This page provides an overview of calibration and validation activities, related documentation, and current validation needs, including updates from the Cal/Val team, such as product calibration and validation reports and links to relevant validation tools.

Instrument Calibration

Calibration activities ensure that the EarthCARE instruments are performing well and to their established baselines.

ATLID Calibration

The in-flight calibration of the Atmospheric Lidar (ATLID) on EarthCARE enhances the most important calibration parameters and corrects instrument drift.
 

Radiometric Calibration

The radiometric calibration is carried out during the normal measurement flow. The calibration parameters are extracted by proper selection of echoes and post processing.

• Earth background light is estimated before and after each echo, which allows an accurate offset subtraction on each echo
• The spectral and polarisation cross-talks are continuously monitored by applying dedicated processing on the stratospheric backscatter (assumed to contain pure Rayleigh backscatter) and cloud/ground echoes
• The lidar constants of each channel are calibrated using the atmosphere backscatter (stratosphere and ground echoes) in conjunction with a backscatter prediction model and selected test site with known ground albedo and atmosphere optical depth
• Provision is made for regular detection of dark signal calibration for potential compensation of dark signal nonuniformity, even if the low operating temperature of the detector (-30°C) makes this offset theoretically negligible
   

Spectral Calibration

ATLID was able to perform spectral calibration in-flight because the laser could be tuned around the emitted wavelength. The frequency calibration approach for ATLID consisted of a laser frequency scan around the etalon central frequency. After launch, a coarse spectral co-registration sequence was necessary to tune the laser frequency around the receiver's peak frequency. The sequence consisted of sweeping the laser frequency over its tuneable range (-12 / +12 GHz), and continuously recording the atmospheric echoes. The atmospheric echo was acquired with the same operational sequence as nominal measurements. Ground or strong cloud echoes were extracted from measurements to derive the maximum receiver response where the laser frequency matched the receiver filtering response.

Additionally, fine spectral calibration of the laser is regularly performed to compensate for laser frequency drifts and other detuning contributors (filters, pointing, etc.).
 

Spatial Calibration

The spatial calibration of ATLID is performed in two ways:

1. Aiming to always keep the laser beam co-aligned to the receiver's field of view. As the instrument is based on a bistatic configuration, the receiver and emitter have separate line of sights. This spatial alignment capability is compensating for potential in-orbit misalignment due to thermo-elastic instabilities during orbit and also for gravity release, units and telescope setting, and moisture release that will occur after launch
2. The second spatial calibration involves correction of the focusing/divergence of the laser spot to ensure the backscatter signal from the atmosphere is fully collected within the receiver's field of view. This calibration is performed by thermal tuning of the Emission Beam Expander, adjusting the emitted beam focussing and acquiring the laser return spot on the Co-Alignment Sensor

BBR Calibration

Long Wave (LW) Calibration: Gain and offsets in the measured voltage signals are removed using hot and cold blackbodies that are autonomously rotated into the field of view every 88 seconds. This calibration frequency is decided by the temperature stability of the Chopper Drum skin and can be modified, if necessary, on orbit. Every six months in orbit the blackbody temperatures are swapped, allowing a check of instrument linearity by recording data over a range of radiances.

Short Wave (SW) Calibration: LW gain is transferable to the SW, with knowledge of the filter spectral response that is measured on ground. SW gain is monitored in-flight via measurements made with the Visible Calibration (VisCal) system, to detect changes in instrument response due to, for example, aging. The wall of each telescope baffle incorporates a set of three Monitor Photo Diodes, in order to monitor in turn aging of the VisCal optical path.

The solar calibration is performed over approximately 30 orbits every two months and occurs in conjunction with the Multi Spectral Imager (MSI) solar calibration, in order to maximise mission availability.

The Point Spread Function (PSF) of the pixels in the microbolometer arrays are also characterised on-ground, in order to formulate the algorithm for summing pixels to form the 10 km synthesised scene-PSF.

CPR Calibration

The Cloud Profiling Radar (CPR) on EarthCARE is a millimetre-wave radar (radar frequency is 94 GHz). It has a large antenna reflector to achieve much higher sensitivity to cloud particles than general meteorological weather radar. This means it can detect most of the radiatively important clouds in the global region.

The minimum radar reflectivity of the EarthCARE CPR is -35 dBZ at the top of the atmosphere (20 km) on the condition of 10 km horizontal integration. The observation range of Doppler velocity is ±10 m/s, and the accuracy is 1 m/s for cloud echoes of more than -19 dBZ when they are integrated over a horizontal distance of 10 km.

To maintain and ensure CPR performance, several types of calibration data is obtained by CPR. The CPR is designed to obtain transmit power by the power monitor diode in the Quasi Optical Feeder. The receiver performance Noise Figure is obtained by referring to the noise sources’ signal.

