Comparison of two Metop-3MI instrument models and implications for on-ground testing in multi-unit space missions

Spaceborne optical payloads must undergo extensive on-ground testing before launch, both to verify performance against design expectations and to generate calibration data for in-flight image processing. Stray light, in particular, has emerged as a primary limiting factor requiring precise calibration to enable correction algorithms to reduce it beyond design limits. Recently, a trend has emerged to build multiple units of the same instrument. These payloads can be launched together to increase total field-of-view coverage or consecutively to extend mission lifetime beyond that of a single payload. Because testing requires significant time and resources, this raises a crucial question: should every model undergo full ground testing, or is it sufficient to test just one and apply the findings to all? We address this question by comparing two units of the Metop-3MI Earth-observation instrument, whose demanding performance requirements set particularly stringent needs in stray-light testing. From this comparison we derive testing-strategy guidelines for multi-unit missions, with implications for both institutional programs developing a few identical instruments and New Space ventures aiming to deploy constellations of hundreds.