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management, and agricultural production. GPS technology is also
important in spatial data acquisition and surveying controls within a
SDI framework. GPS is basically a type of radio-navigation system.
Any device that uses radio waves can be used to determine distance

and/or direction.
There are currently 4 GPS systems operational or being built:
NAVSTAR GPS (U.S., operational) GLONASS (Russia, being

rebuilt) Beidou (China, experimental/under construction) Galileo
(Europe, experimental).
In general, GPS consists of 3 segments: space, control, and user.
Space segment contains GPS satellites itself. There are currently

24 operational GPS satellites in orbit, with 6 in-orbit spare satellites.
GPS satellites orbit around the Earth. Satellites orbit the Earth at
20,200 km altitude. The high altitude insures that satellite orbits are

stable, precise and predictable, and that the satellites' motion through
space is not affected by atmospheric drag. Each satellite orbits the
Earth every 11 hours 58 minutes, so the GPS satellites cross over any

point on the Earth approximately twice per day.
There are four satellites in each of 6 orbital planes, and each plane
is inclined 55 degrees relative to the equatorial plane (the satellite path

crosses the equator at 317º, 17º,77º, 137º, 197º and 257º degree right
ascension angle). It also insures satellite coverage over large areas.
Each GPS satellite transmits multiple signals on multiple
frequencies. There are two main frequencies used by GPS Satellites

(L1 (1575.42 MHz) and L2 (1227.60 MHz). L1 contains C/A code, P
code, and Navigation Message. L2 contains P code only. The L3, L4,
and L5 frequencies for transmitting navigational signals already exist

or are proposed.
The satellites also provide two levels of service: Standard
Positioning Service (SPS) and Precise Positioning Service (PPS).
These satellite services, signals and codes are discussed in greater

detail below.
GPS satellites transmit signals at extremely low power levels.
Satellite signals require a direct line to GPS receivers (“line of sight”),

thus signals cannot penetrate water, soil, walls or other obstacles.
The signals can pass through clouds, glass and plastic but cannot
go through most solid objects, such as buildings and mountains.

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