AU595982B2 - Communication link connection means - Google Patents

Description

S F Ref: 12669 FORM COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

This document contains the amendments made under Section 49 and is correct for printing.

4 4i q z;~ Class Int. Class Application Number: Lodged: FOR OFFICE USE: PI0176 4 February 1987 PATENT Accepted: Published: Priority: Related Art: ON OL TWENTY DOLLARS TEN DOLL Name and Address of Applicant: Actual Inventor: Address for Service: The Overseas Telecommunications Commission (Australia) 32-36 Martin Place Sydney New South Wales 2000

AUSTRALIA

Nilliam Michael Woods, Adam Gajda, Spruson Ferguson, Patent Attorneys, Level 33 St Martins Tower, 31 Market Street, Sydney, New South Wales, 2000, Australia Complete Specification for the invention entitled: COMMUNICATION LINK CONNECTION MEANS The following statement is a full description of this invention, including the best method of performing it known to me/us 51

ATTACHED

MAIL F CER 5815/2 r i;: is; br Z 5805q/3 ii__

ABSTRACT

Means for automatically establishing a communication link between a base station and one of a number of mobile out stations which is transmitting a sideband call signal including a predetermined audio tone f, comprising a scanning receiver to detect and lock onto the calling out station and recover an audio tone f 0 from said sideband which may vary in frequency from the tone f transmitted by the mobile out station, clarifier means mixing the recovered audio tone fo with a reference frequency Af and applying the three resultant tones fo, fo+Af and f -Af to individual detectors tuned to provide an out put signal when the applied signal falls within a narrow tolerance range of audio tone f, and means responsive to said output from any one of said detectors to connect said communication link through that detector thereby achieving clarification of all audio tones transmitted over said link.

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This invention relates to means for establishment of a communication channel between any one of a plurality of outstations and a base station, especially where the outstations are moving vehicles, such as aircraft.

For example, maintenance, or other, facilities controlled from a base station may be made available to incoming aircraft at an aerodrome with individual communication carrier frequencies allocated to different aircraft, or the airline to which it belongs. At present it is necessary to employ a radio operator 24 hours a day to monitor all the possible calling frequencies and, when he receives an aircraft calling signal for the base station, he is required to clarify the signal (to correct for Doppler, and other frequency, shift) and connect the aircraft to the base station to permit a two-way conversation to take place between the aircraft and the base station.

It is the principal object of the invention to provide a means for initial establishment of a communication link between outstations and a base station, which is automatic and reliable.

In accordance with the invention there is provided connection means for a communication link between a base station and a calling mobile outstation transmitting on a single sideband constituted by a predetermined carrier frequency modulated by a predetermined audio tone f, said connection means comprising means for receiving and demodulating said sideband to obtain an audio tone f 0 which may differ from the frequency of audio tone f due to frequency shift in or between transmission and reception, clarifier means for mixing said audio tone fo with a reference frequency Af to obtain audio tones fo+Af and fo-Af, three individual channels for conveying said audio tones fo 0 fo+f and f 0 -Af, three detectors each one being connected to a respective one of said channels with each of said detectors able to produce an output upon detection within a predetermined tolerance of audio tone f, and means responsive to an output from any one of said detectors for establishing a communication link between the calling out station and the base station via that one of said channels connected to said detector producing said output, whereby automatic clarification occurs of any audio tones subsequently conveyed on said sideband from said calling outstation.

The invention will be described in more detail with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of connection means for a communication link according to this invention; and Fig. 2 shows by frequency graph how automatic detection and clarification is obtained.

GMG/296r 3 d The following description of a preferred embodiment of this invention, for the purposes of explanation, is confined to the application of the connection means for a communication link between a plurality of aircraft approaching or departing an aerodrome whereat a maintenance facility is provided. Oft-times an approaching aircraft will require to arrange for maintenance upon arrival and a specific communication system is provided at the base station, at the aerodrome, and all aircraft serviced by the airport facilities. To this end a number of frequencies, in one instance six fequencies, is allocated in a band say of from 4 to 22 ;Hz. Due to the movement of ihe incoming aircraft, and consequently the Doppler shift effect, as well as the frequency drift in the aircraft transmitter and base station receiver, clarification of the audio signals passing between the transmitter and receiver is necessary in order to maintain reasonable audio fidelity. The present invention achieves this in an automatic manner as well as providing for automatic connection between a calling aircraft and the base station.

It is a characteristic of the invention that an aircraft calling signal includes upper sideband transmission composed of its allocated carrier frequency modulated by a fixed audio tone, or tones. It is quite common when a carrier frequency of 20 MHz is utilised, for a Doppler frequency shift to occur of ±20 Hz, while a frequency drift in the electronics might account for 20 Hz. Thus a total distortion of 40 Hz may be present in the received sideband signal and therefore the modulating audio tone.

