GPS and the Width of Nations: How Bock's Spatial References Measure Up

Dedication of the California Spatial Reference Center
Dedication of the California Spatial Reference Center on February 20, 2001. From left to right: Charlie Challstrom, Director of NOAA's National Geodetic Survey; Yehuda Bock, CSRC Director; Bill Young, League of California Surveying Organizations; and Charlie Kennel, SIO Director.

11.12.02 -- Pythagoras, the Greek philosopher, mystic, and mathematician, once pointed out that change was the norm for humans. "You can never step in the same river twice," he was quoted as saying. Seeing California through the eyes of Dr. Yehuda Bock, a faculty member at the UCSD Scripps Institution of Oceanography and researcher at Calit², you may feel that you can never step on the same California twice.

California's land mass experiences a vast number of shifts, with frequencies ranging from fractions of a second to centuries. Forces big enough to change measurements include earthquakes and other tectonic motion, land subsidence, mud slides, volcanic deformation, large construction projects, accidents, storms and hurricanes, erosion, and even terrorist actions.

What Bock is working on is a novel sensor, communication, and information technology system that can measure all these three-dimensional shifts in time with high accuracy. His work is one of the defining projects in the Calit² SensorNet living laboratory.

Bock serves as director of the Scripps Orbit and Permanent Array Center and the California Spatial Reference Center. In this context, he and colleagues in government, academia, and industry have set up a network of 260 base stations, anchored to the solid earth, that receive location and timing information via spread spectrum transmissions from the 28 Global Positioning System (GPS) satellites, which have become amazingly accurate with the removal of the Select Availability signal scrambler. A growing number of these base stations use CDPD wireless technology to continuously (every second) send their location and related data to a central computer, then to GPS users.

The result is a level of precision that allows Bock to claim, for instance, that he can measure changes of only a few centimeters in a dam after an earthquake. But Bock says his center's capabilities actually have a much wider set of applications.

Bock explains, "Over the last decade, we've been installing GPS tracking stations throughout southern California. These stations track the satellites continuously, and every 24 hours we recalculate the position of the sites relative to all other sites. By monitoring their positions over time, we can determine if the crust is moving in response to seismic activity. The ground is moving all the time because the plates are moving with respect to each other. But by monitoring over time, we can see how the movement relates to earthquake physics. Assessing position to study crustal deformation is our main goal."

Elaborates Bock, "While we collect and maintain the data, others are able to use it." Many people from government and industry use the stations as references to determine position. Government users include state and federal surveyors, regulators, zoning authorities, and Caltrans, which is responsible for improving traffic flow in California and for all the highways and bridges. Commercial users include property surveyors, assessors and appraisers, and mapmakers.

"We distribute one to two million pieces of information a month," he says. "We also process raw data for non-scientific users so that they can use GPS capabilities easily. And we've developed network technology and Web site tools to enable users to do measurements - all at no cost to the user."

Bock points excitedly to a new capability: CSRC in collaboration with Orange County's Public Resources and Facilities Division have upgraded the GPS network run by the SCIGN project to enable real-time collection and dissemination over the wireless Internet. "In the past," he says, "we collected the data by downloading just once every 24 hours with a sampling rate of 30 seconds." Using dedicated radio modems, the data is now deposited via a server in Santa Ana, California. Then the data gets checked and put on the Internet so that someone with access privileges can get to it - all with a latency of only about one to two seconds!

Orange County Real-Time Network
Orange County Real-Time Network. Left: Map of GPS site locations for the Orange County Real Time Network; Upper Right: Typical GPS site; Lower Right: Control center radio antennas used to receive data from the GPS sites.

"We're now getting readings every second," continues Bock. "And receivers are available that sample 10 times a second, with others capable of 20 to 40 times a second on the way."

The Hector Mine earthquake, which occurred October 16, 1999 and had a magnitude 7.1, helped Bock's group focus their activities. "We were able to observe and develop the technology to get a fix with one single measurement," says Bock. "If you can observe five satellites out of the seven or eight visible at any one time, you can instantaneously determine your position within a centimeter or two."

During the earthquake, Bock's group was able to 'see' the wave as it propagated through the crust because of the change it caused in the positions of the stations. The sample rate was only once every 30 seconds but, with more than 25 sites active in Los Angeles, they could see the L.A. basin literally resonating up, down, and sideways for several minutes after the quake.

"This isn't possible with seismic instruments," explains Bock. "While they sample 50 times a second, their frequency range is much more limited so they don't have the temporal resolution our system provides. Furthermore, seismic instruments measure site accelerations and velocities, which need to be converted to displacements. GPS does this directly."

The next step is to upgrade the system to accommodate high data rates. Says Bock, "We want to get the data rate as high as possible to provide data from the field, right when it's collected, to support applications that need to determine position extremely quickly."

Bock's GPS receivers, which use two frequencies, are considered "geodetic-quality." As a result, they come at a high price tag - something like $10-20K each. However, with the expanding list of applications and obvious public interest, the growing demand is causing manufacturers to build lower-cost receivers, more on the order of $100 apiece and even available at Wal-Mart.

"While they're not designed for high-precision work," says Bock, "they still can be put to good use. Our system, based on multiple cross referencing and the fact that errors can be determined and accounted for, allows for expansion using these kinds of less sophisticated components. So people with these receivers could leverage the accuracy of our system."

