ESA Science & Technology - Cluster interference campaign successfully completed.

"We are not aware that any previous missions have done this but we felt it was essential in order to ensure that Cluster produces top quality science," said Cluster project scientist Philippe Escoubet.

"As expected, we have experienced a few minor difficulties involving one or two instruments, but we are confident that we can work around these and that there are no 'show stoppers' to prevent the start of the Cluster science programme early next year."

"Everything is now set up so that we can operate all of the instruments at the same time," he continued. "Everyone's hard work has paid off and we are delighted with the successful outcome."

The potential operational conflict is largely due to the complex set of 11 instruments on each spacecraft. Most experiments are 'passive' and simply measure the space environment through which the spacecraft are passing, but some experiments (EDI, WHISPER, EFW, ASPOC) actively probe near-Earth space.

All of the active instruments were designed so that they should be compatible with their neighbours. However, it was not possible to precisely forecast or test for all of the possible interference effects until the spacecraft were in orbit and the experiments were commissioned.

During the interference campaign, the scientists investigated "worst case scenarios" by looking for any unforeseen effects that could perturb the space plasma (electrified gas) and so adversely affect the rest of the science payload. Once these were found, the challenge was to find out how to operate the instruments in question successfully while limiting their impact on the overall science data.

One source of minor interference involving the Electron Drift Instrument (EDI) has been identified. EDI measures electrical fields around the spacecraft by firing beams of electrons into space and measuring the return time for each beam. These electron 'packets' appear to be causing some interference with the wave instruments, especially WHISPER, WBD and STAFF.

"We are continuing to look at the data to see whether EDI is influencing these instruments in all of their operational modes, commented Dr. Escoubet. "However, we can reduce the interference by reducing the current sent by EDI, which is fine for normal operations."

Two additional minor interactions between instruments have been uncovered during the campaign. One of these involves EDI's electron beams, which travel along a circle for distances ranging from a few tens of metres up to about 10 km and then come back to the spacecraft.

"The electrons from EDI are detected when they are fired in the direction of the PEACE instrument on the same spacecraft," said Dr. Escoubet. "But this was done on purpose for our campaign. Usually the beams go in all directions, so it is most unlikely that this will happen in normal operations. And even if it does occur occasionally, we can easily remove the effect during analysis of the data."

The other side-effect involves WHISPER, an instrument that measures plasma density around the spacecraft by emitting pulses of electromagnetic waves and detecting the 'echoes'.

"We see that these pulses excite the plasma, and this causes the PEACE instrument to register false counts of electrons at regular intervals," explained Philippe Escoubet. "However, this problem has been recognised and we are now looking at ways of working around it. For instance, WHISPER can operate in a synchronised mode which so far has shown no interference."

Meanwhile, after some small "constellation manoeuvres" on 21 December, the Cluster quartet are now in the correct orbits to form a perfect tetrahedron for the cusp crossings next February. At this time, the separation distance between each spacecraft will be almost exactly 600 km. Taking advantage of this unusual spatial spread, the suite of instruments on each satellite will provide a unique three-dimensional set of data as they sweep through the cusp regions over Earth's magnetic poles.