by Gary Allen
It might seem easy--where are you? But it can be a difficult question when you're following a suspect in an unfamiliar city, performing reconnaissance for a drug raid or tracking a pursuit from the communications center. Fortunately, technology is providing some very accurate answers.
Automatic vehicle location (AVL) has evolved slowly, but now it includes satellites and high-frequency radio networks that can pinpoint a vehicle or officer to within inches. Combined with sophisticated software, these systems are being installed in vehicles, slipped into drug cargoes and even carried in briefcases.
Satellites Track Vehicles
In February 1992 the Schaumberg, Ill. Police Department rolled out 40 patrol cars with an AVL system based on Global Positioning System (GPS) satellites. Now, dispatchers know exactly where the cars are every moment.
"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 computer-aided dispatch (CAD) software, convert to 800 MHz trunked radio and mobile data terminals (MDTs), and build a new communications center.
Their AVL system integrates equipment from three companies--GPS by Trimble Navigation, CAD software by Integrated Computer Concepts, Inc., and MDTs and radios by Motorola.
GPS receivers in each patrol car receive signals from a network of overhead satellites, compute the car's location and transmit the data to the communications center via the MDT radio link, where it's displayed on an electronic map of the city. The CAD software then uses the data to determine which unit is closest to an incident.
Ostermann says that commonly used police beats just can't account for adjacent beat officers when making CAD recommendations. With AVL and GPS, the dispatchers are always certain they're sending the closest unit, making an apprehension more certain.
As for accuracy, Ostermann says they have never noticed a case where the map showed a patrol car far from its actual position. "I'd say they're within 50 feet of that intersection without a problem."
Besides providing real-time tracking of patrol cars, Ostermann says the system can store vehicle locations and play them back later at higher speed for training classes, examination of critical incidents or response time analysis. The software can even compute a vehicle's speed and the distance it covered between any two points.
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.
GPS Technology
The Global Positioning System (GPS) was developed by the Department of Defense (DOD) for $8.5 billion as a way to absolutely pinpoint locations anywhere on earth, in three-dimensional space. It improves upon other navigational systems designed primarily for mariners, such as Loran-C and Omega.
The 19th satellite was launched into orbit 10,898 miles above earth in early 1992, providing 24-hour coverage to the entire globe. Each of the satellites, controlled by a ground station in Colorado Springs, is expected to stay in orbit 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 derive the receiver's location.
If you venture beyond this fundamental explanation, you get caught up in ephemeris, orbits, differential processing and trigonometry. But there are some complex GPS issues that are essential to understand.
Accuracy is the most-asked question by law enforcement agencies, but it has an answer with the most explanation.
"It's not a simple answer," says John Wenzel, Trimble Navigation's product manager for public safety. He's not trying to hedge, hoping to make a sale. There are lots of considerations here, not the least of which is the Department of Defense (DOD).
When the GPS system was first conceived, two signals were provided--one coded signal just for the military (P-code) and one uncoded signal for civilians (C/A). The two are broadcast on different frequencies but provide similar accuracy.
As more sophisticated GPS receivers became available for the civilian market, the DOD realized that even the C/A signal was accurate enough for military purposes by unfriendly nations. So they ordered so-called "selective availability" (S/A) of the civilian signal--they randomly vary the accuracy of a particular satellite, and turn it on and off at intervals.
You're not supposed to know when S/A is turned on or how much the variance is, although sources say the signal change is easy to detect and the variance can be corrected out of the final fix. But even so, civilian users must always assume a lower accuracy, even though the signal you're receiving may not be degraded at that particular moment.
With S/A turned off, Wenzel says civilian GPS systems in vehicles typically have an accuracy of about 20 meters anywhere in the world. With S/A turned on, accuracy jumps to about 100 meters. However, there's a trick to improve both of these figures--differential processing.
Differential is a way of taking GPS readings from a known location, then applying any error readings as corrections to readings made by other mobile GPS units. The Coast Guard is already building a network of six coastline differential stations that will provide error data to anyone within a 1000 kilometer range.
"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. Want even more accuracy? If you're not mobile and can take a fix every second or so, accuracy is improved to two meters.
And, using a properly equipped GPS receiver, and taking fixes every second over 15 minutes, you can obtain readings accurate to a centimeter--about one-third of an inch. "Comparing it with any other technology that's ever been available, it's an order of magnitude better," Wenzel says.
Trimble Navigation's newest GPS receiver includes dead reckoning software, so a patrol car driving into a garage or blocked by an overpass doesn't interrupt the AVL computations. The unit reads the speed, distance and direction of the vehicle from sensors and continues to compute the car's location. When the satellite signal resumes, it updates the location data from the GPS satellites.
