US2379362A - Radio guidance system - Google Patents

Description

June 26, 1945. w. P. LEAR RADIO GUIDANCE SYSTEM Filed Aug. 25, 1939 9 Sheets-Sheet l B INVENTOR.

wiualam g Bear ATTORNEY.

June 26, 1945. w. P. LEAR 2,379,362

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AMPLIFIER W. P. LEAR RADIO GUIDANCE SYSTEM Filed Aug. 25, 1939 UL ATUR SUPPLY MODULATED RF. TRANSMITTER R.F'.TRANSMITTER 9 Sheets-Sheet .5

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June 26, 1945. w. P. LEAR RADIO GUIDANCE SYSTEM Filed Aug. 25, 1939 9 Sheets-Sheet 5 KU'P Il mwdxl MZOLI )(JUE INVENTOR.

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June 26, 1945. w. P. LEAR RADIO GUIDANCE SYSTEM Filed Aug. 25, 1939 9 Sheets-Sheet 8 AUTOMATIC DIRECTIONAL FILTER I N VEN TOR 8? ,6 car ATTORNEY.

AUTOMATIC DIRECTIONAL SYSTEM FILTER l i will 11am M/fl M June 26, 1945. w. P. LEAR 2,379,362

RADIO GUIDANCE SYSTEM Filed Aug. 25, 1939 9 Sheets-Sheet 9 DIRECTION/Ail. AUDIO h mule/non UMT h RECEIVER FILTER FILTER UNIT I UNITE L 53/ 5 .730 S 1 F4 1: JMML J A 1 13x162 L IN VEN TOR. Uilli am 5. Ioear WQ- W ATTORNEY.

Patented June 26, 1945 RADIO GUIDANCE SYSTEM William P. Lear, Dayton, Ohio, minor, by mesne asigmnents, to Lear, Incorporated, Piqua, Ohio,

a corporation of Illinois Application August 25, 1939, Serial No. 291,807

12 Claims.

This invention relates to radio guidance systerns for mobile craft and more particularly relates to novel radio blind approach systems and methods for guiding an aircraft to a landing runway, a marine vessel into a slip, and the like.

Radio direction finders have made it possible to guide on aircraft or ship directly to the vicinity of a radio transmitting station at its destination. During adverse weather conditions such as rain, fog or snow, it is impractical to land the aircraft or vessel without further aid. The so-called equisignal runway localizing beacon was developed about 1930 for guiding an aircraft to a landing runway. In this system directional transmission of radio energy is concentrated along the ap proach to the runway. Upon locating the equisignal path, the pilot must keep on the approach to the runway.

Various forms of the runway equi-signal localizer system have since been carried out in practice. The pilot arrives in the vicinity of the airport using a radio compass or flying on a l of a radio range beacon. On reaching the trans. mitter station, as denoted by a cessation of the radio signal, he maneuvers in a predetermined manner in order to locate the equi-signal localizing beacon. It is then necessary for him to fly in the narrow path of the beacon. This often results in weaving and loss of the beacon. Marker beacons are employed at one or more points along the equi-signal path to apprise the pilot of his distance from the runway.

It is generally relatively diillcult to locate the equi-signal beacon, and it is also diflicult to stay on it for any reasonable distance due to wind currents and drift The system is inflexible as to direction of arrival, and no determinations are aiforded the pilot of his position with respect to the airport in general. Descent to the runway is v performed with a radio glide path for vertical guidance, or by reliance on a sensitive altimeter.

A diflerent radio approach system was developed in 1934 by Hegenberger and adopted by the U. S. Army \Ail' Corps. The Hegenberger system employs two spaced radio transmitter stations, each radiating an independent non-directional radio wave as well as an ultra short wave vertical marker beacon signal. The two stations are mobile and placed in line with the desired direction of approach. The pilot uses an ordinary radio loop compass, such as a right-left indicator, and also a marker beacon indicator.

For approaching the vicinity of the airport the pilot tunes the right-left compass to one of the transmitter stations and flies directly towards it He is informed of his arrival over that station by the marker beacon indicator. He then quickly tunes his right-left indicator to the second transmitter station and flies directly towards that, knowing of his arrival thereover by a. marker beacon indication. The pilot in this manner makes several trial flights along the line of ap proach between the two transmitter stations, and adjusts a directional gyroscope to compensate for drift.

In his maneuvers, the pilot using the Hoganberger system is required to quickly change the tuning of his radio compass between one and the other of the two radio stations as he passes over them several times. In order to ascertain his position with respect to the runway, the busy pilot must use his judgment and interpret his several maneuvers as well as the readings of several trial approaches before he is reasonably sure of his position and direction prior to gliding down to the runway. For descent, this system makes use of a sensitive altimeter. Drifting oil the course is not directly apparent from the radio system, hence the reliance on the directional gyroscope.

In accordance with my present invention I provide a radio blind approach system which avoids the shortcomings of the prior systems. The system of my invention continuously indicates to the pilot his exact relative lateral position with respect to the airport in general, and his exact position with respect to the approach path to the runway. Two spaced non-directional transmitters are placed in line with the desired runway. The receiver system is simultaneously tuned to both transmitters and has two needle indicators arranged to continuously and independently point out the direction to each of the transmitters. The

pilot is thus continually mad aware of his position. No study or interpretation of the indications is necessary. No training period is required since the meaning of the indications is apparent almost by intuition.

With the system of my invention, the pilot may approach the runway directly on the shortest route and without any trial flight since he clearly knows his orientation with respect to the airport and runway course. The two indicator needles of the receiver are in line with the centerzero index when the pilot is on-course with the runway. As he passes over the outer transmitter, the out indicator needle reverses by This serves as a marker indication to apprise him of his exact distance from the edge of the runway. As the pilot continues on-course to the runway,

the second or in" needle will reverse by 180 when he passes over the irmer transmitter station. No separate marker beacons are thus necessary. The pilot is then ready to glide down to a landing using any suitable vertical guidance means.

Drifting from the predetermined approach course is accurately detected through the dual indicator readings, and is readily compensated for by the pilot. No reliance or directional gyroscopes or other flight instruments is necessary. The results are foolproof and independent of adverse weather conditions. There are no successive receiver tuning changes or other distracting duties required of the pilot with the system of my invention. After the initial tuning operation, the dual indicator arrangement is continually eifective in guiding the pilot in the manner set forth.

In a preferred form of my invention I employ two transmitter stations having the same carrier frequency. The radiation by the stations is successively switched on and off at predetermined intervals. The two stations are differentiated with either separate audio frequency tone modulations, or with one station being tone modulated and the other unmodulated. A dual automatic directional indicator system is used aboard the mobile craft. In one form the receiver system has two separate rotatable directional antennae, one corresponding to each of the two field transmitters. An automatic radio control unit is used to individually orient the two directional antennae to their null signal positions with respect to the radio transmitters.

The directional antennae are operative over a 360 range. A separate automatic control radio unit may be used for each of the rotatable loop antennae. In a preferred embodiment, I use a single automatic radio control unit connected successively to each of the two rotatable directional antennae in correspondence with the intervals of radiation of the transmitters as controlled by the audio tone modulations thereof. A composite indicator is used, having two needles, one coupled to each of the rotatable antennae. The two needles point out the actual direction towards each of the field transmitters, and apprise the pilot of his exact lateral position with respect to the airport and the approach path to the runway as will be described in more detail hereinafter.

