During routine operations (phase E2), the Sentinel-1 SAR Mission Performance Cluster (SAR MPC), a team of experts contracted by ESA, is responsible for the end-to-end instrument and S-1 product performances (https://sar-mpc.eu/).
This includes the daily quality control of the products, the calibration and validation activities, the maintenance and evolution of S-1 Level-1 and Level-2 algorithms, and the reporting on product performance and anomalies.
For the porpoises of calibration, the Sentinel-1 SAR instrument also operates in RF Characterisation mode, Azimuth Notch mode and Elevation Notch mode. While notch modes are typically acquired during commissioning phase, RF data are acquired routinely for antenna characterization. These dedicated products are only used internally and not distributed to the general public.
Calibration is the process of quantitatively defining the system response to known controlled signal inputs. Calibration tasks are executed throughout the mission to ensure the normalised radar cross-section and phase of imaged scene are provided with stability and accuracy. Calibration of the entire Sentinel-1 system is critical to guaranteeing product quality for operational demands. The SAR system must perform within an absolute radiometric stability of 1 dB (3σ) in all operation modes. This is a more stringent radiometric stability than previous ESA SAR missions.
Calibration can be divided into two forms:
Internal calibration provides an assessment of radar performance using internally generated calibrated signal sources.
Internal calibration uses calibration signals which are routed as closely as possible along the nominal signal path. The calibration signals experience the same gain and phase variations as the nominal measurement signals. The ground processing then evaluates the calibration signals to identify gain and phase changes and correct the acquired images accordingly.
Transmit power, receiver gain and antenna gain are subject to instrument noise due to temperature changes or other effects over time. Internal calibration provides corrections for changes in the transmit power and the electronics gain as well as validating the antenna model. The resulting calibration data are used in ground processing to correct image data.
Internal calibration also covers the signal phase. The overall phase of the echo signal depends on two major elements: measurement geometry and instrument internal phase stability. As the hardware cannot generally provide the required phase stability, it is a task of the internal calibration scheme to cover the internal phase variations by adequate measurements. All internal calibration measurements, either for gain or for phase, are used in ground processing to correct data products and achieve the required stability.
Internal calibration uses a Pulse-Coded Calibration (PCC) technique to embed a unique pulse code on a signal such that it can be identified and measured when embedded in other signals. This allows the amplitude and phase of individual signal paths to be measured while operating the complete antenna. The PCC technique is implemented by sending a series of coherent calibration pulses in parallel through the desired signal paths. The individual successive signals are multiplied by factors of +1 or -1. Factor -1 is implemented by adding a phase shift of 180°, while factor +1 means no additional phase. Each path is identified by a unique sequence.
The PCC technique can be applied if:
The receiver detects the signals coherently.
The whole sequence is executed in a sufficiently short time such that the parameters to be measured are stationary.
The system is linear with respect to the individual signals.
The PCC technique can measure the signal paths via individual Transmit (TX) /Receive (RX) Modules (TRMs) or via groups of TRMs (either TX or RX paths, either polarisation).
The average properties of rows or columns of TRMs can be measured by a short PCC sequence. The length of a PCC sequence is always a power of two. There are 20 rows of waveguides, therefore the PCC sequence has a minimum of 32 pulses. Although the 14 columns (14 tiles) could be measured by a PCC sequence of 16 pulses, it is assumed that a sequence length of 32 pulses is also used. All 20 rows are operated together, meaning the antenna is in a full operational state. The overall signal from all rows is received, digitized and packed into calibration packets. These packets are evaluated (on the ground) to determine the properties of the individual rows. The approach for measuring the average azimuth excitation coefficient is similar to the elevation pattern, using columns of TRMs instead of rows.
