With the launch of PROBA-V’s Companion CubeSat (PV-CC), ESA supports a future where clusters of cheaper, small satellites could complement full-scale missions for Earth observation. 

The technology miniaturisation emerging from New Space has enabled a rapid growth in CubeSats - small, low-cost satellites built up from standardised 10 cm boxes. Now, part of an instrument flown on the successful PROBA-V (Project for On-Board Autonomy – Vegetation) satellite, will be tested on its CubeSat Companion, the PROBA-V Companion CubeSat (PV-CC). Due to be launched on a Vega flight on 6 October, PV-CC aims to understand what is required to fly a large known instrument on a small CubeSat.

PROBA-V was already itself a forerunner in using scaled-down technology. Following the success of the long-established Vegetation instrument (VGT) flown on the French SPOT-4/SPOT-5 satellites in the early 2000’s, and despite being only the size of a washing machine, PROBA-V was tasked with a full-scale mission to map land cover and vegetation growth across Earth daily. It used the VGT instrument to distinguish between different land cover types and vegetation growth, acquiring data for seven years (2013-2020) and serving as a “gap-filler” between SPOT-4/SPOT-5 and the Copernicus Sentinel-3 mission. 

Following PROBA-V’s development, a spare unit of the three spectral imagers (telescopes), composing the VGT instrument, inspired this companion mission. PROBA-V houses its three telescopes, on a satellite 1 m3 large, while PV-CC will fly the spare spectral imager on a platform the size of a shoebox. The idea is to test how well this cut-down version of the VGT instrument performs on a smaller, low-cost platform. 

“We hope to evaluate whether it is possible to mount a similar sensor on a much smaller satellite and still have the same data quality. This idea is very interesting, since the cost of a small satellite like PV-CC is much less than PROBA-V, or any other large satellite. So, if this works, we can think about how clusters of small satellites could make daily remote sensing at 100 m resolution possible, at a rather low cost,” says Roberto Biasutti, PROBA-V operations manager at ESA. 

PROBA-V provides nadir 100 m resolution, while PV-CC will be injected on a lower orbit (foreseen around 565 km altitude), promising a VNIR resolution between 70-90 m along track (final value will depend on the optimal line rate) and around 70 m across track, which is slightly better than Sentinel-3, albeit with a much smaller data acquisition swath (around 340 km). 

The development of PV-CC was supported by ESA’s General Support Technology Programme, based on the initiative of Belgium’s VITO research institute. It has been built by Belgian prime contractor Aerospacelab, who will also perform initial operations. 

Mission data reception services will be provided by the Swedish Space Corporation (SSC), while acquisition planning and data processing will be done by VITO in Belgium. ESA ESEC is the entity currently tasking PROBA-V (calibrations and EPT instrument), and receiving data from the satellite via S-band. 

Following the launch on 6 October in Kourou (French Guyana), there will be three months of commissioning phase, followed by one year of operations and the possibility to extend operations by a further year. 

Initially, it was hoped that PROBA-V and its companion PV-CC could perform joint observations, however, launch delays have hindered this. Instead, cross-calibration experiments will be done with Copernicus Sentinel-2 and Sentinel-3. Specifically, coincident nadir overpasses with these Sentinel missions will be analysed to estimate radiometric cross-sensor biases. 

Additional calibration activities, planned for the commissioning phase, include the use of the CEOS-WGCV RadCalNet network and of well-characterised Earth’s targets for assessing the radiometry, and the use of a set of precise ground control points, derived from Sentinel-2 Global Reference Image (GRI), to evaluate the absolute geometric accuracy of PV-CC.  

As well as testing the performance of the instrument, the mission will also facilitate learning about how to overcome the challenges associated with shrinking satellite platform size, such as thermal and orbit stability issues. 

Xavier Collaud, PV-CC Project Manager at Aerospacelab says: “The main challenge was to implement the spectral imager made for a microsatellite in the context of a 12U CubeSat. The team had to build the platform’s hardware and software around the payload, without any modifications. We are very proud to showcase our development on units, operational aspects, and manufacturing processes on this mission. We were also able to implement lessons learned from our first satellite Arthur-1 launched in 2021.” 

Lars Johansson, Business Development Director at SSC, says: "SSC is excited to have our global ground station network SSC Connect selected to support ESA and Aerospacelab for this exciting mission, which will provide key learning to improve models and measurements of planetary vegetation growth. It is also an exciting mission in terms of pushing the boundary and validating the performance of new, exciting and efficient small-form factor satellite platforms, which will help expand the overall space business."