Geography Reference
In-Depth Information
Anatomy of the Acronym: GPS
Global Anywhere on Earth — Well, almost anywhere, but not (or not as well):
Inside buildings
Underground
In very severe precipitation
Under heavy, wet tree canopy
Around strong radio transmissions
In “urban canyons” among tall buildings
Near powerful radio transmitter antennas
or anywhere else that does not have a direct view of a substantial portion of the sky. The radio waves
that GPS satellites transmit have very short lengths—about 20 cm. A wave of this length is good for
measuring because it follows a very straight path, unlike its longer cousins, such as AM and FM band
radio waves that may bend considerably. Unfortunately, short waves also do not penetrate matter very
well, so the transmitter and the receiver must not have much solid matter between them, or the waves are
blocked as light waves are easily blocked.
Positioning —Answering brand-new and age-old human questions: Where am I? How fast am I moving
and in what direction? What direction should I go to get to some other specific location, and how long
would it take at my current speed to get there? And, most importantly for GIS, where have I been? To collect
GIS data with a GPS, one moves the receiver antenna around areas of the Earth, leaving a tracing of
points in the memory of the receiver, which one later transfers to GIS software.
System —A collection of components with connections (links) among them. Components and links have
characteristics. GPS might be divided up in the following way. 2
The Earth
The first major component of GPS is Earth itself: its mass and its surface, and the space immediately
above it. The mass of the Earth holds the satellites in orbit. From the point of view of physics, each
satellite is trying to fly by the Earth on a horizontal path at 4 kilometers per second. The Earth's gravity
pulls on the satellite, so it falls vertically. The trajectory of its horizontal movement and its vertical fall is a
track that parallels the curve of the Earth's surface, so it never crashes. All satellites, including our moon,
are subject to the same principle.
The surface of the Earth is studded with little “ monuments ”—carefully positioned metal or stone
markers—whose coordinates are known quite accurately. These lie in the “numerical graticule ,” which we
all agree forms the basis for geographic position. Measurements in the units of the graticule, and based
on the positions of the monuments, allow us, through surveying, to determine the position of any object
we choose on the surface of the Earth.
2 Officially, the GPS system is divided up into a space segment, a control segment, and a user segment. We will look
at it a little differently. One of the many places to see official terminology, at the time this topic went to press, is
http://tycho.usno.navy.mil/gpsinfo.html#seg.
 
 
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