The linearity and bias of the logarithm amplifier (Log Amp) are obtained during the internal calibration mode by using the internal Intermediate Frequency signal source, the step attenuator, and the data acquisition with terminated Log Amp.

The overall performance of CPR is checked by the Active Radar Calibrator (ARC) equipped on the ground (CPR in External Calibration mode).

The ARC is used to check the CPR’s transmitter performance (ARC in receiver mode) and the receiver performance (ARC in transmitter mode) as well as the overall performance (ARC in transponder mode with delay to avoid contamination with ground echo). In addition, ARC will have the capability to change the transmit frequency for the evaluation of the Doppler processing. Data obtained during the external calibration will also provide antenna pattern information.

The EarthCARE satellite will perform a roll manoeuvre regularly (e.g. once a month) for the sea surface calibration (CPR in Sea Surface Calibration mode). Since the normalized radar cross-section of the sea surface echo (σ0) has a clear incident angle dependency and its shape is dependent on the sea surface wind, σ0 for various incident angles is useful to evaluate the CPR performance.

Other natural targets such as σ0 over a desert (a desert has an advantage due to a smaller amount of water vapour and virtually no clouds) will also be helpful to evaluate the CPR performance.

MSI Calibration

On the EarthCARE satellite, the Multi Spectral Imager (MSI) response is linear and is calibrated using a high and a low radiance reference point.

Thermal Infrared (TIR): The optics arrangement is designed such that the detectors receive radiation that is reflected from the filters and mounting frame, coming from a common, internal reference body, whose temperature tracks that of the structure and rear optics by means of a thermal link to the housing. Signals from the reference areas will be averaged and subtracted from the signal in the scene area on the detector, accounting for short term, relative temperature drifts.

Absolute calibration is performed once per orbit against an internal, warm blackbody and by observation of deep space. The line of sight is redirected to the sources via an internal, rotating mirror.

Visible/NIR/SWIR (VNS): The two radiance reference points for the VNS are:

1. Observation of solar light through a diffuser that can be rotated into the field of view
2. Instrument response with the aperture closed

Two pairs of Quasi Volume Diffusers (QVD) are carried, within each pair one QVD is for the SWIR-2 and one is for the other VNS channels. One QVD pair is routinely used once per orbit. The second pair is used approximately once per month, in order to monitor degradation of the nominal pair.

The solar calibration is carried out in the South Polar Region, when the satellite has crossed the terminator and is flying over a shadowed Earth, but still with the Sun in the line of sight. The dark calibration is performed on the dark side of the orbit.

EarthCARE Validation

Validation activities ensure that the EarthCARE data and products are meaningful and well characterised with respect to uncertainties due to sensitivity or resolution.

The geophysical validation of the ESA EarthCARE data products involves a suite of correlative instruments and methods. This page provides an overview of the validation activities.

The activities are a result of the responses to the 2017 ESA Announcement of Opportunity (AO) for the Validation of EarthCARE, complemented with more recent gap-filling activities. A further component of the validation programme is the quality assessment through assimilation of EarthCARE products in the European Centre for Medium-Range Weather Forecasts (ECMWF) Numerical Weather Prediction (NWP) Model. 

JAXA issued its own Research Announcement regarding validation of the JAXA data products.

Scientists in the ESA EarthCARE Validation Team (ECVT) collaborate with the mission’s algorithm and instrument experts. They have also been provided with pre-operational EarthCARE data products as soon they were available, i.e. well before the public release of the consolidated data products.

EarthCARE validation is now in full swing and as of November 2024, 59 underflights of 52 EarthCARE orbits have been carried out by 9 aircraft involved in 8 campaigns. Many further airborne campaigns are planned, complemented by campaigns and routine measurements on the ground and at sea.

If you are interested in collaborating on EarthCARE validation and would like to join the ESA ECVT, we invite you to contact esa-ecvt@earthcare.esa.int.

Useful resources for EarthCARE Validation are provided below:

• The eoPortal EarthCARE site*
• The JAXA EarthCARE website*
• American Meteorological Society scientific paper on the EarthCARE mission*
• Atmospheric Measurement Techniques special issue on EarthCARE Level 2 algorithms and data products*
• The restricted collaboration environment for the ECVT Principal Investigators (PIs)
• The EarthCARE Scientific Validation Implementation Plan
• The ESA Atmospheric Validation Data Centre (EVDC)
• The AO for the Validation of EarthCARE documentation

*(The information provided is sourced and updated by external entities. For further details, please consult our Terms and Conditions page.)

Learn more about EarthCARE Validation

• Validation Activities page
• Validation Needs page
• Validation Workshops page