How do GPS stations monitoring Yellowstone measure such small movements?

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In Yellowstone, deformation of the ground surface can be measured to fractions of an inch.  Specialized methods of processing GPS data make it possible to achieve this amazing resolution.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Scott Johnson, Communications Associate for UNAVCO.

Satellite navigation in your car (usually) gets you where you want to go, a fitness tracker knows the route you biked, and a handheld GPS device might give you your coordinates to about plus-or-minus 10 feet. None of that would be good enough to monitor subtle surface movements at Yellowstone or other volcanoes, though. We might need to be able to measure a change of less than 1 centimeter (less than half an inch) to record the signs of moving magma or fault motion underground. And that’s exactly what the scientific GPS stations operated by UNAVCO in the park do. So how does that work?

Cutaway of a GPS antenna

This display model of a GPS station shows the innards of the GPS antenna through a cutout.

(Credit: Scott Johnson, UNAVCO. Public domain.)

These stations consist of solar panels, batteries and electronics in a steel cabinet, and a distinctive antenna with a dome-shaped cover. The antenna is mounted on legs that are driven or drilled into the ground, providing a rock-solid base that only moves when the Earth does.

That specialized antenna does an excellent job of picking up signals from navigation satellites—whether from the US-operated GPS system or others, like Russia’s GLONASS and Europe’s Galileo satellites. (The general term is GNSS, which stands for Global Navigation Satellite System.) Inside the cabinet, a small device called a receiver uses those signals to calculate the antenna’s distance from each satellite, working out its exact position in three-dimensional space.

Part of the increased accuracy of this setup comes from the simple fact that the station is, well, stationary. The error bars on your position (like plus-or-minus 10 feet with a handheld GPS) are caused by several factors, including atmospheric conditions that refract the satellite signal, delaying its arrival at your antenna. By averaging many position measurements over time, however, the error bars are progressively reduced. Using multiple navigation satellite constellations speeds this process up, since there are more satellite signals to use at a time.

The data can also be processed to improve accuracy. For example, tiny corrections to the synchronization of the clocks onboard the satellites and to the satellites’ exact orbits can tighten position calculations. The satellite signal includes the time it was sent and the satellite’s expected position at that moment. Because distance from the satellite is calculated based on the time it took the signal—traveling at the speed of light—to reach your antenna, just a single nanosecond difference between the satellite clocks and ground clocks can throw your estimated position off by about 30 centimeters (or 1 foot). Adjusting any such clock mismatches can remove those errors. And the same is true for corrections that account for atmospheric water vapor and ionospheric effects on the signal.

Some of these corrections can be made in real-time, with final corrections later improving the accuracy even further. But even without waiting, daily position measurements for the station are accurate to about +/- 1 centimeter. That means we can easily detect events like Norris Geyser Basin rising and falling by 10 centimeters or more over months to years due to the movement of magma and water beneath the area.

Vertical movement over time recorded at GPS station NRWY, 2005-2021

Vertical movement over time recorded at GPS station NRWY, near Norris Geyser Basin, during 2005-2021.

(Public domain.)

These stations don’t just monitor volcanoes. They measure the relentless motion of tectonic plates, the sudden fault movement of earthquakes, surface uplift or subsidence due to changing groundwater levels—they can even measure atmospheric water vapor, as a part of those error-bar-shrinking calculations. But importantly for Yellowstone, the volcano can’t keep secrets thanks to their presence in the park. When things move, we notice.

If you would like to check out the Yellowstone GPS data for yourself, head over to UNAVCO’s network map (https://www.unavco.org/instrumentation/networks/map/map.html), where you can scroll to the Yellowstone region, click on any site, and see deformation that has occurred at the site since the station was installed!

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