Sensor

Sun-synchronous orbit and LEO

Why Earth observation satellites all live in similar orbits at similar altitudes — and why that drives revisit times.

SUNNorbit plane ~98°sun directionSUN-SYNCHRONOUS ORBITtypical altitude 500–800 km orbital period ~90–100 min passes per day 14–16 precession rate ≈0.986°/day (matches Earth's orbital rate)local time at descending node is constant: ~10:30 AM for Sentinel-2 ~10:00 AM for Landsat
Fig. 1 Sun-synchronous orbit precesses at exactly the rate Earth revolves around the sun (≈0.986°/day), so the orbit plane maintains a fixed angle to the sun. Every pass over a given latitude happens at the same local solar time — consistent illumination across acquisitions, essential for analytical comparability.

Most Earth observation satellites sit in sun-synchronous low Earth orbit, circling the planet every 90 minutes or so at altitudes between 500 and 800 km. Sun-synchronous means the orbit is timed so the satellite passes over each point at roughly the same local solar time every day — usually mid-morning. This makes illumination conditions consistent across acquisitions, which is critical for time-series and analytical work.

Why this constrains revisit

From sun-synchronous LEO, any given point on Earth only sits under the satellite's accessible swath every few days, sometimes longer at the equator. Off-nadir pointing — tilting the satellite to look sideways — squeezes more opportunities out of each pass, but extreme off-nadir angles degrade image quality. A single satellite will almost never give you frequent monitoring; you need a constellation, and constellation size is the single biggest lever on how fast a provider can respond to tasking.