GIMP
Use GIMP to process satellite images
Satellite photos are familiar, their underlying technology less so. Mike Bedford reveals how to process their spectacular images.
OUR EXPERT
Mike Bedford never tires of studying satellite imagery, whether it’s of the Earth or other objects in the Solar System. Processing Landsat images is right up his street.
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The technology of the first imaging satellites seems archaic – they shot on oldfashioned film. The film had to be returned to Earth in an escape capsule, so when all the films were used – and the first satellite had just the one – the satellite was useless.
Photographs of the Earth taken from space are a part of everyday life, appearing on TV and newspaper weather reports. This state of affairs dates back little more than 60 years, though, to when TIROS-1, the first weather satellite, was launched. How things have changed in the intervening period. By the end of April last year, 971 satellites were in orbit for the purpose of Earth observation. And unlike the early days, these satellites aren’t just operated by governmental organisations: several commercial enterprises now supply photography of the Earth’s surface. But publicly owned satellites are still important.
If you want to learn about the technology, the good news is that these satellites have the undeniable advantage of free access to raw data. We saw this in LXF273 when we looked at how to receive data from America’s NOAA weather satellites and display that data as a map. The NOAA satellites are great for amateur experimentation because they orbit at a low altitude. This means they can easily be received using simple equipment, even though we took the approach of receiving the signal through a WebSDR, an online software-defined radio.
On the reverse side of the coin, NOAA satellites transmit just two black and white images: usually one in the visible spectrum and the other in near infrared. Therefore the scope for producing coloured images – even false colour – is severely limited. So, here we’re investigating a different class of satellites that enables us to produce spectacular colour images, with plenty of scope for choosing between true colour and the many different false colour alternatives.
Introducing Landsat
Landsat is a joint programme between NASA and USGS, that’s the United States Geological Survey. Back in 1972 the Earth Resources Technology Satellite was launched with the purpose of providing multi-spectral images of the globe. It was later renamed Landsat 1, becoming the first of the longest-running earth observation satellite programme, and one that continues to the present day.
Currently Landsat 7 and 8 are operational, having been launched in 1999 and 2013, respectively, and Landsat 9, which was launched in September 2021, is about to come online. Here we’ll concentrate on Landsat 8, but when Landsat 9 data becomes available, the same guidance will apply because it carries almost identical imaging instruments. The principles are the similar for earlier satellites, but you’ll have to read up on their instruments.