GB2277658A - Arrangement for transmitting information to aircraft - Google Patents

Description

4 2277658 1 PROCESS AND ARRANGEMENT FOR TRANSMITTING INFORMATION The

invention relates to a process in accordance with the preamble of claim 1. The invention also relates to an arrangement in accordance with the preamble of claim 9.

There is a difference between various call connections within the context of air traffic control; namely a ground-ground connection and a groundaircraft connection. The ground-ground connection is necessary., for example, for transfer of an aircraft from one air traffic controller to another, for example upon changing from one sector to another. The groundaircraft connection is particularly required for transmission of air traffic instructions from the air traffic controller to the crew.

In order to cover a defined airspace optimally in terms of radio engineering, unmanned transmission stations are spatially separated from the air traffic control centre, and the distance between the air traffic control centre and the respective transmission station can easily be several hundred kilometres. The connection between the air traffic control centre and the transmission station is provided using digital or analogue lines; as an 4 2 exception, directional radio links can also be provided to this end. The lines provided are generally post rental lines in which various safety requirements with regard to line defects and flight radio transmitter defects have to be provided for currently existing speech-transmission systems which are required to carry out air traffic control; additionally, considerable requirements are made of such arrangements with regard to rapid readiness for calls and availability for call connections.

In this type of arrangement a particular transmission frequency is provided for a respective single line with a connected radio transmitter; a number of reserve lines and/or reserve transmitters are also provided in case the lines and/or radio transmitters fail. In this type of configuration, for example, given defective lines, rigid switching can take place to an equivalent, up to the total number of available reserve lines; "rigid equivalent switching" is to be understood as permanent reservation of a line with a connected transmitter for a particular transmitting frequency, independently of whether a call is taking place on the line or not. If there are more defective lines or transmitters than there are reserve lines, it is not possible to use all the necessary transmission frequencies, as each further line failure also causes the failure of a transmitting 3 frequency or a frequency channel.

A special case makes provision for the number of reserve lines and/or reserve radio transmitters to be equated with the number of lines or transmitters required; however, this arrangement of reserve lines and reserve transmitters entails a considerable outlay on lines and transmitters, and consideration must also be given to the fact that in currently available systems only 30% of the transmitting capacity is taken up or used, even in cases of heavy demand.

In accordance with the invention, a process with a simplified yet increased operational safety is characterised by the features listed in the characterising part of claim 1. An arrangement in accordance with the invention is characterised by the features listed in the characterising part of claim 9.

In the method of the invention, one of the number of available free line paths is made available precisely when it is required; as a rule a number of unused line paths are generally available for use. Availability is made possible independently of the frequency on which the call is to take place, as multichannel transmitters are used, with which transmissions can be made on any frequency or any frequency channel. Hence in the ground 4 station a line pAth With g connected multi-channel transmitter is associated with a call made by an air traffic controller, in that the computer in the ground station, or a corresponding control unit, selects a free line path from the number of line paths available and sets the corresponding frequency information on this line path for a radio transmitter. The said line path can be selected at random by a selection unit from the number of available and free line paths, so that transmissions can be made via any free line path at any frequency, so long as one single line path remains available for transmitting.

In contrast to current speech transmission systems in which the line path and the transmitter frequency are rigidly associated, or only the flight radio transmitter is switched up by actuation of the microphone button or PTT button (push-to-talk button), a priori neither the line path or the transmitting frequency is established before actuation of the microphone button or PTT button, but instead the line path is established by the ground station computer or the control device, and the transmitting frequency is established by the air traffi controller before the start of the call and is conveyed to the respective radio transmitter by actuation of the microphone or the PTT button.

1 1 In the usual process, the speech and the switch-up signal in the transmitter are inseparably connected, whereas in the inventive process the speech, the switch-up signal and the transmitter frequency information are to be considered, or can be linked to one another, provided that the transmitter frequency information is not transferred with the switch-up signal to the transmitter upon commencement of the speech transmission; this is advantageous as the air traffic controller firstly actuates the microphone or PTT button and does not begin to speak for some time; if he begins his call then the transmitter is already prepared to transmit on its predetermined frequency, which was conveyed together with or directly after conveyance of the switch-up signal. Hence it is advantageous to link transferral of the frequency information with the actuation of the microphone or the PTT button.

