# Volcano Watch — Global Positioning System accuracy improved for volcanologists and all civilians

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On May 1, 2000, President Clinton announced the United States' decision to stop degrading Global Positioning System (GPS) accuracy. The act has made it possible for civilians to obtain positions as precise as 15 meters (50 ft) using handheld portable GPS receivers. It benefits us in the volcano world, too.

On May 1, 2000, President Clinton announced the United States' decision to stop degrading Global Positioning System (GPS) accuracy. The act has made it possible for civilians to obtain positions as precise as 15 meters (50 ft) using handheld portable GPS receivers. It benefits us in the volcano world, too.

GPS is a satellite-based system that provides accurate location and timing data to users worldwide. Its origin was for military defense purposes. The Department of Defense had a long-standing policy of intentional degradation of the satellite signals, a process called Selective Availability.

GPS works by a system of precise clock-based radio signals. Selective availability works by allowing the clocks in the satellites to drift. At a stationary ground-based receiver, the tiny variations in the time signals make it appear as though the distance to the satellite is changing. Special codes were required to compensate for the clock drift. Anyone using the system without codes had to operate dual receivers in order to correct for the clock variations.

We use GPS in several ways at the Hawaiian Volcano Observatory. Foremost is for our surface-deformation studies. Permanently sited and temporary GPS receivers transmit data about their position to HVO, and small movements in ground surface, as little as 1-2 mm (one-tenth of an inch), are detected. These changes may indicate magmatic changes in the shallow crust that lead to changes in eruption status at Kīlauea and Mauna Loa volcanoes.

We have always surmounted the problem of degraded GPS signals by operating numerous sites and sampling the data simultaneously. Drift in the satellite clock is canceled out by this method. Consequently, we'll see no substantial improvements to our detailed deformation monitoring from these recent changes.

Probably the greatest immediate enhancement will be in our geologic mapping and sample-site information. We map new lava flows using portable GPS receivers. Since 1996 we've had special Department of Defense-authorized GPS receivers that possess the precise codes needed to maintain position despite degradation of satellite signals. But these receivers could only be used by our salaried employees.

Volunteers on our staff could use commercially available receivers, but the precision of these instruments was no better than from 50 to 100 m (160-320 ft). Now these same receivers maintain precision of about 15 m (50 ft), and relative positioning from point to point is commonly much better. Thus, we're able to use our volunteer staff to much greater advantage in helping us monitor the distribution of new lava flows erupted at Kīlauea volcano.

We and Earth scientists worldwide have been using portable GPS receivers to record the position of drill holes, outcrops, and samples collected for radiocarbon dating or chemical analysis. When satellite signals are intentionally degraded, these location data are rarely much better than the reckoning we customarily do using map and compass or air photos. With degradation stopped, handheld GPS positions used in open country are now as precise as the topographic maps of the region. Under forest canopy, especially the dense Hawaiian rain forest, all portable GPS receivers have poor reception and, consequently, poor positional data.

Many hikers and hunters in Hawaii have handheld GPS receivers. A typical unit costs about \$150 and may be ordered from any outdoor-equipment store. These users will realize a dramatic improvement in GPS accuracy now that signal degradation has been stopped. They won't be able to detect ground surface changes of a few millimeters, but they could map a new lava flow with as much precision as we can.

### Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated during the past week. Lava is erupting from Puu Oo and flowing through a network of tubes toward the coast near the eastern boundary of Hawaii Volcanoes National Park. The tube system is now well-established, and surface flows from breakouts of the tube system are seen uncommonly on the coastal flats these days. Lava is entering the ocean mainly at Wahaula and at a site 1.1 kilometers (1200 yards) to the west. The Wahaula site is explosive intermittently. The ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

No felt earthquakes were reported this week.