Global Positioning System Reference
In-Depth Information
Figure 6.9
Miniature GPS module (left) with patch antenna (right).
6.2.4
GPS Positioning
The satellites broadcast a number of signals on the two (current) frequencies in the
microwave band at 1575.42MHz (L1) and 1227.60MHz (L2). The former is used
by the vast majority of public users and at this frequency, small patch and rod
antennae are possible, which make portable equipment convenient. The timing of
each satellite's transmissions is governed by an on-board cesium beam atomic
clock, the performance of which is maintained by the ground stations that monitor
system quality. Code Division Multiple Access (CDMA) modulation is used,
which is a form of digital communications. It employs a set of unique
pseudorandom sequences of binary pulses to differentiate between the satellites
(which all transmit on the same radio frequency channel). In the receiver, after
amplification, although all the available signals are present together, a correlation
process selects the unique codes by comparing them with known locally generated
replicas. This process results in a coding gain that effectively amplifies the level
of the wanted signal and rejects the unwanted. The timing of the pulses can then
be used to calculate satellite distances.
Each satellite visible therefore has a locus of equal distance that is spherical
and where a number of these spheres intersect, the receiver location can be
calculated. Systems such as GPS that use signal time differences are known as
circular lateration systems (and can be contrasted with the systems using
hyperbolic lateration based on signal phase differences).
Knowing the satellite distances is important but to find local position it is
necessary to know (with respect to the center of the Earth) the precise position of
each satellite. The ground stations measure and calculate this information that
takes the form of a set of parameters (the GPS ephemeris), which contain the
orbital elements (or Keplerians). In addition to the timing codes, the satellites also
relay the ephemeris data to users on a low bit rate data channel also modulated
onto the carrier as the broadcast message. Before the positioning algorithms can
