Using a cubesat to improve irrigation: an innovative thermal imager

Water management has become one of the most important issues since 70% of the fresh water available on Earth is used for irrigation. The growing food demand and the scarcity of water resources lead to the need to carefully monitor water use, considering agricultural fields of thousands of km².

The intrinsic hydric stress of crops is an indicator of their water needs. Better water management and crops healthiness could be achieved if this stress could be measured quickly. Hydric stress can be retrieve by comparing the ground temperature (reference) and the leaf surface temperature (LST) which also depends on the transpiration ability of the plant. Yet, this measure is very unpractical without airborne/spaceborn sensors with good resolution.

This finding has led to the birth of the OUFTI-NEXT mission. The recent advances in the field of nanosatellites and the rising attention they get from the space agencies have convinced the University of Liege to develop its own CubeSat mission to image the LST with thermal infrared light above extensive crops to provide data for irrigation schedule. The aim of the long term mission will be to fly a constellation of CubeSats to ensure daily coverage over various fields with resolution of 50m. In addition, hot singular events data can be retrieved such as forest fire, volcanoes activity, pollutant leaking, etc…

Each CubeSat is a dual-band imager in both the middle wave infrared (MWIR) and the long wave infrared (LWIR). The scientific value of combining these bands is huge since LWIR gives accurate temperature measurements around 300K but with bad contrast, the latter being compensated by the MWIR which allows fine resolution. Each band is also sensitive to different atmosphere condition (humidity, clouds) and using both brings robustness to the mission.

The current step of this ambitious project is to fly a single band 3U technology demonstrator to validate the use of MWIR technologies without space heritage and the scientific value of MWIR images for LST determination. This spectral band is very challenging, as it hardly allows diffraction-limited performances: it requires fast optics, more sensitive to aberrations. For this demonstrator, resolution of 100m without daily coverage is chosen, resulting from is a trade-off between science demonstration and mission size. The “new” infrared technologies include: high operating temperature detector, compact optics, passive athermalization and recent infrared materials.

Solutions to make this challenging mission feasible are promising: The MWIR camera achieved diffraction-limited performances and uses compact hybrid lenses made of chalcogenide materials to reduce thermals effects and manufacturing costs. A very wide range of suitable detectors have been reviewed and the possibility to customize their integration is studied with their manufacturers. The orbit is sun-synchronous to optimize the thermal design and in accordance with both the radiometric budget and the observation strategy.