October 24, 2023

Satellite

The satellite will fly over the same point on the globe at the same viewing angle every eight days. However, with a field of view of 34° on either side of the ground track, it will be able to observe the same point every three days at the equator, and more frequently at temperate latitudes. Observations will be made from several angles, with the sequence repeating every eight days, thus covering all land surfaces and coastal strips in less than three days. Trishna’s cameras will operate at a resolution of 57 metres at nadir to 90 metres at the edge of the swath. Data products will offer a resolution of 60 metres. The multisensor satellite will observe simultaneously in the thermal, visible and near-infrared. This unprecedented combination of spatial and temporal resolution will enable global observation of thermal phenomena at fine scales, notably to aid land planning. Not much will escape the satellite’s beady eye compared to previous missions.

Spacecraft bus

The bus accommodating the instrument payload will supply propulsion, power and orientation to control the satellite’s trajectory. It will be built by ISRO from an existing IRS-1k model adapted to the requirements of the Trishna mission.

  • Mass: 770 kg (out of a total satellite mass of approximately 1 tonne)
  • Mass storage: 1.4 Tbits of data
  • Telemetry data rate: 640 Mbps (satellite-to-ground X-band link)
  • Solar panels: 2 kW

Instruments

The payload will comprise three instruments:

1) Thermal InfraRed instrument (TIR)

CNES is in charge of the TIR instrument developed by Airbus. It covers four spectral bands in the thermal infrared to measure the temperature and surface emissivity of Earth’s land surfaces and coastal oceans.

  • Mass: 210 kg
  • Range: –20°C to +30°C
  • Precision: 0.3°C
  • Spectral bands: four (8.65 µm, 9.0 µm, 10.6 µm, 11.6 µm)

The instrument features an entrance mirror mounted on a scanning mechanism to obtain a swath of +/-34°, and a three-mirror telescope followed by a cryostat housing the cooled focal plane consisting of a detector and spectral filters.

The instrument relies on two onboard radiometric references—a cold space target and a warm black body—for calibration.

Tir instrument

Functional diagram of the Thermal Infrared (TIR) instrument

Functional diagram of the Thermal Infrared (TIR) instrument

Tir instrument scanning principle

TIR instrument scanning principle

2) Visible and Near InfraRed/Short Wave InfraRed instrument (VNIR/SWIR)

Developed by ISRO, the VNIR/SWIR instrument contains six visible cameras for identifying crop patterns and growth status. For example, amounts of vegetation and its growth status will be visible in each pixel.

  • Mass: 90 kg
  • Spectral bands: four VNIIR (B1: Blue (485 nm), B2: Green (555 nm), B3: Red (670 nm), B4: NIR (860 nm), B5: WV (910 nm)) and two SWIR (B6: Cirrus (1,380 nm), B7: SWIR (1,610 nm)

The VNIR/SWIR instrument will operate in pushbroom mode, building up images line by line in the satellite’s direction of motion. The +34° field of view is obtained by three visible cameras and three SWIR cameras.

The VNIRSWIR instrument

The VNIR/SWIR instrument will operate in pushbroom mode, building up images line by line in the satellite’s direction of motion. The +34° field of view is obtained by three visible cameras and three SWIR cameras.

Illustration courtesy of ISRO

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