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GPS Used To Monitor Continental Drift
metz2000 writes "BBC News is reporting that a team of scientists from Nottingham (UK) are using GPS to measure sea levels and continental drift. The team has around 50 stations across the UK, and use GPS technology to track miniscule changes in altitude and location. This allows the team to gain an understanding of how the UK landmass is likely to change over the coming centuries. They have discovered that the British Isles are tilting, with the north of the country gaining altitude and the south of the country 'sinking'."
I guess that they are using differential GPS, by which the time delay at a known location is compared to the time-delay at the location of interest. This enables for very accurate estimation of where you are.
On the other hand, at least in California (where they have a GPS network for earthquake monitoring), the network might well be permanent, hence you can do a nice sort of averaging over time. We have found that even with normal GPS, you get nice accuracies over a time period.
Sounds to me like this tilting is just the land settling down after the last ice age. The north of the country used to be covered in ice, while the south was clear. Now that the weight of the ice has gone, the land is just seeking a point of equilibrium.
They are probably not using differential gps as the base stations calculating the difference are on the very landmass they are measuring the movement off....
They probably use a scheme similar to dgps: They don't have to know their exact location, they have to know their exact location in respect to the other measurement points around the country. Which is relativly easy to do.
Jeroen
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The fact that the northern part of Europe is rising and the southern part is sinking (for a rather broad definition of southern: Holland is sinking too), has been known for a long time. I was told in highschool (think before 1983) that this is due do the northern part having been pushed downwards during ice age(s) due to the massive weight ot the ice. When the ice last retreated, the current tilting movement was initiated.
Linux user since early January 1992.
..in how they use GPS to make such precise measurements you can read about it here:
Using GPS to Separate Crustal Movements and Sea Level Changes at Tide Gauges in the UK
Application of the Dual-GPS Concept to Monitoring Vertical Land Movements at Tide Gauges
Whenever the offence inspires less horror than the punishment, the rigour of penal law is obliged to give way...
"They have discovered that the British Isles are tilting, with the north of the country gaining altitude and the south of the country 'sinking'"
GREAT DISCOVERY *sarcasm*
Since the late pleistocene the big icesheets on top of Northern Europe disappeared by global climat change from glacial to interglacial (cfr. Iversen model). As a concequence of this loss of mass on top of these plates they began to bounce back up. Imagine taking a piece of drifting wood, push it down. If you lift your finger it will rise up again. The same principal goes for continental plates and is called isostatic uplift.
So, since the beginning of the holocene and end of the pleistocene countries now known as Sweden, Norway began to rise and Belgium and the Netherlands for example began to sink, because the y once were uplifted by the weight on the northern part of europe.
It seems normal that these consequences aren't just limited to european main land but also influence the UK. In fact Scotland has had a big icecap during the last iceage so the isostatic uplift of Scotland and the drowning, if you may call it that, of the south of the UK isn't exactly new.
They may claim having measured it, but they certainly may not claim the discovery of these changes because that's veeeeeery old news.
Lots of Earth Scientists around the world are using GPS for precise plate motion measurements. It's all differential processing with respect to global reference frames defined by a number of sites and other space-geodetic methods (like VLBI). Check out the graphs from this page for some real numbers. They've been doing this sort of thing for the last 10-20 years now.
Back in the late 80's I had some college rooommates who did this for a living. After moderate earthquakes in southern California, groups of geophysics graduate students would be sent to the channel islands off the coast with huge old clunky GPS receivers. They would align the GPS receiver over a benchmark and camp out for 3 days collecting data. Similar groups would do the same thing all over SoCal. Combining the data makes for a super differential GPS data set. As each receiver is at a known location (well sort of, it is what they are there to determine), each has the accuracy of a single diff. GPS receiver. However, what the scientists cared about was not the aboslute positions of the receivers, but their relative positions. As I recall, 0.5cm resoultion was routinely achieved event back then. I'm sure todays systems are automated, and remotely read out. Today's grad students won't have stories about being buzzed by navy jets or herds of ferrel cats.
From the article:
GPS measurements have also allowed scientists to show that the UK is drifting about 2-3 cm each year in a north-easterly direction.
All I want is a secure system where it's easy to do anything I want. Is that too much to ask ~~ Randall Munroe
For continental drift, they need mm level data. I guess, they just leave the station for a longer time to get even more passes.
See my journal, I write things there
The tilt, at least, has been known for a quite a while; I remember joking with a friend from London that London might be horrible, but if we just waited a few million years the problem would be solved (we were in Scotland). That was back in the late eighties.
I think its called differential carrier phase gps
measurements below 1cm can be taken by looking at the wavelengths of the signal
my associative arrays can kick your hash - TCL
This link has the best introduction to mapping and GPS I have ever read.
All I want is a secure system where it's easy to do anything I want. Is that too much to ask ~~ Randall Munroe
This is hardly news, I was taught about this 10 years ago at school.
This was already common enough knowledge for those interested in the subject ... the south east & east anglia are sinking, the north west rising.
