Science Interviews

Interview

Thu, 6th Jun 2013

Using GPS to track volcanic ash clouds

Kristine Larson, University of Colorado

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Dominic -   GPS might be best known as navigation system, but researchers have also found it a very powerful tool to measure environmental changes on Earth.  Weíre joined by Kristine Larson from University of Colorado whoís developed a novel way to use GPS detectors to monitor volcanic plumes such as the ash cloud which grounded flights across Europe when the Icelandic volcano erupted in 2010.  Kristine, how do people traditionally monitor ash clouds coming out of volcanoes?

Kristine -   I think itís a little bit like the way we think about monitoring weather storms.  They use satellite imagery.  You'll see the photos of the plumes moving and that works fine if the weather is good and you have a good satellite image.  But of course, you donít have those images all the time, depending on the cloud situation.  Another powerful tool for studying plumes is radars.  The problem with that is just you donít have radars where you have volcanoes.  So, you have to really be lucky and have the equipment you need to study that plume, and we just donít have enough equipment around all the active volcanoes in the world.

Dominic -   Now, youíve been working with GPS sensors around these volcanoes.  First of all, why were the sensors put there in the first place?

Kristine -   Right, I've been working on data from Alaskan volcanoes and those were put there by colleagues of mine at the University of Alaska in the Alaska Volcano Observatory and they were using GPS in what I would call the traditional sense. I realise the last couple of people you interviewed talked about the real-time navigation that you think of when you think about using GPS in your phone or in your car.  But there's a lot of people in my business, which is science, that use it to study how the ground deforms, and we use that to very accurately monitor Ė in the case of a volcano - whether there's an eruption likely to occur. Before an eruption, the ground starts to inflate.  The magma chamber inside the Earth starts to inflate and that causes the ground to move.  You have to have very precise GPS receivers to detect that.  So, I had colleagues that had put GPS receivers there for a completely different purpose.  They were going to measure the magma chamber inflating.  I just recently had the idea of seeing if we could see the plume by looking at the data from a different vantage point.

Dominic -   So, how can that data from those sensors tell you about ash which is actually in the atmosphere, above those sensors?

Kristine -   Right.  I mean, you have to look at things backwards and I want to credit my colleagues at the University of Alaska.  They had noticed that they thought they could see the effect of a plume on their positioning data.  So, they were measuring latitude, longitude, and height.  They noticed that during the eruption, the ground moved a lot.  Well, thatís not too surprising.  You'd think, "oh, an eruption causes the ground to move". But they were 5 or 6 kilometres away, and the ground should not have moved as much as it did.  And so, they correctly hypothesised that the plume was making the GPS positions wrong, just because it was an error that was unaccounted for when they calculated positions.  So, they had first seen this, but when I looked at the data, I just said, ďWell, you know, calculating position and then inferring that there was an eruption because the positions look bad, it sort of seems like a difficult way to look at the problem.  So, I sort of looked at it differently and said, ďIf I were a GPS signal travelling through an ash cloud, that would cause the signal to be reflected around. Not as much as of the signal would get into my antenna because it would be, if you will, deflected by all these particles, these ash particles.  GPS was not meant to work in ash plumes and so, I hypothesised that that would cause signal power to go down.  So, instead of looking at the positioning data which is what the rest of us use GPS for, I looked at the signal strength data and that doesnít tell you anything about position.  It really only tells you how good your reception is, just the same way your cell phone tells you how good your reception is too.  So, I looked at the signal power and I could very clearly see when there was a plume.

Dominic -   So, youíve got this sensor on the ground and itís hooked up to various satellites in the sky. And you can see the signal strengths from those various satellites, and you can predict what those signal strengths ought to be.  If thatís less than you expect, then you know there must be something in the way so you infer that there must be an ash cloud there?

Kristine -   Thatís right, because you know these satellites run 24 hours a day, 365 days a year.  Signal strength does not change.  The military run them to specifications. And so, I compared it to the day when there was the eruption, and I noticed the signal powers were perfectly fine in most directions, i.e. the directions that didnít go through the plume.  So, it turned out only 2 or 3 satellites were being affected by the plume.  So that tells me where the plume was, and if I made certain assumptions about where the volcano was, which we know - where it was, or at least I looked on Wikipedia and found out where it was - you could infer how high the plume was and things like that because we know where the satellites are.  One of your previous speakers talked about that.  We know where they are and that tells us which satellite is affected by the plume.

Dominic -   So, youíve got this sensor on the ground and itís hooked up to various satellites in the sky. You can see the signal strengths for the various satellites, and you can predict roughly what they ought to be.  If thatís less than you expect then you know that there is something blocking it so you infer that there must be an ash cloud there.

Kristine -   Thatís right because you know, these satellites as you all know, they run 24 hours a day, 365 days a year.  Signal strength does not change.  The military run them to specifications. So, I compared them to the day when there was an eruption, and I noticed the signal powers were perfectly fine in most directions, i.e. the directions that didnít go through the plume.  So, it turned out only 2 or 3 satellites were being affected by the plume.  So that tells me where the plume was and if I made certain assumptions about where the volcano was, which we know - where it was, or at least I looked on Wikipedia and found out where it was - you could infer how high the plume was and things like that because we know where the satellites are.  One of your previous speakers talked about that.  We know where they are and that tells us which satellite is affected by the plume.

Dominic -   Now, I think this isnít just about ash?  You've actually managed to measure snow and vegetation as well?

Kristine -   That's exactly right, but I do it a little bit differently.  So, in the case of the volcano, I look and see if signal strengths has been degraded if you will, less signal getting into my antenna because of the ash that is basically disrupting the signal transmission.  To measure snow, what I do is again, I sort of look at a normal day when there is no snow.  I look at the effect of GPS signals that bounce on the ground and reflects back up to my antenna.  That has a very characteristic frequency and then I compare it to a day when there is snow and if the reflective signal has a different frequency then because there's these snow layers, so I basically look at the patterns of reflections to tell me whether the Ė basically, if the ground is moved.  If there's snow on the ground, that makes it look like the ground is higher and then when the snow melts, it gets back to where it originally started.  Similarly, if there's vegetation on the ground, that changes the balances of the ground as well.  But itís all using signal strength data and itís all using the same equipment that other people use to measure latitude, longitude and elevation.

Dominic -   At the moment, it sounds like you're using arrays of sensors which are being put down for other purposes, for looking at ground movements.  But given how much you're managing to infer from this, it sounds like it might be worth building arrays in areas where you're interested in monitoring snow and vegetation, and so forth.

Kristine -   Thatís exactly what weíre going to try to do.  Since we are doing something pretty simple, weíre trying to make the GPS sensors cheaper because for example, the reason we all have GPS sensors on our phone is because people have come up with very cheap GPS sensors to use there.  For the kind of thing I'm talking about, I think we could do the same thing.  So, what weíd like to do is weíve shown that you can use these GPS receivers that were put out to measure plate motions and volcanic motions.  Weíve shown you can measure snow with that like you said.  But weíd like to do it in arrays with cheaper sensors and thatís what weíre working on.  I have some students working on that and some colleagues working on that so that we can provide a cheaper environmental sensor.

Dominic -   Thanks very much, Kristine.  I'm sure thatís something weíll be keeping an eye on in months ahead.  That was Kristine Larson from the University of Colorado.

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