Latitude

The New copy of this book will include any supplemental materials advertised. Please check the title of the book to determine if it should include any access cards, study guides, lab manuals, CDs, etc.

The Used, Rental and eBook copies of this book are not guaranteed to include any supplemental materials. Typically, only the book itself is included. This is true even if the title states it includes any access cards, study guides, lab manuals, CDs, etc.

INTRODUCTION

* * *

Hundreds of millions of people around the world marveled as American astronauts bounded across the surface of the moon, held their breath as a tiny robotic vehicle roamed the surface of Mars seeking signs of life, and stared in awe at images captured by the Hubble space telescope that revealed the birthing of billions of stars in distant nebulae. In contrast to these spectacular space exploration missions conducted by NASA under the watchful eye of the international news media, development of the NAVSTAR Global Positioning System began under the cloak of military secrecy. The goal: to create a navigation and positioning system that would support military operations of the United States and its allies twenty-four hours a day, in all weather conditions, anywhere in the world-from beneath the surface of the oceans outward to the realm of near-earth satellites. Equally important, enemy forces were to be denied access to GPS.

By the late 1970s work on the secret multibillion-dollar GPS program was progressing well, with about a third of the planned twenty-four satellite constellation already in orbit, when an unexpected turn of events occurred. Academic researchers at the Massachusetts Institute of Technology (MIT) who had been working for many years on the development of Very Long Baseline Interferometry with no knowledge of GPS, proposed the launching of a constellation of artificial satellites that could be used to accurately determine the coordinates of points anywhere on earth. Because the signals received from nearby satellites would be many orders of magnitude more powerful than those received from natural radio sources located billions of light-years from earth, the huge radio telescopes used for VLBI would not be required. First-generation satellite receivers would be no larger than a small suitcase, and, if the demand proved sufficient to warrant the manufacture of special integrated chips, receivers might some day be small enough to fit into a wristwatch. It did not take long for the idea to gain favor with earth scientists, particularly those seeking a faster and more economical way to measure the motions and deformations of the giant plates that form the surface of the earth.

When Department of Defense (DOD) personnel learned of the MIT proposal, they realized that GPS already possessed many of the capabilities the scientists sought, but worried that providing access to civilian researchers might compromise national security. The DOD was particularly concerned that the signal-processing techniques developed by the VLBI experts would effectively bypass the GPS Selective Availability (SA) system, making it impossible to deny access to unfriendly forces. The Defense Mapping Agency (DMA) (now the National Imagery and Mapping Agency) hosted a quickly organized meeting to explore the issue at their offices on the grounds of the U.S. Naval Observatory.

The MIT researchers started the discussion by summarizing their proposal, and then representatives of the National Geodetic Survey, the Air Force Geophysics Laboratory, and the NASA-California Institute of Technology Jet Propulsion Laboratory outlined the potential applications and national benefits that might be gained from civilian access to such a constellation of satellites. DMA personnel responded by revealing only the "barest bones" of the engineering and operating specifications of GPS, and expressed their concerns that the security of the nation could be compromised if too many details were made known.

A period of rather awkward and disjointed discussions followed. Without revealing the details of how SA worked, the DOD personnel tried to ascertain if in fact the scientists could bypass SA; without knowing exactly how the system worked the scientists were unable to say how difficult it would be to bypass the effects of SA. There was no doubt that techniques that had been developed for VLBI could be used to record the satellite signals for later processing, but the delays involved would prevent their use in real time navigation, of highest concern to the military. The meeting ended with vague commitments on both sides for further discussions. While it was not the most encouraging of starts, the meeting marked the beginning of a sometimes friendly, sometimes contentious tug-of-war between the civilian and military communities over access to and control of GPS. Eventually GPS emerged from the shadows of military secrecy and blossomed into perhaps the single most important achievement of the American space program.

Over the past twenty-plus years GPS has evolved into a truly global "utility" that provides position and navigation services to users in all nations. Today it would be hard to find a mariner or weekend boater who ventures beyond the sight of land without a GPS receiver close at hand. Search and rescue workers depend on GPS to get help to those in need as quickly as possible, whether at sea or in wilderness areas. Farmers use GPS to guide their tractors and to map areas of crops that require special treatments of fertilizer and insecticides. Trucking companies equip their fleets with GPS to assist the drivers in getting to their destinations, and to maintain time-tagged records of their fleet operations. GPS-based systems that display maps and provide directions to the driver are becoming a more common feature in rental cars, and are beginning to get the attention of new car buyers.

Scientists have installed dense networks of continuously operating GPS receivers that are used to monitor crustal strain and motions occurring along faults in earthquake-prone areas such as California and Japan. GPS observations also are used to track the motion of masses of water vapor in the atmosphere, information that scientists hope will one day enable meteorologists to predict not only when and where, but how much it will rain. Still other scientists use GPS to track the motion of major glaciers and the Arctic and Antarctic ice caps, to monitor the inflation and deflation of active volcanoes, and to measure post-glacial rebound. GPS is used to navigate and position a wide variety of remote sensing platforms, ranging from vans filled with instrumentation for scanning and recording the surface conditions of highways to ships mapping the bottoms of the ocean with multibeam sonar, to aircraft using scanning laser-ranging systems to map hurricane damage to beaches and sand dunes.

The day-to-day operation of GPS remains under the control of the DOD, but executive orders signed by Presidents George H.W. Bush and Bill Clinton now provide strong assurances that, except during times of extreme national emergency, the civilian community will have access to the system. Future generations of GPS satellites are being designed with special features to better serve civilian users.

