The Goddard Laser for Absolute Measurement of Radiance (GLAMR) laboratory at Goddard Space Flight Center (GSFC) is a state-of-the-art spectral and radiometric sensor characterization facility. The work is based on the calibration methodology developed by National Institute of Standards and Technology (NIST) called Spectral Irradiance and Radiance Calibration using Uniform Sources (SIRCUS). The GLAMR facility consists of a set of tunable lasers that have combined continuous scan capability across the 340-2500 nm spectral range; 1-meter integrating sphere with selectable 203-mm, 254-mm, and 305-mm diameter exit ports; wavemeters; laser spectrum analyzers; monitor radiometers; transfer radiometers; various control electronics and computers. The whole GLAMR system can travel to the sensor vendor facility for measurements. GLAMR combines spectral and radiometric characterization into one measurement sweep by creating a monochromatic extended source by fiber-coupling the tunable lasers to the integrating sphere. Spectral position of the light is measured with tens-of-picometer accuracy, and the radiometric scale can be transferred with 0.24 – 1.4 % (k=2) uncertainty depending on spectral region. This talk will provide a description of the GLAMR system, operation, spectral and radiometric traceability, and examples from recent (Landsat 9 OLI-2 and JPSS-4 VIIRS) and upcoming (PACE Ocean Color Instrument and CLARREO Pathfinder) deployments.

The objective of the SPAR@MEP project is to derive a consistent long-term data set of Aerosol Optical Thickness (AOT) and surface Bidirectional Reflectance Factor (BRF) using the SPOT- VGT and PROBA-V MEP archive with the Combined Inversion of Surface and AeRosol
(CISAR) algorithm. This archive contains more than 20 years of observations starting in 1998 acquired in four spectral bands (blue, red, NIR and SWIR) by three satellites: SPOT-VGT 1 (04/1998 – 04/2012) and 2 (01/2003 – 06/2014) and PROBA-V (04/2013 – 10/2021). The first step of this activity consists in the verification of the temporal consistency of the MEP time series obtained from the three different sensors that are considered in some sense compatible because of their similar design. This continuous time series is harmonised with respect to a common calibration reference. For that purpose, time series of clear-sky Top-Of-Atmosphere BRF has been extracted over the pseudo invariant calibration site (PICS) Libya-4.

Our calibration reference consists in simulated TOA BRF and is referred to as Libya-4 Rayference Calibration Reference (LRCR). Four different radiative transfer models have been used for the generation of LRCR. The estimation of LRCR uncertainty relies on a two-fold approach. Firstly, LRCR has been compared with observations acquired over Libya-4 by well-calibrated radiometers such as MODIS or S2/MSI for close-to-nadir viewing conditions and Sun zenith angles not exceeding 30◦. Secondly, the LRCR angular consistency between nadir and large viewing conditions have been estimated. LRCR, the mean accuracy of which has been estimated to about 2%, is used as an absolute reference to correct differences between the tree satellites.

This correction accounts for the sensor spectral differences between the radiometers, i.e., the time series is harmonised but not homogenised. Each sensor is calibrated to the common reference in a way that maintains the characteristics of that individual sensor such that the calibration radiances represent the unique nature of each sensor. This means that two sensors which have been harmonised may see different signals when looking at the same location at the same time where the difference is related to known differences in the responses of each sensor such as differences in the sensors spectral response functions. In the present case, only minor corrections are applied to PROBA-V data. For SPOT-VGT1 and 2, there corrections are in the range of 1 to 4%. Additional work is still needed to characterize the radiometric uncertainty at the pixel level.

For more than 15 years, the Moon has been evaluated as a valuable radiant calibration source for Earth observation sensors. The absence of an atmosphere, its temporal stability and high and predictable availability makes it an ideal target for the vicarious calibration of optical sensors. The photometric stability of the surface of the Moon is estimated to be of the order of 10 9 per year.
In 2017, ESA launched an initiative to derive the new “Lunar Irradiance Model of ESA” (LIME). The model is based on automated measurements of the total lunar irradiance, taken at high altitude using a multispectral CIMEL CE318-TP9. The instrument was calibrated and characterized at the National Physical Laboratory, against SI-traceable standards. The University of Valladolid mounted the instrument on the Pico Teide in Tenerife, where it continues to perform nightly lunar irradiance measurements for lunar phases angles between -99° to +99°, at a frequency of about 100 quality controlled measurements per year.
A follow-on project is currently on-going to take new measurements and develop further improvements to the model with better the spectral resolution, improved spectral interpolation between model wavelengths and a more detailed uncertainty analysis. It is also planned to develop, within the project, a reference implementation to be released in the form of an open source repository. The current status and achievements of this project are demonstrated in this presentation.
Up to now, the model has already been applied to calibrate different spaceborne sensors, mostly on an ad-hoc basis (Nemenam et al.). The PROBA-V was routinely calibrated during its mission life-time using vicarious calibration methods. As part of the operational radiometric calibration, the PROBA-V instrument has been taking lunar acquisitions (Sterckx et al.). For eight years, the moon was imaged twice a month at a constant phase angle, 7 degrees before and after full moon. This dataset has been compared with the LIME model during the instrument operational phase.
Since mid-2020, PROBA-V operational phase was transferred to the ‘experimental’ modus. With the instrument recording only about 25% of the nominal earth-view acquisitions, the freed capacity has been used to record extra lunar data. Up to now, 7 full lunar cycles of -90 degrees to 90 degrees phase angle with a granularity of 10 degrees have been acquired. This data is being be used to support the development and further understanding of the LIME model.
With the nominal calibration acquisitions of the moon at constant phase, only a small part of the model is exploited. This presentation will show how LIME is applied to calibrate the PROBA-V sensor and how the results of the experimental acquisitions contribute to the model development.


