US2028722A

US2028722A - Airport control and signaling system

Info

Publication number: US2028722A
Application number: US376032A
Authority: US
Inventors: Laurence J Lesh
Original assignee: Associated Electric Laboratories Inc
Current assignee: Associated Electric Laboratories Inc
Priority date: 1929-07-05
Filing date: 1929-07-05
Publication date: 1936-01-21
Anticipated expiration: 1953-01-21

Description

Jan. 21, 1936. J, LESH AIRPORT CONTROL AND SIGNALING SYSTEM 3 Sheets-Sheet 1 Filed July 5, 1929 QQmQl mvzsl Jangzl, 1936. LESH 2,028,722

AIRPORT CONTROL AND SIGNALING SYSTEM Filed July 5, 1929 s sheets-sheet 2 Jan. 2l, 1936. L. J. I EsH AIRPORT CONTROL AND SIGNALING SYSTEM Filed July 5, 1929 3 Sheets-Sheet 5 Laur'amce Pateated Jan. 21, 1936 y 2,028,722

UNITED STATES PATENT OFFICE AIRPORT CONTROL AND SIGNALING SYSTEM Laurence J. Lesh, Chicago, Ill., assignor, by mesne assignments, to Associated Electric Laboratories, Inc., Chicago, Ill., a corporation of Delaware Application July 5, 1929, Serial No. 376,032

40mm. (c1. 177-352) The present invention relates in general to airrectangle in the upper left-hand corner of Fig. port control and signaling systems. The main 2 indicates a radio transmitter such as ls located object of the invention is the provision of an in each of the planes operating over the route. automatic airport lighting control and fog alarm Each of these radio transmitters is equipped with 5 System. an impulse sender suitable for sending impulses 5 According to one feature of the invention, the at a predetermined frequency. In the drawings, hangar lights and the correct fioodlight for landa Vibroplex key has been illustrated for this ing, governed by wind direction, may be conpurpose. When the key is operated to close controlled by the pilot of an airplane approaching tacts |60 the weighted portion is caused to vil the landing eld. brate at its natural period of vibration and 10 Another feature of the invention lies in the closes contacts |60 at each vibration, thereby provision of receiving and recording apparatus at transmitting a series of dot impulses at a definite the terminal airports of the route whereby such frequency. The frequency of these impulses may terminal stations are notied of the landing of a be adjusted by varying the position of the weight l plane and the particular field at which the plane ISI on the lever of the key. Dash impulses may 15 is landing. be transmitted by operating the key so as to According to another feature of the invention, close contacts I5I. These contacts are mounted the presence of fog at a particular field will opersuiciently near the pivot point of the key that ate indicators at the two adjacent flelds to warn the vibrations of the key lever do not open these pilots approaching from either direction of this contacts and a continuous dash will be sent out 20 fog condition. as long as the key is held in this position.

Still another feature lies in the provision of The rotary switch CS may be of the rotary selective receiving apparatus at the terminal lineswitch type well known in the art of autostations which is arranged to operate recording matic telephony. The switch comprises two rows devices to make a permanent record of the fog of stationary bank contacts which are wiped over 26 conditions at each eld and the time such conby the wipers 92 and 93. These wipers are ditions prevailed. mounted on a shaft which is rotated under con- Numerous other features of the invention will trol of the rotary magnet 89. The oscillator become apparent from the following speciiicashown at'O, Fig. 3, is of the type disclosed and tion when read in connection with the accomdescribed in my co-pending application- Serial 30 panying drawings, comprising Figs. 1 to 3, inclu- No. 353,283, filed April 8, 1929. The arrangement sive. 'I'hese figures show by means of the usual of the lights and fog indicators at the flying field schematic diagram, an air route consisting of is shown within the dotted rectangle in the leftn two terminal stations and four intermediate iiyhand portion of Fig. 3. The lights LI to L4, in- 3" ing fields. elusive, are the floodlights used in landing. NI 35 Fig. l shows in detail the apparatus and cirand N2 are arrow-shaped neon lamp indicators cuit arrangement of the equipment located at which are used to warn pilots of fog conditions terminal station No. l. The recorder or register at iiying fields F2 and F4, respectively. PH is CR may be any of the well known types in which a photo-electric cell and may be any one of a the recording tape is wound on reels which are number .of well known types. 40

rotated only during the time that signals are In the right hand portion of Fig. 3 are indibeing received. The recorders or registers FRI cated the fourth intermediate iiying eld F4 and and FR2 are of the type with a continuously terminal station #2. The equipment and circuit rotating cylinder making one revolution in arrangement at eld F4 is similar to that shown 4 twenty-four hours and on which signals are rein detail for F3 while the apparatus at terminal 45 COlded aCCOlding t0 the time at Whichl they are station #2 is similar to that shown in detail for received. Fig. l also shows two intermediate terminal station #1.

