RU2286030C1 - High frequency system and method for exchanging packet data - Google Patents

Abstract

FIELD: automatic adaptive high frequency packet radio communications.

SUBSTANCE: each high frequency ground station contains at least one additional high frequency receiver for "surface to surface" communication and at least one additional "surface to surface" demodulator of one-tone multi-positional phase-manipulated signal, output of which is connected to additional information input of high frequency controller of ground station, and input is connected to output of additional high frequency "surface to surface" receiver, information input of which is connected to common high frequency receiving antenna, while control input is connected to additional control output of high frequency controller of ground station.

EFFECT: prevented disconnection from "air to surface" data exchange system of technically operable high frequency ground stations which became inaccessible for ground communications sub-system for due to various reasons, and also provision of possible connection to high frequency "air to surface" data exchange system of high frequency ground stations, having no access to ground communication network due to absence of ground communication infrastructure at remote locations, where these high frequency ground stations are positioned.

2 cl, 12 dwg, 2 tbl

Description

The invention relates to an automatic adaptive packet RF radio communication.

An analysis of the features and development trends of high frequency (HF) radio communication systems (3-30 MHz) shows that ensuring guaranteed reliability, efficiency and reliability of HF radio communication is possible only with the introduction of technology for the automatic management of adaptive HF radio communication, which involves the integrated use of several technologies:

- technologies for automatic communication channel compilation (ALE) with automatic selection of the working frequency (ACS);

- technologies for high-speed data transmission using adaptive reception methods with channel alignment, direct error correction (FEC) and automatic request for repeating packets received with uncorrected errors (ARQ);

- technologies for multi-parameter adaptation of a radio line in terms of data transfer rate, types of modulation and coding (ALM);

- frequency control technologies in a communication system (ionospheric monitoring) based on long-term forecasting and operational forecasting of radio wave propagation conditions using vertical and oblique sounding of the ionosphere, as well as special software for calculating ionospheric paths and selecting communication frequencies;

- spatial diversity technology for terrestrial HF radio centers (stations).

There are no Russian standards for technology for the automatic management of adaptive RF radio communications. This impedes the development of Russian HF radio communication systems, which lag behind foreign characteristics in terms of basic characteristics, and leads to the lack of interaction between different HF radio communication systems.

In the world, there are two standard technologies for automatic RF radio communication, namely:

- HFDL technology (High Frequency Data Link), based on the ARINC 635 specification [3], ARINC 753 specifications [6], ARINC 634 guidelines [2], RTCA standards DO-265, DO-277 [8.9];

- military standard MIL-STD-188-141B [11] and its civilian version FED-STD-1045.

The MIL-STD-188-141B technology supports a radio link (correspondent-correspondent communication), a correspondent-to-many correspondent radial network, and a one-to-one fully-connected network and has universal application, i.e. can be used for communication "Earth-Earth", "Air-Earth", "Air-Air", "Shore-Sea", "Sea-Sea", "Air-Sea". It describes the procedures for automatically composing a communication channel with auto-selection of the operating frequency. MIL-STD-188-141B uses two sets of frequencies - one for composing a channel (for a calling network) and the other for communicating. The frequency resource can be redistributed between these sets depending on the nature of the traffic and the intensity of the application flow. Moreover, there are complex ambiguous relationships between the parameters of the call network and the communication network. In the MIL-STD-188-141 B communication system, the duration of the connected message is not fixed, it depends on the amount of transmitted data, which makes it difficult to organize the TDMA protocol in the communication network and to predict system characteristics for network optimization. However, it is possible to predict performance in a call network using standard command messages and the TDMA protocol. Protocols for automatic communication with multi-parameter adaptation are not specified by this standard, as well as aspects of the network. This makes the MIL-STD-188-141B technology more flexible in use, but complicates the design of specific systems and the calculation of system characteristics.

