A Sun-synchronous orbit is a near-polar orbit tuned so its plane rotates once around Earth per year, keeping the satellite crossing the equator at the same local solar time every day. That constant lighting makes it the standard orbit for Earth-imaging satellites.
Understanding SSO
How oblateness drives the orbit
Earth is about 21 km wider at the equator than pole-to-pole, and that extra mass tugs asymmetrically on any inclined orbit, causing the orbital plane to swivel slowly around Earth's axis — nodal precession. The rate depends on altitude, eccentricity and inclination; at 98° inclination and ~700 km it comes out to +0.9856° per day, one full lap per year. An SSO is simply the family of altitude-inclination pairs that hit this magic rate: higher orbits need slightly more inclination.
Why imaging missions care
Change detection is the heart of Earth observation — comparing today's image with last month's. Consistent Sun angle removes the biggest confounding variable: shadows fall the same way in every scene, radiometry stays comparable, and mosaics stitch cleanly. Constellations spread satellites across several crossing times (early morning for calm atmospherics, mid-morning for balanced light) to build a coherent daily picture of the planet.