| remote sensing PROTECTING RIGHTS |

targeted as they fly into remote areas in Sudan. Satellite imagery means that we can find out what’s happening in these areas without risking people’s lives.’

But analysing remote satellite imagery isn’t like looking through a series of CCTV tapes. There’s still a lot of detective work involved, explains US geographer Lars Bromley of UN unit UNOSAT, who was brought in by the UN to strengthen its human rights monitoring work through the use of remotely gathered data. ‘A lot of people think that there’s a satellite above their head every moment of the day, and that it’s simply a matter of calling in these images to find out what’s going on, but there are only about five or six satellites that we can use for this sort of work and they only cover a very finite area of the Earth every day,’ he says. ‘The resulting satellite images are really only a snapshot, taken in a picosecond [one trillionth of a second], so you don’t get “smoking guns”, such as a burning Sudanese village surrounded by a group of militants in government jeeps.’

And there are plenty of other challenges involved in the process. ‘The first thing we have to do when we’re asked to investigate an event that has occurred in a particular place is to find out where that place is,’ says Wolfinbarger. ‘And that’s not always as easy as it sounds.’

Recently, when the AAAS team, at the time led by Bromley, was asked to look at a problem in Myanmar, they had to resort to using old Russian topographic maps because the military regime had changed all of the country’s place names. ‘It has become a lot easier now thanks to Google Earth, which includes some really remote places but, in the past, sometimes we’ve had to say, “Sorry, we can’t investigate such and such a place because we can’t find its coordinates,”’ says Wolfinbarger.

Once a place has been located, analysts usually hope to acquire a set of before and after images that they can compare

Clearing the air Astrophysicist Chris Bishop, who works at the University of Plymouth with Dr Chris Lavers, usually spends his time looking at star births in images produced by a state-of-the-art telescope based in Hawaii, but the techniques he uses to compare images of stellar newborns can also be applied to satellite imagery of the Earth.

‘In order to work out whether anything has changed between before and after satellite images, you need to subtract one image from the other,’ says Bishop. ‘If nothing has changed, the resulting image will be completely blank, but if something has changed, it should immediately be revealed.’ It sounds simple, but the problem with looking at both stars and the Earth from a long way away is that the Earth’s atmosphere often gets in the way. ‘Between the time of the first image being taken and the second image being taken, the conditions in the atmosphere often change, making it a lot more difficult to compare the images,’ says Bishop. ‘The astronomy community uses an experimental program called ISIS to balance out the blurring caused by the atmosphere so that two images can be compared,’ he continues. ‘It had never been used in a non-astronomical context before, but Chris and I are now using it to compare satellite images of the Earth.

context before, but Chris and I are now using it to compare

It makes a real difference. Not only does it improve the

Not only does it improve the resolution of the image by about ten to 15 per cent, but it also means you can spot something such as a damaged hut in Sudan that might resolution of the image by about ten to 15 per cent, but it also means you can spot something such as a damaged hut in Sudan that might previously have been invisible.’

previously have been invisible.’

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