We can understand the distinction between phase and frequency by considering a column of soldiers marching to a drummer's beat. If the soldiers step in time to the beat, they will all be walking at the same rate, or frequency; but they will not be in phase unless all left feet move forward together. Power companies have developed devices that allow them to make certain that a new generator is connected to a system only after it is running with the correct frequency and phase.

In power pools, frequency helps to monitor and control the distribution and generation of power. Based on customer demand and the efficiency of various generating components in the system, members of power pools have developed complex formulas for delivering and receiving electric power from one another. But there are unexpected demands and disruptions in the system-a fallen line, for example-that produce alterations of these schedules. To meet scheduled as well as unscheduled demands, electric power operators use a control system that is responsive both to electric energy flow between neighboring members of a pool and to variations in system frequency. The net result of this approach is that variations in both frequency and scheduled deliveries of power are minimized.

Time and frequency technology is also a helpful tool for fault location-such as a power pole toppled in a high wind. The system works somewhat like the radio navigation systems described later in this chapter. (See page 104.) At the point in the distribution system where the fault occurs, a surge of electric current will flow through the intact lines and be recorded at several monitoring stations. By comparing the relative arrival time of a particular surge as recorded by the monitors, operators can determine the location of the fault.

The East Coast blackout a few years ago brought to attention the vital role that coordination and control-or the lack of themplay in the delivery of reliable electric power. Today many power companies are developing better and more reliable control systems. One of the requirements of such better systems will be to gather more detailed information about the system-information such as power flow, voltage, frequency, phase, and so forth-which will be

fed into a computer for analysis. Much of this information will have to be carefully gathered as a function of time, so that the evolution of the power distribution system can be carefully monitored. Some members of the industry suggest that time to 50 microseconds and even better will be required in future control systems.

MODERN COMMUNICATION SYSTEMS

Time and frequency technology is, if anything, even more basic and vital to the operation of communication systems than it is to the control of electric power systems. Time and frequency information is used to help keep track of messages and to make certain that they reach their intended destination.

One of the most familiar applications of frequency for message identification is "tuning" our radio or TV set to a desired station. What we are really doing is telling the set to select the correct frequency for the station we wish to tune in. When we turn the dial to channel 5, for example, the TV set internally "tunes" to the frequency that is the same as channel 5's broadcast frequency; the set thus selects just one specific frequency from all of those coming in, displaying it and screening out all others.

Of course, channel 5 will have a number of different programs throughout the day, and we may be interested in watching only the program scheduled for 8:00 P.M. We select this program by consulting our watch or clock. Thus we use frequency information to help us select the correct channel, and time information to help us select the desired program on the particular channel.

This use of time and frequency information is routine, but there are other, newer kinds of communication systems that make heavier demands on time and frequency technology. Let's consider a communication system which will provide eight distinct message channels. We might use these channels to connect eight pairs of people each pair consisting of a person at the "send" end of the communication link and another at the "receive" end. At the send

end we have a device which scans each of the eight channels in a round-robin fashion. At any particular instant, only the message from one channel is leaving the scanning device, but during the time that it takes the pointer of the scanner to make one complete revolution, the output signal will be made of the parts of eight different messages interleaved together. The interleaved messages travel down some communication link, such as a telephone line, and are then fed into another scanning device which sorts the interleaved messages into their original forms. As the sketch shows, the scanning devices at the two ends of the communication link must be synchronized. If they are not, the messages leaving the scanning device on the receive end will be garbled. In some very high-speed communication systems the scanning devices must be synchronized to a few microseconds. This kind of communication system where the signals are divided into time slots is called "time division multiplexing."

As another possibility, we could send the eight messages simultaneously, but at eight different frequencies. Now we must know which frequency to tune to. This kind of scheme is called "frequency division multiplexing." Many systems combine time and frequency division multiplexing so that the senders and users must have clocks that synchronize in both time and frequency.

RECEIVER

SENDER

Since no clocks are perfect, they will gradually drift away from each other. So it will occasionally be necessary to use the communication system itself to make certain that all of the clocks involved show the same time.

One of the ways this can be done is for one of the users in the system to send a pulse that leaves his location at some particular time say 4:00 P.M. Another user at a different location notes the arrival time of the 4:00 P.M. signal with respect to his clock. The signal should arrive at a later time, which is exactly equal to the delay time of the signal. If the listener at the receiving end records a signal that is either in advance of or after the correct delay time, he will know that the sender's clock has drifted ahead of or behind his own clock, depending upon the arrival time of the signal.

