Surprising uses for GPS


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With your smartphone by your side, you may believe you are a traffic-navigation pro. To navigate the terrain, you might even hike with a GPS device. The capabilities of GPS, the global positioning system that powers all modern navigation, would likely still astound you.

The constellation of satellites that make up GPS transmit signals to the surface of the Earth. A basic GPS receiver, like the one in your smartphone, uses the arrival times of signals from four or more satellites to pinpoint your location to within roughly 1 to 10 meters. Scientists are able to determine their locations with a centimeter or even millimeter accuracy with sophisticated (and more expensive) GPS receivers.

Researchers are learning that GPS can tell them much more about the planet than they initially believed it could using this fine-grained information as well as new methods of signal analysis. Scientists have been able to shed light on how the ground shifts during strong earthquakes during the past ten years because of faster and more precise GPS equipment. Better warning systems for natural disasters like volcanic eruptions and flash floods have been made possible by GPS. Additionally, some GPS receivers have been modified by researchers to serve as snow sensors, tidal gauges, and other unexpected tools for measuring the Earth. Kristine Larson, a geophysicist at the University of Colorado Boulder who has led several of the discoveries and written about them in the 2019 Annual Review of Earth and Planetary Sciences, recalls that “many thought I was crazy when I started talking about these applications.” Well, it turns out we were successful.

Here are a few surprising uses for GPS that scientists have only recently become aware of:

1. Feel an earthquake

Seismometers, which measure how much the ground trembles, have been used by geoscientists for centuries to gauge how powerful and destructive an earthquake is. A different use for GPS receivers was to study geologic processes that occur on considerably slower time periods, such as the speed at which the Earth’s large crustal plates collide in a process known as plate tectonics. As a result, seismometers can monitor the ground trembling when the Californian San Andreas Fault ruptures, while GPS can tell scientists how quickly the opposing sides of the fault are creeping past one another.

The majority of scientists believed that GPS simply couldn’t measure positions accurately and quickly enough to be helpful in evaluating earthquakes. However, it appears that scientists can extract more data from the signals that GPS satellites send to Earth.

There are two components to those signals. The code, or distinct set of ones and zeros, is what each GPS satellite broadcasts. The second transmission is a “carrier” signal with a shorter wavelength that relays the code from the satellite. The carrier signal provides a high-resolution method to pinpoint a location on the surface of the Earth because it has a shorter wavelength than the code—only 20 centimeters—in comparison to the code’s greater wavelength, which can be tens or hundreds of meters. It only takes a more complex GPS receiver for scientists, surveyors, the military, and others to need an extremely exact GPS location.

As a result of improvements in engineering, GPS receivers can now update their location as frequently as 20 times per second or more. When scientists discovered they could quickly obtain accurate readings, they began using GPS to look at how the ground moved during an earthquake. In one of the first studies of its kind, Larson and her colleagues examined how the ground changed as seismic waves from an earthquake in Alaska measuring magnitude 7.9 spread over the western United States in 2003. In 2011, scientists were able to use GPS data on the 9.1-magnitude earthquake that wreaked havoc in Japan to demonstrate that the seafloor had moved an incredible 60 meters as a result of the event.

Today, scientists are more generally examining how GPS data can assist them in rapidly evaluating earthquakes. To test if they could predict an earthquake’s size seconds before it occurred, Diego Melgar of the University of Oregon in Eugene and Gavin Hayes of the US Geological Survey in Golden, Colorado, looked back at 12 significant earthquakes. The scientists were able to estimate whether an earthquake would be a catastrophic magnitude 7 or a magnitude 9 within 10 seconds by using data from nearby GPS sites.

Even GPS has been incorporated by US West Coast researchers into their developing earthquake early warning system, which detects ground shaking and alerts citizens in far-off cities as to whether shaking is expected to strike them soon. And Chile has been expanding its GPS network to have more timely access to reliable information that may be used to determine if a near-coastal earthquake will likely cause a tsunami or not.

2. Observe a volcano

Beyond earthquakes, the speed of GPS is enabling authorities to react to other natural disasters as they develop more quickly. For instance, many volcano observatories have GPS receivers scattered throughout the mountains they keep an eye on because when magma starts shifting beneath, the surface frequently shifts along with it. The location of the flow of molten rock can be better understood by experts by tracking how GPS stations near a volcano rise or sink over time.

Researchers utilized GPS to determine which areas of the Kilauea volcano in Hawaii were shifting the fastest before last year’s major eruption. That data was utilized by officials to determine which regions needed resident evacuations. On the shores of Kachemak Bay in Alaska, a GPS station is located (top). This receiver shows how GPS signals can be used to track changing water levels by tracking data with data from a nearby National Oceanic and Atmospheric Administration tide gauge (bottom).

Even after a volcano has erupted, GPS data still has value. The signals must pass through whatever the volcano is spewing into the air because they travel from satellites to the earth. When various study teams analyzed GPS data from the Redoubt volcano explosion in Alaska four years earlier in 2013, they discovered that the signals started to degrade very quickly.

