Why, Strategic Air Communications
December 1950 Radio & Television News Article
You could take this article from 1950 and substitute 'drone' for 'intercontinental bomber' and it would do a good job of describing present-day capabilities for global defense based on modern communications. Get a load of the photo of an Air Force technician doing repair on a radar chassis - using a Weller soldering gun! Compare that to today's method of circuit repair that often requires a hot air tool, tweezers, and an eye loupe. Actually, I can remember replacing fried resistors in my USAF group's radar system using just such a soldering gun. Most of the chassis had point-to-point connections where components connected to bifurcated terminals or terminal tab strips. The Ground Controlled Approach (GCA) mobile trailer pictured looks a lot like the Equipment Trailer for our MPN-14 search radar system, except the search radar antenna was larger. That tall box on the left is a mechanically scanned x-band precision approach radar (PAR) antenna for elevation; the azimuth antenna is similar and lies horizontally lower on the trailer.
December 1950 Radio & TV News|
of Contents]These articles are scanned and OCRed from old editions of the Radio & Television News magazine. Here
is a list of the Radio & Television News articles
I have already posted. All copyrights (if any) are hereby acknowledged.
See all available vintage Radio News articles.
Why, Strategic Air Communications
By Major Gen. F. L. Ankenbrandt
Graduated from the U. S. Military Academy in 1926 and commissioned second lieutenant of Signal Corps. Attended Sheffield Scientific School at Ya'e University receiving his MS in 1927. In June of that year reported to Fort Monmouth where he served in various capacitIes until he entered the company officers' course of the Signal School in September 1928, which he completed a year later. He subsequently served as an instructor in chemistry and electricity at West Point where he remained for five years except for a short period of study at Columbia University. He held various commands during the war and in 1945 returned to the United States to become air communications officer at Air Force Headquarters in Washington. He served in other capacities and received his present post in 1947.
Director of Communications, Headquarters, U. S. Air Force
From take-off to "bombs away" - at the flip of a switch - our giant global air armadas are in constant contact with their headquarters.
In the few years since the end of World War II the development of an aerial concept of strategic national defense has presented the public with a new family of terms - terms that inspire vision of a shrinking globe and suggest the elimination of boundaries previously thought insuperable. The sonic barrier has been penetrated and supersonic speeds have been attained. We have devised aerial tankers for in-flight refueling. We have indirectly brought every spot on the globe within range of USAF strategic bombers. Concepts of tactics and strategy have necessarily been revised to ensure that new capabilities are exploited to the utmost.
Central to the concept and to the revision of our strategy is the super-bomber - the intercontinental land-based bomber that can deliver its bomb-load from a base in this country to an enemy-held target anywhere in the world. This bomber can pulverize an industrial target; it can support our Army; it can eliminate the source of an attack directed against our Navy. It is a weapon new to the history of warfare, and unique in its application. The bomb-load it carries spreads destruction beyond the capacity of any other weapon. Its range is unrestricted. It introduces problems previously unknown to military strategists and tacticians. But the principles directing its use are as old as warfare itself, because they are the principles of command and control. And command and control mean communications.
It makes little difference to the commander of a strategic air force where his command post is located as long as he is in communication with the aircraft he dispatches. The world, to him, is an onion and the vast distances that face a land or sea commander are just so many hours - or minutes - in span. He can dispatch aircraft from a number of points in the world to far-distant targets with almost push-button ease - but when the aircraft are out of sight of their bases, does he still have the command and control that was exercised in making them airborne?
If he has good ground-air radio communications, his command is extended directly to the aircraft in flight and he can redirect it to alternate targets or recall it to its base at will. However, if the communication link to aircraft in flight cannot be established or maintained, his command ends the instant the aircraft becomes airborne and the aircraft is committed irrevocably to the destruction of the target, regardless of sudden political changes that may make recall or diversion a matter of grim urgency.
Skilled technicians repair the USAF's electronic equipment.
On paper, the problem of keeping the strategic air commander in touch with his pilots is simple. To illustrate, take a number of fixed points and draw a number of radial lines in all directions at random. Draw another family of lines across the radials at random to indicate the possible flight paths of aircraft. The radials now are communications circuits. Let's take a closer look at them.
Applying scale to our drawing, we find that the circuits are from a few hundred to a few thousand miles in length. We consult the frequency prediction charts furnished by the Central Radio Propagation Laboratories and make a series of calculations. The optimum working frequencies will vary from about five to twenty megacycles depending upon which circuit is selected, the time of day, and the distance from the station. We select a frequency from the five, eight, twelve" eighteen, and twenty megacycle bands and send them out to the half-dozen or so ground stations associated with the relay stations of the main point-to-point communications axis.
