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Art In Space - Or, How To Set Up A Formation
by Ralph Kahle
Bonn, Germany (SPX) Jul 19, 2010

Movement of TanDEM-X relative to TerraSAR-X (in crosshairs) as viewed by an observer flying behind TerraSAR-X. The Earth is 514 kilometres below TerraSAR-X. The relative ellipse alters its shape with every manoeuvre. Between 18 July (turquoise path) and 21 July (red path), TanDEM-X will be performing five orbital manoeuvres.

On its launch date, 21 June 2010, roughly 16,000 kilometres separated TanDEM-X from its twin satellite, TerraSAR-X. Now, that distance has shrunk to just 2000 kilometres. The time has come for the relative movement between the two satellites to be slowed down, and for them to be set up for formation flying.

To accomplish this, the Flight Dynamics Group at the German Space Operations Center (GSOC) will carry out a total of 10 orbital manoeuvres over the next eight days.

The orbit of TanDEM-X was set shortly after its launch to an altitude some 4.6 kilometres lower than that of TerraSAR-X, meaning that TanDEM-X has a slightly higher orbital speed.

This means that the two satellites have been approaching one another at a rate of approximately 630 kilometres per day - not much, when you consider that they are flying at a speed of 27,000 kilometres per hour. Without alterations to its orbit, TanDEM-X will overtake its twin within the next three days.

However, the radar instrument team wants to achieve a large-separation formation; one in which TanDEM-X follows TerraSAR-X at a distance of 20 kilometres, covering the same track across the surface of the Earth. This formation will be retained until September, to allow the radar to be calibrated.

Setting up the formation must take place in stages. On the one hand, the satellite only has four one-newton propulsion units - which equates to the force of four bars of chocolate on a set of kitchen scales. Given that it weighs in excess of 1300 kilograms, it follows that these propulsion units have to operate for a long time to effect small changes in speed.

The manoeuvre, for example, will last for 421 seconds, and will alter the orbital speed by 1.14 metres per second. That might not sound like much, but it is enough to raise the mean orbit altitude by 2000 metres. A longer manoeuvre would have a negative impact on the effectiveness of the orbital correction.

On the other hand, it is necessary to eliminate any risk of the two satellites colliding with one another. To accomplish this, the relative speed will be reduced in small steps. This means that by 18 July the satellites will only be approaching one another at the rate of about 140 kilometres each day.

In the event that it proves impossible to carry out the final deceleration manoeuvre on 18 July, as a consequence of problems on the ground or on the satellite, the two satellites would move past one another at a safe distance of more than 250 metres.

After this deceleration process, a further five manoeuvres are envisaged to precisely adjust the orbit's plane and ellipsoidal shape. If everything runs on schedule, it should be possible to start the next calibration phase for the radar instrument on 22 July.


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