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Onboard Autonomous Navigation (OBAN)
In recent years, spacecraft destined to explore the Solar System have become more compact and complex. One of the key elements in the general trend of 'Faster, cheaper, better' is the principle of autonomous operations.
When missions to other planets often hundreds of millions of kilometres away take years of precise navigation to be successful, the more a spacecraft can take care of itself, the easier it is for the mission teams back on Earth.
One of the important components of onboard autonomous operations is navigation, the determination of a spacecraft's trajectory and the computation of the corrections required to reach a target.
Teams managing spacecraft operations are today often mobilised for months monitoring a spacecraft as it progresses. A course manoeuvre can be preceded by weeks of intensive calculations at ground level. Such operations can be costly.
To date, the only interplanetary mission to feature an onboard autonomous navigation system is the NASA asteroid fly-by probe, Deep Space 1 (DS1), launched in October 1998. Following in its steps, SMART-1 will be contributing to a European capability in this field with OBAN, an experiment on OnBoard Autonomous Navigation for interplanetary missions.
OBAN is part of a study on 'Autonomous Onboard Navigation for Interplanetary Missions', of ESA's Technology and Research Programme. It is being managed by Finn Ankersen, Guidance, Navigation and Control analyst at ESTEC, Noordwijk, the Netherlands, in cooperation with the European Space Operations Centre (J. Fertig), ESOC in Darmstadt, Germany.
Some basic geometry
SMART-1 will not itself be relying on OBAN for its own guidance and navigation which will be managed in a conventional way from the SMART-1 Mission Operations Centre at ESOC. The OBAN experiment will function in what is termed an 'open loop', obtaining all the data an autonomous navigation system would require, but instead of being processed onboard, this information will be sent back to be processed on Earth.
The experiment will involve the spacecraft looking at certain celestial objects or 'beacons', taking images of them with the AMIE camera. These time-tagged pictures, together with information from SMART-1's Attitude and Control Systems will allow the OBAN ground system to determine the lines of sight to the beacons, and calculate the precise trajectory. Subsequently all the parameters that would be required to carry out a correction in the spacecraft's course can be obtained. The picture analysis algorithms and computations will be extremely complex, but the principles of the experiment are those of elemental triangulation geometry.
"Although we will have carried out simulations before the launch with synthetic images, the challenge of the OBAN experiment will be to get the algorithms to work properly using real images obtained in flight," says Finn Ankersen.
Onboard autonomy in navigation does not mean that a spacecraft will be able to "do its own thing", capable of deciding to radically change its route. It will be programmed to act within certain constraints to reach its destination, but with the advantage of better handling any flight disturbances, such as precise gravity fields and solar flux that may be encountered.
Autonomous navigation will never eliminate the need for ground operations, if only to receive the science data and because spacecraft housekeeping will always be necessary. But onboard autonomy will eventually allow less frequent contact periods and hence considerably decrease mission costs.
The OBAN experiment will be conducted several times whilst SMART-1 spirals out from Earth to reach the Moon. Since the required pictures will mean slewing the spacecraft to point towards one beacon body, then a second, the experiment will take place when the solar propulsion is not working. The beacons to be used will be the Earth, the Moon and perhaps asteroids.
With the OBAN experiment on SMART-1, European onboard autonomous navigation will be passing from a preliminary study to an experimental phase. Its full operational use on future missions is not far off. ESA's projected mission to Mercury may be too far advanced to benefit, but it is reasonable to believe that all interplanetary missions in the future will feature such a system.
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