The PCC-32 measurements described above need approximately 129 pulses. Additional warm-up pulses may also be needed. Such a large number of calibration pulses represent a significant interruption in image generation when operated within the image acquisition of the Stripmap mode. For intermediate calibration pulses in Stripmap mode, and also for calibration pulses related to each sub-swath measurement in the TopSAR modes, a shorter sequence is needed. The Sentinel-1 acquisition timelines foresee slots of 20 calibration pulses block wise interleaved within the imaging operation (before each burst for TopSAR modes). In order to derive the combined calibration signals 5 or more of these slots shall be processed together. Moreover, due to the high stability of measured internal calibration parameters, since May 2015 it has been decided to avoid interleaved calibration sequences in stripmap data, demanding them only to the pre and post-amble acquisition sequences.
For the antenna model, the reference patterns of all beams are derived for radiometric correction of the SAR data. The active antenna of the SAR instrument allows a multitude of different antenna beams with their associated gain patterns. All these patterns are described by the mathematical antenna model which provides the antenna patterns as functions of the commanded amplitudes and phases within the front end EFEs and within the tile amplifiers. The quality of the patterns is ensured by the on-board temperature compensation controlled by the tile control units. The internal calibration signals measure the actual phases and amplitudes and allow verifying the correct function and performance of all included elements. The antenna model is established on-ground, based on pattern tests at various integration levels up to the complete antenna.
RF Characterisation Mode
The RF characterisation mode is a self-standing mode and is not associated with the individual imaging data-takes. It is operated at least once per day during a convenient point within the long duration of wave mode.
The RF characterisation mode verifies in-flight the correct function and characteristics of the individual TRMs. Operating it two or more times at different temperatures during the cool-down phases between the high dissipating imaging modes can provide in-orbit characterisation versus temperature where necessary. The RF characterisation mode performs measurements with internal signals and is designed to achieve a number of goals. The RF calibration mode will:
Cover all those measurements needed in-orbit but which are not required for each individual data-take.
Provide data sets to assess the instrument health and performance as far as possible.
Verify the correct function of the individual TRMs, both within the front-end and the tile amplifiers.
Verify the excitation coefficients for the TX and RX patterns to ensure the validity of the antenna model.
This mode is based on the same measurement types as the internal calibration. The mode has to address the individual TRMs while operating the full antenna in representative thermal conditions and with nominal power consumption. This can be achieved using the PCC technique. As a standalone mode, it is not forced to use the signal parameters of a dedicated imaging mode, but instead an optimised set of parameters can be used. The calibration mode is to be operated for both TX polarisations. The receiver will measure both polarisations in any case.
External calibration makes use of reference targets to ensure an end-to-end calibration of the SAR system. Typically the process involves the use of well characterized corner reflectors and transponders for absolute radiometric calibration and geometric calibration. Additionally, external calibration can also be done using natural targets with a certain expected behavior to support, e.g., relative radiometric calibration and tuning of the elevation antenna pattern, typically done using rain forest areas, or to support calibration of noise measurements, typically done over calm see.
According to the Working Group on Calibration and Validation (WGCV) of the international Committee on Earth Observation Satellites (CEOS), validation is the process of assessing, by independent means, the quality of the data products derived from the system outputs.
Level-1 Product Verification and Validation
The verification, validation and the routine performance monitoring of L1A (SLC) and L1B (GRD) products consists in assessing geometric, radiometric and polarimetric parameters. It includes the verification of Impulse Response Function (IRF) parameters over strong point-like targets (like active transponders and trihedral corner reflectors) and comparison with the expected values, but also as assessment of the dynamic range over different point/distributed targets and the verification of the geolocation accuracy. The verification of the absolute radiometry is mostly performed exploiting active transponders, but also large corner reflectors can be used. The absolute geometric accuracy of the products is verified comparing the location of known point-like targets as measured form the SAR products with the values obtained measuring the accurate location of the target backscattering phase center by means of geodetic techniques.
Level-2 Product Verification
Level-2 product verification consists of verifying the main quality of the engineering parameters prior to the inversion steps. Ocean Swell Spectra (OSW) verification consists mainly of measuring the performance parameters related to the input imagette or the cross-spectra. Ocean Wind Field (OWI) verification consists of verifying the calibration constant obtained from the transponders against a geophysical calibration constant and deriving the noise equivalent radar cross-section (NESZ). Radial Velocity (RVL) verification is mainly to verify the accuracy of the Doppler anomaly estimation.
