Sentinel-3A/B SLSTR Pre-Launch Calibration of the Thermal InfraRed Channels

Abstract

The first two models of the Sea and Land Surface Temperature Radiometers (SLSTR) for the European Copernicus Sentinel-3 missions were tested prior to launch at the Rutherford Appleton Laboratory space instrument calibration facility. The pre-launch tests provide an essential reference that ensures that the flight data of SLSTR are calibrated to the same standards needed for surface temperature measurements and to those used by shipborne radiometers for Fiducial Reference Measurement (FRM). The radiometric calibrations of the thermal infrared channels were validated against accurate and traceable reference BB sources under flight representative thermal vacuum environment. Measurements were performed in both earth views for source temperatures covering the main operating range, for different instrument configurations and for the full field-of-view of the instruments. The data were used to derive non-linearity curves to be used in the level-1 processing. All results showed that the measured brightness temperatures and radiometric noise agreed within the requirements for the mission. An inconsistency that particularly affected SLSTR-A was observed which has been attributed to an internal stray light error. A correction for the stray light has been proposed to reduce the error. The internal stray light error was reduced for SLSTR-B by replacing the coating on the main aperture stop. We present a description of the test methodology and the key results.

Benefits and Lessons Learned from the Sentinel-3 Tandem Phase

“…Results are generally found to be within the expected uncertainties, except for channels S8 and S9 in the oblique view, which show differences vs. temperature ( Figure 6). These results are consistent with the presence of an internal stray light effect observed during the SLSTR pre-launch calibration campaign [25]. The results of this tandem-phase analysis show the potential benefits of a stray-light correction.…”

Comparison of the Sentinel-3A and B SLSTR Tandem Phase Data Using Metrological Principles

“…The calibration of these channels is based on the observation of two internal warm blackbody calibration targets that are each viewed once per scan [2]. The data for these channels are provided as brightness temperatures, and come with the information required to evaluate independent (or random) and common (or systematic) components of uncertainty per pixel [10].…”

“…Such a result indicates there is an error effect in one or both of the instruments that should be corrected for, or an additional component should be added to the uncertainty budget. Such an effect has, in fact, been observed since the pre-flight calibration in both instruments, though primarily SLSTR-A, in channels S8 and S9 in the oblique view [10] and is thought to be the result of an internal stray light effect caused by a non-black coating of an optical stop within the instrument that was modified for SLSTR-B.…”

“…Here, we now propose to apply the correction originally defined in [10] to both SLSTR-A and B data, ∆L, as…”

“…The resultant temperature change for both sensors is shown as solid lines in Figure 6 for reference. The spectral response functions (SRFs) of SLSTR TIR channels have been shown to be temperature sensitive [10], and so the change in the radiometric response is likely to be linked to the change in instrument temperature. Temperature sensitivity as the source of step change is further supported by the fact that channel S9 is the most temperature sensitive channel and shows the largest step change.…”