Aircraft Radio

It is undoubtedly true that without the aid of radio, aerial navigation would not have advanced to the stage where giant airliners arrive and depart keeping time schedules unheard of only a few years ago.

As a matter of fact the airplane is rapidly closing in on such proven means of transport as trains, ships and buses in regards to exact time schedules in all sorts of weather.

It is a recognized fact that the actual value of aircraft radio installations depends entirely upon radio facilities provided on the ground. Hence aircraft radio aids may be roughly divided into two classes, namely:

1. Equipment located on ground

2. Equipment located in the aircraft.

Radio aids to aerial navigation maintained by the Bureau of Air Commerce are of the following three types:

1. Communication stations broadcasting weather information, also available for radio telephony.

2. Radio range stations, radio beacons which work the airway routes by signals which are particularly valuable when clouds, fog, haze or smoke obscure landmarks or lights which would sferve as guide in clear weather.

3. Radio marker stations which indicate the locations of - strategic points on airway routes and are frequently

0 established at important intermediate landing fields.

Point to point radio is part of the ground communication system. Each airway in the United States has a weather reporting service consisting of Weather Bureau and Bureau of Air Commerce observation station, together with a communication network for collecting and discriminating these reports which on some airways consists of teletypewriter circuits and on others of point to point radio stations.

How the Airway Radio Beacon Operates.—With reference to fig. 1, the directional radio range station operates principally as follows: When the pilot is flying by radio guidance, he listens for the Morse Code letters "A" dot dash and "N" dash dot. If the two letters are heard in equal volume, they blend together into a continuous monotone, which is the signal to the pilot that he is on the true course. If he moves to one side of the course he hears the letter "A" predominantly, if to the other side he hears the letter "N".

By knowing the orientation of the off-course signals, or determining it by reference to his air navigation map, he changes his direction to get back to the line of flight where the previously described "on course" is heard.

For station identification the directional signals are punctured at intervals of about 30 seconds by another Morse code combination. distinctive for each radio range. This identification signal is usually transmitted twice, first into the "N" quadrants and then into the "A" quadrants.

Flying on course, the pilot hears the two transmissions of the identifying signal with equal intensity. As he moves away from the course one of them begins to fade into the background until finally only one is heard. This provides another guide for quadrant identification.

If the pilot moves into the "N" quadrant, say, the "N" signal predominates as the "A" fades out, and in addition the «

first of the two station identifying signals becomes more prominent—both indicating to the pilot that he is off course in an "N" quadrant. This system of landing aid is known as the aural radio range as compared with the visual radio range described on page 468.

FIG. 1—Schematic illustrating aircraft landing aid by means of radio j,, beacon. ♦

FIG. 1—Schematic illustrating aircraft landing aid by means of radio j,, beacon. ♦

FIG. 2—Illustrating aircraft radio receiver. The pilot receives code and messages through head phones which are interconnected to receiver as shown.

(Courtesy Western Electric Co.)

FIG. 2—Illustrating aircraft radio receiver. The pilot receives code and messages through head phones which are interconnected to receiver as shown.

(Courtesy Western Electric Co.)

Radio Marker Stations.—Radio markers indicate the location of definite points on airway routes, and are in operation at a number of important intermediate landing fields. A marker station's signal Consists of dot and dash combinations identifying the station which may be heard for a distance of from 5 to 7 miles.

FIG. 3—Interior view of traffic control tower of modern airport. Dispatcher is directing the landing and take-off of aircraft by radio. The cabinet at the dispatchers right is the airport's transmitter which puts his voice on the air. Other cabinets contain air line receivers, each one responding to the frequencies allotted to the respective lines. One receiver is utilized for reception of calls from itinerant flyers. On the desk are the loudspeakers Working with the receivers and controls for lighting at the field, etc. (Courtesy Western Electric Co. Inc.)