In practice, the receiver installation for the radio guidance system of the present invention is essentially an automatic 360 radio directional indicator or compass having one or two directional antenna systems and two indicators arranged for intermittent connection to the radio unit during the blind approach operations in response to the predetermined field transmitter tone signals. The pilot employs the general radio directional indicator in the usual manner for approaching the vicinity of the airport from a distance. When he reaches to say within 'or miles of the airport, he tunes the directional radio unit to the frequency .common to the two field radio transmitters. The dual indicator and second directional antenna if present are thereupon immediately made effective to operate the directional receiver as a blind approach or guidance system in accordance with the present invention by the field station signals.

The present invention is applicable for guiding a marine vessel into its slip, such as during fog conditions. In this case, the two radio transmitters are placed on land in line with the direction which the vessel is to enter the slip. The pilot employs the same approach equipment described in connection with the aircraft guidance and is accurately guided into the slip despite river current or zero visibility conditions. Anticollis ion devices can be used for safety reasons when there is sole reliance on the radio guidance in docking the vessel.

It is accordingly an object of my present invention to provide novel methods of and means for radio approach guidance of mobile craft to a predetermined line of destination.

v Another object of my present invention is to provide a novel lateral position radio indication ,method and system comprising two separate directional indicators.

Still another object of the invention is to provide a novel radio lateral position indicator system continuously efl'ective in apprising the pilot of his exact relative position with respect to two spaced transmitter stations.

A further object of the present invention is to provide novel radio blind approach systems employing a plurality of rotatable directional antennae automatically controlled from a unitary directional control receiver.

Still a further object of the present invention is to provide novel systems for indicating the lateral position of a. mobile craft with respect to two spaced radio transmitters comprising a single directional antenna and radio receiver.

These and further objects of my present invention will become apparent in the following description of exemplifications thereof shown in the following drawings in which:

Fig. 1 is a diagram illustrating the operation of the radio guidance system of the present invention in guiding an aircraft to a landing runway.

Fig. 2 is a schematic diagram illustrating one form which the receiver system of the invention may assume in practice comprising two separate receiver units. v I

Fig. 3 is another form which the receiver system may assume in practice comprising a single automatic receiver unit.

Fig. 4 is a schematic electrical diagram of the arrangement at one of the transmitter stations of the present invention.

Fig. 5 is a schematic electrical diagram of the arrangement at the second transmitter station.

Fig. 6 is a group of curves illustrating the electric switching operation for the two transmitter stations.

Fig. '7 is a diagrammatic arrangement of a complete receiver system incorporating the principles of the present invention.

Fig. 8 illustrates one form of the dual indicator mechanism, being partly in section.

Fig. 9 is a schematic electrical diagram of a preferred circuit arrangement for the automatic directional receiver equipment corresponding to .the diagrammatic showing in Fig. 'I.

Fig. 10 is a group of curves used in describing the theory of operation of the directional receiver system.

Fig. 11 is a schematic electrical diagram of g the electronic motor control system for the directional receiver.

Fig. 12 is a group of curves used in describing the theory of operation of the electronic control system of Fig. 11.

Fig. 13 is a diagrammatic illustration of another form which the receiver system of the aamssa Aircraft guidance method m. l is a diagrammatic showing of the applica of the present invention in guiding an aircraft to a landing runway R. T1 and T: are

transmitter stations aligned with the runway R chosen for the landing of the aircraft. Station T1 may be positioned about a half mile from the edge of the runway and station To spaced about four miles from station T1. The spacing of transmitters T1 and T: from runway R. is ntional, but is preferably prearranged in practical awlication. A receiver for the signals from stations Ti and T: is aboard the aircraft, and indicator needles I and 2 are automatically controlled by the receiver to point towards the respective transmitter stations.

Needle I, corresponding to the in position is arranged to point towardstransmitter T1. Needle 2, the out" needle, 'is arranged to point towards transmitter T2. The operation of. the transmitter and receiver-systems, to be described in detail hereinafter, is such that needles I and 2 individually point to their respective stations T1 and Ta, and give an accurate indication, without directional ambiguity, to these stations. A zero index I on the indicator dial corresponds to the longitudinal or axial position of the aircraft, being the direction of flight of the aircraft with no cross-winds.

Referring to position A of the aircraft. which is behind the outer station '1: and towards the left of it, we note that "out" needle 2 pointing towards '1: and "in" needle I pointing towards T1 give a rapidly comprehended indication of the planes lateral position. A pilot at position A notes that both stations T1 and T2 are ahead and that he is heading directly towards T: and towards station T1, from the left of the approach path.

Ti-TI. As he moves towards the right, for example to position 3, needle I moves into the index I position.

At position B, which is inline with the runway and transmitters T1 and Ta, both needles I and 2 are on-course and point to index I. If the airposition by the orientation of needles I and 2 with respect to the zero-center index. In position C. needle I' is deflected towards the right of needle 2 indicating the relative position of the plane towards T1 and runway B.

At position D, the aircraft is at the left side of transmitter T: as ascertained by the near 90 position of needle 2, and the position of needle I to the right of index I. When the aircraft is at the right side of station Ts, such as at position E, the indication of needles I and 2 thereat readily signifies this fact. If the pilot is in alignment with the runway or is otherwise on-course, "out needle 2 reverses by 180 as he passes over station Ts. Position 1" shows the "out needle 2 in the 180' position, and needle I at the position.

The pilot then knows that he is properly heading on-course and is between transmitters T1 and Tr, having just passed station Ta.

At position 0 shown in dotted, the aircraft is heading transverse to the on-course direction as denoted by the position of index I corresponding to' the flight direction of the aircraft. Needles I and 2 point towards transmitters T1 and T: practically in the orientation of adjacent position D. The general orientation of needles I and 2 at D and G apprises the pilot that he is somewhere between stations T1 and T: and to the left of the approach path between them. The index I at position G, lying between needles I and 2 apprises him that he is heading directly towards the approach path T1Tz rather than along the path as at position D. Similarly dotted position H indioates that the aircraft is headin towards the approach path from the right.

Index I as well as indicator needles I and 2 coact to apprise the pilot of his relative lateral position and as well as direction with respect to the required radio course. The pilot, with negligible consideration or interpretation immediately realises how to maneuver his plane to arrive at the proper path and correct altitude. At position I, the plane lies on the approach path, needles I and 2 being 180 apart as they are at position F. However, the direction of flight of the plane at I is transverse to the on-course path as denoted by index I. At position I, the reading of needle I on the scale is and needle 2 is 270. At position F however, needle I reads 0 and needle 2 reads 180, corresponding to the oncourse position between stations T1 and T2.

At position J, the aircraft is intermediate between stations T1 and T2 and clearly to the left of the on-course path between them. The position of index I at J informs the pilot that he is heading in a general direction from station T: towards station T1, and that he is to the left of the approach path T1T2. Symmetrical position K to the right of the path has a similar indication to J, with needles I and 2 both showing that stations T1 and T: are to the left of the direction of flight indicated by index II.

