SunSynchronous

Sun-synchronous orbit is a near-polar orbit designed so that the satellite passes over the same portion of Earth's surface at roughly the same local solar time on every pass. The key feature is a deliberate regression of the orbital plane caused by Earth’s equatorial bulge (the J2 term), which makes the orbital plane rotate westward about 1 degree per day. This precession keeps the Sun’s angle relative to the orbit nearly constant for a given latitude, producing similar lighting conditions for successive observations.

The stability of illumination makes sun-synchronous orbits ideal for Earth observation and remote sensing, where consistent shadows and solar angles improve image comparability over time. To achieve this, the orbit is typically near-polar with a retrograde inclination of about 97.5 to 98 degrees and an altitude in the range of roughly 600 to 800 kilometers. The local solar time of the ascending node is chosen to suit mission needs, often aiming for morning or early-afternoon lighting.

Common uses include imaging, environmental monitoring, agriculture, and disaster assessment, with several prominent satellites and missions operating in sun-synchronous orbits, such as Landsat, SPOT, and the Sentinel series. Limitations include long-term drift in repeat cycles due to natural perturbations and atmospheric drag at lower altitudes, which can require occasional reboosts. Ground tracks repeat only approximately, and mission design must account for weather and illumination constraints.