Most Earth observation satellites fly in a sun-synchronous orbit, or SSO. It is also the most common destination for rideshare missions. This guide explains what makes an orbit sun-synchronous, why imaging missions depend on it and what operators need to specify when they book a launch to SSO.
What sun-synchronous means
A sun-synchronous orbit is a near polar orbit whose plane slowly rotates by about one degree per day, which matches the Earth's yearly path around the Sun. As a result, the satellite passes over any given point on the ground at roughly the same local solar time every day. The rotation is caused by the slight bulge of the Earth at the equator, and it only works for a specific combination of altitude and inclination. For typical altitudes between about 500 and 800 kilometres, the inclination is slightly above 97 degrees.
Why Earth observation needs it
- Consistent lighting: images taken on different days show similar shadows and sun angles, which makes changes on the ground easier to detect.
- Global coverage: the near polar path lets the satellite see almost the entire planet as the Earth rotates underneath.
- Stable power and thermal conditions: the geometry between the satellite, the Sun and the Earth stays similar throughout the year.
An example: Landsat 9
NASA and the US Geological Survey operate Landsat 9 in a sun-synchronous orbit at an altitude of about 705 kilometres. It crosses the equator in the morning and, together with Landsat 8, provides repeat coverage of the Earth's land surface every eight days.
Local time of the node
The key parameter for an SSO mission is the local time of the ascending or descending node, meaning the local solar time at which the satellite crosses the equator. Morning orbits, often around 10:30, avoid afternoon clouds in many regions. Dawn dusk orbits, which follow the boundary between day and night, give the satellite almost constant sunlight on its solar arrays and are popular with radar satellites.
What to clarify when booking a launch
- The target altitude and the tolerance the satellite can accept.
- The local time of the node and whether the rideshare mission offers the right one.
- How the satellite or an orbital transfer vehicle will correct differences between the drop off orbit and the target orbit.
Frequently asked questions
What altitude is a sun-synchronous orbit?
According to ESA, a satellite in a sun-synchronous orbit usually flies between 600 and 800 km. At 800 km it travels at approximately 7.5 km per second.
What is the local time of the node?
The local solar time at which the satellite crosses the equator. It stays the same every day and is a key parameter when booking an SSO launch.
Is heliosynchronous orbit the same as sun-synchronous orbit?
Yes. Heliosynchronous is another name for the same orbit, from the Greek word for sun. See our article on the heliosynchronous orbit.
Related reading
- Heliosynchronous Orbit: What It Means and Why Earth Observation Depends on It
- LEO, MEO and GEO Satellites Compared: Altitudes, Uses and Examples
- Europe's launch sites: Kourou, Andøya, SaxaVord and Esrange compared
Sources
Cover image: The Landsat 9 satellite during launch preparations, 2021. Credit: NASA (KSC-20210720-PH-RNB01_0022), public domain.