The HFDL technology (ARINC 635) optimizes, in terms of communication reliability, spectral and economic efficiency, the Air-to-Earth packet communication system in which a large number of aircraft (up to 2500) are served by a small number of frequency channels (up to 48-60) and ground stations (up to 16) in the multiple access mode with time and frequency division. HFDL defines both channel compilation procedures with auto-selection of the operating frequency, and all other automatic communication procedures at all levels (physical, channel and subnet) with multi-parameter adaptation of the radio line in frequency, transmission speed, types of modulation and coding, as well as spatial diversity of terrestrial stations guaranteeing reliability (residual probability of error) not worse than 10 ^-6 . The HFDL system uses the same set of frequencies for channelization and communication. High spectral efficiency of the system is achieved through the use of the combined protocol of multiple access to the channel with frequency (FDMA) and time (TDMA) separation. The frequency separation protocol is ensured by the fact that different frequency channels (from two to six) are assigned to different HF ground stations (HF NS). The TDMA protocol is ensured by the fact that the usage time of each frequency channel is divided into 32-second frames, and each frame is divided into 13 temporary access slots with a duration of 2.461538 s, equal to the transmission time of one data packet 2.343888 s plus 117.65 ms per propagation delay time uncertainty and desynchronism in a radio link. At all HF frequencies, ground stations periodically (in the first slot of each frame) emit marker signals, the quality of which is estimated by airplanes when choosing a communication frequency. The aircraft selects for communication any channel whose marker signal quality is acceptable or best, is recorded on this channel at the ground station and communicates on it until the channel quality meets the required level. Multiple aircraft can choose one communication channel and register on it. Each HFDL HF channel of a system is used by all aircraft registered on it in time division multiple access mode. The TDMA protocol is controlled by the HF ground station, transmitting slot assignment signals reserved on-board, random access slots, and ground transmission slots in signals. An HF ground station predicts system characteristics (packet transmission delay) on each of its frequency channels and sets the channel busy flag in the marker when a critical number of planes have registered on the channel to stop new correspondents from accessing it and guarantee the specified system characteristics (packet transmission delay of not more than acceptable). The simplicity of predicting system characteristics in the HFDL system is largely determined by the fact that all messages in the system (ringing and communication) have the same standard duration equal to a slot and are transmitted using the same TDMA protocol on a common set of frequencies. Depending on the quality of the channel and the amount of data transmitted in the message, the optimal type of multi-position phase manipulation and coding is selected. At the same time, the user data rate changes, but the message duration and the symbol rate of 1800 Baud do not change.

Thus, HFDL technology maximizes the efficiency of using a limited set of frequencies allowed for communication, since it provides services for a large number of aircraft on one frequency channel (on average up to 26 correspondents) and uses one set of frequencies, both for communication and for calling. This technology of automatic adaptive RF radio communication provides the integrated use of all promising RF communication technologies, which makes the RF radio channel an attractive and effective means of long-distance communication in terms of reliability, simplicity, ease of use, and bandwidth comparable to a satellite channel, but one or two orders of magnitude cheaper [ 10].

Taking into account the advantages of the above-described HFDL packet data transmission system [1-7] for organizing Air-to-Earth communications in comparison with all currently known communication systems for authors, it is taken as a prototype of the proposed aviation RF packet data exchange system.

The HFDL Air-to-Earth HF packet data exchange system, which provides communication between long-range airplanes and air traffic control (ATC) and air traffic control (UAL) control centers in the interest of air traffic safety, is described in detail in [1, 2, 4, 5 , 6]. The method of data exchange in the HFDL system is described in detail in [3, 4].

