Sun-synchronous orbit

In yesterday’s post we suggested that the reason for the near polar orbit of most imaging satellites was to improve coverage. After a bit more research it turns out to be more interesting than that. Apparently there is a special orbit called Sun-synchronous orbit, which is designed such that the satellite always crosses a given latitude on the ‘day’ side of the Earth at the same time of day. This has two benefits:

• if two neighbouring images, or images of the same location are captured on consecutive passes of the satellite, they will have the same lighting (both the overall brightness as well as the length and direction of shadows), which makes it much easier to stitch them together in the case of side by side images or comparing for changes in the case of images of the same location.
• the orbit can be positioned such that the satellite is always overhead as close to noon as possible (or whatever time is considered the best for imaging) for the latitudes of greatest interest. A non Sun-synchronous near polar orbit would result in some days when the satellite is orbiting in a plane at 90 degrees to the direction of the sun, which would make it constantly sunrise or sunset.

A Sun-synchronous orbit has a particular inclination depending on the altitude of the satellite. The lower the altitude, the closer the orbit is to the north-south direction. For a table of altitude vs. maximum latitude, and other technical details, see Wikipedia.

As far as we can tell almost all imaging satellites are in Sun-synchronous orbit, including SkySat-1 and SkySat-2. They also all have altitudes in the 500-900 km range, meaning they should all have similarly tilted orbits with a maximum latitude of around 82 degrees.

Thank you to GEB readers franksvalli2 and Vasilis for letting us know that the mystery image in yesterday’s post is almost certainly from one of the SkySat satellites. See this PDF file for details on those satellites, including their sensor arrangements which creates the distinctive ‘Y’ pattern. Also thank you to GEB reader Daniel Plant for bringing our attention to TeLEOS 1, which, as you can see here has a very different orbit.

We used our circle drawing tool to estimate the orbit of the satellite that took yesterday’s image and it came remarkably close to the expected 82 degree maximum latitude.

Although DigitalGlobe imagery in Google Earth is typically in both vertical and horizontal stripes, we believe that the imagery is actually captured by a satellite following a Sun-synchronous orbit very similar to the Sky-Sat orbits. We believe that the almost perfect north-south or east-west alignment of the DigitalGlobe strips is for some reason other than orbit. We have noted in the past that they line up with degrees of latitude and longitude.

For more interesting reading about orbits see this article from NASA.