In usual systems, each flight radio transmitter in a transmitting station is set to a particular transmitting frequency, which means that a signal present at the input to the flight radio transmitter is radiated at the single transmitting frequency predetermined for the radio transmitter. If the connection to this radio transmitter is interrupted or if the transmitter is defective, then operation is not possible at this frequency if no equivalent line, or equivalent 6 transmitter matched to the same frequency, is available. In the process of the invention none of the flight radio transmitters are associated with a particular transmission frequency (or vice versa); instead the transmission frequencies are set for the radio transmitter immediately prior to the call, and it is irrelevant which of the flight radio transmitters is to transmit on the transmission frequency, as each flight radio transmitter is oriented as a multichannel transmitter to transmit at any frequency.

The choice of transmitter frequency is dependent upon circumstances or flight traffic regulations, which, for example, state which frequencies are to be used in which regions or areas. Hence it is possible for an air traffic controller to make a call at a first frequency via a first line path, and for his next call to be made on the same line path or a different line path at a different frequency. It would also be possible to carry out a conversation on a different line path at the same frequency.

The result is substantially improved use of the line paths and radio transmitters to be installed. The problems in relation to equivalent line paths and/or transmitter-failure can be minimised simply, as upon failure of a line path or a transmitter, all the other 1 7 line paths and transmitters present are fully available; finally, the transfer capacities, for which experience has shown a low level of use, can be better exploited. The process of the invention and/or the arrangement of the invention thus provide considerable savings in line paths and radio transmitters without resultant losses with regard to safety.

In accordance with the invention, it is possible to connect a number of line paths in a ground station, the paths having their own respective radio flight transmitter connected thereto. Hence there is the possibility of selecting the line paths with connected flight radio transmitters by means of a corresponding selection unit in the ground station.

Another possibility is that optionally, additionally to the above possibility, a number of transmission stations each comprising several flight radio transmitters are connected to a ground station. A transmission station of this kind can comprise a number of flight radio transmitters. Each one of these transmission stations can be connected to the ground station via a certain number of line paths, the said number of lines not having to correspond to the number of flight radio transmitters provided in the transmission station. The number of line paths provided can be more than, the same 8 as or less than the number of flight radio transmitters provided in the transmission station. It is advisable for selection of the line paths to take place in the ground station in configurations of this type, but for the selection of the flight radio transmitter to take place in the transmission station. To this end a separate control centre can be provided in the transmission station to associate a flight radio transmitter with the line path selected by the ground station, the flight radio transmitter having been selected from the number of available flight radio transmitters by the control centre in the transmission station.

It is also important for the radio transmitter to be switched up rapidly, and to be set simultaneously for the selected transmitting frequency. In accordance with post standards in force, band filters are to be used to prevent unwanted intermodulation products, the filters being set rapidly, in accordance with the invention, to the selected frequency range.

Preferred embodiments of the invention can be seen from the following description, drawings and claims.

The invention will subsequently be described in greater detail with reference to the drawings. Figs. 1 and 2 4 9 show preferred embodiments of the invention; figs. 3 and 4 show preferred embodiments of transmission stations.

The drawing shows, quite generally, an arrangement for conveying information from a ground station Z to an aircraft 19. The ground station is an air traffic control centre, generally designated as Z, to which lines or line paths 8 are connected, and are generally designated as.L, and produce the connection to flight radio transmitters, which are generally designated as S.

In this case six microphones 1 are provided in the ground station Z for air traffic controllers and are supplied with corresponding microphone buttons or PTT buttons 11. The number of microphones 1 provided can vary and is adapted to requirements or to the size of the ground station. Each of the microphones 1 is connected to a speech transmission system or a computer or a control device 3, via operating-position electronics 2. The said control device 3 at present comprises, quite generally, a selection unit 4, a frequency pre-setting unit 5 and a linking unit 6.

It is to be noted that instead of, or as well as, the microphone(s) 1, other devices for issuing signals can be provided, the signals being for transfer to an aircraft; in this case the speech transmission system 3 is set so as to forward corresponding data and/or signals. This forwarding, however, takes place in the same manner as the forwarding of speech information. A number of line paths 8 are connected to the speech transmission system 3 via a number of line interfaces 7, the paths leading to a respective transmitter S positioned at a distance from the ground station Z. These line paths 8 can be formed by cables or directional radio links; each line path 8 can, for example, have an overall capacity of 64 kBit; each line path 8 can be divided into line channels, which each have a transfer capacity of 16 kBit. The transfer of speech in PCM mode and the control signals for the transmitter S can thus be divided over different parallel channels.

