GPS Plots the Public Sector
- By Bob Brewin
- Aug 09, 1998
During the Age of Exploration, the difference between a good map and a bad one often meant the difference between a successful voyage and ending up feeding the fishes. As a result, accurate maps and cutting-edge navigation tools were treated as closely guarded state secrets.
They still are. But instead of compasses and sextants, today's cutting-edge tools are Global Positioning Systems that are used to precisely locate any position on Earth using data transmitted from a network of satellites. The Defense Department designed GPS to help its forces accurately traverse uncharted territory and guide precision munitions. GPS really came of age during Operation Desert Storm in 1991, when U.S. Army tank forces used the system to find their way through rain and sandstorms that reduced visibility to zero, leading top U.S. commanders to hail it as "one of the major technology advances...in modern war."
The good news is that state and local agencies-as well as private business-are rapidly getting the benefits of trickle-down technology from the more than $10 billion that DOD has spent developing the GPS infrastructure. In the years since Desert Storm, GPS has emerged into what one of the leading receiver manufacturers, Trimble Navigation Ltd., has dubbed a new information "utility," with users in the civil sector far outnumbering military users.
Improving Accuracy, New Uses
GPS has been in limited use for several years by surveyors, who could be seen hauling around tripods and receiver units in backpacks. But the development of smaller, easier-to-use and more accurate equipment has greatly broadened the uses of GPS for state and local agencies as well as the private sector. Transit systems throughout the country have started to field GPS-equipped buses, trains-and in Washington state, ferry boats-which automatically radio their position back to a central dispatch point.
Increased portability and ease of use have resulted in other interesting applications of GPS. The Spartanburg County coroner's office in South Carolina, for example, uses a handheld Magellan Systems Corp. GPS receiver to mark the location of accident victims, murder victims and other deceased individuals for later reference. And the Orange County, Calif., Sheriff's Department is using handheld units from Magellan in search-and-rescue operations. When searchers find a missing person, they take a reading from the GPS receiver and radio coordinates to the rescue helicopters.
The high end of the technology is still closely guarded. The Pentagon reserves for military and qualified government civil users a signal that provides accuracy within 25 meters or better. But in a recent test of four units that receive the civil signal, I found they all provided position locations with comparable accuracy. And post-processing can improve accuracy even further. For example, California, in cooperation with the Jet Propulsion Laboratory in Pasadena, operates a statewide network of GPS sites that, with computer enhancement, delivers millimeter-level accuracy and is used to predict earthquakes by tracking the movement of the Earth's tectonic plates.
Easier to Set Up, Easier to Use
I tested small, commercial, handheld GPS units from Garmin Corp., Magellan, Trimble and the civil version of the standard military handheld from Rockwell Collins Government Systems. Compared with today's receivers, an old Magellan unit that I tested during Operation Desert Storm looks like an underpowered 1960s Volkswagen Beetle in a race with the supercharged 1998 version.
The advances in receiver technology became apparent when I first turned the test receivers on and began the process of initialization, in which a receiver collects an almanac from one of the 24 satellites in the network and then stores it in memory. This almanac is a continuously broadcast timetable of the satellites in the constellation, identified by number and orbital location. Receivers tap into the almanac to help determine distance from the satellites and, hence, the user's location. An almanac must be collected every time the receiver is moved more than 500 miles or if the memory has been cleared.
Back in 1991, I made sure I had a book to read as the old Magellan collected an almanac; it took 15 to 20 minutes. The receiver slowly displayed messages about "scanning the sky" and "collecting data" as I paged through my book and pondered the low-tech utility of a map and compass. Finally, after what seemed an interminable wait, the receiver finally flashed the latitude and longitude of my location in a simple alphanumeric display.
This time, thanks to smarter chips in all four receivers I tested, initialization took less than five minutes.
After initialization, the Desert Storm-vintage Magellan often took more than five minutes to provide me with my first "fix," even if I had not moved more than 100 feet after turning the receiver off and then on again. Anticipating a similar wait for this test, I lined up all four of the new receivers on my backyard table, pushed the power buttons and walked about 20 feet into the kitchen to check the clock. When I returned about two minutes later I found that all four receivers had locked in the GPS constellation and displayed my position: 38 57 635N, 78 57 091W, as reported by the Garmin. (Next to the butterfly bush in the backyard of a home in a quiet Maryland suburb, for the map-impaired.)
Not only had all four test receivers locked on to the three GPS satellites needed to record my latitude and longitude in less time then it takes to surf through all the channels on a cable TV system, they had acquired the fourth bird needed to determine altitude. (That is, 228 feet above sea level here in Hyattsville, which abuts the far lower and, hence, strangely named Mt. Rainier, Md.)
Unlike geostationary communications satellites, which "sit" over the same spot on Earth, the GPS satellites rise and set above any given position, which means you derive the best position information from birds with the best and highest angle to the receiver. Two of the commercial receivers-those from Garmin and Magellan-featured circular graphic displays that made it easy to determine the "best" satellites (and their automatically broadcast constellation number) within view, along with bar charts that depict the relative strength of their signals.
The simpler Trimble unit display shows the number of satellites acquired with small icons on the left-hand side of the location screen and scrolling alphanumeric messages, which report the number of satellites in view of the receiver-from zero to four-as well as an ominous "poor satellite geometry" message that means you may end up with a bum position. The no-nonsense Precision Lightweight Ground Receiver (PLGR) from Rockwell Collins also uses an alphanumeric display to identify satellites in view and indicate their health and the elevation angle.