The structural diagram of the HFDL packet data exchange system is shown in FIG. 1, (see [7], p. 3, [5], p. 10), where it is indicated:

1 - HF airborne station (HF BS);

2 - HF ground station (HF NS);

3 - control center (CC) HF data exchange system;

4 - air traffic control and air traffic control center (ATC and UAL);

5 - subsystem of ground communication used by the HFDL RF system;

6 - interface HF NS with the subsystem of ground communications;

7 - the interface of the control center with the subsystem of ground communications;

8 - the interface of the air traffic control and UAE points with the ground communication subsystem;

9 - HF radio channel "Air-Earth" between the HF NS (2) and HF BS (1).

HF airborne stations (1) are connected via HF radio channels (9) to HF ground stations (2) connected via interfaces (6) to the ground communication subsystem (5), which in turn is connected via interfaces (7) to the control center (3 ), and through interfaces (8) with air traffic control units and UAL (4).

The users of the HFDL system are air traffic control and UAE control posts (4) on the ground and avionics on-board equipment connected to the HF airborne station via the airborne router.

The structural diagram of the HF airborne station (1) is presented in figure 2, where it is indicated:

10 - on-board HFDL RF transceiver;

11 - on-board antenna matching device (ACS);

12 - onboard HF antenna;

13 - control panel of the transceiver (PU);

14 - onboard router (BM);

15 - control device RF data exchange;

16 - RF transceiver ARINC 719.

Moreover, the on-board HFDL RF transceiver (10) consists of the ARINC 719 RF transceiver (16) and the RF data exchange control device (15), which contains a modem and an HFDL data exchange protocol controller.

In this case, the RF transceiver (16) is connected on the one hand to the ACS (11), and on the other hand to the RF communication control device (15). The RF communication control device (15) is connected on the one hand to the RF transceiver (16), and on the other hand to the control panel (PU) (13) of the radio station and to the on-board router (BM) (14). ACS (11) is connected on one side to the onboard RF transceiver (16), and on the other hand to the RF onboard antenna (12).

Structurally, the RF data exchange control device (15) can be made both in the form of an independent unit, connected externally to the RF transceiver (16), and in the form of a subunit or boards built into the transceiver (16).

The RF airborne station (1) is connected to the airborne router (BM) (14) (a communication management unit that complies with the ARINC 724 or 758 specifications), which sends (receives) packet messages to airborne sources / receivers of information (avionics) such as a multifunction console control, display, printer, computer, maintenance system, airborne navigation system, electronic indication and alarm system, navigation system, etc. The on-board router (14) is connected not only with the HF on-board station (1), but also with the on-board stations of other frequency ranges.

The structural diagram of the HF ground station (2) HF packet data exchange system HFDL (see [7], p. 60; and [5], p. 12) is presented in figure 3, where it is indicated:

17 - HF radio transmitter;

18 - RF transmitting antenna;

19 - HF radio receiver "Air-to-Earth";

20 - RF receiving antenna;

21 - RF modulator single-tone signal of multi-position phase shift keying;

22 - RF air-to-ground demodulator of a single-tone signal of multiposition phase shift keying;

23 - the controller of the HF ground station (2);

24 - receiver of signals of a single time;

25 is an interface device with a terrestrial communication network;

26 - a receiving antenna of signals of a single time.

HF ground station (2) The HFDL HF packet data exchange system includes:

- N RF transmitters (17) associated with N transmitting RF antennas (18), a controller (23) of the RF ground station (2), and N RF modulators (21);

- N RF receivers "V-3" (19) associated with a common RF receiving antenna (20), a controller (23) of the RF ground station (2) and N RF demodulators "V-3" (22);

- N RF modulators (21) associated with N RF transmitters (17) and the controller (23) of the RF ground station (2);

- N RF demodulators "V-3" (22) associated with N RF receivers "V-3" (19) and the controller (23) of the RF ground station (2);

- a controller (23) of the HF ground station (2) associated with N HF receivers "V-3" (19), N HF transmitters (17), N HF modulators (21), N HF demodulators "V-3" (22 ), a single-time signal receiver (24), an interface device with a terrestrial communication network (25);

- an interface device with a terrestrial communication network (25) connected to a HF NS controller (23) on the one hand and, on the other hand, to a terrestrial communication network through an interface (6);

- a receiver (24) of signals of a single time associated with the controller (23) of the HF NS (2) and with a receiving antenna of signals of a single time (26);

- a public RF receiving antenna (20) connected to V-3 RF receivers (19);

- a receiving antenna (26) of a receiver (24) of signals of a single time, connected to a receiver (24) of signals of a single time;

- N transmitting RF antennas (18) connected to N RF transmitters (17).

