USGS' seismic shift

The U.S. Geological Survey upgrades its earthquake monitoring system in Northern California to digital, operating in the newly available upper half of the 1.7 GHz radio frequency band

• By William Jackson
• Dec 15, 2008

THE U.S. GEOLOGICAL SURVEY has moved its seismic network in the San Francisco Bay area into a new slice of the radio spectrum.

The agency moved to the upper half of the 1.7 GHz band, becoming one of the first agencies to turn over newly vacated spectrum in the bottom half of the band to commercial users who bought rights to those airwaves in 2006.

The new network expands the microwave backhaul capacity of the earthquake monitoring system from 1.5 megabits/sec to 24 megabits/sec.

“That comes at just the right time,” said Gray Jensen, supervising electrical engineer in the USGS office at Menlo Park, Calif. Although 1.5 megabits/sec has been adequate, the network was reaching its capacity as the monitoring system expanded.

That development also came at just the right time for T-Mobile, which bought licenses in the vacated spectrum in northern California.

“We knew the cellular phone operators were eager to move in,” Jensen said. “We still have things to do,” such as integrating network management systems into the new frequencies. However, the network now is operational in its new home in the radio frequency spectrum. “Our main goal was to get the basic system installed, and then take care of the other issues later,” he said.

USGS dodged a bullet in the transition. It was originally thought that the network would have to be moved out of the 1.7 GHz band entirely, all the way up to the less-crowded 8 GHz band, which could have delayed the move for years and substantially increased the network’s cost as a new physical infrastructure was engineered and installed. However, when frequencies adjacent to the original network became available, the Alcatel- Lucent government service subsidiary LGS was able to provide a new radio system that would work on the existing infrastructure.

Meanwhile, it wasn’t simple to plan and install a new system on the existing towers without interrupting network operations.

“Logistics-wise, it was a Herculean task,” said Tony Boykins, LGS civilian agencies account executive.

But the work was substantially completed four months ahead of schedule, and digital seismic data now is streaming into Menlo Park over the new links.

Real-time warning

The network links about 450 stations that monitor ground motion along northern California fault lines, about half of them connecting with eight mountaintop microwave hubs to aggregate data and backhaul it to the central office.

“It’s the backbone of the system,” Jensen said of the microwave backhaul. But satellite links and even DSL lines for nearby stations are also used where microwave hops are not practical or necessary. “We use a lot of telemetry paths” to keep down costs and provide diversity so that a strong quake will not knock out the entire system.

The stations continuously monitor ground movement in fault areas to provide real-time warnings about earthquakes and data for researchers. Many of the stations are located in remote, rugged areas where communications infrastructure is not easy to come by.

“We decided in the early 1980s to put together a microwave system of our own when the cost of telephone lines started to go up after deregulation,” Jensen said.

USGS built its own system of microwave relay towers for backhaul south from the Bay Area and connected with an Army Corps of Engineers network using the same Motorola equipment to the north. When the corps gave up its network, USGS took it over and now operates the entire system from north of Eureka south to San Luis Obispo.

Originally it was an analog system. Stations sent data via FM radio to the microwave hubs, which passed it along via Motorola radios operating in the 1.710 GHz to 1.755 GHz band. Data was gathered at the Menlo Park office and was digitized. The microwave links provided the equivalent of as many as 120 channels of analog telephone lines.

“It’s an audio tone,” Jensen said of the data. “If you listen to it, it sounds like a whistle, and it warbles when the ground moves.”

Several years ago, USGS began changing the configuration, putting modems into the microwave sites so they could send more efficient digital signals to the central office. At the same time, it began upgrading the monitoring stations to send data in digital format.

“We were living in a dual world,” with digital and analog signals, Jensen said. “That was the situation we were in when the transition came along.”

From Sonet to Ethernet

The transition resulted from the Federal Communications Commission’s Advanced Wireless Services (AWS) Spectrum plan, intended to accommodate the rapid growth in commercial wireless services by licensing additional government spectrum for commercial use.

Originally, it was expected that the entire band from 1.710 GHz to 1.850 GHz would be sold off, which would have been bad news for USGS and others in similar positions.

“We had been eyeing the 8 GHz band for some time,” as an alternative location for the network, because it is less crowded, Jensen said. “But that was not a very desirable prospect.”

“The higher you go in the spectrum, the shorter the distance the signal will go,” Boykins explained.

That meant additional relay towers would be needed for the added hops. Finding adequate sites, doing environmental impact studies and constructing the facilities would have been expensive and time-consuming.

Fortunately, when FCC auctioned AWS spectrum in 2006, it sold only the 1.710 GHz to 1.755 GHz band, leaving the upper end, up to 1.850 GHz, in government hands. Unfortunately, there was no commercial equipment operating in this band.

“For the past five years, they had been saying that this spectrum was going away,” Boykins said, so vendors were not building for it.

But when USGS began asking for help, LGS was able to modify the Alcatel- Lucent MD-8000 series radio with existing equipment to operate in the required lower band.

Because the upper 1.755 GHz band is in the same propagation range as the band originally used by USGS, LGS was able to use the same infrastructure for the new radio system, allowing a relatively simple changeover.

USGS began talking with LGS about a new radio system in January 2006, before the FCC auction. It took the company about six months to test prototypes of its new MD 8502 model radio and establish that it was commercially viable. USGS released a request for proposals in September 2007, awarded the contract in December and gave the company 12 months to get the new system in place. It took about 45 days to design the new system.

Installation on the existing infrastructure began in early 2008 and was completed in August.