Invisible Stars - GPS Declassified: From Smart Bombs to Smartphones - page 73

technology. They take for granted electricity, radio waves, wireless communi-
cation, video screens, atomic clocks, spaceflight, satellites, solar power, micro-
computers, and software programming. All of these are essential enabling
technologies for gps.
The Visible Parts
Of course, gps satellites are visible, but because they orbit at an altitude of
about 12,550 miles, seeing one requires a telescope. This distance is sometimes
expressed as 20,200 kilometers, using the metric system favored by science
and most nations, or 10,900 nautical miles, a traditional navigation unit based
on the circumference of the earth. (A one-minute arc of latitude or one-minute
arc of longitude at the equator is one nautical mile. This equals 1,852 meters,
or about 6,076 feet, making a nautical mile 1.15078 times longer than a statute
mile.) Regardless of the unit of measure, this distance amounts to roughly half
the circumference of the earth, a comparison Captain Cook would have under-
stood, or a bit more than one-and-a-half times the diameter of the planet, a
more useful mental image for modern readers accustomed to viewing photos
of the earth from space. Illustrations of the gps constellation, such as the offi-
cial image posted on the government's website (gps.gov; fig. 5.1), usually exag-
gerate the size of the satellites and trace invisible orbital paths but convey its
overall scale.
The constellation contains several generations of gps satellites with some-
what different designs. Each weighs around two tons, their heights range from
about 6 feet to 11 feet, and their wingspans range from 17 feet to 116 feet. 8 The
solar-panel wings charge backup batteries on board the satellites and power
their clocks, signal transmitters, and other circuitry. While the height and
weight of successive satellite designs have varied, the wingspan has grown
steadily, raising the solar power generated from 800 watts to 2,450 watts. 9 Like
the wingspan, the life span of the satellites keeps increasing. Engineers designed
early satellites to last seven and a half years, and the newer ones have a twelve-
year design life. However, many have lasted much longer than expected. Among
older satellites designed to last seven and a half years, those remaining
“healthy” at this writing include one launched in 1990, one launched in 1992,
and six launched between 1993 and 1997. 10 The U.S. Coast Guard Navigation
Center maintains a website ( www.navcen.uscg.gov) where users can see the
status of every gps satellite. Each satellite has a unique space vehicle number,
or svn, but higher numbers are only generally indicative of age; the satellites