FIG. 3—Interior view of traffic control tower of modern airport. Dispatcher is directing the landing and take-off of aircraft by radio. The cabinet at the dispatchers right is the airport's transmitter which puts his voice on the air. Other cabinets contain air line receivers, each one responding to the frequencies allotted to the respective lines. One receiver is utilized for reception of calls from itinerant flyers. On the desk are the loudspeakers Working with the receivers and controls for lighting at the field, etc. (Courtesy Western Electric Co. Inc.)

Receiver Circuit Operation Power
FIG. 4—Schematic circuit diagram of marker beacon receiver set for operation on 10,000 k.c.

They are transmitted on the frequency of the adjacent radio range station. Since the power and range are limited, the pilot flying in or above the clouds and hearing these signals knows that he is close to the transmitter and can determine his approximate position by reference to the location of the station as shown on his air navigation map.

All three types of radio facilities, radio communication 'stations, radio range stations and radio marker stations are used by the airman in a radio-directed flight. Keeping on his course by following the radio range signals, he listens to the periodic broadcasts of weather information and from time to time checks his progress and position by reference to the signals of the marker stations. He may also call a radio communications station or an intermediate field for additional weather information.

Radio Compass Navigation.—The radio compass may be utilized in two different ways. (1), To obtain the direction of a transmitting station, and (2), to obtain position by means of radio bearings from two or more stations.

When it is desired to obtain the direction, the pilot merely turns on the radio compass receiver and moves the station selector to the frequency of the station he desires to pick up.

This may be a commercial broadcasting station, a Department of Commerce radio range station or any other type of transmitting station that sends out signals either continuously or at frequent intervals, and within the receiving range of the compass.

He tunes on this station until the maximum volume is received and from that time on he merely watches a needle on his instrument board which is pointed vertically at zero. If the plane veers off to either side, the needle will move accordingly and hence provide a warning that the plane is deviating from its course. Therefore, the aviator's only duty is to fly the plane in such a direction that the needle constantly points to zero. His course will lead directly to the radio station and from there he points himself to the landing field.

FIG. 5—Simplified representation of needle deflection vs. flight direction with respect to the location of radio transmitting station.

The Radio Compass as a Position Finder.—When it be desired to establish the position, the pilot establishes the direction toward a radio station as previously described, or by rotating the loop antenna obtains a bearing from his magnetic compass and draws a line on his navigation chart to indicate this.

FIG. 6—Illustrating method of determining position by means of radio compass. As the surrounding territory shown on air navigation chart is laid out in scale, it follows that the intersection of the two lines gives the approximate location of the aircraft. It is evident that the accuracy of position so obtained will depend on such factors as: flying speed; angle and distance between aircraft and stations; accuracy of instruments; skill of navigator, etc.

FIG. 6—Illustrating method of determining position by means of radio compass. As the surrounding territory shown on air navigation chart is laid out in scale, it follows that the intersection of the two lines gives the approximate location of the aircraft. It is evident that the accuracy of position so obtained will depend on such factors as: flying speed; angle and distance between aircraft and stations; accuracy of instruments; skill of navigator, etc.

When he takes a bearing on another station and draws the corresponding line on his chart, the airplane's position is at the intersection of the two lines. The utilization along the airways of the radio compass, in conjunction with the radio beacon stations greatly simplifies the problem or orientation.

For example, if the pilot is off the radio beacon course and wants to get back to" it, the radio compass is of assistance. It also simplifies the process of proceeding to the landing field when the pilot has reached the radio range station.

Federal Airway Beacon

LOOP ANTENNA WITH

ANTENNA MOUNTING

FIG. 7—Radio compass attachment assembly diagram. (Courtesy Western Electric Co. Inc.)

LOOP ANTENNA WITH

ANTENNA MOUNTING

FIG. 7—Radio compass attachment assembly diagram. (Courtesy Western Electric Co. Inc.)

Since it may be used in conjunction with any radio station within its receiving range, the radio compass is an aid for off-airway flying—for flights by various commercial operators and private flyers when these follow routes which are not marked by Federal Air Navigation Aids.