For anyone suffering from digititis like me, the graphic displays were far easier to comprehend. One quick look at the Magellan display showed me I was getting very poor signals from all three satellites in view. This indicated I should move from under the porch overhang, where the graphic displays quickly indicated a much more solid satellite lock.
Differences That Make a Difference
To test the accuracy of all four receivers, I bicycled over to a favorite known, surveyed point near my house: College Park Airport (38 58 50N, 76 55 22W), where in 1909 the Wright Brothers introduced the Army to a spiffy new technology. During the trip, I fastened the Magellan unit to my handlebars with some rubber bands. I locked the unit into positioning mode, which also features speed and trip distance readouts. (The result was undoubtedly the most expensive bike speed-ometer and odometer on the Anacostia River Northeast Branch Trail.)
Exactly 3.2 miles later (according to the Magellan), I arrived at the airport and set up all four receivers on a picnic table overlooking the runway and roughly 50 feet from the surveyed geographic center of the airport. Despite the fact that the literature provided by all three manufacturers warned that accuracy of their gadgets was limited to 100 meters, this test showed all receivers locking in within seconds of the surveyed position. One second translates into 100 feet.
Running all the receivers in a stationary mode-which allows them to continuously recompute the errors deliberately induced into the GPS system by the Pentagon-I managed to refine the accuracy even more, with the Magellan reading out an Estimated Position Error of 30 feet. The Trimble won this accuracy test by a slight margin, reading out a position just two seconds off in latitude and three seconds in longitude.
Based on this test, the commercial GPS receivers of the late 1990s offer greater accuracy, improved displays and more functionality. But they're not perfect, especially when the average user tries to employ their more sophisticated navigation functions. That's because to tap into these functions the user must turn the receivers into data-entry devices. Any gizmo that requires the average person to use rocker arms, poorly identified buttons or even the numbered keyboard on the PLGR as a data-entry device, flunks the "easy to use" interface test faster than anything ever hatched by Microsoft Corp.
Each of these receivers has its own strong and weak points.
Garmin GPS III
The Garmin GPS III is the all-around best value of the units we examined. For starters, this receiver's graphic display outdoes all the other units tested. Besides the satellite-status display, it features a powerful interactive map page of the United States, which allows a user to zoom in from a region (such as the Washington, D.C., area) to a matter of feet. This feature makes initialization quick and easy. Instead of inputting approximate latitude and longitude (as suggested for the other receivers) all the user has to do to initialize the GPS III is to move the map cursor to an easy-to-find point on the map-in my case the intersection of the Capital Beltway and Interstate 95-press Enter, and within minutes, up pops the location.
This powerful mapping tool is also the GPS III's curse. The graphic display causes this receiver to eat batteries (four AA cells) in a matter of two to three hours, an expensive proposition, even if you buy batteries by the 12-pack at Price Club, as I do.
Magellan ColorTRAK Satellite Navigator
The Magellan runs a close second to the Garmin unit as an all-around value. But don't believe the hype about the unit being the first color receiver. It is, but the color really doesn't add much. Instead, focus on the unit's ease of use and accuracy, which it offers aplenty.
Although lacking the map that is packed into the Garmin, the receiver features a graphic display that makes it easy to initialize (which you can do by selecting a region and a state on a pull-down menu) as well as to track satellites. I found its eight-button keyboard and "arrow pad" more logically arranged and easier to use than the Garmin keyboard. At $280, the ColorTRAK also offers a great value for the money.
Trimble ScoutMaster GPS
As well as being the accuracy winner, this receiver also is an ergonomic delight, with its curved contour fitting smoothly and easily into the hand. The internal and invisible antenna seems to do a better job than the external and movable antennas on the other receivers.
The seven-button keypad on the ScoutMaster does not delight, however, requiring constant consultations with the manual. What's more, the procedures for setting waypoints and managing routes are frustratingly nonintuitive. You can perform just about any mapping operation with the ScoutMaster that the other units can perform, but it will take you more time and requires a good deal more patience.
Rockwell Collins PLGR
If you're going scouting in the desert and need to have the receiver work after you've thrown it in the back of your Hummer while making a quick getaway from enemy patrols, the Rockwell Collins PLGR is the unit most likely to stand up to the abuse.
This Army-standard receiver-which as far as I can determine from observation is bolted into every Humvee in Bosnia-features a rugged case that ensures the receiver will continue to function even if it is thrown to the ground and stomped upon, which is exactly what I wanted to do when confronted by the PLGR manual, which was seemingly written to obfuscate rather than enlighten. The manual is one only a military mind could produce. It took me 20 minutes just to figure out how to put the batteries in.
Unfortunately, as rugged as the Rockwell unit is, it's also big and bulky, measuring nearly 10 inches long with its antenna retracted. The large alphanumeric keypad makes this the best data-entry device of all four receivers tested-an important factor to consider if you plan to input many waypoints.
The Rockwell unit is accurate, though slightly less so than the other units we examined. And its routing tools are strong, but many users will be disappointed to find that it offers no graphic display features. Instead, all the users sees are readouts of locations and headings.
Bob Brewin is the editor at large for Federal Computer Week.