A satellite's to-do list
An Earth-imaging satellite in low orbit circles the planet about every 95 minutes, at roughly 7.5 km/s. In that time it has to keep its batteries charged, hold its orbit, point at the right places, store what it sees, and hand the data to the ground, often all at once.
Soak up the Sun
Solar panels turn to face the Sun while the satellite is in daylight, charging the batteries. For up to about 35 minutes of every orbit the satellite passes through Earth's shadow. The panels go dark and everything runs on battery.
So the power budget is planned orbit by orbit. A camera, a radio and the heaters can't all run at full power in eclipse.
Stay on track
A perfect orbit only exists in textbooks. Real ones get nudged, or perturbed:
- J2, Earth's bulge. Earth's equator sticks out about 21 km farther from its center than the poles do. That extra mass tugs on the orbit and slowly swings its plane around Earth's axis. Engineers name it after its term in the gravity model, J2 (about 0.00108).
- Air drag. Even 500 km up there's a trace of air, and it slowly lowers the orbit.
- The Sun and Moon pull too, and so does sunlight itself (solar radiation pressure).
Stationkeeping is the small, regular thruster burns that undo the drift. J2 isn't always the enemy, though. Sun-synchronous orbits are tilted so J2 turns the orbit about one degree a day, exactly keeping pace with Earth's trip around the Sun. Every pass then sees the ground at the same local time.
Pick what to photograph
Customers ask for far more pictures than one pass can take. The satellite has to slew, turning its whole body to aim the camera, then hold steady while it images. Every slew eats seconds while the ground races past below at about 7 km/s.
So each pass is a puzzle: which targets to take, and in what order, given turn times, priorities, sun angle and clouds? Planners solve it with scheduling algorithms. Some targets wait for the next pass.
Store it, then send it home
Images go onto an onboard recorder until the satellite passes over a ground station. In low orbit a pass lasts only about 10 minutes, and a station sees the satellite just a handful of times a day.
The radio has to empty the recorder fast, at hundreds of megabits per second or more, before the satellite drops below the horizon. If it can't, the oldest or lowest-priority data waits, or new pictures get skipped.
One orbit, every job
A simplified 95-minute orbit. The playhead sweeps across while each job takes its turn, and some overlap. Flight software and the planners on the ground juggle these every orbit, all day.
Times and rates are typical for a small Earth-imaging satellite in low orbit and vary a lot between missions.