Commercial Radio Compass.—Among the several commercial radio compasses at present available, the compass shown in fig. 7 consists of the following units: A loop antenna with attachments; a control unit from which the loop antenna is controlled

Antique Aircraft Loop Antenna

by means of a flexible shaft; an input transformer connected between the radio receiver and the loop antenna and which facilitates the timing and permits the separation of the loop antenna and the receiver of a distance of up to 25 feet.

This system of coupling the loop to the radio receiver offers a great advantage to transport air lines. Because of the unique coupling arrangement two or more receivers may be tracked Into the same loop and the transmition line between the loop and the receiver switched by means of a plug and jack relay or ordinary toggle switches.

Xenia Ohio Tornado 1974 Radar

FIG. 9—Aircraft loop-antenna control unit. (Courtesy Western Electric Co.

FIG. 9—Aircraft loop-antenna control unit. (Courtesy Western Electric Co.

The fact that one loop may be used with either beacon receiver is of great value and this is only possible because of the coupling system employed in the radio compass attachment.

The radio compass loop is surrounded by an electrostatic shield which enables the pilot to fly through atmospheric disturbances such as rain, snow, sleet or dust static which ordinarily paralyzes his receiver when used with a simple wire antenna.

The gear ratio between the control unit and the loop as shown in fig. 7 is 264 to 1, which permits extremely accurate adjustment of the loop. The position of the loop is indicated on the face of the control unit by the position of the needle and by means of a removable cord it is possible for the user to correctly calibrate the control unit for his particular plane.

FIG. 10—Aircraft position finder, facilitating orientation when used in conjunction with the radio compass.

Aircraft Position Finder.—An aircraft position finder to be used in conjunction with the radio direction finder has recently been put on the market by the Air-Track Mfg. Corp., Inc.

This instrument shown in fig. 10 consists of a circular metal frame 8^ ins. in diameter containing three movable transparent discs. The top disc is removable and contains an airway map, accurately scaled down from a full size map by photographic process, showing all radio range stations within an area 520 miles in diameter. The entire airway system in the United States is covered by 16 such discs, allowing ample overlap.

The other two discs are marked with parallel lines, scaled 10 miles apart. The discs are rotated by three knobs through gears cut in the circumference of the discs. The frame is graduated in 360°.

In operation, the map is first oriented to the compass course being flown. Radio bearings on frwo stations are then obtained, and transferred to the instrument by setting the two discs with parallel lines to show the bearings in degrees. Position of the airplane is then given by the intersection of the lines passing through the selected radio stations. ■■

When using the instrument for off-airway flying, a special blank matt disc is provided. On this the radio range stations may be traced in pencil from Bureau of Air Commerce radio facilities maps. The matt disc is sufficiently transparent, and can be used repeatedly after erasures.

Visual Radio System Landing Aid.*—To land an aircraft safely at an air field completely blanketed by fog, it is necessary for the pilot first to find the vicinity of the air field and second to reach a suitable point of landing.

Radio range beacon system previously described and now in operation on the civil airways in the United States, renders the solution to the first part an accomplished fact.

Here the pilot following the beacon signals as described is guided directly over the beacon stations which ordinarily are

*The system briefly described is developed by the U. S. Department of Commerce, Bureau of Standards. Other systems of commercial application of instrument landing aid in this country are: The Air-Track, Bendix and 1.1. & T. Lorenz.

located within a ijew miles of the airport, and thereby learns the exact position with respect to the airport.

The second part of the problem requires information on the position of the landing aircraft in three dimensions. Lateral and longitudinal guidance are required to determine the direction of landing and the boundaries of the landing field, while vertical guidance is necessary for the determination of altitude and of the freedom of the landing path from obstacles. In the system of radio landing aids three elements are utilized to give the necessary guidance in three dimensions.