Now that our frequency problems seem to be taken care of, let's pause a moment and examine the ground plant. Making a tremendous arc around the globe somewhere between thirty and sixty degrees north are a number of ground terminal stations that form the main line. The circuits have a maximum capacity of 300,000 groups a day, and associated with each radioteletype tape-relay facility is a high powered ground-air station capable of transmitting simultaneously on three of the five selected frequencies. Modern, professionally engineered, and capable, the lash-up seems foolproof enough and we turn with confidence to an examination of the airborne terminal.
We walk along the ramp to an actual terminal, admiring the sleek aerodynamic shape that looms higher against the sky as we approach, our eyes search for the familiar fixed-wire antenna and the fairlead with its trailing antenna weight tucked tightly in the cup. But our eyes search in vain. A closer inspection reveals no sign of insulators or sky-wires.
A non-commissioned officer steps down a ladder from the bomber and looks quizzically in our direction. "Sergeant," we ask, "where are the antennas?" The sergeant gives us that look reserved for mere laymen and points at the wing that seems to reach the vanishing point as we scan the trailing edge for the hitherto invisible antenna array. "I'm afraid that you'll have to point it out" we say in embarrassment, and the sergeant signals us to follow. We walk in his wake to the tip of the wings. "That's the insulator for the wing cap," he says with a pitying look.
"And that's the antenna?"
"There's one just like it at the other end."
"Mind if we go inside and look at the radio operator's position ?"
"It's all right I guess; anything special you want to see?"
"Yes," we reply as the radio operator sergeant leads the way into the cavernous interior, "everything."
The sergeant sits down at a table and moves a headset to one side. At the back of the table is a panel with a collection of unidentifiable knobs and firmly attached to the table is the usual radiotelegraph key. "Well" he says with a wave of his hand, "this is it."
"But where is the transmitter and receiver?"
The crew chief says "In the wing though all you can see is a tank that looks more like it holds gas or oil or something. Doesn't look like a radio set. Anyhow, it works, wherever it is."
We smile a little to ourselves as the sergeant goes on to explain that the radio set and all of its immediate complexities have been removed from the radio room and stored in some remote portion of the aircraft. The maintenance section is responsible for keeping it a serviceable set, but even so, the radio operator still has his hands full handling his briefing folder (containing frequencies, propagation data, schedules, and call signs) and the plain business of getting a message through.
"I hear a rumor," the sergeant tells us, "that the 'long hairs have cooked up a 'black-box' that will put me out of business one of these days. Some gadget that prints the message on a roll of paper - something simple enough for the co-pilot to operate."
Interior view of the Ground Controlled Approach equipment.
"I wouldn't worry, sergeant," we assure him, "we can make 'black-boxes' do everything but use common-sense. That's where the human element comes in."
"Right," says the sergeant; "'black-boxes' can work out problems that would take me weeks, but when a channel's jammed or the frequency predictions don't work out just right the 'black-box' just quits. A sharp radio operator won't quit until he's got the message through."
We leave the aircraft trying to absorb the effect of this airborne terminal station on the probabilities of maintaining the chain of command between the Strategic Command and this bomber once it becomes a striking force. The efficiency of this airborne terminal is certainly far below that of the ground-air station with its high-power, its high-gain directive antenna arrays, its diversity reception, and its surface-level operating conditions. The airborne radio operator, encumbered with clothing and protective devices against the cold and low pressures of the sub-stratosphere (in case cabin pressures are lost) certainly cannot be expected to come up with the same degree of performance that another operator, sitting at the control console of the ground-air station, can. Furthermore, the airborne sergeant can't entirely free himself from certain anxieties over the hours that he is airborne. The fatigue of his unrelieved attention to his job mounts at a faster rate than that of the ground operator who can say: "Hey, Joe, take my position for a minute, I wanna run over and getta cuppa coffee."
We can't escape the fact that the airborne operators of our strategic bomber force loom up as the single element in the whole chain that must come through with perfect performance against heavy odds. But now that we are acquainted with the problem and the mechanisms involved, let's look at the conditions that the airborne sergeant will have to surmount before he can assure us that the command line is intact.
