A sun-synchronous orbit is a near-polar orbit whose plane slowly turns with the Sun, so the satellite passes over each place at about the same local time every day.
Picture a satellite that crosses the equator at 10:30 in the morning, local time, on every pass, every day of the year. Shadows on its photos always point the same way and have about the same length. That is what a sun-synchronous-orbit gives. NASA's Terra satellite, for example, crosses the equator at about 10:30 a.m.
Most sun-synchronous satellites fly between 600 and 800 km up, in near-polar orbits.
Earth is not a perfect ball. It bulges slightly at the equator. That extra mass tugs on a satellite's orbit and slowly swings the whole orbit plane around Earth's axis. NASA calls this a walking, or precessing, orbit.
Engineers choose the height and tilt so the plane swings round exactly once a year — about 1° a day eastward — to keep pace with Earth's journey around the Sun. For a satellite a few hundred kilometres up, that means an inclination a little over 90°, so the satellite travels slightly against Earth's spin.
Earth takes 365.256 days to go round the Sun, so the Sun's direction, seen from Earth, shifts by 360° / 365.256 = 0.9856° each day.
If the orbit plane did not turn at all, it would fall out of step with the Sun. After three months it would be about 90° out — and since Earth turns 15° an hour, that would move the satellite's equator crossing by about six hours. A satellite crossing at 10:30 a.m. in January would be crossing around dawn or mid-afternoon by April.
Cartosat-3 was launched on PSLV-C47 from Sriharikota on 27 November 2019 into a sun-synchronous orbit. ISRO gives its mean altitude as 509 km and its inclination as 97.5°. Repeat images taken at the same local time can be laid side by side to spot changes on the ground.
The orbit does not stay perfect forever. NASA notes that sun-synchronous satellites need regular adjustments, with their inclination tuned every year or two.
Because Cartosat-3's inclination is 97.5°, its path never quite reaches the poles: the track turns around at about 82.5° north and south. Track Cartosat-3
The equatorial bulge is described by the coefficient J2 = 1.08263 × 10⁻³. To first order it makes the ascending node of an orbit drift at the rate
dΩ/dt = −1.5 · n · J2 · (R/a)² · cos i / (1 − e²)²
where n is the mean motion (degrees per day), R = 6,378 km and a is the semi-major axis.
For sun-synchronous motion the node must advance 360° per year: 360 / 365.256 = +0.9856° per day. Since everything else in the formula is positive, cos i must be negative — the orbit must be retrograde, i > 90°.
Worked numbers for Cartosat-3 (circular, 509 km):
ISRO quotes 97.5° and the CelesTrak catalogue lists 97.41° — a close match for a first-order formula.
The same formula shows why sun-synchronous orbits have a ceiling. The factor (R/a)² · n falls quickly with height, so higher orbits need larger tilts. Eventually |cos i| would have to exceed 1, and no sun-synchronous orbit exists.
The orbit plane must turn eastward about 1° per day. Earth's bulge turns a retrograde orbit (inclination above 90°) eastward, so a tilt a little past 90° gives the right rate.
They pass over each place at the same local time, so lighting and shadows are similar on every image. Pictures from different dates can then be compared fairly.
180° − 97.5° = 82.5°, north and south.