The dotted position of the aircraft at L indicates a reverse flight direction. By observing that "in needle I points to his left and that "out needle 2 points nearer to index I, the pilot knows that he is flying generally towards outer station '1: and accordingly will have to reverse his direction of flight to approach runwaylt. At dotted position M, the pilot is on the approach path T T: and between stations T1 and T: as denoted by a 180 displacement of needles I and 2. How ever, since on needle 2 points towards index I, and "in" needle I to the 180 scale position, the pilot realizes that he is flying in a direction away from the runway even though he is on-course. At dotted position N which is symmetrical with position L, the pilot similarly ascertains that he is fly nfl generally away from the runway and is to the right side of it. 1

The interpretation of the indications by index I and needles I and 2 are natural and readily comprehended to any pilot without a study or training period. The meaning of any reading is apparent to him practically by intuition. No unnecessary maneuvering with respect to stations T1 and T: is required with the system of my invention. As thepilot approaches the vicinity of the airport from any direction he can, by the lateral position indictations, ascertain exactly how to maneuver his plane to reach head position B which is on-course with the runway and behind outer transmitter T2.

Should the pilot approach the airfield from position A or C he will know that he is properly behind transmitter T2 and need merely direct his plane nearer to the right or left as the case may be toreach the approach path position indicated to him by an indicator reading as at B. Similarly, should the pilot approach the airfield from the rear as from position P or Q, the relativ indications of needles I and 2 with respect to the index will inform him that he is nearer transmitter T1 than transmitter T2; that he is flying generally in a direction away from the runway; and that he should continue flying until he is beyond station T2 and reverse his flight direction in order to reach position B at the head of the approach path.

Should the pilot arrive at the airport in a direction transverse to approach path T1T2, it will be readily apparent to him. Positions G and H, for example, indicate such direction of arrival. The orientation of needles I and 2, on opposite sides of index 0, apprises the pilot that he is somewhere between stations T1 and T2 and approaching towards the path between them from the left or right side as the case may be. Should he approach the path from position H, a 180 displacement of the needles I and 2 when he reaches position I informs him that he is crossing the approach path T1--T2- From the indications, the pilot can readily tell the most direct manner in which to maneuver his aircraft towards head-on position B for approaching runway R.

In accordance with the present invention, the two synchronized transmitters T1 and T2 are arranged in line with the approach path to the chosen runway R upon which the aircraft is to descend. Theinner station T1 is spaced 8. predetermined distance from the edge of runway R, for example, 500 feet or a half mile. The outer transmitter T2 is spaced a predetermined distance from station T1, say 4 or 5 miles. In view of the fact that in my system marker indications are given the pilot when the aircraft passes over either station T1 or T2, predetermined spacing from the edge of the runway R is a marked advantage in practice. The pilot thus is given a clear indication of his position prior to landing and it assists his proper maneuvering for descent to the runway. As hereinabove explained, the relative indications between needles I and 2 and index 0 give the pilot a clear aspect of his lateral position with respect to the airport, stations T1 and T2 and runway R, enabling him to directly maneuver his aircraft to head-on position B behind outer transmitter T2 in preparation for an approach to the runway.

With the system of the present invention, the

pilot thus quickly determines his lateral position and attitude from any direction of approach, and moves to head-on positionB where needles I and 2 both point to index 0 if there is no cross-wind or drift and the attitude of the aircraft is oncourse. The pilot then continues flying along path T1Tz. As he passes over station T2 out needle 2 reverses by 180, the needles assuming the 180 displacement such as shown at F. The

distance of station T2 from the runway, and is properly on-course. The flight is continued towards T. As soon as the pilot passes over station T1, the "in needle I reverses its indication by 180 and reads as shown at position 0. The pilot then knows that he is the predetermined distance asvasaa of T1 from the edge of runway R, and is in line with the runway.

An important feature of my present system resides in the accurate apprisal of drift of the mobile craft in its movement along the predetermined on-course path. Whenever needles I and 2 are 180.apart such as shown at positions F, I and M, or aligned and 0 apart such as at positions B and 0, the craft is directly on-course. As lon as the pilot is flying along path T1-Tz needles l and 2 are either zero or 180 apart. Cross-wind or wrong attitude of the aircraft tending to move it ofl path T1T2 is readily made apparent by a corresponding change in the 0 or 180 displacement of needles T1 and T2. The concentric mounting of needles T1 and T2 and accurate bearing indications affords a sensitive drift indication, since needledisplacements of even 1 are easily read. It is to be noted that the on-course indications by the 0 or 180 needle displacements are independent of the position of index 0 with respect to needles land 2. The relative position of index 0 with respect to needles I and 2 gives the relative position of the axis of the aircraft in its movement along the path T1Tz.

In utilizing this significant characteristic of the system, the pilot can readily and accurately adjust his controls to keep the plane on-course despite variable or unknown drift due to cross-winds and the like. It is merely necessary for him to jockey or otherwise crab the aircraft on path T1T2 and maintain needles I and 2 zero or 180 apart to insure his arrival to runway R in the exact predetermined direction as determined by stations T1 andTz. As he passes over station T2, out needle 2 reverses 180 from its showing at position B to that at position F, without affecting t e drift indications noted, yet serving as an accurate marker indication of the distance of the aircraft from the edge of runway R. Similarly, as the aircraft passes over station T1 to position such as at 0, "in needle I' reverses to the 180 position and in alignment with reversed needle 2, still sensitive to any drift indications and serving as a marker indication of the aircraft with respect to the edge of runway R.

Descent to the runway may then be effected by 1 orientation of which is automatically controlled pilot knows that he then is at the predetermined by the directional receiver unit resulting in a straight line glide path indication direct to the landing field. The angle of the descent path to the landing field is optional and is determined by the altitude of the aircraft prior to the descent.

Transmitters T1 and T2 may be independent of each other and have separate radio frequencies. The receiver system shown and described in connection with Fig. 2 is indicated for such arrangement. However, in commercial operation, where weight and bulk of equipment placed upon an aircraft is significant, I prefer the form of receiver equipment hereinafter disclosed using a single automatic receiver unit tuned to one frequency. Such equipment contemplates transmitter stations T1 and T2 using the same carrier frequency arranged for synchronous and successive transmission. When using the same radio freasvasoa carrier, transmitters Ti and T: are disby an audio frequency signal modulawhmbothltatlons'lilnd'haremodua distinguishingaudio frequency signal, unit, double filter receiver system dein connection with Fig. 3 is indicated. In simplified arrangement, only one transthe other transmitter is made to radiate an unmodulawd carrier wave. Receiver systems operative with the latter form of transmission are shown and described in connection with Figs. 7 to 15, and use a single automatic receiver unit.

General receiving system Two separate automatic directional receivers having individual indicators. or a combined sysautomatic directional receivers, each corresponding to an automatic 360 radio directional indicator such as for example disclosed in my copending application, Serial Number 286,733 filed on July 27, 1939, entitled Automatic radio direction indicator, now Patent No. 2,308,521, issued January 19, 1943. The directional control receiver units 23, of the system each have an associated rotatable loop antenna 2| and 2|, as well as non-directional antennae 22 and 22 respectively. A

Loop antennae 2|, 2| are rotatably mounted upon their respective shafts 23 and 23', and are connected to receivers 20 and 20' through slip rings and connections 24 and 24'. Both directional systems 23 and 23' are individually and simultaneously tuned to the field transmitters T1 and Ta. System 20, for example, may be made responsive to the radio signals emanating only from transmitter T1 while system 23' is made responsive to signals from transmitter T2. The loop motor 25, connected to output of control system 2. drives loop shaft 23 through an electromagnetic clutch 2t and gearing 21, 23.