Calit² provides a formal context to facilitate partnerships across traditional disciplinary boundaries - some might call them barriers - and Bock, clearly a believer in the benefit of this philosophy, has followed up an introduction from Calit² director Smarr to team with transportation engineers at UCI.

"I'll be putting low-cost receivers in Will Recker's ZEVNET wirelessly enabled vehicles to identify their position to within centimeters," says Bock. That means both the car itself as well as Recker's control room will have that information, which, according to Bock, could be used for navigation or a better collision-avoidance system.

"Onboard navigation today is only accurate to 100 meters," says Bock, "but ours is accurate to within 10 centimeters. You could even turn your cell phone into a collision-avoidance device." Taking that one step further, one could imagine that, if Bock's vision is realized, insurance companies might insist their customers have state-of-the-art GPS-enabled mobile phones with them while driving - "just don't use them to talk!" they are likely to warn.

Other vehicles - on land, sea, and in the air - could make use of this system, according to Bock. "If you had this system at airports, you could use it to land aircraft. We are testing a CDPD service with only one to two seconds latency, but we want to minimize the latency even further. An alternative is to do some kind of predictive calculation to lower the latency."

The key to Bock's approach is that it takes advantage of the full signal structure of the GPS satellites by using the more precise carrier phase measurements taken by modern receivers at two GHz frequency bands (commonly known as L1 and L2). Pseudorange is an unambiguous, but less precise, measure of the radio signal's travel time from the satellite to the receiver and is the basis of the common hand-held receivers. Carrier phase is a much more precise measurement of the time of travel, but only its fractional part can be measured. The number of cycles is ambiguous but can be resolved by taking measurements at two sites and analyzing the data differentially. This is the basis of the so-called real-time kinematic (RTK) GPS that is used widely by surveyors.

However, Bock's innovative approach, developed through his company, Geodetics, Inc., in La Jolla, allows the ambiguities to be resolved instantaneously at the sampling interval of modern geodetic receivers. This allows precise and instantaneous positioning of not only static but also dynamic platforms. The lesson here is that algorithms and software are just as important as hardware in obtaining the maximum accuracy out of a device.

In addition to the bread-and-butter surveying, Bock sees three applications areas as particularly relevant for his spatial referencing system.

Weather forecasting is one. GPS signals travel in a vacuum at the speed of light. But, when passing through the atmosphere, they move at slightly different rates. "You could measure the delay in the GPS signal as it passes through the atmosphere," says Bock, "and this can tell you the amount of water vapor in the air. That information, if incorporated into NOAA models, could improve short-term weather forecasting. While there are only 100 stations around the U.S., over the next year or two this network will be expanded will and this data will become more comprehensively available."

Currently, the state of the art in weather forecasting, notes Bock, is to launch weather balloons once or twice a day, which provides at most one reading every 12 hours. By contrast, Bock is proposing continuous measurement.

This dramatic increase in sampling rate is a good example of a Calit² theme to use innovative technology to achieve a large improvement on standard practice. For a similar instance, look at Prof. John Orcutt's "Moorings for Environmental Monitoring", which measure oceanic/marine information continuously, instead of sailing out a few times a year and dropping buoys in the water. Bock is way ahead of me on this application "connection" with marine monitoring and admits with a laugh, "In fact, Dave Chadwell and I just wrote a paper on applying GPS to buoys and sea platforms."

The weather is too short term for Bock, though. "You can see changes in water vapor every half hour or less. Over the long term, you could see changes in global climate." Bock's work could have serious implications for many citizens of the world if he could apply GPS to study weather patterns and link that information to policy related to climate management and water husbandry.

Dams comprise a second application. Diamond Valley Lake, operated by the Metropolitan Water District, is the largest reservoir in southern California, holding a six-month emergency reserve of water. Three dams contain this lake - two large and one small. "If one of the dams were to collapse, it would put tens of thousands of people in danger and significantly curtail our emergency water supply," says Bock. "So we put GPS on the tops of the dams and sensors on each of the dams and on stable rock. We take measurements every few seconds to determine, literally instantaneously, the positions of the dams."

Homeland security is a third application. Bock thinks that the State of California could put GPS on each police, fire, and emergency medical vehicles to know the positions of all.

"You could track motion of the vehicles, all the data, all the positions in a central facility, relative to each other," says Bock. "If you wanted to map an area affected by a chemical attack and what emergency responders were on site, you could survey the area through GPS."

Clearly, geolocation is going to become a big business, says Bock. Cell phones, as indicated above, will become geolocators. You can also imagine, as unfortunate as this situation may be, a market for geolocation to track children's whereabouts when they're abducted.

"The only constraint we've got is that the geolocator has to be able to see the satellite," says Bock. "So we've still got work to do to make this work inside a building, for example."

Calit² researchers often find that, by integrating across multiple fields, they are not only surprised but delighted with the creativity that emerges.

"I'm excited about this," says Bock, "because this area is a classic case of scientific research spun off into practical applications. The economic, disaster prevention, and hazard mitigation applications have exceeded what we imagined, so our research has gone further in these unexpected directions. And that's where all the fun is."


1.National Oceanographic and Atmospheric Administration, the federal government's weather man and global warming testers, is a part of the Department of Commerce.