Formation flying satellites

23 May 2011 | By Ellie Zolfagharifard

In June 2010, two satellites, Mango and Tango, were launched from Yasny, Russia, by the Swedish Space Corporation. Their mission: to undertake the dangerous act of flying in close formation a task so difficult that a slight drift could damage one or both satellites, scattering billions of pounds’ worth of technology across space.

The satellites, jointly developed by German, French and Danish engineers, were part of the PRISMA project, one of a growing number of initiatives looking at the development of formation-flying satellites.

It’s an area with a unique set of challenges, explained Luis Gomes, head of Earth observation at Surrey Satellite Technology (SSTL), which is also investigating formation flying. ’When we get these very close formations, tens of metres away, implementing control can be a major problem,’ he said. ’Satellites flying in this way are extremely sensitive. You need very low thrust power and precise sensors. It’s almost like having a 200kg spacecraft that you move by blowing on it to gently nudge it in a certain direction.’

Nevertheless, a number of global space organisations are investigating the technology. Germany’s TanDEM-X satellite was launched last year to join TerraSar-X in a formation mission to capture detailed radar imagery. The European Space Agency’s (ESA’s) Proba-3, designed to validate control techniques for formation flight, will launch in 2013. Meanwhile, the US Defense Advanced Research Projects Agency plans to fly a six-satellite mission, the F6 (Future, Fast, Flexible, Fractionated Free-Flying Spacecraft United by Information) in less than three years.

As well as precise control systems, these satellites are equipped with optical and GPS sensors that can detect small distances between spacecraft. A basic structure is to have a single master satellite with ’slave’ satellites positioned around it. The structure can be mathematically difficult to implement, said Gomes, because orbits do not follow straight lines and the chances are that the satellites’ paths will cross.

The data will help develop future close formation-flying satellites and swarms of nano-satellites

To prevent this on PRISMA, a GPS receiver the size of a credit card is used within the navigation system. Data from the Tango GPS receiver is sent to the main satellite, Mango. By comparing them with the GPS data from the receiver on Mango, the distance between the satellites can be determined to within 10cm in real time. Mango can use this information to control its path in relation to Tango. Any slight inaccuracy could lead to the destruction of both satellites.

So, why go to such lengths to fly satellites in formation when the risks are so high? ’There are some things that you cannot do without formation-flying satellites,’ said Gomes. ’Telescopes are an interesting application because the deployment of a set of satellites can combine the light from all of them, providing a better resolution. Some of our large telescopes are now reaching the limits of mirror size. So this can be one of the ways that we will find other planets, particularly those that are like Earth.’

A similar technique for radio astronomy has been proposed by the Dutch Orbiting Low-Frequency Array (OLFAR) project. Taking inspiration from the movement of a swarm of bees, the project is hoped to place swarms of nano-satellites in orbit around the moon. These nano-satellites will receive frequencies in a band ranging from 30kHz to 30MHz.

For scientists, this is of huge interest for the discovery of planetary and solar bursts in other solar systems and obtaining a tomographic view of space weather.

Gomes predicts that formation-flying satellites could also be important in space warfare. ’When your enemy is trying to interfere with your equipment, if you have distributed systems it becomes much more difficult. You can also leave your system operating in a loose formation and only bring it together when it’s needed. That reduces the perceived threat the enemy will not realise what the distributed system is until it’s formed and operating.’

“When your enemy is trying to interfere with your equipment, if you have distributed systems it becomes much more difficult”

One issue with developing formation-flying satellites is the power required to maintain formation. Dr Mini Saaj at the Surrey Space Centre is looking at the use of hybrid electric propulsion systems to help solve this problem. The system would scavenge electromagnetic power from drifting space plasma or the Earth’s magnetosphere to maintain position and reduce on-board power requirements. ’I’m also interested in how swarms can be shaped into formations while avoiding collisions,’ she said. ’Techniques borrowed from terrestrial robotics, such as artificial potential fields, could help steer the system.’ Like the OLFAR project, this approach would look to the natural world to animal swarms and herds using shared rules to direct flexible group behaviour.

While these long-term projects may make swarms seem like an exciting proposal, Gomes is less enthusiastic. ’I’m not a fan of close formation flying, especially when it comes to swarms,’ he said. ’There is a guy at NASA whose opinion of swarms is that they are just debris. He has a point. Imagine you have 14 nano-satellites in constellation. If one of those fails, it could knock out several others, if not the entire constellation, causing a cascade of destruction and debris.’ However, Alberto Tobias, head of ESA’s systems, software and technology department, does not believe this would be a problem as long as adequate control systems are in place. He said: ’Intrinsically, formation-flying satellites do not create debris as they have actuation technologies, so they can be controlled.’

Scientists have proposed limiting the flight of swarms to orbital altitudes to reduce risk. Gomes agrees this could be a good solution, and added that the most exciting application for swarms lies in the exploration of unknown environments out of Earth’s orbit. With the emphasis in Europe moving away from human space exploration, satellite swarms could be the future. Whether the technology helps us to understand our universe, or merely ends up as space debris, remains to be seen.