Since the moment Malaysian Airlines flight MH370 was determined by authorities to have crashed into the Southern Ocean some 2000km south west of Perth, I’ve been asked the same question repeatedly: How can satellites help find the crash site?
The purpose of this blog is to acquaint the readers with some facts about the strengths and limitations of satellite imagery.
Several governments have come together to form a massive search operation using satellite imagery, search aircraft and naval vessels.
Satellites have a large footprint and they can cover large amounts of area in short amounts of time. If they do find some interesting objects in this imagery, the next task will be to deploy air and sea resources to confirm the sightings.
Satellite imagery provides the starting point – a useful place to gather compelling evidence.
So what can these commercial satellites ‘see’?
There are two kinds of satellites which can be used in the search for debris in the ocean – optical satellites and radar satellites.
Making the assumption the plane has actually crashed in the search area, there are likely to be some parts of the plane scattered and floating on the ocean surface.
Optical and radar satellites will see the floating objects slightly differently.
As you can probably determine from its name, an optical satellite can take images – or photos – of the Earth as it orbits.
Some optical satellites can see objects as small as 0.5m in size, while for others the objects must be larger to detect – say in the order of 5-10m.
The wingspan of the Boeing 777 is just over 60m and if it is still mostly intact, and floating, then the satellite with 0.5m ‘lens’ will surely be able to spot it without too much difficulty. If this is the case and there are identifiable letters on the wing, then these can even be read.
If the satellite lens was ‘10m’ then the wingspan will appear just as a collection of 5-6 dots on the image and will therefore be harder to accurately pinpoint.
Optical satellites are good at picking up the colour, shape and size of the objects – even large letters or writing on objects – but they cannot ‘see’ at night.
Radar satellites, on the other hand, will pick up reflections from the metallic part of the plane and pick up damping of the water surface due to an oil-slick. Even if the object is only meter in length, it appears as a bright blob on the radar imagery and will assist the analyst in tracking it.
These radar satellites will determine if there are objects in an area, but cannot reveal many details about them.
Neither can identify debris more than half a metre under the surface – at best.
Realistically, those involved in a search effort have to closely co-ordinate the data gathering and analysis effort from both optical and radar satellites data to make the most of the imagery.
Naturally the question has been asked as to why the satellite images have not been able to pinpoint the debris even after so many weeks?
One of the first issues with satellites is many of them are not looking at the same area of the Earth‘s surface all the time. At best, a satellite may fly over the area once a day. Some satellites fly over the search area once every few days.
Of course, a big challenge with mapping debris in the open ocean is that it is always in motion – currents, winds and other environmental factors. If the debris was on a land surface it would be static and satellites could centre in a particular area, not so for drifting debris. So even if an image clearly identified MH370 debris the next time imagery is acquired – in a day or two – it would have moved to another point in the ocean.
Like the photos we take with our cameras, optical satellite images are snapshot in time. If the area is covered with cloud, of course we cannot take the picture and the opportunity is missed.
The very first sighting of debris reported in the media was from optical satellites called Worldview (US) and Gaofen (China).
I believe there would have been several attempts to take further pictures but cloud cover could have been a problem or potentially the debris would have drifted to another part of the ocean and the satellite would have to look at another location quite far from the original.
Radar satellites can ‘see’ through the clouds and the third reported sighting was believed to be using a German radar satellite (Germany) operated by a French company.
But again, the sightings only constitute the first key step and these images need to be verified.
We hope something is found soon for the sake of the families of the passengers and flight crew, the enormity of the task ahead of the searchers should not be underestimated.
Dr Dipak Paudyal
Remote Sensing & Imagery, Esri Australia