GPS to the Rescue
by Mike Fillon
| For the past 14 years, Mike Fillon has written hundreds of technology and business articles for a score of national publications. He lives south of Lawrenceville, GA near Atlanta. |
| Photographs: (select thumbnail to view detail image) |
 Rescue volunteers in Minnesota and Wisconsin are using Magellan GPS receivers to guide air ambulances to remote accident sites. Magellan Systems Corp. |
 Working screen for Vision Software's VisionMobile. Using the Global Positioning System, users in patrol cars can graphically identify a call's location on a computer map, as well as access all the benefits of a desktop program right in the field. Vision Software |
 Public Safety Fleet Management uses GPS satellites and Intelligent Vehicle Highway Systems (IVHS) to track and guide patrol vehicles to incident scenes. GeoResearch |
 Patrol officer in Lawrenceville, Georgia. Ruggedized laptop computers receive dispatch information as well as a graphical map display of the call's location. Lawrenceville PD |
 GeoLink PowerTrak from GeoResearch provides continuous tracking of multiple vehicles, two-way messaging, peer-to-peer in-vehicle tracking and GIS feature data collection in a Windows 95/NT-based system. GeoResearch |
This article can be found on
page 38 of the May/June 1998
issue of 9-1-1 Magazine.
In Lawrenceville, Georgia, police dispatchers can send an audible and a visual alert to officers for 9-1-1 calls. The officer can look down and immediately see the call's location, and what the nature of the emergency is. At the same time, an in-car map displays the location of the call.
In Saginaw, Michigan, the dispatch center for St. Mary's Medical Center can tell an air medical helicopter its latitude and longitude from 150 miles away.
And at Ft. McCellan, Alabama, Army Military Police have tested an in-vehicle event triggered automatic vehicle location (AVL) system for the Military Police School. At the December Warfighters Symposium for military police, the system demonstrated its benefits in traffic investigation, game control, and fire rescue operations before an audience of senior military command personnel
What do these three systems have in common? They all use the Global Position System (GPS) to find the locations of vehicles in real time. GPS is on the verge of creating a new and exciting market potential in emergency response, rescue and transport services throughout the U.S.
So far, Lawrenceville Police Chief L.R. Johnson is happy with his police department's system. "The officers in the field now have a wonderful tool to assist in their duties," he said. "The entire department will feel the benefits of this program."
The GPS system was developed by the Department of Defense (DOD) to provide the military a means to determine land, sea, and air positions anywhere on earth, and to determine exact velocities and time, thus giving guidance to troops, vehicles and weapons movements. Worth some $40 million in 1989, the market for GPS receivers and related equipment grew to $1.2 billion in 1995 and is expected to reach more than $8 billion by the year 2000. Industry observers predict GPS will soon change navigation forever and eventually could influence nearly every phase of our lives.
GPS relies on 24 satellites, orbiting about 32,000 miles above the earth that transmit data via radio frequencies to ground-based GPS receivers. Until recently, for technical and financial reasons, GPS devices have been marketable primarily to high-end users such as surveyors, and to special-purpose users such as hikers and boaters.
GPS is based on a technique called triangulation. Simply, the system
relies upon three points to determine distance, with two points being known
and the third being unknown. Using the coordinates of the known points,
it's possible to find the distance to the third, unknown point. GPS satellites
are positioned so that at least six of them are visible theoretically from
any point on Earth. Since a GPS receiver must have an unobstructed line
of reception to each satellite, however, all six are not visible at once
in many instances.
The GPS Constellation
The GPS Operational Constellation consists of 21 navigational satellites (SVs) and 3 active spares which orbit the earth in 12-hour orbits. These orbits repeat the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day).
There are six orbital planes (with essentially four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight SVs visible from any point on the earth.
The Control Segment consists of a system of tracking stations located around the world. The Master Control facility is located at Falcon Air Force Base in Colorado. These monitor stations measure signals from the SVs which are incorporated into orbital models for each satellites.
The GPS User Segment consists of the GPS receivers and the users. GPS receivers convert SV signals into position, velocity, and time estimates. Four satellites are required to compute the four dimensions of X, Y, Z (position) and Time. Each satellite broadcasts its exact position via high frequency radio signals and the exact time with atomic-clock accuracy.
Navigation in three dimensions is the primary function of GPS. Navigation receivers are made for aircraft, ships, ground vehicles, and for hand carrying by individuals. By measuring the time interval between transmission and reception of a signal from one satellite, the electronics in a GPS receiver on earth can compute its own exact distance from that satellite. When that information is combined with the distance from at least three other satellites, the location of the receiver is triangulated and computed.
Depending on the specifics of the system and the processing of the data,
position can be fixed with accuracy from about 100 meters to less than a
meter. Velocity is computed from change in position over time.
Selective Availability
One criticism of GPS is its accuracy. Not that it isn't and can't be, but that commercial application accuracies are purposely limited by the Air Force. Here's why.
GPS satellites send out two different signals, one highly encrypted and difficult to jam for secure military use (P-code) and one for commercial users (C/A-code), which like any radio signal can be picked up freely by anyone. To discourage hostile use, the commercial signal is purposely made less accurate by the US Air Force by adjusting the perceived accuracy of the time signal sent by the satellites. Accuracy from the C/A-code commercial signal is 80 to 100 meters, while the military P-code signal provides accuracy to less than a meter.