flying fields FI and F2 which have been indi- The operation of the system will now be excated by dotted rectangles. The equipment at lplained in detail in connection with the equipeach of these elds is the same as is shown in ment illustrated in the drawings for flying field r detail for flying field F3. F3 and terminal station #1. It will be rst as- Fig. 2 and the left-hand portion of Fig. 3 sumed that a plane is approaching the flying show in detail the apparatus and the circuit arfield F3 and wishes to land. As he nears the rangement of` the equipment located at flying fie1d,the pilot will operato theVibropleX key 43,

eld F3. The apparatus RT Within the dotted associated with the radio transmitter RT. Fig. 2, "5

located in the plane, so as to close contacts |60. The Vibroplex key vibrates at a denite predetermined frequency, closing contacts |60 at each vibration, and causes a series of impulses to be transmitted to the airplane antenna 46. This series of vimpulses is received at the 4flying field radio receiving set RR through its antenna 41. As the receiving set RR may be any one of a number of well known makes, the detailed operation will not be explained. It is thought suilcient to state that each impulse received by the receiving set RR is amplified by the tube 48 and operates the plate relay P. Thus the plate relay P will be energized and deenergized in synchronism with the vibrations of the Vibroplex key 43. Each time that relay P operates it closes obvious circuits for relays 5|, 53, 64, and 11.

It might be Well to explain at this time that each company whichA operates planes over the route including field F3 may be assigned a different Vibroplex key frequency. If this is done, all the planes of one company will be equipped with Vibroplex keys of the same frequency. A corresponding tuned relay, such as 53 or 64, is provided for each frequency in use. A separate hangar, such as H| or H2 of Fig. 3, is also provided for each company operating planes over the route.

Continuing with the explanation of the system, it will be assumed that the plane which is preparing to land is owned by Company A whose Vibroplex frequency is fl. It will further be assumed that hangar H| is owned by Company A. As described above, plate relay P will energize and deenergize at a frequency fl in response to the operation 0f the Vibroplex key 43. Relays 5| and 11 are operated each time the contacts 50 are closed. The function of these relays will be explained later. The circuits of all tuned relays, such as 53 and 64, are also closed at each impulse and these relays tend to attract their weighted armatures, such as 54 and 65, respectively. Each of these weighted armatures is tuned to vibrate at a frequency equal to the Vibroplex frequency assigned to the plane of one of the companies. It will be assumed that armature 54 of relay 53 is tuned to respond to frequency fl.

The energization of relay 53 causes the weighted armature 54 to vibrate at its natural frequency and, when the vibrations reach a certain amplitude, the armature operates the contacts 55. The closing of these contacts completes a circuit from ground, contacts 55, back contacts 59, winding of relay 56, normally closed contacts of the release key 16, back contacts 80 of relay 19, winding of relay 8|, to battery. Relay 56 and relay 8| operate in this circuit. At contacts 58, relay 56 closes a locking circuit extending from ground, contacts 58, winding of relay `60, winding of relay 51, winding of relay 56, normally closed contacts of the release key 16, back contacts 80, winding of relay 8|, to battery. Relays 60 and 51 are energized in this circuit and are locked up in series with relays 56 and 8|. At contacts 59, relay 56 opens its initial energizing circuit to prevent the shunting of relays 51 and 60 should the tuned relay53 be again operated by another plane belonging to Company A. The result of the operation of relay 8| and the reason for its slow to operate feature will be explained later.

