TRISHNA: AN INDO-FRENCH SPACE MISSION TO STUDY THE THERMOGRAPHY OF THE EARTH AT FINE SPATIO-TEMPORAL RESOLUTION
J.-L. Roujean (1), B. Bhattacharya (2), P. Gamet (1), M.R. Pandya (2), G. Boulet (1), A. Olioso (3),
S.K. Singh(2), M. V. Shukla(2), M. Mishra(2), S. Babu(16), P. V. Raju(15), C.S. Murthy(15), X. Briottet (4),
A. Rodler (5), E. Autret (6), I. Dadou (7), D. Adlakha(2), M. Sarkar(2), G. Picard (8), A. Kouraev (7), C. Ferrari (9), M. Irvine (10), E. Delogu (11), T. Vidal (12), O. Hagolle (1), P. Maisongrande (11), M. Sekhar(14), K. Mallick(13)
(1) CESBIO, Toulouse, France (9) IPGP, Paris, France
(2) SAC, ISRO, Ahmedabad, India (10) INRAE, Bordeaux, France
(3) INRAE, Avignon, France (11) CNES, Toulouse, France
(4) ONERA, Toulouse, France (12) ACRI, Toulouse, France
(5) CEREMA, Nantes, France (13) LIST, Luxembourg
(6) LOPS, Plouzané, France (14) IISC, Bengaluru
(7) LEGOS, Toulouse, France (15) NRSC, ISRO, Hyderabad
(8) IGE, Grenoble, France (16) SPL, ISRO, Trivandrum
TRISHNA (Thermal infraRed Imaging Satellite for High-Resolution Natural resource Assessment) is a cross-purpose high spatial and temporal resolution thermal infrared Earth Observation (EO) mission that will provide observations in the domains of terrestrial and marine ecosystems, inland and coast, urban, cryosphere, solid Earth and atmosphere. It is an Indo-French innovative polar-orbiting mission that will overcome the limitations of TIR-optical observations from Landsat series and ASTER. The high-quality optical-thermal imagery will be used to provide precise surface temperature, emissivity and albedo of vegetation, manmade structures, snow, ice and sea. Atmospheric fields such as cloud mask and type, aerosol load, water vapour content will be well described. TRISHNA products will improve our knowledge of radiative and heat transfer to quantify evapotranspiration, fresh water discharge, snow-melt runoff, bio-geochemical cycle, urban heat island.
Energy transfer and exchanges of water and carbon fluxes in the soil–vegetation–atmosphere system need to be well described to enhance the role of environmental biophysics. Climate indicators include the surface temperature, the ocean heat, the glaciers and the Arctic and Antarctic sea ice extent. Land surface temperature (LST) and land surface emissivity (LSE) are Essential Climate Variables (ECV) (Global Climate Observing System/GCOS). LST is defined as the radiative skin temperature and is useful in agriculture (plant growth, water stress, crop yield, precision farming, early warning, freezing scenarios), hydrology (water cycle, catchment water, etc), and meteorology. LSE differentiates the surface attributes (vegetation, soil, snow, water, rock, manmade material) composing the landscape.
Water use in agriculture represents 70 % of global resources, making sustainable irrigation a key issue. Automatic detection and mapping of irrigated farmland area is vital for many services in charge of water management. In that respect, TIR signal brings, in addition to visible and near infrared, key information on irrigated areas that display lowest LST values at the peak of growth. The global change imposes an implementation of more efficient irrigation practices at the scale of an agricultural plot for better control. The decrease of moisture within the soil after water supply can be evaluated from the surface moisture estimated by radar but TIR observations remain better-suited to monitor vegetation water stress and irrigation at the agricultural plot to adapt the proper needs of each of the cultures. With a pixel size of 57 m, a revisit of 3 days at noon-night time, 6 VNIR (blue, green, red, NIR, water vapor, cirrus) and 4 TIR Bands in 8 – 12 m, TRISHNA will bring new insights. High resolution thermography is of broad interest for manifold domains like coastal area, inland water, urban areas, cryosphere, solid Earth and atmosphere. With a launch in 2025, TRISHNA will pioneer fine and routine collection of TIR scenes of the entire Earth owing to its instrumental design which proposes acquisitions based on across track scanner for 4 TIR channels (8,6, 9.0, 10.6 and 11.6 µm) and on push-broom.for 7 VNIR channels (485, 555, 670, 860, 910, 1380 and 1610 nm).