The runway localizing beacon gives indications of the lateral position of the aircraft with respect to the airport and permits keeping the aircraft directed to and over the desired landing runway. A 200 watt transmitting set of the visual beacon type, operating in the beacon range of frequencies (200 to 400 kc.) and feeding two small, multi-turn loop transmitting antennas, is employed.

One of the beacon courses produced is oriented to coincide with the desired landing direction, depending on the wind conditions. On the aircraft the receiving set normally used by air transport operators for the reception of radio range-beacon signals and airways weather broadcasts is employed for receiving the runway-beacon signals. This set is supplemented by a reed converted to convert the beacon signals to pointer type course indications, and also by an automatic volume control unit whose function is to relieve the pilot of the burden of continuously adjusting the sensitivity of the receiving set as the distance between the aircraft and the ground station changes.

The course indicator consists of the vertical pointer of a combined instrument shown in fig. 12. This pointer is pivoted about its lower end and swings left or right of a vertical index line depending upon whether the aircraft is on one side or the other of the runway course.

A revefsing switch is provided so that the deflection of the pointer and the direction of deviation of the aircraft coincide whether the aircraft is flying away from or toward the beacon.

Vertical guidance of the aircraft is given by a horizontally polarized ultra-high frequency landing beam directed at a small angle above the horizontal and used in such a way as to provide a very convenient gliding path for the landing aircraft. The frequency of operation is 90,800 kilocycles (3.3 meters).

FIGS. 11 and 12—Indicating instrument on the aircraft for utilizing the radio system of landing aids.

On the aircraft a simple ultra-high-frequency receiver is used, fed by a transmission line from a horizontal half-wave receiving antenna which is located in the wing slightly ahead of the leading edge. The rectified output from this receiving set operates the horizontal pointer of the combined instrument shown in fig. 12. The receiver sensitivity is so adjusted that the line of constant received signal below the inclined axis of the beam, corresponding to half-scale deflection of the horizontal pointer, marks out a landing path which is suitable for the aircraft and airport considered.

The horizontal index line across the face of the combined instrument represents the point of half scale deflection and corresponds to the proper landing path. The horizontal pointer represents the position of the aircraft relative to this path. A rise of this pointer above the horizontal index line indicates that the aircraft is above the proper landing path, while the reverse is true if the pointer falls below the index line.

FIG. 13—Typical course indications with combined instrument. The positions of the aircraft as indicated on the instrument; 1, on proper spatial landing path; 2, to the left of desired landing direction and too low; 3, to the right of desired landing direction and too high.

Consideration of the operation of the combined .instrument will show that the point of intersection of the two pointers represents the position of the aircraft relative to the desired landing runway and the proper landing path. Fig. 13 shows three typical readings on the combined instrument. The co-ordination of the two sets of course indications into a single reading is of utmost importance to the pilot, relieving him of the need for considerable mental effort. Deviations from both courses may be corrected simultaneously . By keeping the pointers crossed over the small central circle on the instrument face, a suitable spatial landing path is followed down to the point of landing. Longitudinal position of the aircraft as it approaches the airport is given by the combination of a distance indicator on the aircraft with the aural signals received from two marker beacons.

The-distance indicator, see fig. 11 consists simply of a direct current milliammeter connected in the plate supply to the radio frequencv amplifying tubes of the beacon receiving set.

FIG. 14—Typical aircraft instrument panel arrangement. This layout is based on the principle of having as nearly as possible all the direction indicating instruments in $ vertical row and all the instruments giving altitude in a horizontal row. In accordance with this arrangement the combined instrument (because of its vertical pointer) is placed in a vertical row with the gyroscopic compass at the top and the magnetic compass at the bottom. The horizontal row contains from left to right: The tachpmeter; the air speed indicator; the gyroscopic artificial horizon; the rate of climb indicator, and the barometric âltimeter.

