by Gary Allen
On a moonless night in late December, an officer of the Royal Canadian Mounted Police (RCMP) stopped a vehicle 32 miles outside Macrorie, Saskatchewan. The Mountie didn't feel isolated, even with no homes or vehicles in sight, because his car was linked by satellite to a dispatcher back in the communications center.
His patrol car had an automatic vehicle location system (AVL) using Global Positioning Satellites (GPS) to pinpoint the location of the car stop and display it on a video screen next to the dispatcher.
GPS is the latest advance in AVL technology and it's exciting public safety officials who need a reliable, 24-hour method of accurately locating vehicles during routine and disaster operations.
During the RCMP trial, the NavTrax system designed and installed by Armata Technologies of Calgary will showcase AVL's ability to locate patrol cars and help coordinate responses over thousands of square miles of Canadian wilderness.
GPS Technology
GPS was developed by the Department of Defense (DOD) for $8.5 billion to absolutely determine locations in three-dimensional space, anywhere on earth. It improves on other navigational systems designed primarily for mariners, such as Loran-C and Omega.
The 19th GPS satellite was launched into orbit 10,898 miles above earth in early 1992, finally providing 24-hour coverage to the entire globe and making AVL marketable to public safety agencies. Each of the satellites, controlled by a ground station in Colorado Springs, will stay in orbit for about 7-1/2 years.
Simply put, GPS satellites generate very accurate time signals and continuously broadcast them over a line-of-sight path in the 1200 and 1500 MHz band. GPS receivers decode the time signal, perform some sophisticated math, and determine the receiver's location.
How accurate are these systems? It's a major issue for public safety agencies, but the technology makes answering this question difficult.
"It's not a simple answer," says John Wenzel, public safety product manager for Trimble Navigation, a major GPS provider. There are many considerations, he says, not the least of which is the Department of Defense.
When the system was first conceived, two signals were created--one coded signal for the military (P-code) and one for civilians (C/A signal). The two signals are broadcast on different frequencies and provide similar accuracy.
But the civilian system is so accurate that the DOD degrades the signal so unfriendly nations can't use the system for strategic purposes--so-called "selective availability" (S/A). The degradation is selectively turned off and on during the year, and it randomly varies the accuracy of a particular satellite. This results in an average accuracy of 100 meters when S/A is activated.
You're not supposed to know when S/A is turned on, although sources say it's easy to tell. So you must always assume a lower accuracy when using the civilian signal, even though the satellite you're using may not be degraded at that particular moment.
With S/A turned off, Wenzel says civilian GPS has an accuracy of about 20 meters anywhere in the world. However, there's a trick to improve this accuracy--differential processing.
Differential is simply a way of taking GPS readings from a known location, then applying any error readings from the satellite as corrections to readings from other GPS receivers. The Coast Guard is building a network of six coastline differential stations that anyone can use within about 600 miles of shore.
"By adding the differential capability, even on the degraded civilian signal, we can get the accuracy down to below 10 meters, so it averages about 5 meters," Wenzel says. And for a typical fleet customer with several hundred vehicles, the added cost of differential processing is insignificant.
Want even more GPS accuracy? If you're surveying and can take a fix every second or so from one location, accuracy comes down to two meters. And, using a properly equipped GPS receiver, and taking fixes every second over 15 minutes, you can improve accuracy to one centimeter--about one-third of an inch.
"It depends upon what your application is and what receiver you're using," Wenzel says. Trimble Navigation's newest version of GPS includes dead reckoning, so even an interruption of the satellite signal doesn't stop the flow of location data back to the communications center.
The last word on accuracy has to be a comparison with other available technologies. "Comparing it with any other technology that's ever been available, it's an order of magnitude better," Wenzel says.
For example, Loran-C uses 90-110 KHz signals from several of 94 stations scattered around the globe to give an accuracy of about 500 meters. Omega, another radio-based technology, broadcasts near 10 KHz and is somewhat more accurate, but you have to know your starting coordinates. For mariners, that is usually not a drawback, but it could be a nuisance to police or fire departments.
Even older systems were based on radio triangulation, radio-emitting signposts and sensors mounted on poles or buried in the street. None of these systems was very accurate and none provided continuous monitoring of a vehicle's location, which is a primary requirement for public safety.
GPS receiver prices are comparable to an MDT and start at about $1,200 for models that transmit one-way to the base station. More useful two-way GPS transceivers that can respond to polling signals from the base station are more expensive and require additional radio equipment.
GPS-AVL systems still need the same radio gear as an MDT--an antenna, base station, cabling and other equipment. If you already have an MDT system, the GPS receiver can share that channel to transmit location data back to the base station, saving some costs.
Focusing on U.S.
The new RCMP system uses a trunked 800 MHz radio system to relay its AVL data back to the base station, along with data from the patrol car's MDT. Patricia Kaiser, Marketing Director of Armata Technologies, says the system has already been trialed successfully at the Calgary Police. Now they're out to show the prestigious RCMP the significant benefits AVL offers law enforcement.