Level-2 Geophysical Verification
The geophysical validation of Sentinel-1 Level-2 products consists of comparing retrieved Level-2 products using a geophysical model function from Sentinel-1 measurements with independent 'equivalent' measurements. This allows assessment of the information content and the quality of the Sentinel-1 Level-2 products.
The geophysical validation of the OSW component consists of characterising the performance of wave spectral parameters derived from the Sentinel-1 SAR with other independent sources (buoys), followed by an estimation of RMS error and bias. The comparison is made possible through the collocation of the buoys with SAR.
OWI validation assesses the radiometric calibration performance for all combinations of mode, swath and polarisation. It also characterises wind retrieval performance as a function of the mode, incidence angle (swath), polarisation and wind conditions:
Geophysical validation is undertaken globally against collocated buoys.
Inter-comparison of wind measurements from the different Sentinel-1 modes.
Inter-comparison with global winds retrieved from scatterometers (e.g. ASCAT).
The RVL component is less mature than OSW and OWI. The experiment performed on ASAR has demonstrated that the operational current extraction requires stringent performance on the accuracy of the Doppler shift anomaly estimation. It has been demonstrated that several biases or trends were present in the ASAR Doppler estimation impeding a straightforward RVL estimation without performing heavy Doppler calibration steps for fixing the biases.
Copernicus Sentinel-2 Calibration and Validation
Radiometric Calibration Activities
The radiometric calibration activities allow determination of the parameters of the radiometric calibration model, which aims to convert the electrical signal measured by the instrument, transformed in digital count, into physical radiance measured at the sensor.
Radiometric calibration activities are the set of methods used to estimate the parameters of the Sentinel-2 radiometric model.
Nominal radiometric activities are based on the exploitation of the on-board sun diffuser images (for absolute radiometric and relative gain calibration) and images acquired over ocean by night (for dark signal calibration). The on-board sun diffuser is a full field/full pupil diffuser, called the Calibration and Shutter Mechanism (CSM), which is integrated with Sentinel-2 instruments for radiometric calibration to guarantee high quality radiometric performance. This On-Board Calibration Device (OBCD) collects the sunlight after reflection by the diffuser to prevent the instrument from viewing the sun directly in orbit and from contamination during launch.
As there is no secondary sun-diffuser on-board, the use of the sun-diffuser is optimised to reduce exposure to sun irradiance as much as possible and its consequent degradation. The stability of the diffuser is monitored in the context of the vicarious and cross-mission validation activities.
Geometric Calibration Activities
The geometric calibration activities allow to ensure the maintenance of the better geometry possible of the images, according to the user requirements. It aims at determining all ground image processing parameters (GIPP) involved in the MSI geometric model.
The parameters of the geometric model are:
orientation of the viewing frames
lines of sight of the detectors of the different focal planes.
These parameters have been estimated before launch. The purpose of the geometric calibration activities is to take into account any updates of these parameters values that may occur during the satellite lifetime, and to remove the effects of navigation errors and surface topography on the ortho-rectified product during standard processing.
In order to meet the multi-temporal registration and the absolute geolocation requirements, a Global Reference Image (GRI) is used for the automatic extraction of tie-points for the systematic refinement of the geometric model.
Validation of Level-1 products includes both radiometric and geometric validation activities. These activities occur once the radiometric and the geometric calibration of the MSI instrument and the data processing algorithms have been performed.
The radiometric calibration and image quality monitoring approach rely heavily on the on-board Sun diffuser. As MSI has no secondary or back-up diffuser, it is therefore mandatory to obtain an independent validation of the radiometry in order to:
Identify any radiometric bias introduced by uncertainties in the calibration method; this concerns absolute radiometry as well as relative radiometry (in the field, inter-band, and inter-satellite).
