Big Earth Imager to Be Tested on Small Vega CubeSat
Big Earth Imager to Be Tested on Small Vega CubeSat
Proba-V Companion CubeSat Continues Proba-V Mission
A briefcase-sized CubeSat launched by Europe’s Vega launcher Friday will gather 340 km (≈221 mile) wide views of Earth’s vegetation growth, employing a spectral imager originally designed for ESA’s decade-in-flight Proba-V.
"In the same way that Proba-V demonstrated useful imagery could be returned from a small platform, we will also be able to test whether this much smaller CubeSat platform can also serve to gather acceptable imagery.”
Iskander Benhadj, VITO
The Proba-V Companion CubeSat has been built by ‘NewSpace’ company Aerospacelab in Belgium, hosting a Proba-V spectral imager provided by OIP Space Instruments and Belgium’s VITO research institute processing and making use of its data. The goal of the mission is to see whether this already well-characterized imaging payload can operate well aboard a miniature CubeSat platform.
ESA’s Proba-V mini-satellite was launched by Vega back in 2013, flying a miniaturized version of the vegetation instrument previously flown aboard the full-sized Spot-4 and Spot-5 satellites. At just a cubic meter in scale, Proba-V achieved a daily continent-spanning 2,250 km (≈1,400 mile) field of view, collecting light in the blue, red, near-infrared and mid-infrared wavebands, ideal for monitoring plant and forest growth as well as inland water bodies. Proba-V’s wide swath was achieved by combining a trio of spectral imagers, with 350 m spatial resolution from its side imagers and 100 m resolution in its central field of view.
“Actually a total of four spectral imagers were manufactured– one stayed behind, intended to help with troubleshooting the mission by reproducing any in-orbit problems on the ground," said Iskander Benhadj of VITO. “So then the idea emerged from VITO: why not try and fly this spare spectral imager as well? We can extend the continuity of Vegetation data, since in 2019 Proba-V’s orbit drifted so much that part of its swath is now in the eclipse side, meaning its working mission has ended.
“In addition, in the same way that Proba-V demonstrated useful imagery could be returned from a small platform, we will also be able to test whether this much smaller CubeSat platform can also serve to gather acceptable imagery.”
CubeSats are low-cost satellites assembled from standardized 10-cm boxes. Proba-V CC is a 12-unit CubeSat, although around half the satellite volume is taken up by its spectral imager and dedicated read-out electronics.
“In practice the mission has been built around the imager. The hardware was taken directly out of storage, where it was kept in stable conditions, double bagged and in an inert nitrogen gas environment," said Xavier Collaud of Aerospacelab. “The imager has been secured to an optical bench along with the startrackers used to precisely measure the CubeSat’s attitudes. The rest of the satellite is taken up with reaction wheels that serve to rotate the mission in the correct orientation, plus the usual battery, power systems and on-board computer.”
Once in its 350 mile altitude orbit, Proba-V CC will perform co-observations of global vegetation with Europe’s two Copernicus Sentinel-2 satellites, which are similarly optimized for land cover and vegetation.
As is standard for Earth observation satellites, the CubeSat will be placed in a Sun-synchronous near-polar orbit, meaning that it retains the same conjunction with the Sun as it orbits, so that the same solar local time prevails under the locations it overflies – Proba-V CC’s initial local time of the descending node is targeting 10:14 in the morning. Its predecessor Proba-V was placed in a comparable orbit, although because it far outlived its planned 2.5 year original working lifetime this orbit ended up drifting earlier in the morning, to below 09:00 at the moment, meaning its images contains zones that are in eclipse, have longer shadows and are no longer scientifically useful.
The satellite itself remains in good working order, so some co-observations with Proba-V CC might be possible for experimental studies. It is also acquiring monthly images of the Moon, whose unchanging surface makes a useful radiometric calibration target.
“Being in a lower orbit than Proba-V means the spectral imager will achieve a higher spatial resolution, down to 70 meters,” Benhadj said. “But that means the satellite is moving faster, so the line rate will be affected, meaning small pixels will end up rectangular rather than square if the nominal Proba-V line rate is used. We can try to accelerate the image to compensate for this, but this will require extra power.
“Quality and operational trade-offs such as the selection of optimal line rate or the area of interests to be monitored, have to be carried all over the Proba-V CC lifetime, which includes the commissioning phase, with the objective to provide optimal products for the end user. “
Proba-V’s CC’s lower orbit will also give daily views of the same locations for multiple successive days at a time, with its ground track moving just over 12 miles westward daily, opening up particular scientific uses.
ESA is supporting Proba-V CC through the Fly element of its General Support Technology Programme, offering early space access to promising technologies. Use of Proba-V CC data is being overseen by ESA’s Earth Observation Program, as was the case with the original Proba-V.