When relay |08 operated and closed contacts |01 as described above, a circuit was completed for the correct flood-light, determined by the wind direction, to be used by the pilot in landing. The lighting of the correct iiood light is controlled by the wind vane V. In the particular case illustrated, it is assumed that the wind is from the north-west as indicated by the pointer of the wind vane V. As is well known in aviation, a plane must be landed against'the wind. It is, therefore, obvious that the floodlight L3 in the south-east corner of the flying field should be lighted so that the pilot will have the light at his back while landing. The abovementioned circuit for the flood-light extends from one side of the power supply, conductor |06, conductor |0, pointer of the wind vane V, stationary contact H2, conductor ||3, through the flood-light L3, conductor |09, contacts |01, to the other side of the power supply by way of conductor |05. All lights required by the pilot for landing have now been turned on in response to the operation of the Vibroplex key 43 in the airplane radio transmitting set.

The manner in which the terminal stations are informed of the landing of a plane at a particular eld will now be described. When relay P is operated in accordance with'the Vibroplex frequency fl in response to the series of impulses received by the radio receiving set RR, it closes an obvious circuit for relay 11 at contacts 50 each time it operates. Accordingly, relay 11 energizes and deenergizes at the same rate as relay P. Each time relay 11 energizes and closes its contacts 18, a circuit is completed from ground, contacts 98 of relay 91, contacts 18; winding of relay 19, land line L, Winding of relay at iiying field F2, corresponding to relay 19, land line L extending to flying field F|, the relay at F| corresponding to relay 19, land line extending to terminal station #1, normally closed contacts of key winding of relay 2, to battery. A branch'of this circuit extends over the land line L to iiying field F4, relay at F4 corresponding to relay 19, land line L extending to terminal station #2, normally closed contacts of release key 4, winding of relay ||5, to battery. Thus a series of impulses, occurring at a frequency governed by the frequency of the Vibroplex key in the airplane which is preparing to land, is transmitted over the land line to each of the terminal stations. The frequency at which these impulses occur indicates to the terminal stations the identity of the company whose plane is preparing to land, as will now be described.

Referring now to terminal station #1, illustrated in Fig. 1, relay 2 is operated in response to each impulse received over the land line. Each time it operates an energizing circuit is closed at contacts 3 for the tuned relays such as 4 and 8. One of these relays is provided for each frequency in use in the system. It will be assumed that the relay 4 is tuned, by means of its weighted armature 5, to respond to the frequency fl. Accordingly the closing of contacts 3 in accordance with this frequency and the subsequent impulses .of current to the relay 4, will cause the 'weighted armature 5 to vibrate at its natural frequency, which is equal to the frequency fI. When the vibrations reach a certain amplitude, the armature 5 closes contacts 6, thereby completing an obvious circuit for the signal lamp 1 and relay |52 in multiple. Relay |52 operates and at contacts |55 closes a locking circuit for itself extending from ground, normally closed contacts of the release key |59, contacts |55, winding of relay |52, to battery. A branch of this circuit extends through the lamp 1 to battery. The relay |52 is vthereby maintained operated and lamp 1 is kept lighted after the series of identifying impulses has ceased and the relay 4 has opened contacts 6. vAt contacts |53, relay |52 closes an obvious circuit for the bell |54.

When the attendant at the terminal station hears the alarm. bell, he knows that a plane is preparing to land at one of the flying fields. As the lamp 1 is individual to the tuned relay 4, the lighting of this lamp identifies the company whose plane is preparing to land. When the attendant has taken note of the fact that a plane is preparing to land and has identified the company to which the plane belongs, he may stop the bell and extinguish the lamp by depressing the release key |59. The opening of the contacts of this key opens the locking circuit of relay |52 and the circuit of lamp 1. Relay |52, in releasing,'opens the circuit of the'alarm bell at contacts |53.

In addition to ringing the'bell |54 to attract the attention of the attendant and lighting the lamp 1 to indicate which plane is preparing to land, the series of identifying 'impulses is also recorded on the pen register CR. When relay 2 is operated in response to the receipt of the first impulse, it closes a circuit for the operating magnet I2 of the recorder CR and the slow-to-release relay`|9 in multiple. Relay I9 operates and withdraws pawl I8 from the ratchet teeth onthe reel I1. Reel I1 is then revolved by means ofspring power 'and starts the tape I5 so that the incoming signals may be recorded. Relay I9 is made slow to release and remains operated during the series 'of impulses and for a short time afterwards to insure the recording of all signals before the tape I5 is stopped. Operating magnet I2 is energized at each impulse and attracts its armature I3, thereby forcing the pencil lI4 into contact with the tape I5. Thus a permanent record is made of the series of identifying impulses and the identity of the plane that transmitted these impulses may be determined at any time by counting the number of dots recorded per unit 'distance of tape, thereby determining the frequency of lthe received signal.

