NASHUA, N.H. — Earth observation satellites can measure more than visible light. Depending on the instrument onboard, they can detect energy at wavelengths outside the range visible to the human eye, including infrared radiation.

That information can help operators measure surface temperatures and monitor changes on Earth. Thermal-infrared observations support applications ranging from agriculture and water management to wildfire and volcanic monitoring.

Putting an infrared instrument in orbit, however, means deciding what the satellite needs to see, how much ground it needs to cover and how often it needs to return to the same location. Those choices affect the payload and, eventually, the spacecraft carrying it.

The problem changes again when an operator wants dozens of satellites making those measurements instead of one.

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Resolution determines how much detail a satellite can see

Spatial resolution describes the detail an Earth observation instrument can distinguish on the ground. A sensor with higher spatial resolution can discern smaller features than one designed to observe larger areas at lower resolution.

That does not mean every mission needs the highest resolution possible. A satellite monitoring broad weather patterns has different imaging requirements from one designed to examine individual buildings or infrastructure.

The mission therefore helps determine the instrument. Its optics, detector and orbit all contribute to what the resulting data can show.

Swath determines how much ground passes through the view

Swath is the width of the strip of Earth's surface that an instrument can observe as the satellite moves along its orbit.

A wider swath lets a spacecraft collect information across more territory during a pass. A narrower view can support greater spatial detail but covers less ground at once.

Real instruments show how differently engineers can approach the problem. ESA's EarthCARE multispectral imager observes a 150-kilometer swath at 500-meter resolution. Sentinel-2, which observes in visible, near-infrared and shortwave-infrared bands rather than thermal infrared, combines a 290-kilometer swath with bands operating at several spatial resolutions.

More satellites can shorten the wait for another look

Coverage also has a time component. Revisit time describes how long it takes a satellite system to observe the same area again. A spacecraft can only see a portion of Earth during a given pass, and its orbit determines when it can return.

A constellation gives operators another way to minimize that wait. Instead of relying on the same spacecraft to come back around, another satellite can provide the next observation.

This can be useful when conditions change quickly. Wildfires, crop stress and other events can develop between observations, making the frequency of measurements important alongside their spatial detail.

Higher spatial resolution generally comes with a narrower swath, according to ESA, which can increase revisit time for a given location unless operators use approaches such as steerable instruments. Adding spacecraft provides another way to increase observation frequency.

Infrared detectors bring thermal requirements

The detector itself adds another engineering problem. Some infrared detectors work best at very low temperatures. Cooling reduces detector noise and can improve the instrument's ability to make sensitive measurements, but providing that cooling aboard a spacecraft requires hardware and power.

For example, infrared detectors on the HIRS/4 instrument used a two-stage radiant cooler to operate at about 95 Kelvin. NASA is also developing compact active-cooling technology specifically to make higher-performance thermal-infrared observations practical on smaller satellites.

The system then becomes part of the spacecraft design. Engineers must account for its electrical demand, mass, volume and thermal behavior alongside the detector and optics. This can become particularly important on smaller satellites, where available power and physical space are limited.

A constellation also has to be practical to build

Designing one payload does not finish the job when the mission calls for dozens of spacecraft. Constellation operators have to build, test and integrate multiple copies of the hardware while maintaining enough consistency for the satellites to perform their intended missions. Cost also takes on a different meaning when a component will appear across a fleet rather than on a single spacecraft.

That puts more emphasis on repeatable manufacturing and testing. A design that works for a small number of specialized spacecraft may be more difficult or expensive to reproduce at the rate required for a larger constellation.

That is part of the problem Lynred and Constellr are now addressing. The companies announced plans in September to work on infrared detection technology for Constellr's future Earth observation satellites, with deployment scheduled to begin in 2028. Constellr's roadmap calls for several dozen infrared satellites.

Their longer-term work is expected to examine new detectors and infrared architectures intended to improve spatial resolution, swath and revisit frequency while keeping cost, size, power consumption and manufacturing compatible with a commercial constellation.