Global Positioning System Reference
In-Depth Information
FIGURE 1.1.
Our planet. This famous photograph of the earth was taken from
Apollo
17
while on its way to the moon, in December 1972.
Photo courtesy of NASA.
The outer surface of the earth is the least dense. This crust is only a few
tens of kilometers thick; it is separated from the outer core by the mantle—
2,800 km of hot rock. The crust, especially, is not uniform, and this in-
homogeneity results in magnetic (and gravitational) anomalies. The dy-
namics of the liquid core create interesting movements of the magnetic
poles: these e√ects have significant consequences for navigation.
This brief overview of the earth's structure, and of its orbit, serves to
show why we expect that traditional celestial and compass navigation will
be a√ected by the physics of our neighborhood: astrophysics, planetary
physics, geophysics. In this chapter I will expand upon the consequences
for navigation, and for the closely-related disciplines of geodesy and car-
tography, of our neighborhood astrophysics and planetary science.
There Is a Tide in the Affairs of Men
The moon orbits the earth at an average speed of about 1 km s
-1
and takes
approximately 27 days, 7 hours, 43 minutes, and 6 seconds to make one
