Whither Small Satellites
Stuart Eves

By comparison with previous small satellite designs, the SSTL-300 has a unique configuration. In part this is to allow the imager to be mounted centrally within the structure, and hence benefit from the extremely stable thermal environment in the centre of the structure. However, it is also a result of a desire to create a design which will make more efficient use of the volume inside a launch vehicle shroud.

In most cases, the limiting factor for a multiple satellite launch is not mass but volume. The above illustration shows how three SSTL-300 satellites can be accommodated, and it is clearly cost-effective to build multiple satellite constellations with as few launches as possible.

There are a number of specific advantages that these surveillance constellations offer:-

• More frequent coverage opportunities
  IIt is evident that satellite constellations can offer much greater timeliness than single large satellites. The latter can typically revisit every three days, whereas small satellite constellations comprising 5 satellites in a single plane can provide global daily access opportunities.
  
• Increased capacity
  IIt is increasingly recognised that “area coverage rate” is just as relevant a parameter for characterising satellite systems as resolution, and clearly satellite constellations perform well in this respect.
  
• Increased robustness
  Satellite constellations show far more graceful performance degradation in the event of a single satellite failure.

These advantages can be enhanced if the constellations are themselves composed of satellites with multiple operational modes. The first satellites to pass over a region of interest can be used to collect wide area data which can then be used to task subsequent satellites employing more localised modes far more efficiently.

There are further advantages that accrue for surveillance constellations when the satellites that comprise them possess high agility. Traditional optical surveillance systems utilise sun-synchronous orbits that optimise the lighting conditions. The objective, for a large traditional satellite, is to collect sufficient light from the scene below in the time that it takes for the sensor to pass over the region of interest. By contrast, small satellites have the ability to pitch backwards as they pass over the target location, slowing the effective ground rate of the sensor significantly and enabling the satellite’s sensor to gather significantly more light. In consequence, a small satellite employing a series of pitch modes can collect usable data at a much wider range of local times of day.