Volcano Watch: An important point — Measuring the volcano’s shape with high precision GNSS

  • SARAH CONWAY/USGS photo Campaign GPS antenna on Kilauea’s south flank at Pu‘ukapukapu inside Hawaii Volcanoes National Park during the 2019 Kilauea GPS campaign. View is to the southwest along the Kilauea Volcano coastline.

When you think of GPS, what typically comes to mind? Your phone, the navigation in your car or maybe your watch?

The global positioning system is a U.S. satellite-navigation system originally designed for military use but now is an extremely popular and widely used technology.


In addition to the U.S. constellation, there are 3 other Global Navigation Satellite Systems (GNSS) — GLONASS (Russia), Galileo (European) and BeiDou (China). New GNSS receivers can simultaneously track multiple constellations of satellites (for example eight GPS satellites, three GLONASS and two Galileo) that provide improved accuracy.

USGS Hawaiian Volcano Observatory operates a 67-station GNSS network spread out around the island but concentrated near persistent deforming features such as rift zones. These high-precision GNSS stations give scientists a 24/7 record (called a time-series) of the precise position of the antenna every second through time.

How does this work?

GNSS satellites send out radio waves that travel at the speed of light and transmit information about the exact position of the satellite and the current time. The antenna on the ground listens to the radio signals from multiple satellites and passes them to the receiver, which calculates the exact location using a process called trilateration.

Handheld GPS such as that in your phone can determine your position within a few meters or yards, but high-precision GNSS equipment and analysis can determine a location down to a fraction of an inch.

Currently, the American GPS constellation has 33 operational satellites orbiting at an altitude of 20,000 km (12,500 mi). To accurately pinpoint the location of a high-precision GNSS station, the receiver must “lock on,” or continuously receive data for six hours as satellites arc across the horizon in view of the station. Only four satellites are needed to calculate a 3-D location, but typically a GNSS receiver will track eight or more to calculate a more precise position.

There are several factors that affect the GNSS signal and accuracy of derived locations. The ionosphere and troposphere, layers of the atmosphere through which the radio waves travel, introduce delays in the radio signals that can be corrected with atmospheric models. Noise from signals reflected off nearby objects, such as tall buildings or trees, is called multipath. This makes it especially important for GNSS antennas to have enough clear “sky view” without object interference. This is also why the GPS on your phone does not work well inside a building!

To get a more complete view of the deforming volcano, HVO also conducts yearly campaign surveys on Mauna Loa and Kilauea. During these surveys, HVO staff place temporary GPS receivers and antennas on benchmarks and leave the equipment in place for a couple of days at each site. Benchmarks are permanent brass disks that were drilled into the ground. The benchmark typically has a cross inside a triangle that serves as a reference point for centering of the antenna.

During each survey, we return to these benchmarks to collect data and determine how the point has moved. Data collected allow us to calculate a horizontal and vertical location, similar to latitude, longitude and altitude, and thus to evaluate the change from prior surveys.

Campaign GPS surveys have been conducted on Mauna Loa and Kilauea since the mid-1990s, providing extraordinary time-series records of volcano deformation. Along with Mauna Loa and Kilauea, Hualalai and Haleakala volcanoes are surveyed periodically (approximately every 3-5 years) as part of our volcano monitoring program. This past October, HVO surveyed the western flank of Mauna Loa to add to the picture of volcano deformation provided by the continuous network.

Measuring the changing shape of the volcano helps us refine models of what is happening beneath the surface, for example the inflation of a magma reservoir. A combination of improved technology and new data processing techniques is providing our best data yet in the history of satellite-based geodesy at HVO.

Volcano activity update

Kilauea Volcano is not erupting and its USGS Volcano Alert level remains at Normal (https://volcanoes.usgs.gov/vhp/about_alerts.html). Updates for Kilauea are now issued monthly.

Kilauea monitoring data continue to show steady rates of seismicity and ground deformation, low rates of sulfur dioxide emissions and only minor geologic changes since the end of eruptive activity in September 2018. Rates of seismicity have been relatively consistent, although at the summit episodic increased rates appear to be coincident with the inflated phase of the DI events.

Sulfur dioxide emission rates are low at the summit and below detection limits at Pu‘u ‘O‘o and the lower East Rift Zone. The pond at the bottom of Halema‘uma‘u, which began forming July 25, 2019, continues to slowly expand and deepen

Mauna Loa is not erupting. Its USGS Volcano Alert level remains at Advisory. This alert level does not mean an eruption is imminent or progression to an eruption is certain.

Mauna Loa updates are issued weekly. For more info on the status of the volcano, please go to https://volcanoes.usgs.gov/volcanoes/mauna_loa/status.html

This past week, about 53 small-magnitude earthquakes (nearly all smaller than M2.0) were detected beneath the upper elevations of Mauna Loa. Most of the earthquakes occurred at shallow depths of less than 6 km (~4 miles) below sea level. Deformation measurements show continued summit inflation. Fumarole temperature and gas concentrations on the Southwest Rift Zone remain stable.

HVO continues to closely monitor Kilauea and Mauna Loa for any signs of increased activity.


Visit HVO’s website (https://volcanoes.usgs.gov/hvo) for past Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake info, and more. Email questions to askHVO@usgs.gov.

Volcano Watch (https://volcanoes.usgs.gov/observatories/hvo/hvo_volcano_watch.html) is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

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