroduced a temperature-dependent bias in the AATSR measurements for lowtemperatures (< 240 K) in the period following this maneuver (Cocevar et al.,2011). Our study also shows that the known AATSR 12 µm channel offsetis in fact temperature dependent, grows up to 0.4 K, varies seasonally andis correlated with instrument temperature and cannot be corrected byshifting the spectral response function (SRF) of AATSR. </br>We also present a set of recommendations to help identify the parametersunder which these instruments can provide the most trustworthy observationsfor the AVHRR re-calibration

Comparisons of IASI-A and AATSR measurements of top-of-atmosphere radiance over an extended period