As described in my said Patent No. 2,308,521, the orientation of loop antenna 2| is automatically brought to the null signal position with respect to the oncoming radio signals from the transmitter it is responsive to, namely station T1 in the present case. Motor 25 is a reversible motor which brings loop antenna 2| in the predetermined null signal position without directional ambiguity. A remote indicator 33 is coupled to loop shaft 23 so that its needle I continually indicates the actual angular position of loop antenna 2|. A flexible mechanical cable 3| interconnects remotely placed meter with loop shaft 23 through coacting gearing 32, 33, shown as spiral gears. An electrical telemetering system may instead be used.

In a similar manner the radio signals emanating from transmitter T2 controls directional unit 23' to operate motor 25' to rotate loop shaft 23' through clutch 2i and gearing 21', 23' to bring loop antenna 2 l in the predetermined null signal position with respect to the signals from the transmitter T1. A separate indicator 3| is mechanically coupled to loop shaft 23 through flexible cable 3| and coasting spiral gears 32', 33'. The position of needle 2 depends upon the angular position of loop antenna 2! and a continuous indication of the position of loop 2| ls accordingly eil'ected at the remote position such as in the cockpit of the aircraft.

is modulated with an audio frequency note The directional accuracy of the bearings by needles I and 2 on the respective transmitter stations T1 and T2 is preferably designed to be accurate to within 1' of arc. directional indicator unit such as described in my Patent No. 2,308,521 already referred to, provides such accuracy. Such accurate dual directional bearings combine to provide a reliable drift indication ior theapproach course which is extremely useful in aircraft landing during adverse weather conditions. Both indicators may be combined in a common housing and their shafts made concentric as shown in Figs. 1 and 8. Alternatively two separate meters such as 30 and 30' may be used, one above the other as shown in Fig. 2 to give equally effective lateral position indications. It is to be understood that quadrantal compensators may be employed between the loop antennae and the respective indicator needles to compensate for metallic masses and unequal radio field distribution about the structure of the vehicle.

In Fig. 3 I illustrate a form of the invention employing a single automatic directional receiver unit 40 in place of the two separate receivers 25 and 23' of Fig. 2. Receiver 40 is tuned to the one frequency corresponding to the carrier frequency chosen for both transmitters T1 andIa. In this case, transmitter T1 may for example be modulated with one audio frequency tone and transmitter T1 with a different tone frequency. The tone frequency for transmitter T1 may for example be cycles; and that of Ta, or 300 cycles. Two filters, I and 2 are connected to the audio output of receiver 40 for segregating the predetermined tone signals and selectively energizing solenoids ti and 42 of four pole relay 43, M, 45, 45.

With the receiver system of Fig. 3, field transmitters T1 and T1 are made to radiate alternately instead of continuously as is feasible in the modiflcation of Fig. 2. Transmitter T1, for example, may be made to radiate for a period of the order of one-third to one-half of a second, and shut off while transmitter T: is energized to radiate for an equal period. Receiver solenoids 4i and 42 will be selectively energized in accordance to which of transmitters T1 or T is radiating.

Rotatable loop antenna 50 is connected to the left contacts of armatures t3 and it through leads 5|. Motor 52, which drives loop antenna 53 through clutch 53 and gearing 54, is correspondingly connected to the leftcontacts of armatures 45, 46 through leads 55. When relay H is energized, relay poles 43 to 45 are in the illustrated (left) position connecting the winding of directional antenna 50 and its associated control motor 52 in a predetermined manner to automatic directional receiver 40. During the interval of energization of relay ll through the radiation of transmitter T1, motor 52 is controlled to move loop 50 into the null signal position with respect to the direction to transmitter T1. Loop shaft 56 is connected to indicator 51 through flexible cable 53 and spiral gearing arrangement 58. The angular position of needle i of indicator 51 is controlled by the movement of loop 50, continuously pointing out the position of loop 50 and correspnding to the direction towards transmitter T1 with respect to the axis of the moving craft.

A second directional antenna system is used to derive the direction towards transmitter T2. A loop antenna 60 is connected to the right contacts of relay armatures 43, 44 through leads ii. A separate motor 82 controls the orientation of loop it through electromagnetic clutch 63 and reduc- An automatic 360 tion gearing 64. Motor and clutch 63 are connected to the right contacts of relay armatures 45, 46 through leads 65.

During the energization of solenoid 42 through radio signals emanating from transmitter T2, relay armatures 43 to 46 are moved to connect with the right contacts of the relay connecting loop 60 and its motor 62 to transmitter 40. During the transmission interval of T2, loop 60 is moved to the null signal position with respect to the direction towards station T2. The orientation of loop 60 is transmitted to needle 2 of meter 51 through flexible cable 68 connected to loop shaft 66 through gearing arrangement 69.

In the system of Fig. 3, loop antennae and 60 are successively moved by their associated control motors 52 and 62 to the corresponding null signal positions towards transmitters T1 and T2, pointing out the direction thereto within 1 of arc. Lateral position indication of the vehicle or aircraft is thus effected with respect to stations T1 and T2 and the field. Indicator needles I and 2 point out the absolute direction towards stations T1 and T2 over a 360 scale. The position of needles I and 2 with respect to the zero index apprises the pilot of his lateral position with respect to the landing field in a manner already described in connection with Fig. 1.

General transmitter system The relation between transmitters T1 and Ta may assume various forms in practice. Two independent continuously energized transmitters may be employed having separate carrier fre-' quencies and containing tone modulations or not. A double receiver system such as described in connection with Fig. 2 is indicated where two such separate frequencies are used. However, for aircraft use and in general it is commercially and practically more expedient to employ the novel selectively operated single receiver um't arrangement such as described in connection with Fig. 3 or one of the equivalent forms to be described.

For such single receiver unit arrangement, a single carrier frequency is preferably employed for both transmitter stations T1 and T2 so that the single receiver unit (corresponding for example to 40 of Fig. 3) may be tuned to that carrier frequency.

An alternative form for the transmitters is to use two carrier frequencies separated by, for example, only 1000 cycles. The single receiver unit is tuned to or near one of the two closely spaced frequencies and selectivly operated through the resultant audio beat note or notes. The selective operation in this form is derived by filtering out the predetermined beat note or notes and operating relays thereby. The operation of the transmitters may be by successive switching on and off, or by continuous operation. Crystal controlled reception is preferably indicated in this form since for a predetermined selective beat frequency operation the radio tuning should be accurate for proper selective filtering action.

A preferred form for the transmitter system is illustrated in Figs. 4 and 5, and forms the subject matter of my copending application Serial No. 495,978, filed July 24, 1943, for Direction radio transmitting system, which is a division of the present application and assigned to the same assignee. Fig. 4 is a schematic arrangement corresponding to one of the transmitters, say Ti; and Fig. 5 to the other, T2. Antenna I0 is a transmitting antenna designed to efflciently radiate the radio frequency signal of T1. Any suiteven ultra-high frequency. An efficient receiver and directional antenna for operation at the selected frequency is provided. A carrier frequency of for example 209 kilocycles may be used for T1 and T2. Similarly, a frequency of, say 1614 kilocycles, may be used, or 4790 cycles, etc.