Because of the distances involved, he must use frequencies from the congested - and somewhat irresponsible - high frequency band. The properties of the band permit us to make an educated guess as to what frequencies will be the most suitable for a given path at various times of day and night. However, when we consult the "Berne Book" to see what other countries may be using our choice of frequencies, and monitor the channel for a firsthand survey of users, the call signs we hear somehow or other don't coincide with Berne listings. The conviction dawns on us that at some other point in space, where we have no monitoring stations the interference pattern may be substantially different.
A glance at the world map and its present political divisions is proof enough that large land masses of the world under a single political philosophy can only be controlled by high-powered ground point-to-point radio communications circuits. Industrial developments require high-traffic-capacity communications, and when that nation is at war the civil and military communications requirements will exhaust almost every communications channel in the high frequency band. Even though certain countries may not subscribe to international agreements, in peacetime, at least, they must operate their circuits on a "live-and-let-live" basis or there will be hopeless confusion.
Not only are the big powers a source of congestion. A host of smaller nations have been urged to develop, and the United States has provided them with high-frequency radio communications and broadcasting equipment, and we have seen to it that they have frequencies for its use. Furthermore, the vast quantity of radio frequencies required for the United States military forces in all parts of the world will further burden the capacity of the high frequency band in providing usable channels.
As a mitigating factor to the apparently hopeless situation, a further study of the radio propagation charts shows that if a number of stations throughout the world are transmitting simultaneously on the same frequency (which they certainly are) they will not necessarily be all heard at a given point at the same time. Short-wave listeners and radio amateurs can verify this phenomenon. It is not at all unusual to hear a radio amateur say:
"That's funny - last night at this time the Aussies were booming in, now all I can pick up is South American stations." These anomalies of the high-frequency band are in our favor, though one can be certain that some interference can be anticipated. Our ability to work through that interference will depend upon the personal skill of the airborne operator and the quality of the receiving equipment he operates.
Exterior view of the Ground Controlled Approach equipment.
If he can get his own signal just a few hundred cycles away from the interference (depending upon its modulation or bandwidth) he can slice away the signal completely. Except for outright jamming, he has a good chance of getting a message out of the hash that meets his ear when he switches to a new channel and listens out for his call sign.
Let us now follow through on a mission and see how well the system works.
Let us imagine that unannounced aggression has committed us to war and a message from the Joint Chiefs of Staff arrives, directing that proper measures be taken by the Strategic Air Force commander in carrying out an effort to insure our defense and end the aggression. Simultaneously, from a number of vastly separated points bombers must take to the air and fly to targets in the enemy's heartland and on his perimeter. The first bomb release is four hours away.
At this instant a flash message is delivered to the commander. A country in alliance with the aggressor has broken its ties with the enemy power in the face of potential bombing attacks, and all targets within its borders must be immediately deleted from the current list of priorities. On the situation map is a steadily lengthening line reaching from the base from which our counter attacking planes were launched to the industrial capital of the now unshackled country. Slicing through the sub-stratosphere, and closing in rapidly upon this country that has sued for peace, is an aircraft capable of indescribable harm unless it is diverted or recalled.
A diversion message with "FLASH" precedence is placed on the main communications axis and addressed to the aircraft. The message is converted to perforated tape and is transmitted outward along the main line from tape-relay station to tape-relay station, branching off to a ground-air facility at key points where it is passed to the operator on duty.
A ground operator takes the message from the hands of the supervisor. When he sees the precedence, the paper suddenly feels hot to the touch. He clips it in front of his position and hurriedly consults the frequency plan that was transmitted with the mission order a few minutes earlier and presses a lever on the intercom. "Hey, Joe - Charlie - get this! - put transmitters four, five, and seven on the antenna that covers zone 'B.' This is it!"
He dials the three transmitters in on frequencies that for the next two hours the airborne operator will be guarding in succession, and starts transmitting the call-sign followed by the message. He repeats, and repeats again. When transmission has been completed the ground operator logs the message and listens out. In a minute or so he will be able to hear another ground operator repeating the transmission from a point a little further along the axis of communications.
On the mission, in the aircraft, the airborne operator is keeping an eye on the clock for frequency changes in accordance with his briefing chart. As he makes his first shift at the appointed time, he hears his call sign deep down in the "hash." He rapidly shifts back to his previous frequency and can barely make out his signal in the noise. He shifts up again to the previous channel and turns his receiver to "sharp" reception. He fishes around a moment then hears his call-sign coming in loud and clear.
He copies the message the first time over and waits for the repeat to verify the text and the authentication. He consults his code book. No, this is no enemy decoy message, it's the real thing. He acknowledges the message and switches the intercom to "call" to pass the message to the aircraft commander.