Monitor the long-term radiometry to prevent any impact of the ageing of the diffuser.
Serve as a back-up calibration source in case of failure of the diffuser.
A large number of independent and complementary methods are combined to ensure a robust validation approach:
The Pseudo-Invariant Calibration Site (PICS) method: based on a long history of MERIS measurements and has been successfully used for a number of sensors, including MSI-A and B, and OLCI-A and B. (see Bouvet et al, 2014)
The Rayleigh method over oceanic sites allows an independent validation for lower radiance levels. It is however restricted to shorter wavelengths.
The DCC method can be used for relative radiometric assessments. Because DCC are high-altitude targets, they are less dependent on an estimation of the radiative transfer through the atmosphere. The large number of measurements available is another advantage of the method.
In-situ measurements over the Railroad Valley vicarious calibration site (U. of Arizona) are provided regularly to ESA through a collaboration agreement with NASA. The OPT-MPC continue to monitor these results.
Inter-sensor comparisons offer an important complement and directly address interoperability issues.
Moreover, some activity includes image quality monitoring and validation activities:
Signal to Noise Ratio (SNR) and Fixed Pattern Noise (FPN) short-term performance monitoring using diffuser acquisitions
SNR and FPM independent validation using ground images for long-term performance assessment
The geometric validation activities consist in:
Absolute geolocation performance validation: evaluate absolute geolocation performance against a set of Ground Control Points using L1C images.
Relative Geolocation performance validation: evaluate geolocation performance relative to GRI and trigger a geometric calibration if necessary
Multi-temporal registration uncertainty validation: evaluation of L1C multi-temporal performance over random and selected sites using image correlation.
Multi-spectral registration uncertainty validation: evaluation of the multi-spectral registration uncertainty for refined and unrefined products.
Concerning Level-2A products, the outputs of the L2A processor (i.e. Sen2Cor) can be classified in two main domains:
Radiometry (RAD), which concerns the Atmospheric Correction outputs of Sen2Cor: Surface reflectance images, Aerosol Optical Thickness (AOT) and Water Vapour (WV) maps.
Cloud Screening and Classification (CSC) outputs of Sen2Cor: Scene classification (SCL) map which assigns to each pixel a class (vegetation, soil, water, clouds, thin cirrus, snow, etc…), Cloud probabilistic mask (CLDPRB) and Snow probabilistic (SNWPRB) mask.
Moreover, a proper characterization and update (if necessary) of the auxiliary data used in the L2A processing is also performed. These auxiliary data are: Digital Elevation Model, Meteorological data (CAMS & ECMWF), ESA CCI Land Cover layers, MSI Spectral responses, Solar Spectrum, Sentinel- 2 viewing angles and solar illumination angles.
The validation activities of the Level-2A products consist in:
Validation of the Aerosol Optical Thickness (AOT) (550 nm) and Water Vapour products processed with Sen2Cor with AERONET measurements as reference,
Quantitative validation of Sen2cor surface reflectance outputs with reference measurements,
Validation of the surface reflectance product using intercomparisons and computed references,
Cloud Screening and Classification performance assessment.
Copernicus Sentinel-3 calibration and validation (cal/val) activities are essential to the quality of the mission. Data quality will be assessed through determination of the radiometric, spatial, spectral and geometric fidelity of the satellite sensor and the accuracy of geophysical products. The calibration and validation of the Sentinel-3 instruments took place in three phases:
Pre-launch phase instrument characterisation and on-ground calibration.
A commissioning phase (E1) that lasted 6 months for Sentinel-3A, where all instrument operation aspects have been verified and in-orbit calibration and validation activities have started. For Sentinel-3B, this phase was reduced to 3 months but a tandem phase with Sentinel-3A of ca. 3 months allowed to perform cross-comparisons between the two units. After a successful IOCR (In-Orbit Commissioning Review), the agencies gave the authorization for the data dissemination to end-users.