Returning now to the point where relay BI of Fig. 2 was operated in series with relay 56 and locked in series with relays 56, and 51, the closing of contacts 82 completes an obvious circuit for the slow-to-operate relay 83. A multiple branch of this circuit extends from ground, contacts 82, back contacts 84 of relay 83, normal bank contact of the rotary switch CS, wiper 92, rotary interrupter springs 9|, winding of rotary magnet 89, winding of relay 91, to battery. `The rotary magnet 89 and relay 91 are energized ln this circuit before relay 83 has had time to operate and open its contacts 84. The operation of relay 91 opens contacts 98. thereby removing ground from the armature spring of relay 11 and preventing any further sending of' identifying impulses by the relay 11. This is done so that the Vibroplex frequency impulses will not interfere with the sending of the field code. f

rIt might be well at this time to explain the adjustments of the tuned relays and the slow acting feature of relay 8|. When the identifying impulses are received at the flying field and cause the operation of one of the tuned relays, such as 53 and 64, these impulses are at the same time repeated to the terminal' station to bring about the operation of one of the tuned relays, such as 4 and 8. As explained above, the operation of the tuned relay 53 brings about the energization of re1ay8| and, in turn, the operation of relay 91. Relay 91, at contacts 98, opens the impulsing circuit of relay 11. It will therefore be seen that the tuned relay` at the terminal station must operate and cause the operation of the locking relay, such as |52, before relay 91 operates and opens the impulsing circuit. 'I'he tuned relays at the flying field are therefore adjusted so that their contacts are a relatively large distance away. from the weighted armatures and will not be closed until the armatures are vibrating at nearly maximum amplitude. It is obvious that a relatively large number of impulses and a correspondingly long interval of time will then be required to operate the contacts of these relays.

On the other hand, the tuned relays at the terminal stations are adjusted with the contacts relatively close to the weighted armature. Thus a smaller number of impulses and a shorter interval of time are required to operate the contacts of these relays. In addition to the adjustments of the tuned relays, relay 8| is made slow to operate, thereby allowing a still greater interval of time to elapse before relay 91 operates and opens the impulsing circuit to the terminal stations. The combined effect of the difference in adjustment of the tuned relays and the slow acting feature of relay 8| insures positive operation of the tuned relay at the terminal station.

Returning now to the operation of the switch CS, when the rotary magnet 89 operated it opened contacts 9|, thereby opening its energizing circuit and allowing it to deenergize and advance the wipers one step. As soon as the wipers are advanced to the second bank contacts, a circuit is closed from ground at the bank contact, wiper 92, interrupter contacts 9|, winding of rotary magnet 89, winding of relay 91, to battery. The operation of the rotary magnet 89 again opens contacts 9| and allows the magnetr to deenergize, thereby advancing the wipers another step. This action continues and the wipers of the rotary switch are advanced step by step.

Relay 91v is made slow-to-release so that it is maintained operated during the stepping of the rotary switch and until the. switch has returned to normal.

When the wipers of the rotary switch lare moved into engagement with the fourth bank contact, a circuit is closed from ground, wiper 83, bank contact #4, winding of relay 95, to battery. Relay 95 operates and closes its contacts 96, thereby sending a ground impulse to each of the two terminal stations. This circuit extends from ground, contacts 96, winding of rellay 19, land line L extending to F2, winding of relay at F2 corresponding to relay 19, land line L extending to FI, winding of relay at FI corresponding to relay19, land line L extending to terminal station #1, normally closed contacts of key I, winding of relay 2, to battery. The other branch of the circuit extends over land line L to flying field F4, relay at F4 corresponding to relay 19, land line L extending to terminal station #2, normally closed contacts of key I I9. winding of relay H5, to battery. Relay 2, at terminal station #1, operates` and closes its con- 5 tacts 3, thereby closing an energizing circuit for the operating magnet I2 of the register CR and the slow-to-release relay |9. Relay I9 operates to start the tape I5 and the operating magnet l2 attracts its armature I3, thereby forcing pencil 10 I4 into contact with tape I5 and recording the irst dot of the field code.