GPS-AVL receiver prices start at about $1,200, to which you must add the cost of mobile radio transmitters, a base station receiver, comm center hardware and software. If you want the dispatchers to poll the mobile units for their location, you'll need to add the costs of a transceiver in each mobile unit and a two-way base station back at headquarters.
Don't Build Your Own
But what if you can't afford the time and expense of bidding, installing and operating your own AVL system? How about buying the service from someone who already has the technology, experience and personnel? PacTel Teletrac offers just such a service in six of the country's biggest metropolitan areas.
If you live in or near Los Angeles, Chicago, Detroit, Dallas/Ft. Worth, Houston or Miami, Teletrac provides an AVL service with an accuracy of 150 feet, comparable to GPS-based systems. They're currently marketing their service to consumers and fleet managers, but their system also has direct applications for public safety agencies.
Their AVL system marries Teletrac's mobile transmitters and radio location network with PacTel's paging and telephone network. When a vehicle tracker is activated either manually or remotely by a paging signal, its 900 MHz signal is detected at several receiver sites. Its location is then calculated and displayed on a digital map at Teletrac's headquarters, along with the unit's direction, speed and ID code.
Teletrac first began marketing this capability as part of a consumer vehicle security service. A small transmitter is installed in your car by a security or electronics store and the vehicle is registered with Teletrac. If the transmitter is activated by a thief, Teletrac immediately begins tracking the vehicle. Their dispatchers notify the vehicle owner and the police by telephone and relay the vehicle's year, make, model, color and license number to help intercept and recover the vehicle.
In some cities, police departments have taken the next step and installed Teletrac terminals right in their communications center so dispatchers can directly view the location of stolen vehicles in real time, which speeds the interception of the vehicle and suspects.
Portable transmitters allow law enforcement to track suspects, contraband, ransom drops or anything else that moves. PacTel maintains location records to support personal testimony in court proceedings. In fact, PacTel says the Los Angeles County Sheriff used the system to track a suspected child molester, then later presented the Teletrac location logs to link him to actual crime scenes.
Now Teletrac is expanding their AVL service with Fleet Director(TM), which combines AVL with two-way mobile data communications. There are two versions: a status-only terminal that allows transmission of pre-defined messages from the vehicle, and a full-text terminal that can send and receive text messages on a 2-line, 48-character screen.
Using Fleet Director(TM), you eliminate the time-consuming approval of radio frequencies--if you can find any--and the expense of setting up your own radio system. You simply install a Teletrac terminal in your communications center and units in each patrol car and begin communicating.
Teletrac's services are priced similarly to cellular--you buy the device, pay a one-time start-up fee, then make monthly payments for the locating service. Fleet Director(TM) customers pay about $2,000 for communications center software, $750 for each mobile terminal, and a $30 monthly fee for each tracked vehicle.
Take It With You
Of course, you're not always tracking vehicles with an AVL system. Sometimes you need to track yourself. In that case, you'll want to tote the PCW-2000, a briefcase-sized unit by EER Systems.
Besides a GPS receiver, this professionally integrated unit contains a 25 MHz 80386 computer, 80 Mb hard disk drive, black-and-white VGA display, 360 dpi plain-paper printer, 2400 baud data modem, Group III fax modem, and 3-watt cellular telephone.
Armed with this suite of technology, you can write, calculate or draw in the field and print-out what you produced. You can connect to headquarters, electronic mail or information services via the cellular phone and exchange mail or transfer data files. You can also send and receive faxes anywhere in the world.
If you're responsible for intelligence gathering, stake-outs, mobile surveillance, drug interdiction, or special operations, you can pinpoint your location to within 10 meters with the GPS unit, and then use the data to plot, calculate, draw or send the data using the PCW-2000.
If your investigations take you beyond the U.S. cellular system, just plug the PCW-2000 into any compatible INMARSAT terminal and you're connected worldwide, with all the same capabilities. If you need to capture and send images, EER supplies the Dycam digital camera, which takes up to 32 black-and-white, 400x300 dpi images. The camera connects to the PC for viewing or image enhancement, and then you can send the photos as data files or fax documents via the cellular phone.
The system fits into a 6"x12"x18" case and weighs about 23 pounds. EER offers many memory, display and power options. The basic PCW-2000 with GPS option costs $8,195.
AVL technology will continue to improve, but even now it provides law enforcement officers with all the capabilities for getting the edge you need to fight crime more effectively.
copyright 1992 911 Dispatch Services, Inc.