The line path 81 is merely for return of checking signals, with which the arrival of a signal or a piece of information at the transmitter S is checked; failure of corresponding responses via the line path 81 leads to the assumption that the line path 8 or the transmitter S has failed.

Each line interface 7 supplies a line path 8 which leads directly to a single transmitter S, which is permanently associated with the said line path 8.

11 It can further be seen that the number of microphones 1 or controller places provided can be selected to be considerably higher than the number of available line paths 8 or the number of radio transmitters connected to the said paths 8. This results from the fact that in the usual systems the capacity exploitation of the line paths 8 and the transmitters S did not even reach 40%, so that the number of line paths 8 and transmitters S can therefore be reduced. However, a further result of the invention is the absence of equivalent line paths and equivalent radio transmitters, as all the information for transmission can be sent on all the line paths 8 via the respective transmitter connected thereto, at any frequency. This is achieved in that each of the transmitters provided, which is supplied by a line path 8, is a multichannel transmitter which in particular can transmit all the necessary flight radio channels.

If an air traffic controller then wishes to make contact with an aircraft 19, he actuates the PTT button 11 of the microphone 1, whereupon the selection unit 4 associates the speech with a free line path 8; if no free line path is available the speech is deferred, or in the case of particular urgency another line path 8 is freed.

12 If a free line path 8 of this type is thus available and associated with the speech, then the frequency information will be transmitted by the frequency determination unit 5, either immediately, because of actuation of the microphone button or PTT button 11, via the line path 8 to the transmitter S, or optionally linked to the switch-up signal, so that the call begun by the air traffic controller can immediately be despatched by the radio transmitter on the correct frequency. To this end multichannel transmitters are used, with a switch-up time T or a setting time for the transmission frequency T < 40 ms, so that even if speech is undisciplined, the beginning of the call is not lost.

For reasons of simplicity only one radio transmitter S is shown in the drawing; however, it is quite clear that each of the three line paths 8 shown are connected to a transmitter S of this type. The ratio of the number of microphones 1 to the number of line paths 8 with connected transmitter S is adapted to circumstances.

A line interface 9 is provided in the flight radio transmitter S, and transmits the information entering on the line path 8 to an interrupt facility 10 which delivers the arriving speech information to a modulation device 12 via a unit 11 for speech preparation. The speech information is transferred to an RF amplifier 16 13 from the modulation device 12.

The transmitter switch-up signal and/or the frequency information are delivered by the interrupt facility 10, via a frequency recognition unit 14, to the synthesiser 15 in the radio transmitter S, which influences the modulation device 12 and the RP amplifier 16. The transmitting aerial 18 is then supplied by a filter 17 coupled back to the modulation device 12.

It is clear that in principle the radio transmitter can also be differently constructed; however, the essential feature is the circuit module for recognition and setting of the conveyed transmitting frequency, and also the fact that the transmitter is a multichannel radio transmitter. It must also be possible for the speech information to be modulated upon the predetermined transmission frequency; preparation of speech for the high-frequency portion substantially takes place in the unit 11.

The frequency information, the switch-up signal and the speech information can be linked in various ways; however, despatch of the frequency information preferably takes place simultaneously with or immediately after despatch of the transmitter switch-up signal, and not at the beginning of transmission of 14 speech information. Because of the very rapid setting time of the radio transmitter, which together with selection of the line path and switching- up is < 100 ms, prompt speech transmission can take place at the desired frequency.

After the end of a call the line path 8 and the radio transmitter can immediately be used for transmission on another frequency.

Responses from the aircraft 19 to the air traffic controller take place via corresponding receivers, which are not causally related to this process, which only deals with transmission of information from the ground station Z to the aircraft 19, and are solved in the usual manner.

The process of the invention can be used on analogue lines (300 Hz - 3400 Hz) or on digital lines (64 kbit) with standardised interfaces. However, in the case of analogue lines, the frequency band width is somewhat limited as a result of in-band signalling, in case no separate line paths can be made available for the transmission of the PTT and frequency information; in digital lines this information can also be transferred to a separate channel, for example a 16 kBit channel, which does not entail any restriction of the speech band width.