FIG. 14—Typical aircraft instrument panel arrangement. This layout is based on the principle of having as nearly as possible all the direction indicating instruments in $ vertical row and all the instruments giving altitude in a horizontal row. In accordance with this arrangement the combined instrument (because of its vertical pointer) is placed in a vertical row with the gyroscopic compass at the top and the magnetic compass at the bottom. The horizontal row contains from left to right: The tachpmeter; the air speed indicator; the gyroscopic artificial horizon; the rate of climb indicator, and the barometric âltimeter.

Since the automatic volume control operates to increase the negative biasing voltage on the grids of these tubes with increasing input voltages to the receiving set, the plate current is approximately inversely proportional to the field intensity of the runway beacon. The instrument may therefore be calibrated

Aircraft Detector Diagram

FIG. 15—Schematic circuit diagram of landing beam receiving set. The set comprises a detector and two stages of audio frequency amplification. The detector is untuned, a simple high-pass filter being interposed between the detector input circuit and the transmission line feeding it in order to minimize interference from service operation on lower frequencies. The output from the receiving'set passes through a mechanical filter tuned to the modulation frequency at the transmitter (60 cycles) which provides further freedom from interfering signals. Adjustment of the sensitivity of the set is obtained by means of a variable condenser shunting the input to the detector grid.

FIG. 15—Schematic circuit diagram of landing beam receiving set. The set comprises a detector and two stages of audio frequency amplification. The detector is untuned, a simple high-pass filter being interposed between the detector input circuit and the transmission line feeding it in order to minimize interference from service operation on lower frequencies. The output from the receiving'set passes through a mechanical filter tuned to the modulation frequency at the transmitter (60 cycles) which provides further freedom from interfering signals. Adjustment of the sensitivity of the set is obtained by means of a variable condenser shunting the input to the detector grid.

in miles from the beacon (say, 0 to 5 miles). The distance indication secured is approximate only, but is sufficiently accurate for all necessary maneuvers of the landing aircraft at distances from the airport of the order of 1 to 5 miles.

Absolute indication of the longitudinal position of the aircraft when near the airport is given by aural signals from two low port marker beacon transmitters. One marker beacon is located about 2,000 feet from the approach end of the airport while the other marker defines the boundary or edge of the landing field.

Different modulation frequencies are employed for the two marker beacons to facilitate ready identification of the one being passed over; the marker beacon at the field boundary having a modulation of about 250 cycles and the approach marker beacon a modulation of 1,250 cycles.

The marker beacon transmitting antennas provide for great flexibility of operation; any portion of the landing field boundaries and approaches may be defined, and any radio frequency may be used in the range of from 200 to 20,000 kilocycles.

Advantages of the Landing Beam.—The advantages of the landing beam are as follows:

1. The landing path may be so directed that a landing aircraft following the glide path is automatically kept above obstruction without requiring exact knowledge of the territory over which he is passing.

2. The average landing path may be made to suit the particular airport merely by adjusting the power of the landing beam transmitter. Each individual aircraft may follow a path departing considerably from the average landing path and more closely suiting its flying characteristics simply by an adjustment of the sensitivity of the landing beam receiving set.

FIG. 16—Depicting the principle of blind landings with the aid of the bent radio beam. The bent beam (shown dotted) gives the pilot approaching the landing field a definite gliding path to follow. Encircled letters indicate: A, glide path; B, runway localizer beam; C, marker beacon at end of runway or edge of filed gives pilot audible signals as he flies through it; D, airport beam transmitter; E, airport control tower containing monitor unit indicating operation of landing system and apparatus for two-way voice communication with pilot. The position of the aircraft as indicated on the combined instrument are: 1, to the left of gliding path and too low; 3, to the right of gliding path and too high; 5, on course and on gliding path, etc.

FIG. 16—Depicting the principle of blind landings with the aid of the bent radio beam. The bent beam (shown dotted) gives the pilot approaching the landing field a definite gliding path to follow. Encircled letters indicate: A, glide path; B, runway localizer beam; C, marker beacon at end of runway or edge of filed gives pilot audible signals as he flies through it; D, airport beam transmitter; E, airport control tower containing monitor unit indicating operation of landing system and apparatus for two-way voice communication with pilot. The position of the aircraft as indicated on the combined instrument are: 1, to the left of gliding path and too low; 3, to the right of gliding path and too high; 5, on course and on gliding path, etc.