Neneman, M.; Wagner, S.; Bourg, L.; Blanot, L.; Bouvet, M.; Adriaensen, S.; Nieke, J. Use of Moon Observations for Characterization of Sentinel-3B Ocean and Land Color Instrument. Remote Sens. 2020, 12, 2543. https://doi.org/10.3390/rs12162543

Sterckx, S.; Adriaensen, S.; Dierckx, W.; Bouvet, M. In-Orbit Radiometric Calibration and Stability Monitoring of the PROBA-V Instrument. Remote Sens. 2016, 8, 546. https://doi.org/10.3390/rs8070546

Planet currently operates a constellation of about 200 high resolution cubesats spanning three major design generations and 6+ years of collecting Earth imagery. In addition to significant improvements in telescope design, the Dove satellites have benefited from three major updates to their imaging sensors, introducing spectral band responses matching those of Sentinel-2 bands and increasing the number of bands from four to eight.
Satellites are launched into sun synchronous orbits (SSO) in groups (called "flocks") of varying numbers and time intervals, a process designed to refresh the fleet by replacing older satellites. The large number of operational satellites, heterogeneous nature of their design, and wide range of operational states present significant challenges to providing reliably consistent radiance products. These products form the basis for higher level Planet analytic product offerings, including surface reflectance, normalized Basemaps, and Planet Fusion data.
Interoperability and consistency between all of Planet’s satellites is essential. We have recently moved to using Sentinel-2 as the radiometric reference for both Doves and SkySats. Sentinel-2 has matching
bands for the more recent Dove satellites and both constellations occupy similar SSO orbits, this allows sufficient crossovers for deriving accurate calibrations. For the older Doves with sensor band responses significantly different from Sentinel, we collect crossovers over well characterized calibrations sites. For the newer Doves, crossovers with Sentinel-2 are collected world-wide, allowing for larger quantities of calibration data and conditions covering a wider dynamic range. These world wide crossovers are randomly split into two collections, one used for calibration and the other for validation.
Planet also conducts a program of monthly Moon image collections by every Dove satellite. Each satellite images the moon over the complete set of phases. Subsequent comparison to the ROLO model, which provides < 1% relative accuracy, allows for general monitoring of the health of the Doves as well as the ability to make small relative calibration adjustments.
Using crossovers collected over the same time period of each calibration update, comparisons with the RadCalNet network of sites provides another independent and traceable source of radiometric validation. This is monitored on a per-flock basis over time during the entire life of each satellite.
In this presentation we will provide an overview of the current process for updating the calibration of Dove satellites while in orbit, from initial commissioning to regular biannual incremental updates. Our program of lunar monitoring will be described, and specific examples shown for both health monitoring as well as relative, per-satellite calibration adjustments. We will then show results of this ongoing campaign along with the validation of the latest radiometry.

The Copernicus Sentinel-2 Global Reference Image (GRI) was originally designed with the main goal of being used by the ESA Sentinel-2 (S2) Ground Segment Level-1 processor to geometrically refine S2 data. The usage of the GRI has shown an actual improvement of the S2 data geometric performance. Indeed, both absolute geolocation and relative (multi-temporal) performances have significantly improved and meet the expectations.

However, for now, the GRI is an internal ESA product not easy to handle and used mostly by expert users.
Therefore, the usage of the GRI as Database of Ground Control Points (GCPs) has been identified as an important improvement to be easily used as the future common layer of reference to refine imagery of several missions (including S2).

The proposed GRI version as database of GCPs is a preselected set of points of interest extracted from the Level-1B images that compose the GRI. The parameters are carefully tuned to provide a sufficient and homogeneous density of points for most applications related to geometry. It will have the same coverage and geometric properties than the current GRI version, and it will be usable also for many imagery data with lower spatial resolution (up to 50m).
Finally, this new GRI version will have a lot of advantages: easier to manage than raw L1B images, lighter, only constituted of validated and qualified points ensuring the repetitiveness of the co-registration. It will also be freely distributed to users for their own geometric issues.

In this presentation, we first recall the concept of GCP and how the GCP database is generated using the current multi-layer GRI version and the new Copernicus DEM at 30m resolution. The benefit of using this new GRI version as database of GCPs instead of the current multi-layer GRI version for geometric refining of S2 data is also presented. The extension of the use of this new GRI version to other missions is also highlighted.