Mary C. Gruszka / 07.06.2005 12:00AM
TV Signals Used for Geo-Positioning
Rosum technology uses TV RF as alternative to GPS
REDWOOD CITY, CALIF.
TV signals can go where no GPS transmission has gone before, like inside buildings or deep into urban geography. And that makes them attractive for terrestrial-based radio location-positioning systems, as an alternative to GPS or complementary to the satellite based system.
At least two entities, Rosum Corp. of Redwood City, Calif., and Communications Research Centre Canada (CRC), of Ottawa, Ontario, have been investigating methods to take advantage of the existing cadre of thousands of TV transmissions.
Rosum already has radio positioning infrastructure in four metropolitan areas: San Francisco and the Bay Area, Washington, D.C., parts of Virginia and Maryland, greater New York City (the five boroughs, parts of Connecticut and New Jersey), and Sacramento, Calif.
"This covers about 11 percent of the country," said Jon Metzler, director of business development for Rosum.
PUTTING THEORY INTO PRACTICE
So far the installations have been used for demonstrations, but the technology will soon find a practical application, incorporated into Trimble's newest version of its TrimTrac locator system. Trimble, a Sunnyvale Calif.-based company that has produced a variety of GPS based products for more than 20 years, will add radio positioning via TV signals to its GPS tracking system for enterprise-wide automobile fleets.
"The Trimble system will be a hybrid of TV and GPS. They are very complementary," Metzler said. "Our system will help Trimble extend its reach to parking lots, chop shops, and the like, since GPS doesn't work inside."
The Rosum system employs a distance measuring technique known as "multilateration," where signals from three or more transmitter sites, with known fixed coordinates, are measured by a remote device (the device being tracked). Distances to each transmitter site are then calculated, simultaneous equations are solved, and the location of the remote device is then determined. (The Rosum system, technically speaking, does not use triangulation where angles, rather than distances, to transmission sources are determined.)
The receiving device, as installed in a car, for example, is pre-programmed to sequentially scan for a certain set of unique frequencies appropriate for the city of operation. Looking at multiple TV signals ensures redundancy in cases of multipath, or if some of the expected signals are not present at all.
"For digital, we are looking for the field segment sync signal, and for analog we are looking for the ghost cancelling reference," Metzler said. "We don't demodulate the signal. We don't care what is on the broadcast. Because we aren't demodulating the picture, we can dig below power levels you'd need to see a picture."
The information gathered at the receiver is not actually processed in the receiver. Rather it is sent via a wireless connection to the Internet to a server that performs the necessary calculations to determine time of travel of each signal, and then the corresponding distance. Stored in the server are the coordinates for each transmission site.
"We clock at what time the signal left the transmitter and clock at what time the signal arrived at the receiver in the mobile device," Metzler said.
What Metzler would not reveal was just how Rosum calculates this signal travel time, saying that's the "special sauce" or trade secret of its system. If that's all the system did, it would not be very accurate because TV signals are not locked to a known time reference. That's why the current, or "phase one" implementation of the Rosum system includes additional receivers at fixed locations which are timed to GPS. "The receivers are small PCs about the size of a construction lunchbox," Metzler said. "Each has two antennas, one for TV and one for GPS."
The receivers are strategically located in each coverage area. "There are three monitors in Washington, D.C., and four in New York," Metzler said. The receivers scan the same frequencies as the mobile device and forward that data, as well as the GPS time stamps of when the signals were actually received, to the server. The server uses this information to calculate the timing offset used to make corrections in calculated transmission travel time of signals received by the mobile device.
Results of tests performed by Rosum indicate that radio location via TV meets or exceeds the FCC benchmark for location tracking for cell phones, i.e. 50 meters, 67 percent of the time, according to Metzler.
For the Trimble project, Rosum is providing a module for the Trimble TrimTrac receiver. The Trimble device is about the size of a large PDA, and is non-evasively installed in the glove compartment of a car. Also included from Rosum are the monitors, server, and other hardware and software, and the "know-how," as Metzler put it.
Phase One "is useful for systems for today, but it's not necessarily what could be used in the future," Metzler said.
ON TO PHASE TWO
In what Rosum refers to as "Phase Two," the monitor would be eliminated if a stable clock reference was added to each TV transmission. Metzler estimated that the cost per transmitter would be around $5,000, and that the clock signal would have no effect on the broadcast content.
But who would pay? "We'll look for homeland security to pay for it," Metzler said. He added that this amount would be less than the annual maintenance cost of the GPS system.
Another emerging technology could also be used to provide a stable reference, the RF watermark or transmitter ID signal making its way through the ATSC standardization process. (Candidate Standard CS/110A for the identification and synchronization of distributed TV transmitters).
The RF watermark was originally developed for synchronizing multiple 8-VSB transmitters, and identifying a particular transmission. It is a low-bitrate spread spectrum signal that is more robust than DTV, according to Yiyan Wu of the Communications Research Centre Canada. "This data is good for emergency alert systems," Wu said. "Even in places that can't receive a DTV signal, it can still receive the transmitter ID sequence. The RF watermark can provide the same function as the DTV field sync [in the Rosum system]. These are complementary to each other."
Wu added that transmitter manufacturers are starting to build systems that can insert the transmitter ID sequence. Wu has met with executives from Rosum.
Dr. Jim Omura, chief scientist at Rosum, had this to say about the RF watermark: "Part of this system includes the embedded pseudorandom noise sequences that are ideal for the robust unique identification of TV transmitters and for sending low rate data that is extremely reliable and can be received at extremely low signal levels. These embedded pseudorandom noise sequences can be employed in both geolocation using ambient TV signals and by Rosum's self-contained positioning system, resulting in a very robust position location system."
Yet another enhancement to the system would be possible when analog TV channels are retired and made available for other uses. One of these uses could be supplemental transmitters, which Rosum calls "Pseudo-TV Transmitters," to achieve greater positional accuracy. These transmitters could produce a regular TV signal or a specialized signal at lower power. Metzler said that the specialized signal could be tailored to be resistant to jamming, something that GPS is susceptible to, and be applied to targeted areas, like spot coverage of a specific building.
But that is in the future. For the present, Rosum has responded to the Presidential Decision Directive of Dec. 8, 2004 regarding U.S. Space-based Positioning, Navigation and Timing Policy that "establishes guidance and implementation actions for space-based positioning, navigation, and timing programs, augmentations, and activities for U.S. national and homeland security, civil, scientific, and commercial purposes."
"Who will be the champion on the behalf of the government?" Metzler asked rhetorically. Rosum is putting forth itself and its technology to be just that.