The power adjustment at a given airport to secure a suitable average landing path and the sensitive adjustment on each aircraft to depart by a desired amount from the average path once made, are permanently fixed. A given aircraft then follows an optimum landing path at» all airports equipped with landing beams.

3. The shape of the landing path is such that the aircraft maintains safe flying speed in following the landing path indications up to the point of receiving the boundary marker beacon signals. Thus the aircraft engine is kept at a safe operating speed during all maneuvers outside the airport boundaries.

4. The landing glide may begin at any desired altitude within a rather wide range. Beginning the use of the landing beam does not, therefore, involve accurate location of any point at specific distance from the airport, but comes automatically so long as the pilot is following the runway beacon course in the correct direction and at an altitude within the prescribed limits.

Sonic Altimeter Principles.—On account of the fact that aircraft must fly near ground for a considerable time in connection with take-off and landing, it is obviously necessary that for safety of these operations some means be provided to guide the operator to instantly and automatically give his correct altitude above the ground.

Under ordinary circumstances it may seem that the visual judgment of distance by a skilled operator would eliminate this problem, however, sometimes visual observations are impossible on account of darkness, fog or a heavy storm, hence some instruments for measuring height above ground is needed.

Various barometric altimeters it seems could be utilized for this service, but only if the existing barometric pressure and the local elevation above the sea level are known. Another detrimental factor is that the ordinary barometric altimeter whose hand makes one revolution for 10,000 feet altitude, will not have sufficient sensitivity to be of service in a landing operation, for example, under any circumstances.

Parallel with the constantly improving design in aircraft and various instruments for their safe operation, a great number of devices for measurement of altitude has been developed.

General Principles.—Such devices, generally called altimeters or Sonic altimeters, depend on their functioning upon the reflection of either an electromagnetic or sound wave from the ground surface.

Various experimenters have been able to produce practical instruments using sound waves, but there is presently no record of similar successful efforts with any other schemes.

The Sonic altimeter has three essential parts:

1. An emitter which sends out a brief sound signal at controlled intervals.

2. A receiver which detects the echo of the signal when it returns from the earth, and

3. A chronoscope which measures the time interval between signal and echo.

Since the velocity of sound in air is substantially constant, the chronoscope is ordinarily calibrated directly in altitude for some average set of conditions.

General Requirements.—With reference to figs. 17 and 18, the essential geometrical features and design of the Sonic altimeter principles will be obtained.

The sound source or emitter E, sends out a sound signal which strikes the ground, reflects upwards again and is picked up by a receiver at R.

The apparatus is completed by a chronometer which measures the time interval between the signal and its echo, which is a function of the airplane speed, the speed of the sound and the altitude.

SENDING RECEIVING

SENDING RECEIVING

Various manufacturers of altimeter instruments differ only in the means used for carrying out the previously mentioned functions.

With reference to fig. 17, observing the horizontal movement of the plane during the time of signal emittance and receipt of echo, the actual altitude derived from the triangle will be obtained as follows:

c=velocity of sound V=airspeed of plane t=time interval between sending and receiving ha=actual altitude.

On account of the fact that the speed of sound is almost constant (being 1,090 ft. per second in air) and the airplane speed has a small effect under ordinary conditions, the echo time is almost proportional to the altitude-of plane.

FIG. 18—Diagram showing co-ordination of apparatus usually employed in Sonic airplane altimeters.
FIG. 19—Typical altitude indicator which may be read directly in feet or meters or with a conventional multiplier, the indicator being located on the
FIG. 20—Schematic circuit diagram of marker beacon transmitter set. The transmitter is enclosed in a weatherproof box and is completely shielded to prevent direct radiation.

CHAPTER 23

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