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One of scientists' best tools for tracking ground deformation was designed to do something else

September 17, 2018

More often than not, unforeseen outcomes are bad news. Requiring a complex password is intended to make your password more secure. And it is – unless you write it down because you can't remember it.

A cyber-strong password scribbled on a scrap of paper under your keyboard is actually less secure than a simpler one tucked away in memory above your shoulders. Good intentions, bad outcome.

Sometimes, though, unforeseen outcomes turn out to be just fine. That's the case for the Global Positioning System (GPS), one of the most effective techniques used to track ground deformation at Yellowstone. GPS was a fortunate afterthought born of a system designed to do something very different.

GPS had its start in 1978 when the U.S. Department of Defense began launching a constellation of NAVSTAR satellites to provide navigation information to its mobile assets – so its personnel in land vehicles, planes, and ships could know where they were and how to get where they were going. The service soon became accessible to civilian users, and now you probably use it to navigate in your car or to find your way around your favorite trail system.

GPS receiver sitting on hill above lake with trees.
GPS monitoring station P709 is located on The Promontory between the South Arm and Southeast Arm of Yellowstone Lake. It was installed in 2005 as part of the Yellowstone component of the National Science Foundation's Plate Boundary Observatory (PBO) under permit YELL-SCI-5546. Photo from UNAVCO station overview page.

Today, in addition to the United States' NAVSTAR GPS, Russia's GLONASS and the European Union's Galileo are operational Global Navigation Satellite Systems (GNSS). The accuracy of such systems varies with sky view and other factors, but generally it's 5–10 meters (15–30 feet) for horizontal position and 10–30 m (30–100 feet) for elevation.

So how do scientists using signals from the same satellites manage to track ground deformation with millimeter (less than 1/16 inch) accuracy? That was an unforeseen outcome that turned out to be very fine, indeed.

The GPS receiver in your car or on your phone uses radio signals from navigation satellites as a virtual clock and ruler. Using a bit of magic (not really) from electrical engineering, it measures the time required for signals to travel from several satellites at a time to the receiver. The signals travel at the speed of light and the satellites' orbits are known. That information, plus the signals' travel time, allows the receiver to calculate its distance from each satellite at a given instant. Using principles of spherical trigonometry, the receiver is able to "fix" its position well enough for you to find your way around.

Now enter some clever geodesists who realized they could do much better. They designed a geodetic-grade receiver that processes signals from navigation satellites in a much more precise way. Instead of using signal travel times to calculate satellite-to-receiver distances, a geodetic receiver counts the number of full and fractional wavelengths between itself and several satellites at a time. The wavelengths are known precisely, and geodetic receivers can count the number of full wavelengths exactly. By measuring the last, fractional part very accurately and doing some simple multiplication, the receiver is able to determine its distance from several satellites instantaneously to within a millimeter or so. So, with a little spherical trigonometry you have a means to monitor ground deformation using a system that was originally designed to track jeeps.

This is a photo of GPS station P711 operated by UNAVCO.
GPS monitoring station P711 is located near Madison Junction in Yellowstone National Park. It was installed in 2005 as part of the Yellowstone component of the National Science Foundation's Plate Boundary Observatory (PBO) under permit YELL-SCI-5546. Photo from UNAVCO station overview page.

At Yellowstone, a network of GPS stations tracks the changing pattern and pace of ground deformation continuously. Combined with information from a network of seismometers and other monitoring instruments, the GPS results help scientists unravel the complex structure and active processes that otherwise remain hidden underfoot.

By the way, another important tool for monitoring ground deformation at Yellowstone from space, InSAR, also was a fortuitous afterthought. For that story, stay tuned to this space.