The radio generator for T1 is schematically indicated within dotted rectangle H. Radio frequency oscillator '12 generates the carrier at the preferred radio frequency. An audio oscillator 13 generates the tone signal for modulating the radio frequency carrier. The tone or audio frequency used is for operating the relay system of the receiver for selectively controlling the operation of the receiver unit with respect to the two directional antennae such as described in connection with Fig. 3, and as will be described in connection with Figs. '7 to 15. The audio-frequency of generator 13 may be '75 cycles, 150 cycles or higher as desired. The radio frequency carrier is modulated by the audio frequency tone at radio frequency modulator 14, the output of which is connected to radio frequency amplifier I5. The output of amplifier I5. is connected to loading coil 16 and in turn to antenna 10. Loading coil 16 is adjusted for most efficient radiation of the radio frequency wave through antenna I0.

An arrangement is provided for maintaining the transmission of the modulated radio frequency wave through antenna 10 for a predetermined period, in the range say of one-third to one-half of a second; extinguishing the transmission of the radio signal corresponding toll at T1 and successively provide for the transmission of the radio frequency signal from the other station T2. A high frequency relaying transmitter is arranged to radiate from antenna 10 to control remote transmitter T2. Transmitter 80 radiates while transmitter H is extinguished, and vice versa.

In the arrangement Of Fig. 4, I employ the same antenna I0 for the transmission of both the modulated radio frequency wave from II as well as the high frequency relaying signal from unit 80. Separate antennae may be used. The output of transmitter 80 is connected to back contact 8| of relay armature 83 through a loading coil 82. When relay armature 83 connects with back contact 8|, high frequency transmitter 80 is connected to antenna I0. Loading coil 82 is adjusted for most eificient radiation of the high frequency wave from antenna 10. To insure the proper extinguishing of the radiation of the radio frequency waves, I prefer to disconnect the positive B potential supply from the transmitters. For this I use relay armature 84 connected to B supply '11 which is connected to radio frequency generator ll through relay contact 85 and lead 18 as shown schematically. Common B supply 11 is also connectable to high frequency transmitter 80 through relay contact 86 when armature 84 contacts therewith upon energization of solenoid 81.

In the described switching arrangements for stations T1 and T2, any time lag due to the action of relays or other components should be properly designed and arranged to give the indicated mode of operation. To simplify the exposition, no lag will be assumed. However. suitable construction in practice to accomplish the desired results willbe apparent to those skilled in the art. The design of the commutators or cams or other switch-over means would naturally account for the lag in the connection and disconnection of the circuits by the relays and associated armatures.

Relay solenoid ll is suitably energised at the proper time intervals to attract relay armatures '8, it from their normal upper position shown, to the lower contact position for correspondingly extinguishing the generation and transmission of the radio frequency signal of unit ll and initiate the generation and transmission of the high frequency relay signal of unit ll through the antenna ll succemively and without overlapping. A preferred transmission and cessation interval is one-third of a second for aircraft guidance. However, it is to be understood that different intervals, such as one-half second or greater are equally suitable, as are intervals of less than one-third second also feasible. For marine guidance longer intervals are useful since the approach speed is smaller.

An arrangement for alternatel energizing relay solenoid to effect the successive switching at predetermined intervals is atically illustrated in Fig. 4. The timing intervals are equal and cyclic, not necessarily absolute in value, but generally within the predetermined timing in- :terval desired. A constant speed motor 80 is energised through a local potential source ll, such as the 12 volt battery. An alternating current motor and supply may also be used. Motor it drives disk I! through suitable reduction gearing I, M. A pin ll projects from the face of disk ll. A star wheel is arranged to coact with pin II on each revolution of disk at. Star wheel 96 is advanced through an are equal to the width of one of its teeth per revolution of disk 92. The speed of motor OI as well as the ratio of the respective gearings l3, l4 and ll, it is designed so as to advance the commutator by one segment corresponding to one switching interval, onethird of a second in the present case.

Communtator Ill comprises switch arm or blade I rotatable over a plurality of equi-spaced contact segments III. Alternate segments III are connected together by connection leads it! and in turn connected to relay solenoid ll through lead ill. Switch arm I is electrically insulated from contacts III and is connected to one side of the potential source, namel battery ll. Swith arm III is shown on one of the isolated contacts. The motivation of switch arm I is by star wheel 96 interconnected through shaft 0!. The battery circuit through relay l1 and ground is open during such position. when switch arm Ill connects one of the four interconnected contact elements ll! of commutator III, relay ll is energized by battery II to attract armatures ll. 84 to the downward or front contact position.

Armatures It, It are normally biased upwards towards the back contacts maintaining transmitter ll energized and connected for radiation through antenna ll. When relay I1 is energized, armatures II, N are attracted to front contacts if and N respectively, energizing high frequency relaying transmitter II and connecting it to antenna It for radiation. During this interval, radio signal transmitter H is incapacitated and not transmitting. Constant speed motor 90 causes star wheel 00 to advance one segment for every predetermined interval. Communtator arm I accordingly is moved to successivelycause energisatlon and deenergization of relay .1 resulting in corresponding alternate of the modulated radio signal from unit H and high frequency relaying si n l from unit 80.

During transmission of the modulated radio frequency signal of transmitter Ii, the corresponding loop antenna of the receiver is automaticaliy moved so that its associated indicator will point towards transmitter T1 in the manner already described. The speed of action of the automatic directional receiver is proportioned to effect the directional indication well within the station transmission interval. In using my system for aircraft approach guidance I have found a one-third to one-half second switching interval preferable. The automatic directional receiver should give a rapid, accurate and stable directional bearing on the transmitter stations, and hold the indications stationary between switching intervals. The loop antennae should be shielded from wind currents to prevent upsetting their inter-period bearings. Streamlined enclosures for the directional antennae are preferred if they are mounted outside the aircraft.

While transmitter unit II is quiescent and high frequency relay transmitter II is radiating from station T1, high frequency receiver I" of Pig. 5

is energized. Antenna m of tuned receiver ill lay. Energization of solenoid I" by receiver I during the high frequency relaying period causes the attraction of armatures H3 and Ill against relay contacts H2 and Ill, energizing radio frequency transmitter Ill with B supply HI and connecting the output of transmitter III to antenna Ill for radiation.

Radio frequency transmitter I II preferably has the same carrier frequency as transmitter unit ll, corresponding to the frequency of oscillator 12. Although audio frequency modulation of radio frequency transmitter ill may be used to actuate a relay of the blind approach receiver, I prefer to employ no modulation for transmitter Ill but transmit a continuous wave signal instead. Flg. 8 shows curves illustrating the relative periods of transmission of stations T1 and T2 to give the radio guidance signals. The equally spaced rectangular curves represent the uniform duration of transmission of the radio frequency signals. Signal transmission period a of station T1 is equivalent to the cessation of transmission period a of station T2. Similarly, cessation of transmission period b of station T1 corresponds to the duration of transmission period b of station Ta. Signal transmission from both stations T1 and T: is at the maximum amplitude during radiation. In the transmission system of Figs. 4 and 5, stations T1 and T: have the same radio frequency carrier, station T1 being modulated with an audio frequency signal such as cycles and station T: being continuous wave. It is not necessary to have stations T1 and T: radiate for the full indicated intervals. However simultaneous on of T1 and '1: should not occur when a common carrier frequency and only one distinguishing modulation signal is used.

