While PROBA-1 has already exceeded expectations by suppling over 21 years of Earth observation hyperspectral data, recent cross-calibration activities are aiming to ensure the data will robustly serve long time-series research, even when the satellite ends operation.

The Project for On-Board Autonomy, or PROBA-1, was originally designed as a two-year technology demonstration mission. It outperformed by becoming ESA’s longest operating Earth observation mission and supplying a dataset, which can be used as a reference for new hyperspectral missions.

As the mission nears its end, ensuring the quality of its long time-series data, is the aim of recent cross-calibration activities.

Jointly operated by ESA and Surrey Satellite Technology Ltd (SSTL), as part of ESA's Earthnet Programme, PROBA-1’s two imaging instruments - the Compact High Resolution Imaging Spectrometer (CHRIS) and the panchromatic High Resolution Camera (HRC) – have provided over two decades of imagery.

While CHRIS ended its operational phase at the end of 2022, archived CHRIS data are available via ESA’s Third Party Mission programme.

Before CHRIS stopped acquiring images, the cross-calibration campaign at targeted sites aimed to validate the quality of its dataset.

The campaign compared CHRIS data with reference data from RadCalNet and other spaceborne hyperspectral missions, such as the Italian-led hyperspectral mission PRISMA, and the German EnMAP mission.

“CHRIS was an old instrument and only calibrated at quite an early stage in its mission lifetime, and yet we’ve been using that calibration all this time. Before its end of life, it made sense to compare the CHRIS dataset with other satellites and ground-based measurements, to see whether this calibration is still okay, or whether we should look at recalibrating the whole dataset when we reprocess it,” says Sam Lavender from Telespazio UK, responsible for quality control for the CHRIS-PROBA dataset in the IDEAS-QA4EO contract.

Cross-calibration campaign focused on three study sites

“We knew CHRIS had limited time left, so we had a small chance to acquire extra data to cross-calibrate. We tried to have more match-ups between PRISMA and CHRIS by pointing both satellites at the same locations,” explains Lavender.

The cross-calibration campaign focused on three study sites, namely, La Crau (France), Barrax (Spain) and Railroad Valley (USA), with data acquired in August 2022.

The data from match-ups at these sites has recently been processed to investigate whether it can reveal information about the calibration.

The initial RadCalNet and PRISMA comparisons against CHRIS/PROBA-1 for the Railroad Valley site appear to confirm that CHRIS/PROBA-1 remains well-calibrated, however further comparisons are needed to quantify the calibration more rigorously.

The team are hoping to gain more information by comparing further with data from the EnMAP hyperspectral mission.

Multispectral satellites acquire data in multiple wavebands – say from 8 to 20 wavebands - whereas hyperspectral satellites can acquire data in the order of hundreds of different wavebands. There are few hyperspectral missions and so leveraging data from newer missions such as EnMAP and PRISMA is vital for cross-calibration.

Dataset quality shall hold for long time-series analysis

“When the PROBA-1 mission eventually comes to an end, we intend to look at the whole time-series dataset and examine whether we can reprocess it to a higher quality,” says Lavender.

PROBA-1’s advantage is its rare combination of being a hyperspectral mission and having multiple pointing angles. Now flaunting a dataset of over twenty years in length, its application areas are multi-fold, ranging from the monitoring of forest, urban areas, glaciers and agriculture, to the advancement of atmospheric correction techniques.

“While the CHRIS/PROBA-1 dataset has already contributed to cutting-edge science, we want to see if we can improve the quality so that it really can be used going forward for time series analysis,” says Lavender.

Long time-series datasets - such as the CHRIS/PROBA-1 dataset - could contribute to valuable climate change research.

Access the CHRIS/PROBA-1 dataset - free for research and applications development.

With the end in sight for this long-lasting satellite, ESA is organising a workshop for the end of the mission. More information about the workshop will be provided in the future.