An exploitation phase (E2) has then started after the IOCR with the routine implementation of the calibration and validation activities for geophysical data products.
commissioning phase Level-2 algorithm verification for all Level-2 'baseline' products
Level-2 algorithm validation starting during the commissioning phase and continued throughout phase E2
quantification of Level-1 and Level-2 product error estimates
long term monitoring for consistency and constant quality of geophysical products
advice on re-processing campaigns.
In addition, the cal/val component of the Sentinel-3 mission (now included in the Optical Mission Performance Cluster) includes maintenance and evolution of Level-2 processors, the generation of all auxiliary data sets needed for Level-1 and Level-2 processing. Sentinel-3 cal/val plan is reviewed and updated on a regular basis as required by the mission.
the Land Validation Report for the release of Sentinel-3 Sea and Land Surface Temperature Radiometer (SLSTR) Level-2 Land Surface Temperature products (SL_2_LST) is available. The report describes the validation of SL_2_LST against in situ observations (Category-A validation), and intercomparison (Category-C validation) of the SL_2_LST product with respect to three independent reference products from the ESA DUE GlobTemperature Project (MODIS, GOES, and SEVIRI). The results of the validation (Category-A) against in situ observations from "Gold Standard" stations show the SL_2_LST product to have an accuracy for all matchups of 0.94 K, thus meeting the overall mission requirement (S3-MR-420) of < 1 K. Intercomparison (Category-C) with respect to other reference products show differences are around 1 K overall.
In-flight calibration of OLCI is a fundamental component of the instrument design.
All OLCI measurements are made via a calibration assembly of a similar design to MERIS that includes a mechanical rotating table. Either a direct view of the Earth (for imaging mode) or one of several calibration targets may be selected by rotating the table: a dark shutter plate (for dark current calibration), a primary Polytetrafluoroethylen (PTFE) calibration diffuser (viewed every 2 weeks for radiometric calibration), a redundant PTFE calibration diffuser (viewed every 3 months to determine degradation of the primary diffuser due to solar exposure) or an erbium doped 'pink' diffuser plate for spectral calibration. During the calibration sequence, a selected diffuser plate is moved into the instrument Field of View (FOV) and illuminated by the sun so that all five cameras can be calibrated at the same time. Characterisation of diffuser ageing is determined through on-ground processing using the two OLCI diffusers in synergy.
The OLCI calibration sequence is carried out before the terminator crossing over the southern hemisphere to maintain a stable internal instrument temperature in a similar manner to that of MERIS. Two successive orbits are required; the first for radiometric calibration and the second for spectral calibration. Each calibration sequence begins with a dark current evaluation. This sequence lasts 45 s and acquires 1 024 measurement frames that are averaged on-ground to reduce noise and used to accurately derive the signal produced under dark conditions.
Figure 1: The OLCI Calibration Mechanism (credit TAS-France)
L1 and L2 Products Validation
Validation of Level-1 products includes both radiometric and geometric validation activities. These activities are performed routinely by the Optical Mission Performance Cluster, and more particularly after a new radiometric and/or geometric calibration of the OLCI instrument, after any change of the processing baseline.
Validation of Level-2 products consists in comparing the OLCI products with in-situ measurements (ideally with Fiducial Reference Measurements) or, or, where these do not exist, by comparing the products with other satellite missions (for example, MODIS).
On-board calibration of the SLSTR is performed continuously, as described below.
For the thermal infra-red channels, provision for their continuous on-board calibration is an integral part of the instrument design. The instrument includes two stable and high precision black-body targets, which are intersected by both the nadir and along track scan cones. Each target is observed by all detectors once per cycle for each scanner. One black body is close to the optics temperature of ~260K and the other is heated to ~302K, so that the interval between the two black body temperatures covers the expected range of sea surface brightness temperatures. The physical temperatures of the two black bodies are continuously monitored, and so for each scan the linear relationship between the infra-red radiance and the measured signal counts can be determined from the measurements of the black body signals.