A variant of the structural diagram of the ground communication subsystem (5) used by the HFDL RF packet data exchange system recommended by the Aeronautical Mobile Communications Committee (AMSR) [5] is presented in Fig. 4, where it is indicated:

2 - HF ground station (HF NS);

3 - control center (CC) HF packet data exchange system HFDL;

4 - air traffic control units and UAF (ground users of the HFDL packet data exchange system);

6 - the interface of the HF NS (2) with the ground communication subsystem (5);

7 - interface of the control center (3) HFDL with the ground communication subsystem (5);

8 - interface of air traffic control and airline (4) with ground subsystem (5);

27 - regional routers of a land communication network (5);

28 is an interface between regional routers (27).

The terrestrial communications subsystem (5) used by the HFDL HF packet data exchange system contains regional routers (27) interconnected by interfaces (28), HF NS (2) interfaces (6), and the control center (3) interfaces (7) , with control rooms (4) interfaces (8).

The HFDL RF packet data exchange system provides a method for exchanging data packets between on-board users of the said system (avionics) connected to the HF airborne station (1) via the on-board router (14) and ground users of the HFDL RF packet data exchange system (air traffic control centers and UAF (4)), as well as the control center (3) of the RF packet data exchange system as follows.

To ensure a given level of communication reliability in the area of responsibility of each HF ground station (2) from the general list of HF frequencies (48-60 OBP channels) allocated to the HFDL system, in the control center (3) an HF data exchange system is assigned for each HF ground station (2) for each time interval of a day with a duration of 1-2 hours, a set of 2-6 active frequencies, optimal according to the conditions of propagation of radio waves and electromagnetic compatibility, bring the assigned set of frequencies along with the time interval of its activation to each HF ground station AI (2) through the ground communication subsystem (5), thus realizing the frequency-division multiple access protocol, divide the time of use of each frequency channel into time frames of 32 s duration, and each frame is divided into 13 time slots of 2.461538 s duration to implement a time division multiple access (TDMA) protocol.

At the end of each frame at each HF ground station (2), for each slot of the next frame, an assignment is made to use this slot for transmission from the ground or for transmission from a specific board upon its preliminary request for an access slot, or for transmissions from any side in random access mode.

From each HF ground station (2), at all active frequencies in the first slot of each frame, marker signals are emitted that contain the destination of each slot in the current frame, as well as receipts for messages received from "Air" in the previous two frames.

At each HF airborne station (1), according to the results of evaluating the quality of the reception of marker signals, the best communication frequency is selected (HF radio channel Air-to-Earth (9)).

Each HF airborne station (1) is registered on the HF channel (9) selected by it on the HF ground station (2) corresponding to this channel, packet data is exchanged in TDMA mode via the Air-to-Earth HF radio channel (9) between the HF ground station ( 2) and the HF airborne station (1), which is registered on it, as long as the quality of the HF radio channel "V-3" (9) exceeds the permissible level. If the quality of the RF radio channel (9) deteriorates below an acceptable level, a new RF radio channel (9) is selected for the airborne station and it is registered on the newly selected RF radio channel (9). Packet data is exchanged through the ground communication network (5) between the HF ground stations (2) and the control center of the HF communication system (3), as well as the users of the communication system - air traffic control and UAL control centers (4).

