Whither Small Satellites
Stuart Eves

Crucially, this means that not only can a small satellite constellation operate in a wider range of near-polar sun-synchronous orbits, but it can also operate effectively in an orbit with an inclination angle much closer to the Equator. The effect of using lower inclination orbits, e.g. 60-70 degrees to the Equator, is that the coverage provided by the constellation is now concentrated over lower latitude regions – which in practice is where the majority of the world’s population lives, and hence where most of the required coverage is needed.

The net effect of using a lower inclination orbit is that an individual satellite in the constellation will pass over a given region of interest more frequently - a typical figure being twice as many usable, sunlit passes over the course of a year, although this will vary with inclination and latitude.

However, due to the inevitable regression of its orbit plane, for a typical LEO satellite there will be periods lasting three or four weeks when it will only pass over the target region at night. This is where the benefits of having a constellation become apparent, since by using multiple orbit planes, a second, third, fourth or fifth satellite can provide coverage when the first is unavailable.

There is a further advantage in using lower inclination orbits. Because the payload mass for a launch vehicle is usually greater to a lower inclination orbit, this allows the constellation to be launched more cost-effectively. It is anticipated that commercial news constellations and systems providing coverage over very wide areas (for climate change, national security, maritime safety, resource exploitation, etc.), will derive particular benefit from such constellations in the future.

By the end of the coming decade, it is anticipated that inter-satellite links will allow these surveillance systems to receive commands and return data in a more timely fashion, further encouraging the use of constellations with more frequent over-flights and a greater range of orbit planes. This is already happening with the SAR-Lupe constellation where the five satellites, which operate in three different orbital planes, interact via inter-satellite links when they are in line of sight with one another to transfer targeting data.

Indeed, we envisage this trend increasing to the point where virtually all satellites will routinely be equipped with inter-satellite links, capable of passing not only command and state of health telemetry, but also imagery and other payload information at much higher data rates. Linked into the terrestrial communication network, this inter-satellite system will form the “Space Wide Web”