This seems like very old news to me.
I seem to recall being told this in the early 1980s at school.
Apperently it's the "rebound" effect. In the last ice age all the ice caused Scotland and Northern England to sink under it's weight.
Since it all melted it's been slowly rising.
I can't remember why southern England is sinking though. Maybe there's a pivet somewhere through Shefield or something ?
I remember an in-depth discussion of the tilting effect on the Open University in the late '80s.
Geologists have been measuring micro-motions of the earth since GPS started in the early 1990s. There are thousands of talks on the subject here .
This hasn't been true for several years. Selective availability was turned off during the Clinton administration.
irb(main):001:0>
In case people don't know, these kind of GPS measurements have been going on for years.
They're doing very high-precision work which doesn't look at the code, but the actual waveform, using static (hours-long) occupations of benchmark monuments. Then custom software is used to work out sub centimeter (often 3-5 mm) locations in post-processing.
This sort of thing has been done in a number of locations. I've been involved with studies like this in Nevada and Italy.
It's hardly suprising that Scotland is rising and England is sinking. The phenomenon is known as 'isostatic rebound' and happens any time a substantial load is removed or added to an area. The massive ice-age glaciers over Scandinavia caused that area to sink and the 'low countries' - especially Holland - to rise. Now that the glaciers are gone, Scandinavia is rising again and the Netherlands are sinking into the sea. The same is probably happening on a smaller scale to Great Britain. In the US, the Appalachian Mountains are eroding away, causing them to rise, and the coastal plains and Mississippi delta, where that sediment is being deposited, are sinking.
This is all a very slow process, millimeters per year, but over time it makes a big difference.
if ($it != $onething) {$it = $another;}
I thought I would include some links to similar projects:
SCIGN -- Southern California Integrated GPS Network
http://www.scign.org/
This GPS array has 250 active stations throughout SoCal continuously monitoring crustal deformation. SCIGN was started after the 1994 Northridge Earthquake and has helped the determination of the velocity field in Southern California produced by SCEC.
An interactive map of station locations can be found at:
http://pasadena.wr.usgs.gov/scign/Analysis/
SCEC -- Southern California Earthquake Center
http://www.scec.org
This is the umbrella organization for EQ research in Southern California and is the parent organization of SCIGN.
PBO/Earthscope -- Plate Boundary Observatory
http://www.unavco.org
One of the most exciting new developments in GPS. Recently the US Congress approved the Earthscope initiative which includes 3 separate parts. The GPS portion is the PBO which will include 875 new stations from the Mexican border through Washington and Alaska. This massive array will be built over the next 5 years.
There are also several other regional deformation arrays include the Basin Range network, The Bary Area network, and a cluster of sites around Parkfield, CA.
Equipment:
The GPS equipment used at these sites are commercially available dual frequency geodetic quality receivers which can recover the military code without needing the military keys. The receivers are capable of good precision when operated alone and are capable of sub-millimeter precision when used in a continuously operating network.
This type of processing requires extremely accurate satellite orbits provided by the International GPS Service (http://igscb.jpl.nasa.gov/).
These large GPS arrays have been deployed throughout the world where there is a seismic hazard. Japan is one of the predominant countries with large GPS networks in place.
We encourage you to look at the various websites and learn about these projects and the science that they produce.
--Keith
SCIGN Network Coordinator
The receivers themselves aren't generating the solution. Rather, they upload raw data to a central server which calculates the position of the receivers sometime after the data was collected (probably once high-precision satellite orbit data is available--this is generated by satellite tracking from several ground stations around the world, updated every 12-24 hrs I think).
Even consumer-level receivers will give you a decent solution when several days of data for a fixed point are averaged (about 5-10cm I think), assuming satellite visibility is good.
Actually, in a differential GPS situation, S/A has essentially no effect. The only noticable difference is that the signal has been dithered. The errors caused by S/A are completely eliminated by differencing the station with unknown coordinates with the station with fixed, known coordinates. Even if both are unknown, you can still get a very good relative position between the two stations.
Also, as far as the time it takes to generate a solution depends on a couple of other things. You were discussing how to get a solution using cheap receivers that only calculate the pseudo-ranges to satellites using the code transmitted on the first GPS frequency (the technique used by consumer receivers). In the case of high-precision GPS, the receiver may not even calculate its own position. Rather, after acquisition of at least 4 satellites (the 10-300 seconds you mention), the receiver will transmit its observables to some server which does the position calculations. With more expensive, phase-locking receivers the accuracy greatly improves. In this case, a receiver will lock onto the first (and possibly second) frequencies, counting the number of cylces it receives. Once the server can calculate the correct integer offset to add to this counter (the number of cycles between the receiver and the given satellite), the position is known to within centimeters of its true location. However, the process of calculating these unknown integer cycle offset values for each observed satellite is rather tedious, usually taking at least 15 minutes for a fixed reciever, longer for a moving one (such as one on a buoy).
The position may be recalculated later once high precision satellite orbit data is available (this is provided by tracking stations around the world every 12-24 hours).