Surveying - The Science of Navigation - page 76

phase of mankind's technological advance stand out: the Internet and GPS.
You may be one of those people who would place cell-phone technology
ahead of GPS in terms of the everyday impact it has had on your life, but
maybe I can persuade you here that GPS has, or will have, a greater impact
further down the line. 11
The Global Positioning System began life in the 1970s as a military
navigation and targeting system and, indeed, is still under the umbrella of
the U.S. Department of Defense. The idea draws on the LORAN radar
system of World War II, in which radar beams guided allied bombers to
their targets in Germany, as we will see. The scope and scale of the GPS
concept was much bigger, however: every point on the face of the earth
would be covered, and the position of every GPS receiver that is visible
from space would be determinable with unprecedented accuracy. This
would be achieved via a constellation of 24 satellites, each of which would
transmit information about its current position and time. 12 A receiver on
or above the earth's surface would need to be able to pick up at least three
such satellite transmissions at any instant in order to calculate its position
from the information; four would be better. The clocks on board each
satellite would have to be accurate to within a nanosecond and be stable for
years. The satellite orbits would have to be controllable from earth, and
their transmissions would have to be unjammable.
The satellites are dubbed NAVSTAR, which stands for ''Navigation Sat-
ellite Timing and Ranging''; one is shown in figure 3.17. The first was
launched in 1989, and enough of them were in place by the time of the first
Gulf War for the growing system to be of significant military value. By the
mid-1990s all 24 of the satellites were in place; this is the number needed
to ensure that every GPS receiver on earth can obtain information from at
least four satellites at any one time. (The actual number varies between 4
and 12.) The satellites are all at the same altitude, 20,200 km above the
surface. At this distance, the orbital period of each is 12 hours, so each
passes over a given spot on the surface twice a day. Currently, there are
30 NAVSTAR satellites in orbit: 24 operational and 6 spares. The life
expectancy of each is about a decade.
The constellation is controlled from Schriever Air Force Base in Colo-
rado Springs and from four monitoring stations across the world. As the
11. For a description of GPS see El-Rabbany (2002); for a user's guide see, e.g., Letham
(2008); for a technical reference in the context of surveying, see Leick (2004). The o≈cial
GPS website at www.gps.gov provides a useful overview of all aspects of the system.
12. For example: ''It is exactly 9:42 a.m., and I am latitude A , longitude B , and altitude Z .' '