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Published 27 March 2013 Referencing Hub media

A Sun-synchronous orbit matches the rate at which the Earth goes around the Sun. It is a low-Earth orbit.

Advantage: consistent lighting conditions of the Earth’s surface enable us to compare images from the same season over several years

Altitude: typically 600–800 km

Satellite period: 96–100 minutes

Satellite examples: Landsat 7, CloudSat

Dr Allan McInnes

Sun-synchronous orbit is a special kind of orbit. Wow, this is where we get into the complexities or orbit mechanics. So orbits are not fixed in space, they tend to change over time, and one of the things that makes an orbit change is the shape of the Earth. And in the case of the shape of the Earth, one of the changes that we see with orbits is something called precession of the orbit, and precession basically means that the orbit moves relative to the Earth over time. So you’re not just orbiting around the Earth – the circle of the orbit is actually shifting in space as well.

Normally that’s something that we either ignore or counter the effects of by manoeuvring the spacecraft. But with a Sun-synchronous orbit, what we actually try to do is take advantage of that. If we pick the right altitude and the right inclination relative to the equator, we can actually get a precession rate at which that orbit changes that just happens to exactly match the rate at which the Earth goes around the Sun.

And what that means is that, if we put a spacecraft into an orbit where, when it initially takes off and is flying around the Earth, it spends part of its time directly over a point that’s seeing midday Sun and the other half of its orbit over the side of the Earth where it’s exactly at midnight, we’re going to maintain that all the way through the year, because as the Earth moves around the Sun, the orbit’s also shifting. If we weren’t in a Sun-synchronous orbit, then we might start out seeing noon and midnight and then later in the year we’d be seeing some other time of day, and it would change over time.

With the Sun-synchronous orbit, we’re locked to the Sun essentially, and so if we start out seeing noon and midnight, we’ll always see noon and midnight. And that can be quite useful for observation and scientific missions where we want to get consistent lighting conditions on the ground. So if we always want to be over something with nice bright midday Sun then we’ll make sure we always see that with the Sun-synchronous orbit.

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