AltiKa Radiometer: Instrument Description and In–Flight Performance

SARAL/AltiKa Absolute Calibration from the Multi-Mission Corsica Facilities

“…This land contamination for the Envisat and SARAL/AltiKa descending pass (#130) overflying the Ajaccio site is illustrated in Figure 7 can reach up to 20 mm due to the large drop that occurs during the Sardinia overflight because no algorithm for decontamination was introduced in the current product; this will be corrected in the next product generation (Picard et al 2014;Steunou et al 2014a). Due to the remaining land contamination in the radiometer, the wet tropospheric correction derived from the ECMWF model, available in the altimeter products, was used to compute the SSH bias in our study (see "Altimeter SSH bias & corrections" section).…”

SARAL/AltiKa Wet Tropospheric Correction: In-Flight Calibration, Retrieval Strategies and Performances

“…Biases up to a few Kelvin ( §5K) between measured and simulated TB have not impact on the actual performances of the radiometer and are even expected due to the limitation in the radiative transfer model, in the meteorological analyses and to the choices made for the on-ground calibration of the radiometer as well (Obligis et al 2006). The standard deviation on the difference between measurements and simulations is mainly the signature of the temporal and spatial inconsistency between the analyses and the measured TB (the instrumental sensitivity being close to C0.15 K; see Steunou et al 2015). Finally, the slope of the linear fit on the scatterplot should be close to C1.0.…”

“…For the cloud liquid water content (CLWC) channel, at 37 GHz for AL-MWR, 34 GHz for AMR, and 31.4 GHz for AMSU-A, the analysis is less straightforward as the three instruments have different frequencies. Moreover the brightness temperatures of this channel for AL-MWR were impacted by a saturation of the hot calibration counts until 22 October 2013 when an on-board database update fixed this issue (see Steunou et al 2015). Corrective solutions are currently under investigation, but it is difficult at the moment to precisely give a starting date from which the saturation has a measurable impact on the TB, even if the impact is obvious on the hottest TB monitoring (see dedicated section below) from the beginning of August (AltiKa cycle 5).…”

“…The impact of the saturation of hot calibration counts on AL-MWR 37 GHz channel is clearly seen on the vicarious calibration metrics with an expected larger impact on hottest TB than on coldest TB (see Steunou et al 2015). The assessment of the 37 GHz channel would not be straightforward even if the saturation was corrected, due to a combination of the different central frequencies used by the different radiometers, their different calibration strategies and the impact of clouds on comparison to simulated TB.…”

“…The microwave radiometer on SARAL/AltiKa mission (AL-MWR) performs measurements of brightness temperatures in both bands at the location of the altimeter footprint. First results at instrumental level exhibit its very good thermal stability, its very fine sensitivity and its sharp spatial resolution, making of AltiKa radiometer one of the best in-flight radiometers (Steunou et al 2015).…”