Sucre: measure infrared radiation, then calibrate it

VRSS-2, named Antonio José de Sucre, launched in October 2017. The archive title Antonio Jose de Sucre identifies that individual flight. The contractor records delivery in March 2018 and work on receiving, processing and distribution systems on the ground. The new mission extended Venezuela’s observation programme while adding instruments with different capabilities from Miranda’s.

CAST describes a one-metre panchromatic — open definition camera, four-metre multispectral — open definition imagery and an infrared camera. Original engineering research describes two long-wave infrared channels with 60-metre sampling. These specifications belong to different measurements. A one-metre visible-band product does not imply that the satellite measures a separate surface temperature for every one-metre patch in the same scene.

Visible and much near-infrared imaging records sunlight reflected by the surface, so illumination matters. Long-wave thermal infrared instruments detect radiation emitted by surfaces and can make useful observations without daylight. The measured radiance — open definition is not automatically a thermometer reading of the ground: atmospheric effects, surface emissivity — open definition, sensor response and the chosen retrieval method all influence the interpretation.

An idealized 60-metre square thermal sample covers 3,600 square metres. It may contain roads, vegetation, roofs and smaller hot areas at different temperatures. The detector records their combined contribution within its response footprint. A small hot feature can influence the signal without being resolved as a separate thermal pixel. This example explains mixed observations; it is not a claimed Sucre measurement of any particular location.

Do not transfer resolution between sensors

VRSS-2 researchers describe combining onboard calibration with cross-calibration — open definition. Calibration relates instrument output to known reference behaviour; cross-calibration compares compatible observations or reference measurements from another system. These approaches help detect biases and track changes in response. Agreement must be assessed under appropriate conditions rather than assuming that two images acquired at different times should contain identical values.

Suppose a hypothetical instrument consistently reports a reference target two units above the expected value. That suggests a bias to investigate, not that every real surface has physically warmed by two degrees. Correcting the bias requires a justified model and a record of the conditions under which it applies. Sucre’s infrared work shows why quantitative Earth observation includes instrument characterization and validation as well as pointing a camera at a useful place.