Obtaining information about long-term environmental and climate trends requires the analysis of decadal-scale time series of observations made by different sensors. To ensure that such comparisons are meaningful, it is essential to determine the radiometric differences between sensors and the uncertainties associated with those differences. This article describes the principles adopted within the FIDUCEO project for harmonising satellite data series to obtain long-term climate data records.
Most sensors are calibrated prelaunch, where calibration means establishing the basic model (measurement equation) for translating a measured signal (e.g. in counts) into the required measurand (e.g. radiance). However, this model may also make allowance for in-orbit factors; for example, it may account for gain changes of the instrument throughout the orbit due to variations in self emission by using parameters that estimate the gain from an in-orbit calibration process (e.g. measuring an internal calibration target). The calibration model therefore typically contains several parameters or corrections (calibration coefficients), some of which are determined pre-launch, others determined in-orbit.
For most of the satellite instruments that are considered in FIDUCEO there are potential problems with using pre-launch coefficients when analysing in-orbit measurements. The pre-launch testing generally had the aim of confirming that the instrument met its design specifications rather than that of determining the optimum set of calibration parameters. The FIDUCEO targets are long-standing historic sensor series. For such sensors, the sensor behaviour in-orbit can be very different from its behaviour during pre-launch testing and more scientific value can be derived from considering the series as a whole, for both the FCDR and the derived CDRs. http://www.fiduceo.eu/vocabulary.
Therefore, some level of adjustment to the initial calibration parameters is required to allow for in-orbit behaviour. Within FIDUCEO we define recalibration as obtaining new calibration coefficients and/or a new calibration model for the sensor from some external information. This may be done by comparing the output of one satellite to a more radiometrically accurate sensor using appropriate match-ups, such as simultaneous nadir overpasses (SNOs).
Recalibration goes beyond the common approach of bias correction, which has the same aim but performs the correction differently. Recalibration adjusts the calibration coefficients, leading to new measured values, whereas, in bias correction, an offset or factor is applied to the existing measured values. Bias correction is more common for an operational update of a sensor providing near real-time data, and is the approach adopted for the current GSICS Corrections. In FIDUCEO, we consider that recalibration is more appropriate and effective for reprocessing historical satellite missions to create improved FCDRs.
When we perform a comparison of two sensors using match-ups we must take into account the fact that those two sensors are not observing exactly the same thing. This is in part due to uncertainties in the collocation process itself, which must be allowed for as part of any sensor-to-sensor comparison. However, a more significant difference is due to differences in the spectral response functions (SRFs) of the two instruments, even when nominally observing the same ‘band’. In FIDUCEO, we do not aim to ‘correct for’ SRF differences by translating the measured values of the test sensor as though they were taken by the reference sensor (‘homogenisation’). Instead, we aim to reconcile the calibration of different sensors given their estimated SRF differences. After recalibration, the sensor series is then ‘harmonised’. We therefore have four different concepts, as summarised in Table 1.
Table 1 Recalibration and bias correction vocabulary
Within the FIDUCEO project our aim is to perform harmonisation. We will obtain new recalibrated L1 products from raw counts, such that the spectral characteristics of each instrument are preserved. The harmonisation process itself will involve refitting the calibration parameters (recalibration) using match-ups, taking into account all error covariances in both the instrument and the match-up process.
This article has also been submitted for publication in the GSICS quarterly newsletter. These terms are also defined in the FIDUCEO vocabulary http://www.fiduceo.eu/vocabulary, where we welcome comments and feedback.