Receiver circuit arrangement Fig. 7 is an electrical diagrammatic representation of a complete receiving system for the radio guidance system employing transmitter stations Ti and T: such as described in connection with Figs. 4 to 6. Two separate rotatable loop antennae I and I20 are mounted upon individual shafts I2I and I2I'. Slip rings I22 connect the winding of loop antenna I20 to leads I23 for con-' nection to the back contacts of relay armatures I24 and I25. Similarly slip rings I22 connect the winding of loop antenna I20 to leads I23 for connection to the front contacts of relay armatures I24 and I25.

Relay armatures I26 and I2! are employed for selectively connecting the control motor drive arrangement for the loop antennae to the receiver. Relay solenoid I30 is arranged to actuate relay armatures I24 to I21 when energized by currents from the automatic directional receiver system connected thereto through leads I23. The position of'loop antenna system I20 is controlled by reversible motor I3I connected to loop shaft I2I through electromagnetic clutch I32 and gearing I33, I34. Motor I3I is connected in parallel with magnetic clutch I32 and in turn to the back contacts of relay armatures I25, I21 by leads I35.

The motor control system of rotatable loop antenna I20 comprises motor I3I connected to shaft I2I' through electromagnetic clutch I32 and gears I33, I34. Motor I3I' and clutch I32 are connected across front contacts of relay armatures I26, I21 through leads I35. Relay armatures I26, I21 are connected to the motor relay system of the automatic directional unit through connection leads I36. The preferred construction of the rotatable 100p antennae I20, I20 and their associated electromotive drives is preferably in accordance with the disclosure of my Patent No. 2,308,521, hereinabove referred to.

The automatic radio directional receiver schematically illustrated in Fig. 7 is preferably similar to that disclosed in my patent modified to perform the radio guidance operation in connection with the two loop systems. Relay armatures I24 through I2l are normally attracted to the upper or front contact position shown, through normal continuous energization of relay solenoid I30.

- With the relay armatures in the upper position,

loop antenna I20 is in circuit connection with the automatic radio directional circuit through lead I28, and its associated motor and clutch I3I', I32 is also connected thereto through leads I36.

The receiver system normally operates in this position as an automatic directional receiver indicating the bearing on any radio station tuned in by the receiver unit. A loop position transmitter unit I is employed to transmit the angular-position of loop antenna I 20' to remotely located meter I45. A direct current Selsyn telemetering arrangement incorporating a battery HI and three-wire cable I42 is used in the embodiment of Fig. 7. Needle 2 of meter I45 is associated with rotatable antenna I20 and indicates its angular position on the scale of meter I45.

When the automatic receiver is to be used for blind approach in conjunction with field transmitter stations T1 and T: such as indicated in Figs, 1, 4 and 5, the directional receiver circuit is tuned to the predetermined frequency of the transmitters. In this case, directional antenna I20 will be moved to point so that its null signal position corresponds to the direction towards station T2, the unmodulated transmitter station, and "out" indicator needle 2 of meter .I45 will point towards station T2. The automatic angular orientation of loop I20 through control motor I3I is effected during the transmission interval of station T2, within one-third of a second in the preferred case.

During the transmission interval of in" station T1, the predetermined audio frequency modulation of the radio frequency carrier wave at station T1 will cause relay I30 to be deenergized In a manner to be described, and relay armatures I24 to I21 will drop to the back contact position. Thus, while station T1 is being received, antenna system I20 is automatically disconnected and antenna I20 is connected to the directional receiver. A separate telemetering system is connected to shaft I2I of loop antenna I20 so that needle I of indicator I45 will accurately point out the position which loop antenna I20 is made to assume, being a bearing on station T1. D. C. Selsyn transmitter I40 is used, energized by battery HI and connected to meter I45 through cable I42. Loop antenna I20 is automatically moved so that its null signal position accurately corresponds to the direction towards in station Ti.

Indicator I45 corresponds to the indicator described in connection with Fig. 1 having in" needle I, and out needle 2. The zero index 0 corresponds to the axis of the aircraft. After adjustment of the tuning of the directional receiver to the carrier frequency of stations T1 and T2, blind approach guidance as described in connection with Fig. l is effected. Alternately radiating stations T1 and T2 cause relay solenoid I30 to correspondingly switch rotatable antenna system I20 and alternately system I20 out of and into circuit relation with the automatic directional receiver. The arrangement is such as to successively move the respective antennae and their associated needles on indicator I45 to point out, preferably to within 1 of arc, the direction towards the respective stations T1 and T2.

The preferred automatic directional receiver, common to both rotatable loop antenna systems, is shown in block diagram in Fig. 7 and corresponds to Fig. l of my Patent No. 2,308,521 al ready referred to. The system of Fig. 7, details of which are shown in Figs. 8 and 9, forms the subject matter of my ccpending application Serial No. 499,754, filed August 24, 1943, for Directional radio receiver, which application is a division of the present application and assigned to the same assignee. The directional receiver is remotely positioned with respect to the rotatable loop antennae and the non-directional antenna, as indicated by the broken lines I38. Non-directional antenna I50 is connected to primary winding I5I which is coupled to secondary winding I52 of the radio frequency transformer coupled to radio frequency receiver and audio frequency detector I53. Variable condenser I54 is shunted across secondary winding I52 and is adjusted to relay .Ilg.

val

tone

asvasca add I30 remains normally energilled attracting annatures' I24 through I21 as indicated in '1. However, during the transmission interof radio transmitter T1 modulated by a relay say of '15 cycles, a corresponding 75 cycle note will appear at the output of audio frequency amplifier Ill. The 16 cycle tone is impressed upon relay filter III which prevents the passage of other frequency signals and passes the predetermined relay frequency of 75 cycles in the present example.

The output of filter III is connected to relay rectifier I". the output of which is connected to solenoid I30. A preferred arrangement for the actuation of solenoid I3. is such that the solenoid is I normally energized connecting loop antenna system I20 to the directional receiver. Upon reception of a radio signal bearing a substantial 75 cycle (or other predetermined frequency) signal, relay I30 is arranged to be deenergiaed and switch the directional receiver from directional antenna system I20 to directional antenna system I20. It is to be understood that relaying action wherein the predetermined (75 cycle) note causes energization of relay I30 instead of deenergization thereof may equally well be employed. Detailed circuit diagram Fig. 9 illustrates the arrangement for effecting the herein described operation of relay solenoid I30 by the '15 cycle signal.

The loop winding relay armatures I24, I25 are coupled to the loop radio frequency amplifier I60 through radio frequency transformer I6I, I62 by W101! cable I26 which is preferably of low impedance. A variable condenser I63 in shunt with the secondary winding I62 is used to tune-in the desired radio station. All the tuning controls such as condensers I50 and I63 ar preferably mechanically ganged together to provide a unitary tuning control. It is to be understood that a plurality of receiving bands may be employed to permit operation of the receiver system over a wide range of radio transmission frequencies. If a particular frequency is used for eilecting the blind approach to an airport, a rapid switch over means to bring the receiver to that frequency may be provided, as will be understood by those skilled in the art.