In the process of exchanging packet data, a packet message for the air traffic controller or UAF dispatcher containing the address of the recipient — the air traffic controller or UAV dispatch point (4), as well as the sender address (ICAO board address), are formed in the on-board communication control unit (on-board router) (14), transmit to the HF airborne station (1), where it is packaged in a package designed for transmission via the HF radio channel (9), then transmit via HF radio channel (9) to the HF ground station (2), on which the HF airborne station (1) is registered where it is packaged in a bag intended for transmission via the ground-based communications subsystem (5), and transmitted through the interface (6) to the ground-based communications subsystem (5), from where they are transmitted via the interface (8) to the air traffic control center or UAL (4).

A packet message for the control unit (3), containing the address of the recipient - control unit (3), as well as the sender address (ICAO board address), is formed in the HF on-board station (1), where it is packaged in a package designed for transmission via the HF radio channel (9 ), transmitted via the HF radio channel (9) to the HF ground station (2), on which the HF airborne station (1) is registered, where it is packaged in a package intended for transmission via the ground communication subsystem (5), transmitted via the interface (6) to ground communication subsystem (5), from where it is transmitted through the interface (7) to the control center (3).

In the opposite direction, a packet message from the air traffic control center or UAL control center containing the address of the receiver - (ICAO board address), as well as the address of the sender - air traffic control center or UAL control center (4), form at the air traffic control center or UAL control center (4), transmit it through interface (8) to the ground communication subsystem (5), from where the packet is transmitted through the interface (6) to the HF ground station (2), on which the HF airborne station (1) is registered - the destination, where it is packaged in a packet intended for transmission via HF radio channel (9), and then transmit via HF radio Nalu (9) to the HF airborne station (1) - to the addressee.

A batch message from the control center (3) for the board (1), containing the recipient address - (ICAO board address), as well as the sender address - the control center (3), form in the control center (3), transmit it through the interface (7) to the ground subsystem communication (5), from where it is transmitted via the interface (6) to the HF ground station (2), on which the airborne station (1) is registered - the destination, where it is packaged in a package designed for transmission over the HF radio channel (9), and transmit on the HF radio channel (9) to the board (1) - to the addressee.

In the event of a malfunction of the interface (6) between the HF ground station (2) and the ground subsystem (5) from this HF ground station (2) inaccessible to the land communication network, broadcast in marker signals for all HF airborne stations registered on it (1) ), on all active Air-to-Earth RF channels (9), commands to change the communication frequency with the cause code “HF ground station malfunction”, and then the packet data exchange through the Air-to-Earth HF radio channels (9) between the HF ground station (2) with faulty int interface (6) and registered on it HF airborne stations (1).

The disadvantage of the analogue to which the invention is directed is that in the event of a technical malfunction of the interface (6) of the HF ground station (2) with the ground communication subsystem (5), i.e. in the event of the inaccessibility of the HF NS (2) for the ground subsystem, the inaccessible HF NS (2) cannot perform its main functions of conveying messages received from the HF airborne station (1) to the air traffic controllers and the UAL (4) to the control center ( 3) and vice versa, and also cannot receive control information (system table) from the control center (3) and transfer to the control center (3) a table of registered aircraft and information about its state of technical serviceability. Therefore, in the event of a technical malfunction of the interface (6) between the HF ground station (2) and the ground communication subsystem (5), the HF NS (2) is disconnected from the HF data exchange system, although it itself remains operational and can communicate via the HF radio channel (9) with sides (1). To do this, in marker signals at all assigned frequencies, this HF NS (2) transmits to all HF airborne stations (1) a message about changing the channel with the cause code "malfunction of HF NS" (see ARINC 635-3, page 57, Appendix 2- 8A (Marker format)) so that all planes registered at its frequencies switch to channels of other HF NS (2).

