Planet Earth: SWARM Satellites to Monitor Earth's Magnetic Field.

In this week's Planet Earth, Sue Nelson has been along to find out about a European Space Agency mission called Swarm to study the earth’s magnetic field from orbit using a...
19 April 2011

Interview with 

Julia Ryan and Ralph Cordey from Astrium UK’s division of Earth Observation Science


 The earth is a giant magnet with its magnetic north pole at the earths south pole and its magnetic south pole at the earths north pole. The magnetised needles north pole is attracted to the earths magnetic south pole so the needle points towards the...


Diana -   Time now for this week's Planet Earth, and Sue Nelson has been along to find out about a European Space Agency mission called SWARM to study the Earth's magnetic field from orbit using a constellation of three satellites.  She joined engineers Julia Ryan and Ralph Cordey from Astrium UK's division of Earth Observation Science inside the "Andromeda" clean room...

Ralph -   When I get used to the typical run of satellites coming through a factory like this, I think of the big telecommunication satellites as being like London buses.  But for me, the SWARM satellites were like long, sleek, dark racing cars.

Sue -   Why was it built in that particular way?  Why does it physically look so different?

Ralph -   Well I guess there are two principal reasons for that.  One is the environment, the orbit, that the satellites are going to go into.  That's a relatively low orbit and it means that they have to be sleek, very slim, to help reduce the pressure of the residual atmosphere at the height it is orbiting at, and the other reason is that it needs to keep its sensors,  its extremely precise and sensitive magnetic sensors, as far away from any disturbance on a satellite as possible.  And therefore it's very long and actually extends its length by a 4-metre long boom.

Sue -   This 4-metre long boom then is the reason that it has been described as a giant mechanical rat, which, considering you and I both see it more as a sleek racing car, seems slightly unfair.

Ralph -   It does seem a bit unfair, but the tail of the rat, the boom if you like, is really very vital.  It's on there so that we support what's called a vector magnetometer, the heart of each satellite, the device that is providing the very precise measurements, not just of the size, but of the very detailed direction of the magnetic field that each satellite is flying through.

Sue -   But because they're measuring the earth's magnetic field, that made the construction slightly different to how you would normally make a spacecraft.

Ralph -   We've had to be careful in every way we've been working - in our choice of materials for these satellites, and in the techniques we use.  For example, we've got to be careful that nothing that we use is going to generate its own disturbing magnetic field.  So the materials we choose are far from being the typical materials we would use for a telecommunication satellite.  We've had to build this out of carbon reinforced plastic, we've have to use ceramic materials to form kind of an optical bench on which to support the vector magnetometer.  And the tools that we used to do work on the satellites, we've have to make sure that those contain no residual magnetism themselves.

Sue -   Julia Ryan, you worked on the assembly, integration and testing part of the campaign for the Swarm satellite.  What sort of testing did it involve?

Julia -   The most environmentally disturbing part of a satellite's life cycle is the launch because being on top of the rocket, it gets shaken around a lot and subjected to a lot of noise. In order to make sure that it's going to survive that, we have to do simulations on the ground using computers and then test it afterwards.

Sue -   Often all the components of a satellite aren't put together until launch.  So how do you test a satellite when it perhaps doesn't have all the instruments onboard?

Julia -   When we're testing the structure, we use mass dummies to represent the different instruments onboard.  These will be blocks of metal or plastic to represent the general size and weight of each instrument, and these bolt into the same insets that we would bolt the actual equipment into and it gives the representative mass and dynamic response of the spacecraft when we test it.

Sue -   You mentioned metal there.  Did the metals and the choice of metals on the spacecraft itself have to be very particular to avoid the induction of magnetism? You don't want that because it would affect the instruments and what they're trying to measure.

Julia -   Yes, that's right.  All of the metal components of the structure, for example the brackets and the fasteners themselves, all have to be made out of titanium whereas we would normally use perhaps aluminium or even stainless steel.  These parts were made out of titanium to reduce the magnetic effects.

Sue -   Ralph, the three SWARM satellites are due for launch next year.  Will they be launched in stages?

Ralph -   No.  in fact, they're going to be launched all together from one rocket from Northern Russia. They're going to be put together into an orbit very close to the Earth's poles, two of them at a slightly lower altitude, flying almost side by side around the earth, with the other one a little bit higher and slowly drifting away over time to separate them in time and space from each other.

Diana -   Dr. Ralph Cordey and Julia Ryan from Astrium UK with Sue Nelson.  There's also a longer version of that interview on the Planet Earth podcast and you can find it at


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