"We have still another bugbear which is characteristic of our cosmic radio problem.
This is the effect of the relative motion between our ships and the station with which we communicate. It makes the frequency actually received different from that transmitted.
This phenomenon is called the Doppler effect. It becomes positive or negative as we approach or recede from the station we are working.
"In actuality, the Doppler effect during our voyage is composed of four independent motional effects. The first is produced by the presently increasing distance of our vessels from Earth and will be reversed during the return trip. The second motional effect is caused by the rotation around the Earth of the station we are working. The third motional effect will be caused by our circling around Mars after we enter the satellitic orbit there.
At that time, the fourth the most powerful motional effect will be caused by the relative motion between Earth and Mars.
"Let me outline for you how the Doppler effect is produced. Let's assume for the moment that the distance between us and the station with which we work is increasing.
Waves emitted by that station must overhaul the ship as it tries to get away from them with its own velocity. Therefore the waves strike our antenna in slower sequence than if we were at rest with respect to the transmitting station. You may think of the simile of an ocean liner: a head sea strikes the vessel more frequently than a following sea.
"The extent of the role played by the Doppler effect is exemplified if you'll forget for the moment our motion relative to the Earth and consider only the periodic rotation of the Lunetta station around the Earth. This produces a very powerful Doppler effect. If, at one moment, the Lunetta station is approaching us at her orbital velocity of 7 km per second, an hour later, it will be receding from us at the same speed. That means that inside of an hour a speed difference of 14 km per second may take place, and thus the frequency which we are now receiving is 140,000 cycles per second higher than that we shall receive within the hour. You remember that we wish to reduce out bandwidth to 1,000 cycles, so you can see how important it is to keep our receiver constantly tuned to the transmitter.
"It's not quite as difficult in practice as you might think. We might use a search receiver sweeping periodically over a wide frequency band in addition to the working receiver. The searching receiver would detect the frequency at which the messages were arriving each time it passed them on its scale, and would periodically tune the working receiver accordingly. Actually, we use a single receiver with "automatic frequency control." The Lunetta station is similarly equipped, and it is not difficult to reestablish communication even after lengthy interruptions."
"How do the Lunetta operators know where to direct their radio beams?" asked Billingsley. "I can see that we simply aim ours at the Earth. But Lunetta surely cannot see our vessels at present."
"Their tables give them our exact bearing in space and since they have our coordinates daily they always know whether we're on our prescribed track or not."
"I'm only a child when it comes to radio," remarked Ross, "and I want to ask what might be a foolish question. Could we communicate by radar?"
"The question is by no means foolish. What you have in mind is obviously "pulse radar," because the simple term "radar" is not clearly distinguished from the term "radio."
We checked and found that it would be quite possible to use pulse radar, although it offered no particular advantages over radio. Perhaps you'd like to hear the reasons.
"Pulse radar technique differs from radio in that the continuous waves used in radio are supplanted by momentary radio flashes or "pulses." The output of radio transmitters is generally limited by temperature rises, mainly in the tubes. In pulse radar we have the advantage of being able to overload tubes mightily during the short times that pulses last, for they can cool off between them. Actually, we can multiply the power of emission on the order of magnitude of 1,000 as compared with continuous waves, if the pulses are short enough. So, if we were working with pulse radar, we could multiply the output of transmitters such as this one by a thousand without any greatly increased effort.
"It sounds quite appealing, but there's a catch to it, for such a powerful emission extends over a considerable bandwidth.
"There's an interesting analogy in acoustics; fire a pistol near a piano when the
damping pedal is depressed, and all the strings will vibrate. Why? Because the sound of the shot is composed of tonal vibrations of innumerable frequencies or tones, as the acousticians call them. Each individual string reacts to the air vibration from the pistol shot that corresponds to its natural frequency, and gets in resonance with it. You might say that the explosion has a wide acoustic bandwidth.
"A radar pulse is just such an explosion, so we'd have to set our receiver for a wide range of frequencies if we wanted to extract a lot of energy from it. That would mean a
wide bandwidth.
"There's the answer to your question. A pulse receiver needs a bandwidth that is greater; the shorter the pulses it is to receive are, the greater the necessary bandwidth. But the noise level increases in the same ratio as the bandwidth. So, as we shorten our pulses for a radar receiver, we get more "grass" as opposed to our continuous wave receiver. On the other hand, we cannot increase our output power without shortening the pulses. So you'll see that what we gain on the swings, we lose on the roundabouts and vice versa.
Transmitter efficiency goes up, but receiving efficiency goes down. So the pulse method has no advantages, and we stick to continuous wave radio for maximum range."
"I say, Francis," said Billingsley, "why did you take the trouble to set up a station in the Lunetta orbit? Why could we not communicate direct with Earth?"
"We thought about it a good deal, and there's not much doubt but that we might have done so under favorable conditions," answered Lussigny. "But there were a lot of practical reasons which induced us to set up the Lunetta station.
"First there's the problem of Earth's rotation on her axis. We could communicate with a single station on Earth for a maximum of a half a day at a time, for that station would be blanked by the Earth's mass for the remaining 12 hours. That would have meant several terrestrial radio stations for continuous contact. These stations would have had to work in relays with continual intercommunication between themselves, and the whole world-girdling organization would have had to be set up for three years. The Lunetta station permits us to work at least once every hour, by reason of its bi-hourly circling of the Earth.
"Then too, radio communication directly to and from Earth would be affected by the atmosphere.
"The lower atmosphere would have made communication hinge on local weather conditions. Sandstorms and thunderstorms might have upset communications. Clouds tend to reflect some of the energy radiated at them by our short waves, and we often would not have known whether we'd been received correctly or why the other station didn't come in at the agreed times.
"When I went to General Braden with this idea and kept going back to him with new problems and demands for perfecting such an involved net of communication stations, he wasn't pleased. Finally he pounded the table and said, 'You'll get one ferry flight to the Lunetta orbit! Fix up something to take up there which will do the job our side of all this atmospheric blockage! We want reliable communication with the expedition, not a scientific circus which will be flopping intermittently so that the wise boys can give a thousand reasons from advanced physics for the flops…'