Disconnecting the HF NS (2) from the HF data exchange system reduces the quality of service for the HF Air-Earth communication system as a whole, since each HF NS (2) is capable of serving an average of 26 aircraft at each of 6 active frequencies (up to 156 aircraft in total on one HF NS at the same time). At the same time, the performance, controllability, and reliability of the Air-to-Earth RF communication system as a whole are reduced, and the number of aircraft served is reduced.

In addition, the ARINC 635 protocols [3] and the HFDL guidelines [2, 4] do not provide the ability to connect an HF ground station to the HFDL system that does not have access to the ground communications network due to the lack of terrestrial communications infrastructure in a remote area (for example, Siberia , Far North) where this HF NS is located. The location of the HF NS in a remote territory can be economically advantageous from the point of view of information support for the safety of flights on distant polar and oceanic air routes.

The objective of the invention is to improve the quality of service provided by the air-to-ground RF data transmission system (survivability, controllability, scalability and reliability of the air-to-ground communication system) by eliminating the effects of interface problems (6) of the RF ground station (2) with the subsystem ground communications (5) for the exchange of data packets between aircraft and the HF ground station (2).

The technical result of the invention is the elimination of disconnection from the air-to-ground data exchange system of technically sound HF NS (2), which became inaccessible to the ground communication subsystem (5) for various reasons (for example, due to a technical malfunction of the communication line (6) " HF NS (2) - router (27) ", technical malfunction of the interface device (25) with the terrestrial communication subsystem that is part of the HF NS (2), technical malfunction of the router (27) ground-based subsystem), as well as the possibility of connecting to AT The Air-to-Earth data exchange system of HF ground stations (2) that do not have access to a terrestrial communication network due to the lack of terrestrial communication infrastructure in remote areas where these HF NSs are located (2).

The specified technical result is achieved by the fact that in the known HF packet data exchange system containing HF airborne stations connected via HF radio channels "Air-Earth" with HF ground stations, which in turn are connected to the control center of the said system and to air traffic control centers traffic and airlines through the ground subsystem, in which each HF ground station contains a HF ground station controller that is associated with N RF transmitters connected to the N HF transmitter antennas, with N RF Air-to-Earth receivers connected to a common RF receiving antenna, is also connected to the information inputs of N monophonic multi-position phase-shifted signal connected to N RF transmitters, is connected to information outputs of N monophonic Air-to-Earth demodulators a multi-position phase-shifted signal connected to N RF receivers, in addition, it is connected to a receiver of signals of a single time, connected to a receiving antenna of signals of a single time, and an interface to the device at least one additional ground-to-ground RF receiver and at least one additional ground-to-ground demodulator of a single-tone multiposition phase-shifted signal, the output of which is connected to an additional the information input of the HF ground station controller, and the input is to the output of the additional RF-Earth receiver, the information input of which is connected to a common RF receiving antenna, and the control input is connected to the additional control yuschemu output controller RF ground station.