This selective availability (SA) of a more accurate signal to the military is of concern to some commercial developers that equate accuracy to market potential. A recent marketing study by Booz, Allen & Hamilton shows a 50 percent increase in potential by the year 2000 if SA was "turned off," making the commercial signal accuracy more in line with the military.
Still, the SA issue is a bit of a moot point, because the most accurate expression of GPS is not P-code, but differential GPS (DGPS), the route many commercial developers are taking. In DGPS, a ground-based transmitter sends out a reference signal from a precisely known location. Comparing that to the GPS-based location, an error or bias - say, five meters northwest - is established for that vicinity. Establishing a communications link with the reference station allows constant refinement of the GPS data, resulting in significant increases in accuracy and less drift.
Last year, the Coast Guard established some 50 DGPS reference transmitters
for coastal U.S. and inland waterways, allowing a stated accuracy of less
than 10 meters within 150 to 200 miles of the transmitters. Also, the Federal
Aviation Administration (FAA) is implementing a DGPS network to aid airline
navigation in the U.S.
GPS and Public Safety
The benefits of using a GPS-based system in public safety include faster response times, improvements in field personnel safety, and better utilization of manpower and vehicles.
It allows dispatchers, attempting to allocate the closest available unit to a call for service, to know the actual locations of their units, not just the last known location. Actual unit locations provide for quicker response in assisting or dispatching units because the dispatcher knows exactly where response vehicles are, thus saving valuable time locating them. Since no intervention is necessary from unit personnel to send location or status information to the dispatcher, response is faster. This can be vital in the case of injury or covert operations.
Vehicle wear and tear, fuel usage, and other metrics can be better managed due to the more efficient allocation and monitoring of resources. Reduced travel distances and monitoring of engine hours versus miles traveled can provide better preventative maintenance planning and savings.
Administrators can use playback capabilities to monitor unit activity, allowing analysis for more efficient resource utilization. Fine tuning boundaries or deployment plans becomes manageable when unit, shift or organization audit trails can be seen graphically. Also, tactical command and control is improved because unit activity is viewed in real time providing timely and necessary feedback.
The basics of a GPS receiver are a power source, antenna, display panel, and the GPS circuitry to make the necessary computations. As recently as 1990, the GPS circuitry itself cost about $1,500. Today, there are semiconductor chips that perform the same function and are the size of a postage stamp that cost $50. As a result, GPS applications have exploded, from helping farmers farm to helping golfers golf. With prices plummeting, more and more public safety organizations are looking at GPS.
There have been some early adopters. One was the Providence Forge Volunteer
Rescue Squad. They have used a Trimble Transpak GPS receiver in an ambulance
since February 1992. The service has used the GPS receiver to guide medical
helicopters to accident sites. This is particularly important in rural areas
where visual navigation landmarks are sparse and terrain is featureless.
The GPS receiver carried in the PFVRS ambulance was used for this purpose
in March 1992, allowing the Medflight helicopter to navigate to a remote
site on the Chickahominy River to pick up a drowning victim. In April 1992,
a patient who had sustained a severe head wound in a boat collision was
picked up from a remote fishing camp. Volunteer rescue squads patrolling
isolated regions of Northeastern Minnesota and Northwestern Wisconsin are
using GPS receivers from Magellan Systems to direct air ambulances to wilderness
incidents.
The Lawrenceville System
In Lawrenceville, Georgia, the police department's 9-1-1 GPS system is from Vision Software's Public Safety product suite. Vision Mobile (MDS), as the system is called, is a client server, integrated desktop system for Public Safety Mobile Computing.
Most everyone in public safety today is looking at Intelligent Workstations in place of dumb terminals in their vehicles. In the Lawrenceville patrol cars, Panasonic ruggedized laptop computers run on Windows 95 with Windows NT at the server level back at the police station. Graphic in-car mapping and automatic vehicle location give officers necessary information immediately.
The heart of the system is the 32-bit Windows NT-based message switch. The core piece of technology is engineered to take full advantage of NT's "multi-threading" capabilities, allowing the Vision Mobile Server to process multiple transactions simultaneously. This allows faster transactions and quicker data processing. This makes imaging, field reporting, and other data intensive tasks possible.
AVL support allows vehicles to update their location as part of every transaction as well as with timed data bursts. The system supports both standard and differential GPS. To secure the system, data can be encrypted with the Data Encryption Standard (DES) algorithm, as set forth by the National Security Agency.
"We experienced somewhat of a challenge in getting the GPS initially set right, and reading the right coordinates, and making sure things were displaying correctly on the map," said Vision Software's National Sales Manager, Marty Hollingsworth. "But they were not that significant and was pretty quickly overcome. It's now doing a good job."
As a result, Lawrenceville dispatchers are able to glance at a computer screen and send the nearest car whenever someone calls 9-1-1.
Law enforcement authorities across Georgia said Lawrenceville is the first police department in the state to get GPS tracking. A handful of departments around the country, like the San Mateo, California, Police Department have been using the technology for years with mixed results.
At Lawrenceville, Vision Software has promised to make the system free of charge in exchange for allowing the department to be used as a police force "showroom." For Vision, it's paid off. According to Lawrenceville Network Administrator, Gary Medders, police departments inquire about the system all the time. "Public Safety is very much a reference type of industry," said Medders. "We constantly have people wanting to know if they can see it in operation."