Thousands of miles away in the operations room of the Strategic Air Force commander the line on the map showing the progress of the bomber from base toward target is now dog-legged toward the secondary target assigned to this aircraft.
The worried Intelligence Officer wrinkles his forehead and mutters, "Well, he got it; there'd have been hell to pay if he hadn't."
Yes, there'd have been hell to pay if he hadn't. These bombers pack a punch - the Sunday punch. And command has to stretch all of the way - clear to the instant of bomb release. But the message has been put through. The airborne operator has functioned more reliably than any little black-box. He had transmitted to the pilot the change in order of the strategic air commander, communicated over the ground-air radio.
Liaison Set Development
After the close of World War II the U. S. Air Force began concentrating great effort toward providing aircrews with the improved radio communications equipments which were so sorely needed.
Queries were sent out to all operational agencies asking for comments on the weaknesses" of existing equipments and a statement of desires in improving radio sets. The replies seemed at first to be demanding the impossible but careful evaluation disclosed many major deficiencies in the currently standard equipments.
Of all the radio communications equipments presented for scrutiny, the liaison set provoked the greatest comment - much of which may have been interpreted as a scathing denunciation. There seemed to be little about it that was right. Although it was a substantial advance over its predecessor, it was a far cry from what could be considered the ideal set.
A recap of the criticisms revealed that they all came to focus at one point: it was not possible to train radio operators to the required degree of proficiency in the limited time available between induction and assignment to a combat aircrew. In other words, aircraft manufacturers could produce radio operator positions in heavy aircraft faster than we could man them. It was difficult enough to teach operators the Morse Code, combined Communications Procedures, radio circuit discipline, and communications security, but when he was placed in front of his liaison radio equipment, the profusion of knobs and switches proved to be just so many statistical opportunities for error.
The radio operator had already been relieved of as many of the knob settings as was possible. The maintenance crews had set up the preset frequencies in the storage mechanisms of the ten-channel transmitter, leaving him only the task of tuning the antenna circuits, a task that accounted for frequent communications failures. His receiver was a bandswitching, "coffee-grinder tuning" equipment containing sufficient permutations and combinations of operator-error to account for additional communications failures when the requisite skills were missing., "coffee-grinder tuning" equipment containing sufficient permutations and combinations of operator-error to account for additional communications failures when the requisite skills were missing.
The plotting board at the USAF Radar Approach Control center.
"Give us a set," pleaded the tactical operators, "where the operator turns it on, sets the exact frequency in a window, and starts beating the key, knowing that his transmitter and receiver frequency could be used for calibrating WWV." "Make the antenna tuning automatic," echoed others, "regardless of whether he's on the fixed-wire or trailing antenna." "Let him store up at least twenty frequencies so that he can QSY within a few seconds to another assigned channel." "Take all of the knobs away from him but a channel selector and a volume control." "Make it so simple that a pilot can operate it."
Five years have gone by since the new liaison set was visualized by the operational commands and several hundred man-years have been expended in developments and monitoring activities. The needs of the two services having major interest, the USAF and the USN, have been combined in Joint Military Characteristics and each service has engaged in separate developments - all with the aim of providing a single, jointly standard liaison radio equipment. The products of these separate developments are nearing the evaluation stage - both having demonstrated that the ideals expressed by the operational commands were attainable. True, there were compromises. Neither of the developments could meet the desired weights or QSY times - and the cost of attaining some of the ideals has caused some last-minute soul-searching on the part of the operational commands.
The sets are not simple. As a criterion, let's consider the number of tubes as an index of complexity. The present USAF standard liaison radio set, AN/ARC-8, contains 23 vacuum tubes. The several developments currently assessable contain 54, 86, and 137 vacuum tubes respectively. Just how much of the automatic operation should be accepted, and how much could be given back to the intelligence of the radio operator with a net saving in circuit complexity?
Representatives of the major USAF commands were invited to observe the detailed functioning of the 137-tube development and the questions were propounded. "Here is the set you've dreamed about" they were told, "you've seen it perform in accordance with your desire of five years ago. We can save a lot of tubes and complex circuits here and there by giving some of the tasks back to the radio operator. Just which of the automatic functions do you feel that you can dispense with ?"
Generals turned to confer with their specialists. Radio operators looked at each other quizzically. Engineers looked expectantly for a sign of weakness - for some indication that the operational personnel were appalled with the complexity of frequency synthesizers, were suspicious of mechanical brains, or were skeptical that the set could function under service conditions. A hand went up tentatively.