It will be noted that the lower bank contacts 9, 5, 1, and 9 of the rotary switch CS are multipled together and connected to the winding of 1;, relay 95. 'I'hus it will be seen that as the switch rotates its wipers, a circuit is closed for relay 95 when wiper 93 moves into engagement with each of these four bank contacts. Each time relay 95 operates it sends another ground impulse to )0 each of the two terminal stations and causes another dot signal to be recorded on the registers at these terminals in a manner already described. The field code for flying field F9 consists of one dot, pause, two dots, pause, one dot, as governed by the wiring of the lower bank contacts of the rotary switch. Ground impulses in accordance with this signal are transmitted to the two terminal stations and recorded on the registers such as CR. Each of the four intermediate flying fields has a different code signal. The attendants at the terminal stations are thereby notified at which particular flying field the plane is preparing to land.

Briefly summarizing what has occurred at each of the terminal stations, the attendant is iirst informed of the arrival of a plane at one of the intermediate ilying elds by the sounding of the bell |54 and the lighting of one of the lamps, such as 1 or One of these lamps is provided for each company operating planesI over the route and the attendant immediately knows which company owns the plane which is preparing to land. The series of identifying impulses which causes the lighting of one of the lamps also operates the recorder CR which records a number of dots, which may be used to identify frequencies. The field code is then received and recorded on the register CR, directly after the identifying signal. A permanent record is thereby kept of :,0 the landing of all planes and the particular ileld at which they landed.

The rotary switch CS continues its rotary movement until the wipers 92 and 93 have been returned to their normal position. During the time that the field code was being transmitted by the rotary switch. relay 93 operated and opened contacts 99, thereby opening the initial energizing circuit of the rotary magnet 99 and relay 91. Thus the rotary switch is prevented t0 from again operating when its wiper 92 is returned to normal and engages the normal bank contact. Relay 99, in operating, also closes its contacts 95, thereby completing an obvious circuit for the slow-to-release relay 91. Relay 91 c5 operates and closes its contacts 99, but this has no particular eii'ect at this time.

Everything is now in readiness for the pilot to land. 'I'he correct ilood-light for landing, as

determined by the wind direction, has been turned T0 on. The light above the hangar owned by the company whose plane is approaching the field has also been lighted. The two terminal stations have been informed that a Company A plane is above flying field F3 and is preparing to land.

It will be assumed that the pilot has landed his plane and has taxled to his hangar. Each hangar is provided with a release key, auch as 19, Fig. 2, all of these keys being in multiple. When the pilot reaches his hangar he depresses key 19, thereby opening the locking circuit of relays 59. 5 99, 51 and 9|. All of these relays release. Relay 59 opens its contacts 59, thereby opening a further point in the locking circuit of the relays, and closes its contacts 59, thereby again preparing its initial energizing circuit. Relay 99 opens 10 its contacts 9| thereby opening the circuit of relay |99 and allowing it to release. Relay |99 opens one side of the power supply to the ilood and hangar lights at contacts |91. Relay 99 also opens the contacts 92, thereby opening the 15 other side of the power supply to the light at hangar HI. All the lights at the flying field have now been extinguished.

Relay 9|, in releasing, opens its contacts 92, thereby opening the energizing circuit for relay 99. 20 Relay 93 releases and at contacts 94 again prepares the initial energizing circuit of the rotary magnet 99 of the switch CS. Relay 93 also opens the circuit of relay 91 at contacts 85 and closes a circuit for the rotary magnet 99 of the rotary 25 lay 91 again opens its contacts 99, thereby open- 35 ing the impulslng circuit controlled by relay 11 to prevent any identifying impulses from interfering with the transmitting of the field code should another plane approach the eld and attempt to turn on the lights by the operation of its Vibro- 4o plex key. Rotary magnet 99 operates and interrupts its own initial energizing circuit by opening contacts 9|. Upon releasing the magnet moves the wipers 92 and 93 into engagement with the second bank contacts'. The magnet is again l5 energized from ground at the second bank contact, wiper 92, interrupter springs 9|, winding of rotary magnet 99, winding oi relay 91, to battery. The field code, consisting of one dot, pause, two dots, pause, one dot. is then again trans 50 mitted to the terminal stations in the same manner as previously described. This field code is again recorded on the register CR and informs the attendant at the terminal station that the pilot of the plane has turned oir the lights at the 55 field F3.

