Geoscience Reference
In-Depth Information
Hofmann-Wellenhof et al. (2001: Chap. 2). For details supplementing the
compact description here, the reader is referred to these topics.
5.3.1
Basic concept
GPS is the responsibility of the Joint Program Oce (JPO), a component
of the Space and Missile Center at El Segundo, California. In 1973, the JPO
was directed by the U.S. Department of Defense (DOD) to establish, develop,
test, acquire, and deploy a spaceborne positioning system. The present nav-
igation system with timing and ranging is the result of this initial directive.
GPS was conceived as a ranging system from known positions of satellites
in space to unknown positions on land, at sea, in air, and in space. The
original objectives of GPS were the instantaneous determination of position
and velocity on a continuous basis, and the precise coordination of time (i.e.,
time transfer).
Based on code or carrier phase measurements, GPS uses pseudoranges
derived from the broadcast satellite signal.
Using the code measurements, the pseudorange is derived from measuring
the travel time of the coded signal and multiplying it by its velocity. Since
the clocks of the receiver and the satellite are never perfectly synchronized, a
clock error must be taken into account. Consequently, each equation of this
type comprises four unknowns: the three point coordinates contained in the
true range and the clock error. Thus, four satellites are necessary to solve
for the four unknowns. Indeed, the GPS concept assumes that - without
obstruction - four or more satellites are in view at any location on or near
the earth 24 hours a day.
Using carrier phase measurements, ambiguities must be taken into ac-
count as additional unknowns. For more details see Hofmann-Wellenhof et
al. (2001: Sect. 6.1.2).
5.3.2
System architecture
Space segment
Constellation
The GPS satellites have nearly circular orbits with an altitude of about
20200 km above the earth, i.e., they are mean earth orbit (MEO) satellites,
yielding a period of nominally 12 sidereal hours. The nominal constellation
consists of 24 operational satellites deployed in six evenly spaced planes (A
to F) with an inclination of 55 against the equator and with four satel-
lites per plane. Furthermore, active spare satellites for replenishment may
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