"I don't think the channel-storage function is absolutely necessary," one general observed. "As a matter of fact, I like the idea of the radio operator being 'frequency-conscious' rather than 'channel-number-conscious.' "
"We like the channel-storage feature," said a representative of another command, "most of our frequencies are firm assignments and they can all be set in."
When the comments from the USAF representation were analyzed, it became clear that no one was willing to forego any of the automatic features, that the suggestions and comments were for additional features and advances. They would like to have a flexible liaison radio set that would answer any of the requirements of the
major operational commands without interfering with the ideal of interchangeability of major components. Even when the engineers made little pleas for the relaxation of certain demands, they were received coldly. The concept of a fully automatic liaison radio sat rather well with the operators in spite of the increased dependence upon tubes and integrity of circuit components.
As an innovation in the unveiling of a new development, the maintenance activities were given an opportunity to assess the impact of such a complex device upon the various echelons from the squadrons to the major repair depots. The representatives of Training Command came to the alert. Just how could a man - an ordinary man - be trained to find the source of a malfunction in such a complicated mechanism within his own lifetime?
"Maintenance of this equipment," began the speaker, "is divided up into three echelons - the organizational, the field, and the depot." As the narrative of maintenance actions unfolded, the Training Command representatives relaxed in their chairs. What had appeared as an insurmountable difficulty now was as simple as A, B, C - the equipment was designed so that it would practically maintain itself - at least, it is possible for a uniformed lad to take a simple test set and be guided unerringly to a faulty section of the transceiver. Employing a number of "go-no go" tests, the organizational mechanic would isolate a faulty removable section and replace it on the spot, placing the set back in service within the hour. The faulty chassis would be sent to the field repair shops where additional test equipment would single out the faulty sub-assembly, allowing the return of a serviceable section back to the organizational supply shelves within the day. In neither case was it necessary to heat up a soldering iron. The sub-assemblies are divided up into two categories: those capable of economical repair and those to be thrown away. The former are returned to the rear areas for depot repair. It seems at first glance that the maintenance activities have simply unloaded their problem onto the already sagging shoulders of the supply organization, however, upon arrival at a realistic level of supplies to be maintained at each echelon, the problem of "not-in-stock" disruption of maintenance operations appears surmountable.
Now that an acceptable piece of hardware appears to be within the capabilities of industry, the whole problem of liaison radio communications is in need of review. The existing developments have produced an equipment that has relieved the radio operator of certain skills that are purely apart from his basic skill of transmitting and receiving intelligence with the dots and dashes of the Morse Code. The time available for his training in these respects is still too short to provide the required degree of proficiency. In the last war, the greatest fault was poor radio discipline - radio operators were under such compulsion to get their own messages through that they transmitted out of turn and caused interference to other aircraft or ground stations by persistent calling. Code speeds were those of the slowest operator - too slow, in fact, to permit clearing the transmissions of several aircraft within a short space of time. An attempt was made to relieve congestion by permitting only the lead aircraft of a formation to transmit operational messages, screening the organizations for the most accomplished operators and assigning them to lead crews. This, of course, caused a deterioration of interest on the part of the remaining operators and they found themselves so far down in proficiency when an emergency did arise that they were unable to place the radio equipment on the air and establish communication with navigation or rescue agencies.
The success experienced in attaining our aspirations in radio transmitting and receiving equipment led us also to believe that the problems of the radio operator could be solved by employing some "visual message presentation system" for transmission and reception of the intelligence contained in a purely operational message. Numerous systems were reviewed against the stated objective of a three to six thousand mile transmission path and each was found to be considerably less than automatic. Considered were standard teletype, facsimile, telautograph, lighted "marble-board" panels, and specialized printing systems. There was one insuperable difficulty. In spite of CRPL assistance in determining optimum working frequencies, there remained propagation phenomena such as selective fading and multi-path reception, as well as a hopelessly congested high frequency spectrum. Who was to "clear" the channels needed for printing systems that can't tolerate radio interference? Who was to predict in advance of a flight the channels that could be presumed to be free of interference at various geographical points far removed from monitoring stations? When would the optimum frequencies be so cluttered with multi-path reception that teletype or facsimile would be out of the question. It seems hopeless to design a machine with the intelligence of a trained c.w. operator - a machine that can selectively "listen" to a desired signal to the exclusion of all others present and put down on paper the desired intelligence and none other.
