Recognizing the need for a more precise method of electronic navigation, the United States
Department of Defense set out in 1973 to develop a new, worldwide system. Some twenty
years and thirteen billion dollars later, the result of this effort was the Navigation
Satellite Timing and Ranging Global Positioning System, or NAVSTAR GPS.
The Department of Defense investment proved its worth many times over during Operation
Desert Storm. On the featureless terrain of the desert, allied troops were able to
navigate and communicate positions with precision and confidence using a wide variety of
GPS systems. Even foot soldiers were able to take advantage of the system using Magellan
NAV 1000M hand-held receivers.
Declared fully operational in July of 1995, the Global Positioning System is comprised of
three components: the Space Segment, the Control Segment and the User Segment.
The Space Segment consists of a constellation of 24 satellites in asynchronous orbit at
approximately 10,900 miles above earth. The satellites circle the earth twice daily with
four satellites in each of the six orbital planes. This arrangement is designed so there
are a minimum of four or five satellites visible around the world at any given time. All
satellites transmit a spread spectrum signal on 1575.42 MHz and 1227.6 MHz.
The Control Segment is made up of five monitoring stations worldwide and a Master Control
Station, operated by the 2nd Space Operation Squadron of the 50th Space Wing at Falcon Air
Force Base, located near Colorado Springs, Colorado. The monitoring stations transmit
their data to the Master Control Station where is it processed to determine the exact
location of each satellite. Since a GPS receiver depends on the satellite positional data
(known as ephemeris data) as part of the method used to calculate its position, the Master
Control Station corrects the ephemeris data transmitted by the satellites as required.
Finally, the User Segment is any receiver, military or civilian, which takes advantage of
the GPS data being transmitted from the satellites. The technical details of how the
Global Positioning System works would fill many volumes, and is beyond the scope of this
article. However, a brief overview of how all of these components come together may be
helpful for the general user.
The Author's "Hands-On" Experience
Perhaps the fastest-growing segment of the GPS market is the entry level systems,
available to consumers starting at about $200. Now hikers, boaters or practically anyone
involved in outdoor activities can take advantage of this incredibly accurate navigation
system for a fraction of the cost from only two years ago!
To get first-hand experience with a GPS receiver, I obtained one of the latest offerings
from Magellan Systems, the GPS 3000. This unit, measuring a mere
6.6"x2.3"x1.3" is an example of the recent extraordinary leaps in
technology.
After about twenty minutes with the manual, I was able to get the receiver configured to
start my test. The first step was to initialize the receiver by taking it to a local park,
where there is as much unobstructed sky visibility as possible. Then, I just turned the
unit on and it to began acquiring data from the satellites. A satellite tracking screen
showed the relative location, along with the signal strength, of all satellites within
range. In about fifteen minutes, the receiver had figured out where in the world it was
and was ready to go to work; all in less than an hour from the time I opened the box.
I began by driving around to various locations in the neighborhood, obtaining a fix, and
setting up waypoints (the GPS 3000 will hold 200 waypoints in memory). I then cycled
through the six different graphical navigation screens to gain a better understanding of
how each worked (See GPS example screens). There was a graphical display for almost any
method of navigating one could imagine.
One of the screens I found most useful was the "pointer" display, which shows
the direction of travel, along with an arrow that points continuously to the selected
destination. Another display, which I found beneficial as a pilot, was a screen that
simultaneously indicated bearing and distance to destination, as well as present bearing
and speed. On the same screen is a Course Deviation Indicator (CDI) that displays in
selectable increments how far off course you are. A word of warning to pilots: some GPS
Course Deviation Indicators work opposite of the VOR CDI, and will take you from "off
course" to "very disoriented" (we pilots don't get "lost", we get
"disoriented") very quickly.
Perhaps one of the most beneficial screens, a moving map, scaleable from one-tenth to
fifty miles, allowed me to track my course with extraordinary accuracy. I could see my
present course as compared to the computed course to the selected waypoint.
Waypoints can be established by either keying in the coordinates ahead of time, or by
storing a present position and assigning it a waypoint name. Numerous waypoints can then
be strung together to create a "route". On the GPS 3000, up to five routes may
be stored. After you reach your destination , the GPS 3000 can then automatically reverse
the route to guide you back to the starting point.
One very annoying problem with this and some other GPS receivers I have used, was the
tendency to give false readings when the unit was stationary. This is due primarily to the
inaccuracies induced by Selective Availability. Since the perceived position of the
receiver is constantly being altered by up to 100 meters, the SA induced deviations can
mislead the unit into sensing it is moving when it is actually stationary. This phenomena
may be manifested by the various display screens pointing the wrong direction to
waypoints, latitude and longitude varying slightly, and the odometer indicating
inaccuracies. However, at even a walking pace, these errors usually diminish, or disappear
altogether.
GPS - Nav Aid for the Outsdoorsman
Hikers, hunters and fishermen are among the many outdoor enthusiasts who will benefit from
the use of a GPS receiver. And though this tool may give the user a new-found sense of
confidence, they are strongly advised not to rely entirely on a GPS receiver in the
wilderness. While most of the units available are rugged and reliable (the GPS 3000 is
even waterproof), GPS receivers are not immune to failure. Also, a heavy forest canopy or
obstructions such as hills can absorb or block the satellite signals, rendering the
receiver is unusable. Finally, extensive continuous use, especially with the backlight
activated, may consume batteries far more quickly than anticipated. Anyone venturing into
deep wilderness would be wise to carry a compass and appropriate topographical maps.