To many the development of a global navigation and positioning system may not seem a particularly challenging space mission goal, but rather something more akin to operating a constellation of satellites to handle cell phone transmissions. Even if it were true that GPS satellites were no more complex than communication satellites, the truly challenging problems that had to be solved to make GPS work concerned the more fundamental issue of developing the means to continuously determine the instantaneous position of each satellite. Satellite motions are subject to the complex and ever-changing gravitational field of earth, as well as changes in the gravitational forces associated with the moon, the sun, and other planets. The continual outpouring of energy and material from the sun results in solar radiation pressure on GPS satellites, particularly on the large wing-like solar panels used to generate electrical power, which in turn affects their motion through space. Not only does this pressure increase the difficulty in precisely predicting the motions of the satellites, but the effects slowly accumulate over time and change the relative locations of the satellites within a constellation. Satellites must be repositioned periodically by using onboard thrusters. As thruster fuel is depleted the weights of the satellites are reduced, which changes their reactions to the many forces they encounter.

Because of the complexities of the motion of the GPS satellites, they must be tracked continuously. Today more than a hundred tracking stations are distributed around the globe, most of them owned and operated by nations other than the United States. Each day these tracking stations send their observations to analysis centers located in the United States and in several other nations. The GPS analysis centers combine all of the "raw" observational data received from the global network of tracking stations, and use computer programs developed and refined over decades to derive the best possible positions and velocities of the satellites with time.

Not only must the analysis centers take into account the motion of the satellites in space, they must also account for the motion of earth in space and of the tracking stations on the ground. Changes in the rate of rotation of earth (length-of-day) result in displacements in longitude of the entire constellation of satellites. Changes in the orientation of the earth's axis of rotation in space change the inclination of the orbits of the satellites. Changes in the relative locations of tracking stations, caused by plate motion, slowly change the overall geometry of the tracking network, while events such as earthquakes result in sudden relocations of one or more tracking stations. New orbital parameters must be uploaded often (at least daily), along with such information as the state of the ionosphere and the functioning of the atomic frequency standards that are used to accurately keep time onboard the satellites. The satellites contain computers that process the data received from the master ground station, and generate the information to be encoded on the signals that they continuously broadcast.

GPS receivers must be able to receive, decode, and process the signals from several satellites simultaneously, then extract information about the location and velocity of each satellite as a function of time and determine the offset of the receiver's internal clock relative to GPS time. The basic unit of length used in GPS is the distance that light travels in a unit of time. The receiver must determine the time intervals required for the signals to travel from the satellites to the receiver, and convert them to equivalent distances. In the process the apparent light travel time intervals must be corrected to account for the effects of the ionosphere and troposphere as the signals travel toward earth.

The knowledge required to make GPS work well enough for scientific applications, or even to land an aircraft in zero visibility weather conditions, took scientists hundreds of years to learn, through a process once described by the thirteenth-century English friar Roger Bacon as "winnowing the truth and the reason from what we see."

Not until the sixteenth century did the Polish astronomer Nikolaus Copernicus (1473-1543) dare to suggest that the earth was not the center of the universe, and that the sun and stars did not revolve around the earth. Copernicus correctly concluded from his studies that the earth rotates daily on its axis, and all of the planets in our solar system, including earth, revolve about the sun. Unfortunately, Copernicus incorrectly assumed that earth and the other planets moved at constant speeds along circular orbits and, try as he might, simply could not fully explain the existent astronomical observations.

In 1609 the German astronomer Johannes Kepler (1571-1630) announced his discovery, based largely on observations made by Tycho Brahe (1546-1601), that the planets actually revolve about the sun in elliptical orbits, their distances from the sun and their speeds varying in such a manner as to create the complex motions observed from earth that had so vexed Copernicus (increasing as they moved closer to the sun and slowing as they moved farther away). A decade later, in 1619, Kepler added another finding: the relative times for the planets to complete their orbits around the sun depended only on their mean distances (more precisely, the semi-major axes of their elliptical orbits) from the sun. Planets farther from the sun took longer to complete one trip around the sun, regardless of whether they were small, dense, rocky planets or huge gaseous planets or whether their orbits were nearly circular or highly elliptical. Kepler's "Laws of Planetary Motion," as they became known, made it possible to compute the expected location of a planet at any given moment of time, once the basic orbital parameters were determined by observation.

Any remaining doubts about the Copernican model of the solar system were quickly dispelled with the invention of the telescope. As viewed through a telescope of even a quite modest aperture, it took little imagination to realize that the "wandering stars" were not stars at all, but other planets. Each of the planets was unique: surface features marked the face of Mars, Saturn was surrounded by rings, and Venus displayed phases (that is, variations in the portion of the planet illuminated by the sun as seen from the earth). Clearly earth was just one of several planets orbiting the sun, the central body of our solar system.

The hierarchy of the Catholic Church was not prepared to see earth so easily displaced from the center of the universe. Italian astronomer Galileo Galilei (1564-1642), who had pioneered the use of the telescope for astronomy and had discovered the phases of Venus in 1610, was put on trial in Rome. In 1633 he was found guilty of heresy and, to avoid being put to death, was forced to recant his belief in the sun-centered solar system. Galileo spent the remainder of his life under house arrest on his estate at Arcetri, near Florence.

The same year that Galileo died, English scientist Isaac Newton (1642-1727) was born. From Kepler's Laws of Planetary Motion, Newton was able to deduce that all objects in the universe are naturally attracted to one another, and that the strength of the attraction between two objects depends only on their masses and the distance separating them.

Continue...

Excerpted from Latitude by Bill Carter and Merri Sue Carter Copyright © 2002 by Bill Carter and Merri Sue Carter
Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.