The experience of our point-to-point communications activities was consulted for a solution. It was found that in spite of a constant transmission path, use of optimum frequencies, erection of large directive antenna arrays, employment of high power, and operation by personnel of skill and experience, there was a constant level of unpredictable and unscheduled circuit outage. Take away from the fixed communications station its constant path, its optimum frequency, its efficient antenna system, its high power, and experienced maintenance attendants and you have the equivalent of an airborne communications terminal. It can be seen that the probabilities of an aircraft, say, at three thousand miles from its ground station, making an initial unscheduled contact employing radioteletype or facsimile is remote indeed. Consequently, for the time being until a suitable system comes along, we are still considering c.w. as the primary emission from the liaison radio equipment. Secondarily, for certain specialized applications, it appears that teletype and facsimile show promise, however, for ranges up to six thousand miles, nothing is in sight that can compete with manual radiotelegraphy that is adaptable to the airborne radio installation.
As a result of this conclusion, attention was redirected toward the development liaison radio set with the aim of tightening up the c.w. features and reviewing its capabilities for this method of communication. The transmitter was found to be at the highest state of development as a radiotelegraph set, having the capabilities of a secondary frequency standard in accuracy and stability. With the aid of automatic antenna tuning systems, it could deliver the maximum amount of available power to the antenna, whether fixed wire, wing-cap, or trailing wire. The emission was pure c.w. with no spurious components or frequency modulation. A study of the receiver showed room for improvement. In the initial form it was - in accordance with the military characteristics - a simple, straightforward receiver (embodying refinements, of course) using the usual beat-frequency oscillator "C.W.-Tone" control. The frequency accuracy was the same as the transmitter (being slaved to the same synthesizer) but the normal passband placed a lot of emphasis upon the skill of the radio operator in reading his desired signal through interference. It appeared that some improvement could be made in the rejection of unwanted signals by inserting the narrowest possible passband in the receiver output, permitting the radio operator to slice away on interfering signal even though it was only 500 cycles removed from the desired signal. The "C.W.-Tone" control then became the "Fine-Tune" control, allowing the radio operator some latitude in netting with stations that haven't the frequency accuracy or stability of the liaison set. Fortunately, the "C.W.-Slot" idea of adjacent channel rejectivityywas attained without adding a single extra control, thus staying within the basic philosophy of a minimum of knobs and switches.was attained without adding a single extra control, thus staying within the basic philosophy of a minimum of knobs and switches.
Considering the liaison set for c.w. communications, certain features begin to take on importance - features that are available for the first time in military communications history. The first, that of frequency stability and accuracy, permits an altogether new concept in netting a number of scattered airborne communications terminals with a group of ground stations on the same frequency. In the first place, there is no possibility of off-frequency operation - a contingency that has plagued us from the beginning- and a listening station employing a receiver of the same accuracy and stability as the airborne liaison set can receive transmissions from any number of aircraft without necessity of making readjustments in tuning to accommodate variations in frequency from one aircraft to another. All aircraft on a given frequency will practically "zero-beat" with one another. This will require radio discipline of a higher order than ever achieved previously since the ground operator will no longer be able to separate two interfering signals by taking advantage of slight frequency inaccuracies between two aircraft calling at the same time. The signals will be inseparable and both will be unreadable. On the credit side of the ledger is the fact that no longer can there be a missed contact caused by an airborne liaison set being off-frequency just enough to fall outside of the passband of the monitoring station receiver. He will be heard in exactly the same tuning position as all previous aircraft on the same channel. As a corollary, of course, ground station receiving equipment must have tuning accuracy and stability adequate to long channel guards on a given frequency without drift or inaccurate setting. Since the airborne liaison radio receiver will have a "broad" position for monitoring and a "fine tune" range of about plus or minus three kilocycles for any given channel setting, netability with existing ground transmitting equipment is assured even when using the narrow "C.W.-Slot."
There will be no further necessity for long call-ups or tuning transmissions with this degree of frequency accuracy nor any necessity of making constant fine adjustments because of receiver oscillator drift. The radio operator will be freed of any adjustments to transmitter or receiver other than selecting a frequency, putting the desired signal in the "slot," and adjusting the volume to a comfortable level. It is altogether conceivable that he will never see any part of the transceiver or antenna - all of his operations being conducted from a control box no larger than a folded pocket-handkerchief.
Until the sets are out of development it is not possible to reveal more of the details, but the feeling is shared in all quarters that the substantial advances in liaison set communications have been attained at a respectable cost and will soon be released to the field.
We used to say that "communications is a function of command." Now we may well say: "Communications is command."