15.5 PRACTICAL APPLICATION: A CASE STUDY 731- Pulse-code modulation and demodulation for processing analog signals.
- Time-division multiplexing.
- Digital modulation and demodulation methods for the transmission of information and the
comparison of digital communication systems.
15.5 PRACTICAL APPLICATION: A CASE STUDY
Global Positioning Systems
Modern communication systems abound in practice: cellular phones, computer networks, tele-
vision satellites, and optical links for telephone service. Communication with instruments has
been made possible between our planet, the earth, and Mars.
A global positioning system (GPS) is a modern and sophisticated system in which signals are
broadcast from a network of 24 satellites. A receiver, which contains a special-purpose computer
to process the received signals and compare their phases, can establish its position quite accurately.
Such receivers are used by flyers, boaters, and bikers. Various radio systems that utilize phase
relationships among signals received from several radio transmitters have been employed in
navigation, surveying, and accurate time determination. An early system of this type, known as
LORAN, was developed so that the receivers could determine their latitudes and longitudes.
Figure 15.5.1 illustrates in a simple way the working principle of LORAN, which consists of
three transmitters (a master and two slaves) that periodically broadcast 10-cycle pulses of 100-kHz
sine waves in a precise phase relationship. The signal received from each transmitter is phase-
shifted in proportion to the distance from that transmitter to the receiver. A phase reference is
established at the receiver by the signal from the master transmitter; then the receiver determines
the differential time delay between the master and each of the two slaves. The difference in
time delay between the master and a given slave yields a line of position (LOP), as shown in
Figure 15.5.1.
If the time delays of the signals from the master and a given slave are equal (i.e., no differential
delay), then the line of position is the perpendicular bisector of the line joining the master and
that particular slave. On the other hand, if the time delay from the master is smaller by a certain
amount, the line of position happens to be a hyperbola situated toward and near the master, as
illustrated by LOP for slave 1 in Figure 15.5.1, and LOP for slave 2, where the time delay from
slave 2 is smaller. The intersection of the LOPs for the two slaves determines the location of the
receiver.
LOP for slave 1LOP for slave 2Master transmitterPosition of receiver Slave 1Slave 2Figure 15.5.1Working principle of LORAN.