Several sets of extra batteries would also be highly recommended. As your travel, use the
coordinates from the GPS receiver to mark waypoints on your map. Then, if your electronic
navigational wizard goes belly-up, you should still be able to find your way home.
Amateur Radio and GPS
Amateur (Ham) radio operators have found a number of innovative uses for GPS receivers.
One of the more interesting variations of the hobby is attaching transmitters and
repeaters to weather balloons and launching them high (100,000+ feet) into the atmosphere.
From this altitude, amateurs can communicate through the repeaters for hundreds of miles!
To track the position of the payload during the ascent and parachute ride down, GPS
receivers may be coupled to "packet" data transmitters. The precise latitude,
longitude and elevation, along with other telemetry, are sent via radio signal to ground
monitoring stations. The coordinates of the balloon are then relayed to the intercept team
at the anticipated touchdown area. In some instances, members of the intercept team, with
the use of a laptop computer running Automatic Packet Reporting System (APRS) software,
and a minimal amount of equipment to receive and interpret the data, can view a graphical
depiction of the balloon superimposed on an area map, right from their own car. With
thousands of dollars of electronic equipment floating with the wind, it's nice to know
within a few yards of where the payload came to rest.
Getting The Right GPS Receiver
New GPS receivers are hitting the market at an astonishing rate. Receivers specifically
designed for the aviation, marine or outdoor enthusiast offer a bewildering variety of
options. And prices range from a low of about two hundred dollars up into the thousands.
The type of application for which you need a GPS receiver will usually be the primary
deciding factor. GPS receivers are available with an assortment of pre-loaded database,
depending upon the application.
For pilots, receivers customized for aviation offer a database of airports, fixes, and
airspace boundaries. Displays designed specifically for the pilot's needs show not only
the current position, but ground speed, altitude, tracking information and estimated times
for each leg of the trip. In the event of an emergency, a few keystrokes will get the
pilot the names and directions to the nearest airports.
Boaters will appreciate the customized GPS receivers designed for this specialized
audience. Again, along with current position data, coastlines, buoys, hazards and other
navigational data are available on moving map displays. Handheld units are specially
designed to handle the challenges of a marine environment, including several which are
designed to float. Many marine models have a "man overboard" feature, which
allows you to mark the position of a person who has fallen into the water, and then will
automatically plot a course back to that location.
For hikers, handheld units are getting smaller and more affordable every day. Clear,
concise graphical display point the way to the desired destination. A handy feature of the
Magellan GPS 3000 will plot the way back home using fixes it has automatically logged
during the course of your journey. Rugged, waterproof models, designed for the rigors of
outdoors, make hiking into unfamiliar territory much safer and more enjoyable.
Future Applications
Advances in Global Positioning System technology will soon begin to touch our lives in
ways we probably could have never envisioned. Already GPS receivers can connect to
portable computers, and coupled with mapping software, indicate your current location and
the most direct route to the desired destination. Soon these "moving maps",
shown on built-in displays, will be available as a low-cost option on many new cars.
One of the most practicable applications of GPS technology is in the field of aviation.
For years, commercial aircraft have had to fly indirect routes to their destinations via
highways in the sky known as "airways", wasting immeasurable time and fuel in
the process. Now that aircraft have the ability to transmit their precise position at all
times without the help of radar, a concept known "free flight" is all but a
foregone conclusion. Aircraft will be able to proceed directly from airport to airport,
coordinated by computer controlled navigation systems which will track all other aircraft
in the vicinity. The results will be a substantial reduction in travel time and an
increased margin of safety. Even for light aircraft operating under Visual Flight Rules,
navigation will be far easier, safer and more reliable than ever before.
For emergency services, the possible applications for GPS technology are almost endless.
For example, it is feasible to track the position of all police patrol cars in a given
district. Then, in an emergency, the dispatcher can route the nearest car to the scene,
saving precious seconds. Likewise, should an officer need immediate assistance, merely
pressing a button on his radio will alert the dispatcher to his situation while
transmitting his position. Backup units could be quickly sent without the officer ever
having transmitted a spoken word.
Soon, advances in GPS technology will allow Emergency Locator Transmitters (used in
aircraft) and Emergency Position Indicating Radio Beacons (used on boats), to transmit
their precise position when activated. Currently, Search And Rescue (SAR) teams have to
rely on a system of approximate location by satellite or mobile radio direction finding
equipment. With the ability of these devices automatically transmit coordinates, SAR teams
will be able to receive exact location information and locate downed aircraft or sinking
vessels in a fraction of time. The resultant savings of life and property should be the
most wonderful benefit of this incredible, new technology.
For additional information about the Global Positioning System, the U.S. Coast Guard
operates a computer bulletin board with numerous text files. The BBS can be reached at
(703) 313-5910. You can also reach them via the internet at the following URL: http://navcen.uscg.mil.
For information about the GPS 3000, or other Magellan GPS receivers, call Magellan Systems
at (800) 707-5221.
Other GPS manufacturers include:
All articles are property of Haskell L.
Moore, and may not be reproduced
in whole or in part without written permission from the author.
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