It is essential to the invention that each line path 8 and each radio transmitter is only associated with a particular transmission frequency during a call, and after ending the call the line path 8 and the connected radio transmitter can be used for an entirely different frequency.

Selection of the line paths 8 can be undertaken by the selection unit 4 according to various criteria, for example the choice can be made statistically or according to probability criteria, or according to a predetermined plan, for example in turn. All lines are of equal value or are considered as of equal value; none of the lines are reserved as equivalent lines. This means that the arrangement in accordance with the invention is provided without reserve lines and without reserve radio transmitters; instead, frequencies and line paths 8 with connected radio transmitters are associated dynamically with the individual calls to be made.

The selection unit 4 recognises and stores the altering state of the line paths 8, i.e. it recognises whether the line paths 8 are defective, engaged or available. Provided that the selection unit 4 recognises or is 16 informed of the actuation of a PTT button 11, it will associate with the call the next line path which its memory shows to be free, and/or control the corresponding interface 7, and/or ensure that the switch-up signal and the frequency information is delivered from the frequency pre-setting unit 5 via the said interface 7 to the selected line path 8 which is reserved for the overall duration of the call, i.e. until release of the PTT button 11.

The linking unit 6 deals with the temporal course of the line association, transmission of frequency information, transmission of switch-up signals and transmission of speech information. These processes can be carried out electronically in various ways. With its arrangement of two flight radio transmitters, the arrangement of the invention is already far superior to a standard arrangement, comprising a used line and a rigid equivalent line path, as substantially more calls can be transmitted via the two alternately used line paths, than is the case using one line path and an associated replacement line path.

With the aforementioned embodiments as a basis, further embodiments of the invention will now be explained, with reference to figs. 2, 3 and 4.

17 Fig. 2 shows an arrangement in accordance with the invention, wherein a number of flight radio transmitters S are supplied from a centre, designated in general as Z, via line paths 8, 81, generally designated as L. The said flight radio transmitters S are combined into individual radio stations FS. The radio station FS shown at the top comprises five flight radio transmitters S which can be supplied via six line paths 8, 81. The radio station shown in the centre comprises four flight radio transmitters S, which can be supplied by four line paths 8, 81, and the radio station FS shown at the bottom comprises four flight radio transmitters S, which can be supplied via two line paths 8, 81.

The individual radio stations FS can thus comprise any number of flight radio transmitters S and are supplied via any number of line paths 8, 81, and the number of line paths can be more than, the same as or less than the number of flight radio transmitters S. The number of line paths and flight radio transmitters is selected according to requirements and/or safety criteria.

Thus, in the case of fig. 2, the line paths 8, 81 in the centre are advantageously selected by a corresponding selection unit 4; association of the flight radio transmitters S located in the radio station FS can also take place by means of this selection unit 4 or can take 18 place by means of a selection unit transferred into the radio station FS, which, for example, can be arranged in the unit 10 or in the control centre SZ of the radio station. Thus, after a piece of information for transmission has arrived in the radio station on one of the line paths 8, 81, at this point there firstly takes place association of a free flight radio transmitter S. In this embodiment, greater flexibility is achieved than is the case with an embodiment in accordance with fig. 1.

Fig. 3 shows schematically the construction of a radio station, comprising a number of flight radio transmitters S.

Pieces of information arrive from the ground station or the centre Z via line paths 8, 81, and are divided at an intelligent line interface 9 and/or an interrupter facility 10 including the said line interface 9, into speech and control data (in digital form). It is to be noted that information data and control data, and, if need be, pictorial information and so forth can be transferred as pieces of information for transmission. The speech or information data is delivered to the digital interface 20 in the flight radio transmitter S, which if necessary can also contain a unit 11 for preparation of the speech and information data. The control data is delivered to the control centre 26, 261 1 19 which for safety reasons is provided in duplicate, via the data bus 25. The following items of information are substantially included in the said control data: channel frequency, transmitter switch-up (PTT signal), priority of information. The control centre SZ and/or the active centre 26 or 261 selects a free flight radio transmitter S, and forwards to the control processor 21 therein the frequency information, the switch-up information and/or the PTT signal and/or the corresponding information, via a bus interface 22. In this case the frequency information is forwarded to the associated filter 17 via a bus interface 22 and a control unit 23 for the filter 17.