The use of two black bodies in this way ensures that the calibration is optimised over the normal range of SST, and the effects of detector non-linearity over this range are minimised, maximising the accuracy of SST measurement. The same two-point calibration approach is used for the fire channels, on the basis of the same range of SST as that used for the other TIR channels.
The calibration scheme for the short-wave, near infra-red, and visible channels is based on a diffuser based calibration VISCAL system of accurately known reflectance which is illuminated by the sun over a short segment of the orbit, and which is intersected by the instrument scans. Using the signals from the VISCAL when it is illuminated by the Sun, provides a calibration reference to convert the measured signal in each channel to the surface reflectance.
Corrections for any detector non-linearity are applied using look-up tables derived from the pre-launch characterisation of the instrument.
Figure 2: SLSTR Visible Calibitration Unit (credit Selex Galileo/TNO)
L1 and L2 Products Validation
Validation of Level-1 products includes both radiometric and geometric validation activities. These activities are performed routinely by the Optical Mission Performance Cluster, and more particularly after a new radiometric and/or geometric calibration of the SLSTR instrument, after a decontamination or after any change of the processing baseline.
Validation of Level-2 products consists in comparing the SLSTR products with in-situ measurements (ideally with Fiducial Reference Measurements) or, or, where these do not exist, by comparing the products with other satellite missions (for example, MODIS).
SYNERGY is not an instrument but a process combining the OLCI L1b and SLSTR L1b products.
There are three main objectives for calibration and validation (cal/val) activities.
To provide a comprehensive initial assessment of product validity and quality at the end of commissioning activities.
To monitor the stability and the quality of the products throughout the operational phase of the mission.
To continuously improve the quality of the products throughout the operational phase of the mission following the evolving user requirements.
The cal/val plan defines the scope of activities that calibrate the on-board instruments and validate the data products generated operationally and disseminated by the PDGS centres.
The calibration activities include the delivery of Cyclic Monitoring Reports summarising the assessment of the SRAL altimeter instrument. The performance of the instrument is assessed by monitoring the instrument calibration modes CAL1, CAL2 and AUTOCAL, as well as its temperatures on-board. Each report covers a cyclic period of 27 days.
One of the major goals of radar altimetry is to provide stable and reliable geophysical estimations of the Earth’s topography over the time. To guarantee this stability, the behaviour of the Sentinel-3 SRAL and the MWR instruments is routinely assessed over the Sentinel-3 mission lifetime, through internal calibrations programmed every satellite cycle. The calibration sequences of SRAL are mostly performed over desert regions such as Sahara, Australia, or south of Africa, to avoid disruption of the key Earth observation goals of the Sentinel-3 mission. The Figure 3 below shows the locations of the SRAL main calibration sequences:
Figure 3: Location of the Sentinel-3 CAL1 and CAL2 calibration sequences (in red) [Credits: IsardSAT]
During calibration mode, the altimeter loops the transmit signal back through the instrument to characterise the instrument’s electronics impacts. The data acquired in calibration mode are transmitted on-ground.
For the SRAL instrument, two main calibration signals are calculated by the Sentinel-3 Instrument Processing Facilities (IPF):
The CAL1 signal, corresponding to the instrument Pulse Target Response (PTR). Several calibration parameters are derived from the CAL1 signal, in particular the time delay induced by altimeter’s electronics (i.e. the internal path delay), the CAL1 power and the CAL1 main lobe width.
The CAL2 signal, or Gain Profile Range Window (GPRW), providing information of the altimetry signal distortion due to several instrumental effects (e.g. intermediate frequency filters gain response).
In addition, “AutoCal” calibrations are also made to test the performances of the two on-board attenuators, which are used together to optimize the power dynamic range of the waveform.
The calibration processing performed on-ground produces several calibration parameters, used as inputs of the level-1 and level-2 processing, either for a direct calibration of the radar waveforms, or for correcting the geophysical estimations derived from the altimetry waveforms. The Sentinel-3 MPC team accurately monitors the evolution of these parameters over the time. Besides, additional instrumental parameters are also monitored, such as the on-board thermal behaviour, the power and phase evolution within a burst, the Ultra Stable Oscillator (USO), additional parameters related to the shape of the CAL1 signal etc.