In describing the operation of the automatic directional receiver section, which lies to the right of broken lines I36, reception by one of the loop antennae is first assumed. With the position of relay armatures I26 throuhg I21 as shown in Fig. '1, loop antenna I20 and its associated control motor circuit are connected to the automatic receiver. The automatic receiver of Fig. 'l corresponds to schematically shown unit of Fig. 3, or to units 20 and 20' of Fig. 2. In accordance with the automatic receiver used in the system of the present invention and described in detail in my Patent No. 2,308,521 referred to, a loop control signal is provided dependent upon the received loop antenna signal, to operate the motor drive for the loop antenna and rotate it to its null position with respect to the oncoming radio signals. The normal or stable position of the loop antenna is at the null or electrical neutral position with respect to the oncoming radio signals from the associated transmitter station, giving an exact angular indication of the direction to the transmitter of the radio signals. In the present case, transmitter T2 is assumed to be radiating for controlling the orientation of loop antenna I20 and needle 2. When station T1 is transmitting, loop antenna I20 instead is connected to the system, resulting in a'similar electrical action.

whentheangularpositionoftheloopantenna I20 is changed from null during approach ma,- neuvers, the radio signal is picked-up by the loop and impressed upon amplifier I60. The magnitude and phase of this signal depends upon the amount of the off-null angular position of the loop and the direction of the transmitter to the right or left thereof. respectivedly. A local generator I60 of an audio frequency current, preferably of the order of one hundred cycles, is used to modulate the radio signals derived from the loop antenna and produce a resultant tone modulated radio signal. I prefer to use a tone signal of 102.5 cycles as indicated in the drawings, but a diiferent frequency may instead be used so long as it is different than the signal for relay I30. Loop signal modulator I66 schematically designates the modulation stage, preferably a balanced modulator, for combining the loop signal of I60 with the tone signal of I64.

The resultant tone modulated radio signal at I" has a magnitude and phase dependent upon the oil-null position of the receiving loop antenna. The tone modulated signal is then suitably combined with the non-directionally received signal from antenna I50 by a coupling means, such as coupling coil I66 linked with secondary winding I62 of th input transformer to radio frequency receiver I53. The nature of the radio signal impressed upon the input of amplifier I63 will be described in more detail, hereinafter, particularly in connection with Fig. 10. At this point it is sufficient to say that the superposition of the non-directional radio signals with the locally modulated loop signals provides a resultant radio signal with the 102.5 cycle tone component having a magnitude and relative phase dependent upon the off-null position of the loop antenna with respect to the oncoming wave. Radio frequency receiver I53 may be a tuned radio frequency circuit or a superheterodyne circuit. The receiver unit I53 contains a demodulator or detector for the audio frequency components of the amplified radio signals. The audio frequency signals at the output of unit I53 comprise modulations of the original radio wave plus the 102.5 cycle tone or control signal obtained when the loop antenna is of! null.

An audio frequency amplifier I55 is connected to the output of receiver-detector unit I53. Amplifler I55 supplies audio unit I56 with sufllcient energy for operating headphones connectible to jack I51 at the output thereof. The output of audio frequency amplifier I55 is also connected to a separate control signal amplifier I61 through a suitable phase shifting net-work I66 and 102.5 cycle pass filter I60. The 102.5 cycle tone signal is thus filtered out from the output of audio frequency amplifier I55 and amplified a substantial degree for use as a control signal to operate the relay control tube system indicated at I10. The relay control tube system is energized by both the control signal from amplifier I61 as well as the correspondingly amplified tone signal obtained directly from generator I and intermediate tone amplifier I1 I. Details of the operation and interrelation of the respective control signals and the relay control tube system I10 will be described in further detail in connection with Figures l1 and 12.

Control relays indicated at I12 comprise solenoids I13 and I1! connected to the relay control tube system I10. Solenoids I13 and Ill are selectively energized from control system I10 in accordance with the angular position to the right through their shafts I 2I and I2I and associated gearing. Energlzation of either relay I13 or I14 is determined upon the direction of the angular deviation of the connectedloop antenna (I20) from its null position with respect to the radiating station (T2) so that the proper counter-rotation of the associated motor (I 3|) will occur to bring the l p antenna to the null signal position.

Relay I13 remains energized until the loop antenna is rotated to reach its null position, whereupon the control signal derived from the loop signal impressed upon amplifier I60 is so reduced in value or obliterated as to cause relay armature I15 to drop back to its neutral or back contact position, deenergizing and stopping the associated motor. Electromagnetic clutches I32 and I32 are electrically shunted across their motor energization circuits to immediately disconnect the associated motor from the loop antenna, insuring a rapid stop of the loop rotation at the time of motor deenergization, and eliminating the possibility of overshooting or overdriving by the motor due to its mechanical inertia. The frictional forces of the gearing and the bearings of the loops are generally sufiicient to quickly stop the loop rotation.

In practice I have constructed systems in accordance with the present invention which automatically operate the loops and therefore the associated bearing indicators at a rate of 180 and more per second. The accuracy of the resultant bearing may readily be made within 1 Marc, 1. e. the actual directional indication on each radio transmitter being correct to within 1 or less. The bearing indications are on a 360 dial and move to the stationary bearing position through the shorter angular path. I prefer a rate of indication of about 125 per second for the one-third'of a second transmitter T1 and T2 radiation periods.

When the loop antenna reaches, or is substantially at, its null signal position a zero or substantially zero magnitude radio frequency signal is-impressed upon radio frequency amplifier I60 for modulation at I65 by the generated tone signal from I64. The magnitude of the control signal from amplifier I60 accordingly is also zero, or substantially zero at that time, and control relays I13, I14 are in the deenergized position shown. The loop accordingly remains stationary when it is at its electrical signal null position with respect to the direction of the oncoming waves. This position corresponds to the geometric position of the loop where the plane of the open face thereof is perpendicular to the direction of the oncoming radio waves.

Should the aircraft carrying the loop antenna deviate from this direction the loop will be energized by the oncoming radio signal from the transmitter, and impress it upon amplifier I60 with a magnitude and phase relation corresponding to the altered direction thereof. Should the deflection of the aircraft cause the loop to receive a signal of phase corresponding to that which energizes, solenoid I13, the above described operation of the loop motor is repeated to bring.

the loop to the new null signal position. Should, however, the aircraft turn so that the loop is deviated in the opposite angular direction, the phase of the control signal impressed upon relay control tube system I10 will be difierent by 180 and energize solenoid I 14 instead.

When solenoid I 14 is energized, its armature I16 is attracted to the front contact to electrically complete the connected loop motor circuit including its associated clutch, ground, and battery I11. Electromagnetic clutch I32 is thereupon immediately engaged and motor I 3| is rotated in the direction opposite to that corresponding to its energization by solenoid I 13 when loop system I20 is in connection with the receiver as shown. Motors I30 and I30 are reversible in the present case, and are not necessarily a direct current type. Relay armatures I15 and I16 are arranged so that the direction of current flow through the connected motor is selectively reversed to cause the motor to rotate its associated loop towards its null signal position in the shorter path of rotation. Thus, when solenoid I14 is energized, the motor will rotate in a direction opposite to that due to energization of relay I13.

