When More Than Maps Are Needed—The Amazing Global Positioning System
BY AWAKE! CORESSPONDENT IN AUSTRALIA
THE young hiker’s face was covered with perspiration from her exertion on such a hot day. Her shoulders wriggled to ease the weight of her heavy backpack filled with provisions needed for survival. She carefully unfolded and examined her hiker’s map, but, to her dismay, nothing in the surrounding terrain looked familiar. Before long, a desperate feeling of uneasiness began to stir deep inside her. “Don’t tell me I’m lost,” she moaned.
But suddenly she brightened. Reaching into her backpack, she pulled out an instrument in a protective case and keyed in some information. Within moments she began to smile. Now, she quickly adjusted her pack and headed off with the assurance of someone who knows exactly where she is going.
Why was this hiker able to find her way out of a seemingly hopeless situation so quickly and easily? Because she made use of a remarkable aid known as the Global Positioning System (GPS). This enabled her to know exactly where she was and showed her which way to go. Just what is this amazing system, the GPS?
The full name is the Navstar Global Positioning System. Navstar is an acronym for Navigation Satellite Time and Ranging System. GPS was first developed for the U.S. military, but it is now available for use by anyone anywhere in the world. The first satellite needed for the operation of GPS was launched in 1978. Now, the complete system utilizes 21 Navstar satellites plus 3 active on-orbit spares. These are sent into 10,898-nautical-mile [20,196 m] orbits, and each orbit is tipped at 55 degrees with respect to the equator. In this configuration at least four satellites are available from any location on the earth’s surface at all times.
Split-Second Timing Vital
The satellites transmit radio pulses at specific, known times, and by measuring the exact time a pulse arrives, the Navstar receiver can determine the distance to the satellite. The pulse takes about one eleventh of a second to travel to the earth. The receiver then multiplies this figure by the speed of light, which supplies the distance to the satellite with astonishing accuracy. However, measurement of time must be exact because a mismatch of as little as one millionth of a second will result in an error of about 980 feet [300 meters]!
How can such unbelievably accurate time be kept? This is achieved by the use of sophisticated atomic clocks within the satellites. In his book The Navstar Global Positioning System, author Tom Logsdon explains: “The Block II satellites . . . carry four highly accurate clocks—two cesium atomic clocks and two rubidium atomic clocks. These clocks are so stable and accurate they would lose or gain only about one second every 160,000 years”!
In practice, a receiver, such as the one used by our hiker described at the outset, picks up signals from four or more satellites and calculates a distance to each of them. The ranges are then used to resolve the current latitude, longitude, and altitude of the portable receiver. The result is then displayed by the GPS receiver. At least four satellites are needed for accurate positioning. The portable receivers are light and inexpensive and are roughly comparable in size and cost to a portable telephone.
Superior to Conventional Maps?
GPS receivers indicate not only the exact location of the receiver but also, if details regarding the desired destination are entered correctly, the direction to be taken. In that respect then, devices such as GPS are superior to the most accurate conventional maps. For example: When finding your way using a map, you can be hindered by such variables as high trees or even dense vegetation. Featureless terrain (especially oceans and deserts), darkness, and fog are just some of the other things that can make using a map difficult or even completely ineffective. Of course, the GPS receiver does not do away with the need for maps, but it works very well when used in conjunction with maps and charts. It can be used to great advantage in guiding ships into foggy harbors and in keeping track of cargo containers in busy ports, as well as for many other commercial uses.
As development of GPS progresses, the following are some of the other uses being found for it.
● Tracking hazardous icebergs.
● Forecasting the weather.
● Precision landing of aircraft.
● Finding sunken ships.
● Tracking and navigational systems for automobiles.
● Accurate distribution of fertilizer.
So it was that our hiker was guided by this unique satellite system, which gave an overview of her exact position and provided precise information when it was most needed. Furthermore, she was given instruction as to the correct direction in which to walk, and she eventually reached safety. Yes, just when she thought she was lost, she was helped by the amazing Navstar Global Positioning System!
[Box/Picture on page 22]
In 1984, Oklahoma businessman Ron Frates used the Global Positioning System (GPS) to help locate the remains of ancient Maya settlements hidden under dense jungle foliage in Guatemala and Belize. Frates combined observation of Landsat photographs and precise navigation by GPS. “We were able to map the extent of the Mayan civilization on the Yucatan in about five days,” reported Frates and his colleagues. “Working on foot it would have taken at least one hundred years.”
[Diagram/Pictures on page 22, 23]
(For fully formatted text, see publication)
WHAT THE GPS CAN SHOW
Your exact location
Longitude and latitude coordinates are displayed
The time and date
A tracking compass
Your altitude
A map reference
This can be zoomed in or out. If driving, you can use this to direct you to a destination you have never been to previously
Your bearing
A compass guide and pointer can lead you back home and display the distance you have yet to travel
Antenna
Actual size
Satellite status
This sky view shows which of the 24 satellites your receiver can “see”
Signal strength
If some satellites get blocked (shaded bar), the receiver uses alternates to maintain your position fix
[Picture Credit Line on page 22]
Globes on pages 21-3: Mountain High Maps® Copyright © 1997 Digital Wisdom, Inc.