The system is also arranged so that the pilot of a plane may turn off the lights without landing. To do this he operates his Vibroplex key 93 so as to close contacts |5| and holds it in this posi- 60 tion to transmit a long dash signal to the ileld receiving set. In response to this signal, plate relay P operates and remains operated for a short interval. 'I'he closing of contacts 59 again operates relay 11 which also remains operated G5 for a short interval. The closing of contacts 19 again completes the circuit from ground, back contacts 99 of relay 91, contacts 19, winding of relay 19, landline L extending to F2, winding of the relay at F2 corresponding to relay 19, land line L Aextending to Fl, winding of the relay at FI corresponding to relay 19, land line L extending to terminal station #1, normally closed contacts of the key winding of relay 2, to battery. In this case the circuit is maintained closed for a 75 sumcient length of timevtol allow the v-slow-to# operate relay19 toA energize. Re1ay'19operat'es and opens its contacts `80, thereby opening the locking circuit of relays' 56, 60, 51 and 8l, and bringing aboutthe turning `olf of the -lights and the sending ofthe field code as described above.

Should the attendant at either terminal station, having been notified that a plane belonging to Company A is over flying eld F3, wish the pilot to continue and not land, he mayv turn olf the lights at the :dying eld from the' terminal station. To do this he depresses the key, suchas I in Fig. 1, thereby closing a circuit from ground, key I in its operated position, land line L extending to FI, relay at F| corresponding to relay 19 of F3, land line L extending to F2, relay at F2 corresponding torelay 19, land line L extending to F3, relay 19, land line L extending to F4, relay at F4 corresponding to relay 19, land line L extending to terminal station #2, normally closed contacts of key ||4, winding of relay H5, to battery. Relay 19 operates in this circuit, and by opening its contacts 80,'brings about the extinguishment of the lights and the sending of the eld code as described above. The turning off of the lights in this manner is a signal for the pilot to continue to the next flying eld and not land at iield F3.

The pilot of a plane over the ying field F3 may also transmit messages over the land line L by means of telegraph signals. To do this, the pilot operates the Vibroplex key 43 so as to close contacts |5| in accordance with the dots and dashes of the telegraph message. These signals are received by the eld radio receiving set RR. which in turn operates plate relay P. Relay P will be operated a comparatively short time for the dots of the signal and a relatively long time for the dashes of the signal. Each time relay P operates, it closes an energizing circuit for the relay 11 at contacts 50. Relay 11 is thereby energized in accordance with the dots and dashes ofthe telegraph message. At contacts 18, a corresponding group of dot and dash impulses are transmitted over the land line L to each'of the two terminal stations. The relay 2 at terminal station #l and relay ||4 at terminal station #2 respondto these impulses and in turn operate the registers, such as CR, to record the complete telegraph signal. The attendants at the stations may then take the telegraph message from the recording tapes.

The signals from the intermediate flying field to the terminal stations may be transmitted by radio instead of over the land line L. This is done by means of the eld radio transmitter FT shown in Fig. 2. When the Vibroplex key 43 of an airplane approaching a eld is set in vibration andthe relay P responds to the resulting series of impulses, a circuit is closed for relay 5| each time relay P closes its contacts 50. As previously explained, one of the tuned relays, such as 53, is operated by the series of identifying impulses. This results in the operation of relays 56, 60, 51 and 8|. Relay 51, upon operating, closes its contact 63, thereby completing the filament circuit of tube 15 of the eld radio transmitter FT. The field transmitter is now in readiness to transmit the signals received by the relay 5| and repeated at its contacts 52. The radio transmitter FT may be any one of a number of well known makes and its detailed operationwill not be explained. It will be seen from the foregoing that the radio transmission of the signals maybe usedas analternative of the `transn'lis'slon. over the land line L.