C

B

SIGNALS LEAVE ALL 3
TRANSMITTERS AT 12 NOON
AND ARRIVE SIMULTANEOUSLY
SHORTLY PAST NOON AT SHIP

By expanding on this scheme all of the clocks in a system can be readjusted to the same time, or synchronized, simply by use of a synchronization pulse every so often. How often the readjustment must be made depends upon the quality of the clocks in the system, and also the rate at which the information is delivered. In a very familiar example-the television broadcast that we receive in our homes-there are about 15,000 synchronization pulses every second, a few percent of the communication capacity of the system. We shall discuss this example more fully in Chapter 16.

If we want to optimize the amount of time that the system is used to deliver messages, and minimize the amount of time devoted simply to the bookkeeping of synchronizing the clocks, then we must use the very best clocks available. This is one big reason for the continuing effort to produce better clocks and better ways of disseminating their information.

TRANSPORTATION

In the second chapter of this book we discussed the important part time plays in navigation by the stars. But time is also an important ingredient in modern electronic navigation systems, in which the stars have been replaced by radio beacons.

Just as a road map is a practical necessity to a cross-country automobile trip, so airplanes and ships need their "road maps" too. But in the skies and on the oceans there are very few recognizable "sign-posts" to which the traveler can refer. So some artificial sign-post system has to be provided. In the early days of sailing vessels, fog horns, buoys, and other mechanical guides were used. Rotating beacon lights have long been used to guide both air and sea travel at night. But their ranges are comparatively short, particularly in cloudy or foggy weather.

Radio beacons seem to be the answer. Radio waves can be detected almost at once at long distance, and they are little affected by inclement weather. The need for long-range, high-accuracy radio navigation systems became critical during World War II. Celestial navigation and light beacons were virtually useless for aircraft and ships, especially in the North Atlantic during wintertime fog and foul weather. But time and frequency technology, along with radio signals, helped to provide some answers by constructing reliable artificial sign posts for air, sea, and even land travelers.

Navigation by Radio Beacons

To understand the operation of modern radio navigation systems, let's begin by considering a somewhat artificial situation. Let's suppose we are on a ship located at exactly the same distance from three different radio stations, all of which are at this moment broadcasting a noontime signal. Because radio waves do not travel at infinite speed, the captain of our ship will receive the three noon signals a little past noon, but all at the same time. This simultaneous arrival of the time signals tells him that his ship is the same distance from each of the three radio stations.

If the locations of the radio stations are indicated on the captain's nautical map, he can quickly determine his position. If he were a little closer to one of the stations than he is to the other two, then the closer station's signal would arrive first, and the other two at later times, depending upon his distance from them. By measuring this difference in arrival time, the captain or his navigator could translate the information into the ship's position.

There are a number of navigation systems that work in just this way. One such system is Loran-C, which broadcasts signals at 100 kHz. Another is the Omega navigation system, which broadcasts at about 10 kHz. Operating navigation systems at different radio frequencies provides certain advantages. For example, Loran-C can be used for very precise navigation at distances out to about 1600 kilometers from the transmitters, whereas Omega signals can easily cover the whole surface of the earth, but the accuracy of position determination is reduced.

What has time to do with these systems? The answer is that it is crucially important that the radio navigation stations all have clocks that show the same time to a very high accuracy. If they don't, the broadcast signals will not occur at exactly the right instant, and this will cause the ship's navigator to think that he is at one position when he is really at another. Radio waves travel about 300 meters in one microsecond; so if the navigation stations' clocks were off by as little as 1/10 of 1 ms, the ship's navigator could make an error of many kilometers in plotting the position of his ship.

There is another way that clocks and radio signals can be combined to indicate distance and position. Let's suppose that the captain has on board his ship a clock that is synchronized with a clock at his home port. The home-port clock controls a radio transmission of time signals. The ship's captain will not receive the noon "tick" exactly at noon because of the finite velocity of the radio signal, as we mentioned earlier. Because the captain has a clock synchronized with the home-port clock, he can accurately measure the delay of the signal. If this delay is 10 ms, then he knows that he is about 60 kilometers from the home port.

With two such signals the captain could know that he was at one of two possible points determined by the intersection of two circles, as shown in the sketch. Usually he has a coarse estimate of his position, so he will know which point of intersection is the correct one.

Navigation by Satellite

We have described a navigation scheme that requires broadcasting signals from three different earth stations. There is no reason, however, that these stations need be on earth; the broadcasts could emanate from three satellites.

Satellites offer various possibilities for navigation systems. An interesting one in actual operation today is the Transit satellite