The scientists were able to determine how much ash had been spat out and how quickly it was moving by examining the aberrations. Larson referred to it as “a new approach to detect volcanic plumes” in a subsequent publication. She has been experimenting with ways to do this using inexpensive smartphone GPS receivers rather than pricy research receivers, along with her colleagues.

That might make it possible for volcanologists to build up a reasonably priced GPS network and track the ascent of ash plumes. Volcanic plumes pose a significant challenge for aircraft, which must maneuver around the ash rather than take the chance that the particles could block their jet engines.

3. Test the snow.

The messiest bits of GPS’s signal, the parts that bounce off the ground, are where some of the technology’s most surprising applications can be found.

The majority of the signals that a typical GPS receiver, like the one in your smartphone, detects originate from the GPS satellites themselves. However, it also detects signals that have been reflected up to your smartphone from the surface you are walking on. For many years, scientists believed that these reflected signals were merely noise—a kind of echo that complicated the data and made it challenging to interpret. However, Larson and others started to consider whether they could benefit from the echoes in scientific GPS devices approximately 15 years ago. She began examining the frequencies of the signals that were reflected off the ground and how they interacted with the signals that had directly reached the receiver. That allowed her to infer characteristics of the surface that the echoes had reflected off of. According to Larson, “We just reverse-engineered those echoes.”

This method enables researchers to gain knowledge of the terrain beneath the GPS receiver, such as the amount of soil moisture or the amount of snow that has been collected on the surface. (The distance between the echo and the receiver decreases as more snow accumulates on the ground.)) In mountainous places where the snowpack is a significant water resource each year, GPS stations can function as snow sensors to detect snow depth. Since there aren’t enough meteorological stations in the Arctic and Antarctica to record snowfall year-round, the method also performs well there. From 2007 to 2017, Matt Siegfried and his colleagues at the Colorado School of Mines in Golden examined snowfall at 23 GPS stations in West Antarctica.

4. Feel yourself sinking

Although GPS was initially developed to determine the position on solid ground, it has now been shown to be helpful in tracking changes in water levels. At the confluence of the Ganges and Brahmaputra rivers in Bangladesh, John Galetzka, an engineer with the UNAVCO geophysics research group in Boulder, Colorado, found himself installing GPS stations in July. The objective was to determine whether the land is slowly sinking and river sediments are compacting, making it more susceptible to floods from tropical cyclones and sea level rise. GPS is a fantastic tool, according to Galetzka, to aid with these and other questions.

Galetzka and his associates set up one GPS station on the concrete roof of a primary school in the farming town of Sonatala, which is situated on the edge of a mangrove forest. They erected a second station close by on top of a rod that was driven into a rice field. The second GPS station will appear to be progressively rising from the ground if the ground is actually sinking. Additionally, the scientists can determine variables like how much water is present in the rice paddies during the rainy season by analyzing the GPS echoes beneath the stations.

By serving as tidal gauges, GPS receivers can even be useful to seafarers and oceanographers. While working with GPS data from Alaska’s Kachemak Bay, Larson discovered this. Larson was interested since the bay also contains some of the largest tidal changes in the United States, even though the station was built to monitor tectonic deformation. She tracked tidal variations almost as precisely as a genuine tide gauge in a neighboring harbor by looking at the GPS signals that were reflecting off the water and up to the receiver. This might be useful in regions of the world without installed long-term tide gauges but with nearby GPS stations.

5. Examine the atmosphere

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Meteorologists tracked monsoonal moisture flowing onshore there during a storm in July 2013 using GPS data, and this knowledge proved to be important for delivering a warning 17 minutes before flash floods occurred. The ionosphere, a region of the upper atmosphere that is electrically charged, has an impact on GPS signals as well. Scientists have used GPS data to track changes in the ionosphere as tsunamis race across the ocean below. (The force of the tsunami produces changes in the atmosphere that ripple all the way up to the ionosphere.) This technique could one day complement the traditional method of a tsunami warning, which uses buoys dotted across the ocean to measure the height of the traveling wave.

Finally, GPS can reveal details about the sky above, in ways that researchers hadn’t previously imagined conceivable. GPS signals can be delayed as they travel through the atmosphere due to water vapor, electrically charged particles, and other factors, which enables researchers to make new discoveries. One team of researchers utilizes GPS to measure the amount of atmospheric water vapor that is available to form rain or snow. Forecasters can now more accurately predict flash floods in areas like Southern California thanks to research that uses these changes to estimate how much water will likely fall from the sky in heavy downpours.

And using GPS, researchers have even been able to examine the impacts of a total solar eclipse. They monitored how the number of electrons in the high atmosphere decreased in August 2017 as the moon’s shadow moved over the nation, suppressing the light that would have otherwise created electrons. They did this using GPS stations all around the United States. GPS is hence suitable for a variety of situations, including shaking ground and snowfall. Not bad for something that was only meant to aid you in getting around town.