The control processor 21 of the flight radio transmitter S sets the synthesiser 15 to the channel frequency and switches the digital interface 20 through to the modulator 12. The modulated frequency signal is delivered to the filter 17 via an RF amplifier 16, and the filter places a strict limit on the frequency range to be transmitted, to prevent inter-modulation products.

Further flight radio transmitters S can be connected via the data bus 25 to the control centre SZ and/or to the line interface 9 and/or the interrupter facility 10, and undertake the corresponding data exchange. The said further flight radio transmitters S are supplied with corresponding speech and/or data signals via the lines 27, the said signals having been separated from the control data in the interrupter facility 10. As previously mentioned, the respective flight radio transmitters S are selected from the control centre or by the interrupter facility 10 to transmit the information.

Fig. 4 shows schematically a radio station comprising a somewhat altered flight radio transmitter S- The difference to the embodiment shown in fig. 3 is that the amplifier 16 is positioned subsequently to the filter 17. Transfer of the transmission frequency takes place from the control centre via the bus interface 22 to the control processor 21, which for its part controls both the synthesiser 15 and the control electronics 23 for the filter 17.

The filter 17 is set to the corresponding frequency within the shortest time (less than 40 ms) so that the flight radio transmitters is ready for use in the shortest time.

Advantageously, provision is made for release, for switching through the speech and/or data information to the aerial 18, to only take place once frequency setting has been undertaken.

21 Key to captions on Figs. 3 and 4 A Speech or data radio equipment B Automatic filter C Aerial D Digital interface E Modulator F RF amplifier G Filter H Speech or data (digital) I Synthesiser J Control unit K Control processor A Speech or data radio equipment B Aerial C Digital interface D Modulator E Filter F RF amplifier G Speech or data (digital) H Synthesiser I Control electronics J Control processor K Bus interface Fi g. 3 Fi g. 4 L Bus interface M Bus interface N Lines 0 Data bus P System A, Central computer unit Q Intelligent line interface R System B, Central computer unit S High-power computer unit (in duplicate) L Further devices M Lines N Data bus 0 Intelligent line interface P System A, Central computer unit Q System B, Central computer unit High-power computer unit (in duplicate) 22

Claims (1)

1. Claims:

   1. A process for transmitting information (signals and/or speech) from ground stations to aircraft, in particular for flight radio control centres, wherein a number of flight radio transmitters for transmission of information to the aircraft are associated with the ground station, and pieces of information or messages (speech, data, signals etc.) for despatch are respectively transmitted from the ground station to the radio transmitters via at least one line path (line, directional radio link), characterised in that a selected transmission frequency, or a frequency pre-determined by the air traffic controller, is associated with each piece of information for despatch or each call to be made, and is transferred to the radio transmitter, and in particular is pre- determined for the duration of the information transmission or the call, and in that any free line path is selected from a plurality of available line paths for the transfer of the information or the call from the ground station to the flight radio transmitter, and is made available for the duration of the transfer of the information or the call.

   A process in accordance with-claim 1, characterised 1 23 in that the selection of the respective line path In the ground station, and the selection of the radio transmitter from which despatch of information is to take place, occurs in a transmission station comprising a plurality of radio transmitters in the transmission station.

   A process in accordance with claim 1, characterised in that the selection of a line path, with a radio transmitter connected thereto, takes place in the ground station.

   4.

   A process in accordance with one of claims I to 3, characterised in that the association of the free line path with the radio transmitter, and the transfer of the selected and/or predetermined transmission frequency to the radio transmitter for transmission of information or a call, takes place or is initiated immediately before or upon the beginning of speech, particularly upon pressing a microphone button or PTT button, and, in particular, the association of the line path and/or the radio transmitter is maintained until the end of transmission and/or the end of the call, more particularly until release of the microphone button and/or the PTT button.

   24 A process in accordance with one of claims 1 to 4, characterised in that the frequency information is transmitted on the selected line path to the radio transmitter before transmission of the speech information, more particularly simultaneously with or immediately after pressing of the microphone button and/or PTT button, or simultaneously with or directly after transmission of the switch-up signal for the radio transmitter, despatched by pressing the microphone button and/or PTT button.

   6. A process in accordance with one of claims 1 to 5, characterised in that the frequency information for the radio transmitter is linked to the switch-up signal or to the beginning of the information when it is transmitted on the selected line path.