TRISHNA mission will look at early warnings of water scarcity and fire risk, including optimization for agricultural irrigation, rainfed crops, water consumption and its partitioning, plus food security. LST is a surrogate for soil water, near-ground air temperature and indirectly for productivity. TRISHNA will help to monitor any reduction and increase in evaporation (E) and transpiration (T). Evapotranspiration (ET) is an ECV that reflects the satisfaction or not of plant water needs. Its accuracy and timeliness is central as ET governs soil moisture at the surface and in the root zone through water extraction by plants, with large consequences for infiltration, runoff and for the whole catchment water budget. The detection of a water stress, deduced from a temporal chronicle, is useful to manage irrigation or to warn of the potential lack of water threatening ecosystems.
A main objective is to describe mixing processes and water quality in the coastal areas and estuarine from high-resolution SST (Sea Surface Temperature), also to enhance productivity and biodiversity assessment on the coast and for rivers and lakes including warning and monitoring of water borne diseases, to estimate energy fluxes in alluvial plains and aquifers, and to describe hazards (river floods, storm surges, inundated vegetation) in link to sea level rise.
Another main objective is to predict and to quantify at short-term the effects of the Urban Heat Island (UHI) on population health (Figure 5). A spectral un-mixing method was developed to provide sub-pixel abundances and temperatures from radiance images. Main goal is to improve LST retrieval due to enhanced footprint, to account for cavity anisotropy and environment effects, and to relate LST to air temperature. Urban areas are formed by complex 3D structures of mixed materials (cement, steel, bituminous roads, stones, bricks, glass, wood, grass, etc) and LST can vary locally by a few K. Topics are the modeling of urban climate, hydrology, building stress, storm water flow and generally UHI.
Key scientific question for the cryosphere concerns the combination of thermal and optical high resolution data to improve the prediction of the energy budget and the melting of snow and ice covered areas. In this regard, LST will be of added-value and how it may capture small-scale variability in mountainous areas is a relevant issue. Other issue will concern the mapping of debris-covered glaciers, lake ice formation and development, and lake water dynamics.
Snow is a good thermal insulator regulating soil temperature and sea-ice growth. Where the soil is permanently frozen (permafrost) presence of snow is critical for the preservation of the carbon storage in the soil. Besides, the evolution of the snowpack is a driver of the hydrological cycle in many watersheds.
Thermal anomalies may serve to anticipate volcano eruption and TIR measurements may help better characterizing volcanic ash clouds, geothermal resources, coal fires. Moreover, topography and roughness affect the surface energy balance of the solid Earth. Main properties are grain size, porosity, water content and composition. Soil temperature is a primary tracer of energy and water exchanges between the surface and the ground.
Downwelling shortwave and longwave radiation at noontime will be quantified under all-sky conditions through retrieval of aerosol optical depth, columnar precipitable water, cloud mask, cloud type / phase and albedo using seven optical and four TIR bands. In addition, surface air temperature and dust aerosol index using TIR data will be developed. These will be used for understanding aerosol-cloud interaction, to improve NWP model skills, agro-meteorological applications and air quality monitoring. Assimilation experiments will be carried out primarily with different land surface and atmospheric products as well as radiance assimilation to evaluate skill of NWP model weather forecast at various space-time scales.
CalVal is an important component of TRISHNA program. It consists in developing strategies to support the validation of TRISHNA Level 1 and 2 products, namely LST, LSE and ET. Efforts concern the data preprocessing to remove or minimize turbulent and directional effects that jeopardize the specification of 1 K on LST. Micrometeorological stations in different climates are equipped of TIR OPTRIS cameras and flown by UAV. Metrics and statistics are proposed as criteria for inter-comparison.
Cloud mask, atmospheric and anisotropy corrections (relief, directionality) are under development. Methods for measuring LST and LSE use consolidated approaches such as TES (Temperature-Emissivity Separation). Such key variables along with ET product will include accuracy assessment and a Quality Flag. ATBD are in preparation, notably the method of LST normalization. TRISHNA products will have global users such as FAO, GEOGLAM, global water watch, for meeting several Sustainable Development Goals (SDGs) as outlined by United Nations. Moreover, TRISHNA activities serve the preparation of future fine resolution TIR missions such as LSTM and SBG.