, The operation of the fog indicating system' will now be explained. Each intermediate ying field is equipped with aphoto-electric cell, such as PH shownwithin the dotted rectangle in Fig. 3. This cell is mounted at one side of the eld and may be located about one hundred feet'in the air. Dl-

rectly acrossfrom the'cell and at thesameheight is mounted a source of light within a parabolic mirror or reector so as to project a very narrow beam of light directly into the telescopic end of the photo-electric cell. In addition, each flying field is equipped with an oscillator, such as shown at O. The oscillator at each eld operates-at a different frequency. In the case illustrated, the oscillator at flying field F| is arranged to trans'- mit a wave at 100 cycles, the oscillator at F2 is arranged to transmit at 200 cycles, the oscillator O at F3 is arranged to transmit at 300 cycles, and the oscillator at F4 is arranged to transmit at 400 cycles per second. In addition, each field is equipped with two tuned receivers arranged to receive the fog alarm signals from the two adjacent elds. Thus the flying .field F3 is equipped with a receiver which responds to the 400 cycle alarm signal received from F4 and also with a receiver which responds to the 200 cycle fog alarm signal received from F2. Each terminal station is provided with a receiver and a recorder for each of the fog alarm frequencies.

'I'he operation of the fog alarm system willl now be explained in detail in connection with the apparatus shown at ymg eld F3 and terminal station #1. Normally, with the beam of light B from the light ||6 directed into the lphoto-electric cell PH, current is allowed to ow through the photo-electric cell and resistance |46 from battery |41. The resistance |46 is of such a value that the voltage drop across the resistance with current flowing through the photo-electric cell maintains the grid of tube ||1 at zero or at a positive potential with respect to the filament. This allows a plate current of suiiicie'nt value to flow in the plate circuit of tube lll to operate relay ||8 and maintain it operated as long as the beam of light B is shining into the photo-electric cell.

It will now be assumed that fog is present at flying field F3 and has cut of! the beam oflight B from the photo-electric cell PH. As soon as the light is cut off from the cell, current ceases to flow in the above described circuit including the photo-electric cell, resistance |46, and the' battery |41. 'I'he voltage drop across resistance |46 drops to zero and the grid of tube ||1 assumes a negative potential with respect to the filament. This negative bias on the grid of tube ||1 reduces the plate vcurrent through relay ||9 toa sufficiently low value to allow relay |8 to re-v lease. Contacts ||9 close and complete the battery circuits for the oscillator O. The oscillator O is a tuned reed oscillator of the type disclosed in my co-pending application previously referred to and a detailed description of its operation need not be given here. It is thought suilcient to state that the tuned reed |22 is tuned to a frequency of 300 cycles per second and a current of this frequency is therefore caused to flow through wind-- vvariable condenser I 2 1, is provided so that none of this induced current passes through coil |23 of the receiving apparatus. A portion of the induced current passes through this artificial line to ground and the remainder passes through condenser I30 to the land line L byway of conductor |04. 'I'he manner in which the induced current is received and identified at the ying fields F2 and F4 will be described later in connection with the receipt of a fog alarm signal at flying eld F3. A portion of this induced'current is transmitted to terminal station #1 through condenser 42, over land line L extending to ying fleld F2, condenser 4|, land line L extending to Fl, condenser 40, land line L extend ing to terminal station #1, condenser 20, winding 2| of the transformer, to ground. A portion of the current is also transmitted to terminal station #2 by way of land line L and condenser |43 to receiving equipment at terminal station #2 similar to that shown at terminal station #1.

Referring now to the receiving equipment at terminal station #1 shown in Fig. 1, the 300 cycle current nowing in the primary coil 2| of the transformer induces a current-of like frequency in the secondary coil 22. 'Ihis induced current varies the potential on the grid of tube 23 and, results in a 300 cycle plate current owing through the tuned relays, such as '21 and 24. Four such relays are provided, one for each of the frequencies used by the four intermediate flying fields. It will be assumed that relay 24 is arranged to respond to 300-cycle current, that is, the tuned reed 25 is adjusted to vibrate at 300 cycles per second. When the reed 25 is vibrating at a certain amplitude in response to the 300- cycle current through the relay 24, it opens contacts 26, therebyopening the circuit of the normally operated relay 30. Relay 30 releases and, by closing its contacts 3|, completes an obvious circuit for the operating magnet 33 of the register FR2 and the lamp 32 inmultiple. Lamp 32, being lighted, immediately noties the attendant at the terminal station that there is fog at flying field F3. Magnet 33 attracts its armature and brings the pointer in contact with cylinder 34, thereby making a record of the fog condition at the iiying field F3V and also of the time at which this condition prevailed.

