Whither Small Satellites
Stuart Eves

A number of these technology trends can be seen in SSTL’s latest surveillance satellite. Called the SSTL 300, because the mass of the system is roughly 300 kg, it will deliver both high-quality 2.5 m resolution imagery and wide-area 32m surveillance data from an orbital altitude of 700 km. It is equipped with a main camera with a primary aperture of close to 40 cm, and an array of smaller cameras which provide the wide-area “peripheral vision” data which will allow the high resolution sensor to be targeted far more efficiently.

The increased resolution on this satellite creates a demand for increased on-board data storage and also an enhanced downlink data rate. The latter demand is satisfied by a pair of steerable x-band data downlink antennas which can track the location of a data reception station on the surface of the Earth, and so achieve downlink data rates in excess of 100 Mbit/s.

The most radical feature of this satellite’s design is its agility. Large satellites tend to conduct their imaging operations at a fixed orientation to the Earth below. This is because, with deployed solar panels and deployed antennas, they require significant settling time following manoeuvres before they are stable enough to generate high-quality images. By contrast, the compact design of the SSTL-300, and the novel optical bench on which its imagers are mounted, allows it to make rapid re-orientation manoeuvres and then collect data almost immediately.

As a consequence, it becomes possible for the platform to support a number of novel modes of operation:

• Point target mode
  IIn this mode, the satellite is able to rapidly switch between geographically separated point targets. This agility allows the satellite to collect a group of relatively closely-spaced targets in a single pass, rather than having to build up the required coverage over a series of passes – a process which can take several days.
  
• Area mode
  AArea mode involves the satellite performing a series of imaging operations as it approaches the target region, then pitching backwards and making a small roll manoeuvre to permit a further strip of imagery to be collected as the satellite passes overhead, and then a further combination of pitch and roll manoeuvres to enable a third strip of imagery to be collected as the satellite recedes from its target.
  
• Line of communication mode
  TThe agility of the platform allows the satellite to follow a linear feature on the ground by interspersing periods of manoeuvring with periods of imaging. Originally designed for following roads, rivers and railways, this mode can additionally be used for following borders, coastlines and man-made features such as oil pipelines.
  
• Stereo mode
  AAn advantage of smaller satellites is that they can achieve the angular pitch rates required to capture two images of a given location in a single pass. This allows three-dimensional data to be collected which has a high degree of correlation from image to image.
  
• Super-resolution mode
  This mode exploits the fact that by yawing the satellite and then pitching backwards, it is possible to oversample the target region both along track and across track simultaneously. With appropriate processing, it is then possible to use this data to improve the effective resolution of the imagery by approximately 40%.