   7. A process in accordance with one of claims 1 to 6, characterised In that the transmitted information is split into frequency information and speech.

   8. A process in accordance with claim 7, characterised in that by means of the frequency information, a settable filter is controlled and is set to the respective transmission frequency.

   9. An arrangement for transmitting information from ground stations to aircraft, particularly for air traffic control centres, wherein a number of flight radio transmitters for despatching information to aircraft are connected to the ground station, and pieces of information for despatch are respectively transmitted from the ground station to the flight radio transmitters via at least one line path, particularly for carrying out the process in accordance with one of claims 1 to 8, characterised in that multichannel transmitters, particularly multichannel transmitters including all the necessary flight radio channels, are provided as flight radio transmitters (S), and in the ground station M or, in the case of a radio station (FS) comprising a plurality of flight radio transmitters, in the ground station (Z) and/or in the radio station (FS), a selection unit (4) is provided, by means of which any respective free line path (8) from the number of available line paths (8) can be selected or can be made available for transmission of information, and a frequency pre-setting unit (5) is provided, via which the frequency information selected for the transmission of information, or specified for the radio transmitter (5) connected to the line path (8), can be transmitted via the said line path (8) which was selected, and the said frequency information can optionally be transmitted 26 simultaneously with the switch-uP signal or with the beginning of the information for transmission.

   10. An arrangement in accordance with claim 9, characterised in that the selection unit (4) and the frequency pre-setting unit (5) are coupled to the microphone button and/or PTT button (11).

   An arrangement in accordance with claim 9 or 10, characterised in that a linking unit (6) is provided in the ground station (7) for the frequency information and the switch-up signal or the speech information.

   12. An arrangement in accordance with one of claims 9 to 11, characterised in that an interrupt facility (10) for the arriving piece of information is provided in the radio transmitter (S) or in the radio station (FS), and with the said facility the frequency information and the switch-up signal can be delivered to the synthesiser (15) and the speech information can be delivered to the modulator (12) in the radio transmitter (S).

   13. An arrangement in accordance with one of claims 9 to 12, characterised in that the frequency pre-setting unit (5) transfers the frequency 27 information on the selected line path (8), upon actuation of the microphone button or the P= button (11), prior to the beginning of the transmission of speech.

   14. An arrangement in accordance with one of claims 9 to 13, characterised in that transmission of the speech information and the frequency information takes place separately, on parallel channels of the selected line path (8).

   15. An arrangement in accordance with one of claims 9 to 14, characterised in that transmission of information on the line paths (8) takes place in compressed PCM form.

   16. An arrangement in accordance with one of claims 9 to 15, characterised In that the number of line paths (8) corresponds to the number of radio transmitters (S), or is less than the number of radio transmitters, and is less than the number of microphones (1) or air traffic controller places or connected call stations.

   17. An arrangement in accordance with one of claims 9 to 1 16, characterised in that multichannel transmitters, preferably for all given air traffic 28 control frequency channels, are provided with a transmission frequency setting time of T<40 ms.

   18. An arrangement in accordance with one of claims 9 to 17, characterised in that between the modulator (12) and the aerial (18) each flight radio transmitter (S) preferably has a filter (17) which can be acted upon by the frequency information and which can be matched to the said frequency.

   19. An arrangement in accordance with one of claims 9 to 18, characterised in that an amplifier (16), more particularly an RF amplifier, is connected before or after the filter (17).

   20.

   An arrangement in accordance with one of claims 9 to 19, characterised In that the filter (17) can be set to the selected or specified frequency range immediately after arrival of the frequency information and the switch-up signal, preferably within a maximum period of 0.08 s, preferably a maximum of 0.04 s.

   21. An arrangement in accordance with one of claims 9 to 20, characterised In that the filter (17) can be set by a control unit receiving the frequency information and the switch-up signal.

   29 22. Use of multichannel transmitters with a transmission frequency setting time of T<40 ms for a process in accordance with one of claims 1 to 6, and/or an arrangement in accordance with one of claims 7 to 13.

   23. A process for transmitting information from ground stations to aircraft, the process being substantially as hereinbefore described with reference to any of the accompanying drawings.

   24. An arrangement for transmitting information from ground stations to aircraft, the arrangement being substantially as hereinbefore described with reference to the accompanying drawings.