Armata Technologies is also targeting the United States, with its 185 million registered vehicles. "The U.S. market is more receptive to AVL than Canada," Kaiser says. In particular, California, Florida and Texas "are really ready for the benefits that AVL has to offer."
Those benefits became obvious, Kaiser says, when Florida's hurricane Hugo blew away street signs and other landmarks. Emergency crews often had no clue what street they were on. Equipped with GPS-AVL, a fire truck, ambulance or police car could have easily navigated to any address in the disaster area.
"We've spent the last couple of years refining the system," Kaiser says, "making sure that it works." Now Armata is pushing ahead to market their AVL in the transit, energy, forestry, trucking, utilities and public safety fields.
Normally, it would be difficult to develop AVL software to mate with applications for such diverse markets. But Armata has taken a unique approach. "One of NavTrax's key features is that it's been designed with a very open architecture," says Kaiser.
This open architecture lets NavTrax easily exchange data with different applications, including any computer-aided dispatch (CAD) software used by public safety agencies. "We can start with a basic AVL package that meets the requirements of a public safety agency," says Kaiser, "and then upgrade and advance the system as their requirements change or as they grow geographically."
Besides coordinate information, NavTrax can also transmit the unit's ID, speed and heading, says Dan Pacholik, Armata's president. The link can optionally carry other vehicle status information, such as the engine temperature, if the siren is operating, or if the lightbar is on or off.
NavTrax can also relay emergency signals from a vehicle-mounted button or from a transmitter worn on the officer's belt. If the officer presses the emergency button, the AVL system immediately transmits an alert and pinpoints the vehicle's exact location on the dispatcher's video display map.
Pacholik says NavTrax combines GPS and dead-reckoning technology to insure the vehicle's location is constantly available, and uses differential processing to improve accuracy to within a few meters.
Dead-reckoning combines distance information from the odometer and heading information from a compass to calculate a vehicle's location, even when buildings or other urban obstructions block the GPS signals. Differential processing helps cancel out GPS errors caused by atmospheric and other timing errors and substantially improves accuracy.
The dispatcher can poll, or interrogate, a NavTrax GPS receiver at any time for its location, or establish polling intervals based on time and distance, or a vehicle's speed and status. Proprietary software developed by Armata allows quick polling that minimizes radio air time.
NavTrax can communicate over almost any type of radio configuration, including single-channel and trunked systems. Pacholik says Armata has successfully used existing UHF and VHF radio bands and he expects cellular and satellite links to be used in the future.
First GPS Site
In the United States, the Schaumberg, Ill. Police Department is using a GPS-based AVL system to track 40 patrol cars. Dispatchers now use AVL to visually determine which units are closest by studying a color monitor that displays units and incidents overlaid on a map of the city. Soon, a software upgrade will automatically display the closest unit when the dispatcher requests a CAD recommendation.
"We went from the stone ages to the future overnight, all in one major step," says Lt. Tom Ostermann, who handles Schaumberg PD's technical services. Schaumberg, a 26-square-mile suburb of Chicago, spent $3.5 million to upgrade their CAD software, convert to 800 MHz trunked radio and mobile data terminals (MDTs), and build a new communications center.
Schaumberg's GPS-AVL was supplied by Trimble Navigation, the CAD software by Integrated Computer Concepts, Inc. (ICC) and the MDTs and radio system by Motorola.
A Trimble GPS receiver was also used at the Oakland Hills fire in October 1991 to plot the extent of the fire so firefighters could devise a strategy for dousing the flames. There were no street signs from which to navigate, and hellish temperatures kept survey teams from entering the fire area to make on-site observations.
So a Trimble technician carried a GPS receiver in a helicopter that flew the perimeter of the fire. The receiver calculated coordinates and stored the data until the helicopter landed. The information was then transferred to a desktop computer and maps of the fire's perimeter, overlaid with the area's streets, were printed out for firefighting strategists.
Ostermann says AVL accuracy at his agency has never been a problem. He can't recall when a patrol car's displayed position was really off-track and says most vehicles are displayed within 50 feet of their actual location.
Despite the possible "big brother" aspects of AVL, police officers didn't complain, Ostermann says. He gave the officers two reasons for using AVL--"One, is to get the best response that we can to the emergency calls, and two, is their safety."
The system isn't complicated to operate or maintain, either. "It's all self-enclosed within that little receiver," Ostermann says. The antenna, about the size of an upside-down soup bowl, mounts to the top of the car or trunk, and the receiver is a compact unit mounted in the trunk.
Beyond all the hoopla of technology, what's the system do for dispatchers? "It's unbelievable," says Ostermann. "When you're in a dispatch center, you've got a blanket over you. It's like lifting that blanket."
But more than just providing the dispatchers with a tool to help speed an emergency response, Ostermann sees a more personal benefit. "When the dispatchers know where those cars are, I sense a lessening of anxiety."
"I think anxiety plays a part in dispatching a lot. This is just one level of anxiety that can be removed from that job," Ostermann says.
Back at the car stop outside Macrorie, there was certainly no anxiety at the car stop. It turned out to be routine, but the officer and dispatcher were relieved to know that GPS-AVL technology was watching over them just in case.
copyright 1991 911 Dispatch Services Inc.