Automatic clarification of audio signals within a total band swing 'of 75 Hz is provided by the arrangement shown in the accompanying drawings. It will be noted from Fig. 1 that aircraft A2 of the six incoming aircraft Al-A6 has transmitted a calling signal upper sideband composed of a frequency which combines the frequencies of a carrier C and an audio tone f(i.e. C+f) which is received at the scanning receiver 10 at the control unit 11. The receiver 10 is arranged to scan for 100 ms a band which includes each of the allocated carrier frequencies so that full cycling occupies 600 ms. The signal from aircraft A2 is then demodulated and the audio tone fo is divided between three channels 12, 13 and 14 which are connected to the inputs of a respective one of three audio tone f detectors 15, 16 and 17. The connection via channels 13 and 14 is in each instance through a clarifier 18 or 19 which mixes the tone f 0 with a reference frequency Af of Hz. The frequency connected to the detector 16 from clarifier 18 is f -Af while that connected to the detector 17 from the clarifier 19 is o GMG/296r 4 fo+Af. Channel 12 conveying the audio tone fo is connected directly to the detector Each of the audio tone f detectors 15, 16 and 17 has a narrow tolerance of 12 Hz whereby, with reference to Fig. 2, it will be seen that no matter where the frequency of audio tone f should fall within a Hz frequency band, one or the other of detectors 15, 16 and 17 will respond and provide a signal at its output 20. Depending upon which output 20, connected to the audio switching unit 21, is activated, the channel 12, 13 or 14 associated with that activating detector 15, 16 or 17 will be by output D connected to the base station 22. Consequently, the communication link now established between the aircraft A2 and the base station 22 will have an audio fidelity within ±12 Hz.

Nhichever detector 15, 16 or 17 becomes activated, via its terminal A it sends a signal to the terminal A of the scanning receiver to cease scanning which in turn causes the receiver 10 to develop an output signal at its terminal B to tune the transmitter for the base station 22 to the carrier frequency C associated with the aircraft A2. Subsequently a geographical sector signal will be sent from the aircraft A2 also to produce an output D at audio switching unit 21 which via the sector detector 23 causes the transmit aerial, which may be of a steerable log-periodic type to face the signalling aircraft A2. The geographic sector signal will be received satisfactorily as the velocity difference between the aircraft and the base station will not change during the brief period before its transmission. Therefore, a communication link is now established between the incoming aircraft A2 and the base station for voice transmission.

It is preferred by this invention that the audio tone f correspond to the decimal digit identification frequencies presently allocated to Dual Tone Multi-Frequency (DTMF) signalling, and more particuarly to the frequencies indicative of decimal These frequencies of 697 Hz and 1477 Hz require detection with a narrow tolerance and have been chosen for greater accuracy in detection. This form of signalling system has been used throughout the Public Switched Telephone Network for many years and press button operated units are readily available as are detectors therefor. Furthermore, the DTMF code and may be used to indicate to the scanning receiver lOthe specific geographical sector in which the calling aircraft is located. The entire system may be re-set by either the aircraft or the base station transmitting a decimal in order to restart the scanning receiver GMG/296r i'_1 A.x Whereas a preferred embodiment has been described in the foregoing passages it should be understood that other forms, modifications and refinements are feasible within the scope of this invention. For example,calls originating from the base station for direction to an individual aircraft may also be signalled through a conventional DTMF signalling unit.

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Claims (9)

1. Connection means for a communication link between a base station and a calling mobile outstation transmitting on a single sideband constituted by a predetermined carrier frequency modulated by a predetermined audio tone f, said connection means comprising means for receiving and demodulating said sideband to obtain an audio tone fo which may differ from the frequency of audio tone f due to frequency shift in or between transmission and reception, clarifier means for mixing said audio tone f with a reference frequency Af to obtain audio tones fo+Af and fo-Af, three individual channels for conveying said audio tones f O fo+Af and fo-Af, three detectors each one being connected to a respective one of said channels with each of said detectors able to produce an output upon detection within a predetermined tolerance of audio tone f, and means responsive to an output from any one of said detectors for establishing a communication link between the calling out station and the base station via that one of said channels connected to said detector producing said output, whereby automatic clarification occurs of any audio tones subsequently conveyed on said sideband from said calling station.

2. Connection means according to claim 1, wherein said calling mobile out station is one of a plurality of mobile out stations to which different carrier frequencies have been allocated and which are capable of making a call, and said receiving means comprises a receiver repeatedly scanning a predetermined frequency band to detect the existence of a transmitted I sideband from any one of said mobile out stations.

3. Connection means according to claim 2, wherein the production of an output from a detector causes a signal to be sent from that detector to interrupt the scanning of said receiver and causes said receiver to tune a o transmitter at the base station to the carrier frequency of the calling mobile out station.

4. Connection means according to claim 3, wherein said transmitter is connected with an aerial of a steerable type, and switching means is provided in said connection means which upon receipt by said receiver of a geographic sector signal from the calling mobile out station ensures that said transmitter aerial is positioned to face said calling mobile out station.

Connection means according to claim 4, wherein each of said mobile out stations is provided with a DTMF (Dual Tone Multi-Frequency) signalling unit of a public telephone network to represent a decimal -I s\ 3T I V T t i a 1 ,b number, and said geographic sector signal is of a frequency corresponding to the frequency transmitted by said DTMF to represent a letter of the alphabet.

6. Connection means according to Claim 5, wherein said decimal number is 3. ac orcA\

7. Connection meansito any one of Claims 4 to 6, comprising also reset means for restarting scanning by said receiver, and wherein activation of said reset means is effected by transmission of a further signal from said mobile out station or said base station.

8. Connection means according to any one of the preceding Claims, wherein said reference frequency is 25 Hz, and said predetermined tolerance of each of said detectors is 12 Hz.

9. Connection means for a communication link substantially as hereinbefore described with reference to the accompanying drawings. DATED this FOURTH day of FEBRUARY 1988 OVERSEAS TELECOMMUNICATIONS COMMISSION (AUSTRALIA) Patent Attorneys for the Applicant SPRUSON FERGUSON IIL SI- GMG/296r 8 i