An important feature of the preferred automatic directional receiver resides in the fact that for any bearing, the loop antennae are at their electrically neutral and geometric null positions, and remain stationary for the duration of the bearing. Furthermore, as will be shown in more detail hereinafter, no sense or directional ambiguity occurs, and the null position which the loop assumes is accurate for any bearing on the transmitters. Pointer 2 of indicator I45 moves in exact correspondence win loop antenna I 20' and is arranged to point to the center zero position 0 shown on the dial, of,loop I 20 is perpendicular. to the longitudinal axis of the aircraft. Similarly, pointer or needle I moves in exact correspondence with loop antenna I20, pointing to center zero when the plane of loop I20 is perpendicular to the flight direction.

The accuracy of indication is independent of the position of indicator needles I and 2 since any reading thereof corresponds to an electrical null position of loop antennae I20 and I20, and no balancing of electrical parameters or signal components are required to maintain the readings. The interpretation of the readings is readily apparent to the pilot, and he effects a blind approach as described in connection with Fig. 1.

In Fig. 8 I have diagrammatically illustrated components of one form of dual indicator I45. A common housing I encloses the direct current Selsyn movements for both needles I and 2 having corresponding torroidal or annular actuating coils I8I and I82. Coil I8I is connected by leads I84 to three-wire cable I84 which in turn is connected to a direct current loop position transmitter of one loop system. Annular winding I82 is similarly connectable by leads I83 to three-wire cable I83, for connection to the direct current loop position transmitter of the second loop system. Three-pole double-throw switch I85 is used to connect cable I83 with cable I83 as shown in dotted during the double indicator blind approach operation of the system as described hereinabove so that both needles I and 2 of meter I45 are independently controlled.

Indicator needle I is connected to central spindle I86 pivoted in end bearing I81. Magwhen the open plane aa'rasca neticcore l8l,shownindotted,issecuredio spindle I88 and arranged to magnetically coact with the interior of annular winding III in the usual telemetering manner. Indicator needle 2 is coupled to the end of tube I88 concentric about spindle I88. Tube I88 is rotatable on diagrammatically indicated bearing I8I for independent rotation with respect to spindle I88 of needle I. Magnetic core I82, shown dotted, is mechanically secured with rotatable tube I88, and is magnetically coactable with annular winding I82. Indicator needles I and 2 are coaxial and independently controlled to assume the angular positlons of the two loop antenna systems during the blind approach maneuvers described. A casing I88, having a flange I84, fitted within the open end of housing I88, encloses indicator needles I and 2 and contains the scale for the indications. A transparent window I85, such as glass, is fitted into the top end of enclosure I83.

when the dual receiver system is to be used for taking normal directional hearings on only one transmitter station and not for the described twostation blind approach maneuvering, three-pole switch I85 is thrown to the position drawn in solid lines'ln Fig. 8 connecting annular windings I8! and I82 in parallel. Both indicator needles I and 2 then act in unison, one above the other. and each assumes the same angular position on the scale of meter I45. For ordinary automatic directional operation, both sections of the meter are connected in parallel with one of the loop position transmitters and give the same reading.

Accordingly, when the dual indicator of Fig. 8 is in circuit with a dual receiver system in, accordance with the present invention, three-wire cable I84 connects to the position transmitter of the loop antenna normally in circuit with the receiver, and not to the loop antenna switched-in by the reception due to the field stations corresponding to Ti and T2. For example, if used in the receiver system of Fig. 7, cable I84 would correspond to cable I42, connecting to position transmitter I of loop antenna system I28 normally connected to the receiver through relay armatures I24 to I21. Three-pole switch I85 is operated to the solid position for both needles of indicator I to give a singel reading when the automatic directional system is used for radio guidance in general, and is connected to the dotted position when the pilot is'ready for blind approach to a runway having stations corresponding to T1 and T: when both needles become independently operative in the manner described.

Receiver circuit details Fig. 9 is a detailed schematic electrical dia gram, partially in block form, of a commercial form of the automatic receiver system constructed in accordance with the principles of my present invention. Relay system I24 through I21 selectively connects the two loop antennae systems I28 and I28 to the receiver as shown in Fig. 7. Fig. 9 is a specific electrical showing of the system shown generally in Fig. 7, and represents a preferred embodiment thereof though not limited thereto.

Signals from the rotatable loop antenna in circuit with loop transmission cable I28 are imprmed upon primary winding ISI of the loop radio frequency input transformer, secondary winding I82 of which is shunted by tuning condenser I 83 and connected to the control grid of radio frequency amplifier pentode 288 for amplification and introduction to the control grids of balanced modulator stage I". The gain of loop amplifier 288 is manually controllable by rheostat 28I connecting the cathode thereof to ground. The anode of amplifier 280 is energized through a shunt radio frequency choke coil 282 connected to the B supply.

The output of loop amplifier 2 is coupled to the control grids of tubes 203, 283 of modulator I through coupling condensers 284. The cathodes of tubes 283, 203 are tied together and connected to ground through by-pass condenser 285 and biasing resistance 286. An audio frequency oscillator I64 comprising two triodes 281, 281' is arranged to generate an audio or tone frequency current of a relatively low frequency. The control grids of triodes 201, 281 are coupled to the anodes thereof by condensers 288, 288. Cathodes of the oscillator triodes are tied together and connected to ground through biasing resistor 288.

Resistors I88 and I88 are coupled between the grid electrodes of triodes 281, 281' and ground. Intermediate taps on the resistors I88 and I88 couple a portion of the available alternating cur,- rent tone energy from oscillator I54 to the grids of modulator triodes 283, 203' through coupling resistances I85 of about one megohm each and through coupling condensers I81, I81. Further resistances I88, I88 normally connect grid coupling resistors I81, I81" to ground to stabilize the grid circuits of tubes 203, 283',

The actual frequency of the tone current generated by oscillator I64 as used in the system is optional, and may for example lie anywhere in the audio frequency spectrum, or even higher. Practically. however, ,the tone frequency should be chosen so as to emciently pass through the respective radio frequency circuits as sidebands, and the audio frequency circuits as well, be audible to the pilot when present and differ from the relaying tone from the field transmitters as T1 or T2. It is also desirable to prevent interference with the intelligibility of the aural messages of the radio signals. An important consideration is to minimize any effect due to the sound modulations of the radio wave upon the control circuit. I have found that a control signal in excess of 200 to 300 cycles contains sound modulation components after filtering out for control purposes.

Modulation kicks: occur when the sound frequencies of the radio signals coincide with the control frequency, and interfere with the stability of the directional indici-tions. A tone frequency of the order of cycles is sufllciently high to efiiciently pass through the radio and audio frequency channels of the system, sufficiently low to not interfere with the intelligibility of the audio frequency modulations of the radio signal, and is not affected by modulation kicks. A practicaltone frequency in this range I found to be a tone of 102.5 cycles, as indicated in the figures. This frequency is satisfactory when the loop relaying frequency of station T1 is 75 cycles. The field transmitter tone should differ sufficiently from the rereceiver control frequency, 102.5 cycles in the present case, to be properly filtered out of the audio output circuit. Thus 75 or less cycles, or or more cycles per second are satisfactory for T1 when 102.5 cycles is used in the receiver.

Control grid electrodes of modulator triodes 203, 283' accordingly simultaneously receive the audio frequency tone signal from oscillator I64 and the radio frequency signal picked up by the connected directional antenna. The electrical interaction of these respective signals is described

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