The CAL1 time delay is the main range instrumental correction impacting the determination of the Sea Surface Height. The CAL2 waveforms are used to compensate the effect of the system transfer function distortions on the science echoes. The figures below display CAL1 time delay for Ku-Band and CAL2 waveforms over the last period and since June 2016 respectively.
Figure 5: CAL1 SAR Ku Time Delay Whole Mission Trend
Figure 7: CAL1 SAR Ku PTR Width Whole Mission Trend
Figure 9: Averaged CAL2 Ku and C waveforms over the last period
For the MWR instrument, in-flight internal calibrations are performed using a dedicated sky horn signal.
Two types of calibrations are required for the on-ground processing. First the noise diode shall be calibrated to be used in the main processing mode, the so-called Noise Injection Radiometer (NIR) mode. Second, the system gain shall be calibrated to be used in the non-balanced mode, the so-called Dicke Non-Balanced (DNB) mode.
In addition, the Sentinel-3 MPC also performs vicarious calibrations over natural targets (ocean, forests…) to monitor the MWR brightness temperatures.
The main SRAL validation tasks developed during the SENTINEL-3 commissioning and routine phases are:
in-commissioning phase Level-2 algorithm verification for all Level-2 'baseline' products (tuning of all relevant processing parameters, regeneration of all Level-2 auxiliary products)
Level-2 algorithm validation starting during the commissioning phase and continuing into phase E2
quantifying error estimates for Level-1 and Level-2 products
validation against in situ measurements
validation against other altimetry missions
wind, wave product validation against models
sea-ice freeboard validation with CRYOSAT
internal calibration sequence validation
tracking mode validation
long term monitoring for consistency and quality of geophysical products
advising on re-processing campaigns.
Copernicus Sentinel-5P Calibration and Validation
The algorithms used for processing from Level 0 to Level 1B require Calibration Key Data (CKD) parameters, which are obtained through different calibration measurements and activities (see L1 ATBD). Some of the activities are systematically performed on the Ground Segment and some are triggered when the need for calibration emerges.
A validation plan specific for the S5P mission constitutes the baseline description for the Sentinel-5 Precursor validation activities and, based on this, validation reports are generated and published on the Mission Performance Centre http://s5p-mpc-vdaf.aeronomie.be/index.php/home website. The plan is based on the following approach:
Figure 12: A validation plan
The https://www.tropomi.eu/ is responsible for the quality control, calibration, validation and end-to-end system performance monitoring of the S5P mission during the routine phase. MPC validation activities rely primarily on the usage of Fiducial Reference Measurements (FRMs), which are a suite of independent, fully characterized, and traceable ground measurements that follow the guidelines outlined by the GEO/CEOS Quality Assurance framework for Earth Observation (http://qa4eo.org/ ). FRMs will be progressively being available on the http://evdc.esa.int/ . In addition, within the framework of its Copernicus missions, ESA opened a call for the https://earth.esa.int/web/guest/pi-community/apply-for-data/ao-s (S5PVT) in 2014, that aimed to engage leading expertise for the Calibration and Validation of the S5P in the mission validation team, providing independent experimental data, analysis and recommendations to critically assess the end-to-end performance of the instrument and its products. This call is permanently open.
All the information is further processed in the Quality Working Group, which provide synthetic results to the Mission Manager for improving the quality and knowledge of the products.
The workshop held on 28 September 2018 via teleconference aimed at presenting Calibration/Validation (Cal/Val) results for a subset of S5p Level 2 products (Total Columns of Ozone – OFFL, Tropospheric Ozone - OFFL, Formaldehyde and Sulphur Dioxide – NRT & OFFL).
These results are used to assess the uncertainty of the target S5p products, and gather recommendations about their release to the public.
These results are used to assess the uncertainty of the target S5p products, and gather recommendations about their release to the public.