The operation of the fog alarm receiving apparatus at neld F3 will now be explained. It will be assumed that fog is present at iiyin'g eld F2 and that the oscillator at F2, corresponding to O at F3, is superimposing a 200-cycle current on the land line L. This current is received at F3 through condenser 42, over conductor |04, condenser |30, right half of coil |25, primary coil |28 of the receiving transformer, to ground. 'Ihe resulting voltage induced in the secondary coil |23 of the transformer varies the potential on the grid of tube |3| at this same frequency. This results in a 20G-cycle plate current which passes through the tuned relays |32 and |33. The tuned reed |34 of relay |32 is adjusted to respond to a current of 200 cycles which is the transmitting frequency of F2, while the tuned reed |36 of relay |33 is adjusted to respond to a current of 400 cycles which is the transmitting frequency of F4. Accordingly the 200 cycle plate current passing through the relay |32 sets` the tuned reed |34 into vibration. When the vibrations reach a certain amplitude, reed |34 opens contacts |35, therebyropening the energizing circuit for the normally operated relay |38 and allowing it to release. The closing of contacts |33 completes a spaanse circuit from one side of the power supply source, conductor |43, contacts |33, primary winding |44 of the transformer Tl to the other side of the power supply source by way of conductor |42. The alternating current passing through primary coil |44 of the step-up transformer TI induces a relatively high voltage in the secondary coil |45. 'I'his results in the lighting of the neon lamp indicator N This neon lamp indicator is in the form of an arrow pointing in the direction of flying field F2,- and, when lighted, informs any pilot ying over the field that there is fog at eld F2. Similarly, the neon lamp indicator N2 is controlled by frequency relay |33. In this manner a pilot approaching any of the four intermediate iiying fields is immediately notified if there is fog present at either of the two adjacent fields. The attendants at the terminal stations are also notifled of the fog condition and a permanent record is made showing the time that such condition prevailed. a

Having described the invention what is thought to be new and is desired to have protected by Letters Patent will be set forth in the appended claims.

What is claimed is:

1. In combination, an air route comprising two terminal stations and a number of intermediate ying fields connected by a land line, each of said fields having an identifying code signal, lights at each field, airplanes having transmitters, means controlled by the transmitter of an airplane over one of said fields for turning on the lights at said field, means for transmitting the code signal of said eld over said land line to inform said terminal stations that an airplane is attempting to turn on the lights at said iield, means including said rst means controlled from said airplane or from either of said terminal stations for turning off the lights at said field, said second means responsive to said last means for repeating the code signal over said land line to notify the terminal stations that said last means is being operated to turn off the lights at said eld.

2. In combination, an air route comprising a terminal station and a number of landing fields, each eld having an identifying code signal, airplanes each having a transmitter for transmitting a distinctive signal, means at each field responsive to the transmitter of any airplane preparing to land at the eld for transmitting a continuous signal to said terminal station to inform the station of the character of the plane, means responsive to said signal after a time interval for transmitting the code signal of the eld to notify the terminal station of the particular eld at which said plane is preparing to land, and means for rendering said rst means inoperative while said code signal is being transmitted.

3. In combination, a flying field having a plurality of fioodlights and' a plurality of hangar lights, a current source, a plurality of tuned relays each responsive to a different frequency,

means in an airplane over said eld for trans.

mitting aseries of impulses at a vpredetermined frequency to "operate one of said relays, means common to all of said relays and controlled by any one of them for closing one side of said current source to all of said lights, means controlled by wind direction for closing the other side of said current source to the proper floodlight, and means controlled by the operated relay for closing said other side of said current source to the proper hangar light as determined by the frequency at which the impulses are transmitted from said airplane.

4. In a signaling system for airplane landing fields, a plurality of planes each having a. transmitter for always transmitting a distinctive signal, a plurality of landing fields each having a receiving device and landing lights, said iields linked together by a land line, the receiving device at each eld responsive to a signal trans mitted from any one of said airplanes to light the lights at a landing eld and to retransmit over the land line a signal indicative of the airplane from which the signal was received and the par- 5 ticular eld at which it was received.

LAURENCE J. LESH.