The specified technical result is achieved by the fact that in the known method of exchanging packet data, based on the fact that marker signals are emitted from each RF ground station in the first slot of each TDMA frame of the channel access protocol at N frequencies, which are periodically assigned and activated in the RF control center a data exchange system for implementing the FDMA protocol for access to communication channels, according to which different HF ground stations have different sets of active operating frequencies, at the corresponding HF ground station register I’m waiting for the HF airborne station at the best HF communication frequency selected by the HF airborne station based on the results of its evaluation of the quality of receiving marker signals, between the HF ground station and the HF airborne station registered on it, they exchange packet data until the quality of the “Air- Earth ", if the quality of the Air-to-Earth RF channel deteriorates below the permissible level on the RF airborne station, a new channel is selected and registered on this channel on the new or old RF ground station, through the terrestrial communication subsystem packet data exchange between each HF ground station and air traffic control and airline control centers, as well as the HF communication center control center, additionally at each HF ground station: select the best frequency for receiving messages from each other HF ground station according to the results of signal reception quality assessment markers using additional RF receivers "Earth-to-Earth" and demodulators "Earth-to-Earth" of a single-tone multiposition phase-shifted signal, form an audibility table according to the results of choosing the best reception frequencies, which indicate the sign of their availability (inaccessibility) for the terrestrial communication subsystem, the identifiers of the ground stations and the corresponding numbers of the best reception frequencies with codes of the recommended maximum allowable data rates, one slot of the access frame is allocated to each frequency channel channel for Earth-to-Earth transmissions, transmit the audibility table simultaneously using N RF transmitters in the slots that are allocated for Earth-to-Earth transmissions, receive audibility tables from other HF ground stations at preselected best reception frequencies using additional HF receivers and Earth-to-Earth demodulators of a single-tone multiposition phase-shifted signal, form the Earth-Earth network connectivity table based on the received audibility tables, which indicate the identifiers of ground stations with signs their availability (inaccessibility) for the subsystem of terrestrial communication and the corresponding numbers of the best frequencies of reception and transmission with codes of the recommended maximum allowable speeds data transmission, the connectivity table of the Earth-Earth network is used to select communication frequencies (reception and transmission) with other HF NS, the data packet received at an inaccessible HF ground station from the HF airborne station registered on it is transmitted simultaneously with the audibility table over HF a radio channel in the Earth-to-Earth slot to another accessible HF NS, from which it is transmitted to the air traffic control center or UAF control center or to the control center via the ground communication subsystem, a data packet from the air traffic control center or UAL control center or from the control center An instrument intended for an HF airborne station that is registered to an inaccessible HF ground station is transmitted via the ground subsystem to an available HF NS, from which it is then transmitted via the Earth-to-HF radio channel to an unavailable HF ground station and from which it is then transmitted via the Air-to-Earth RF channel to the HF airborne station, the data packet from the HF data exchange control center addressed to the inaccessible HF ground station is transmitted via the ground subsystem to the available HF ground station, from where first broadcast on HF radio "Earth-Earth" available to the RF ground station.

Enumeration of figures.

Figure 1. Block diagram of the HFDL RF packet data exchange system.

Figure 2. Block diagram of the HF airborne station HF packet data exchange system HFDL.

Figure 3. Block diagram of the HF ground station HF packet data exchange system HFDL.

Figure 4. A variant of the block diagram of the ground subsystem used by the HFDL packet data exchange system.

Figure 5. Structural diagram of the inventive RF packet data exchange system.

6. Block diagram of the RF ground station of the inventive RF packet data exchange system.

7. The structural diagram of the subsystem of terrestrial communications used by the inventive RF packet data exchange system.

Fig. 8. An example of the structure of the time-division channel access frame in the inventive RF packet data exchange system.

Fig.9. Timing diagram of emissions of marker signals and "3-3" packets for the network topology "each with each" of 6 HF NS.

Figure 10. Variant 2 of the time diagram of the emissions of marker signals and ZZ packets for the network topology "each with each" of 6 HF NS.

11. Timing diagram of emissions of marker signals and "3-3" packets for a network topology of the type "12 stars over 5 HF NS".

Fig. 12. Timing diagram of emissions of marker signals and "3-3" packets for a network topology of the type "10 stars over 4 HF NS".

The structural diagram of the inventive RF packet data exchange system is shown in figure 5, where it is indicated:

1 - HF airborne station (HF BS);

3 - control center (CC) HF packet data exchange system;

4 - air traffic control and UAL control centers (ground users of the RF packet data exchange system);

6 - HF NS interface (30) with the ground communication subsystem (34);

7 - interface of the control center (3) with the ground communication subsystem (34);

8 - interface of air traffic control and air lines (4) with the ground communication subsystem (34);

29 - HF radio channel "Earth-Earth" between HF NS (30);

30 - HF ground station (HF NS);

34 is a subsystem of terrestrial communications used by the inventive RF packet data exchange system.

Moreover, the HF airborne stations (1) are connected via HF radio channels (9) to HF ground stations (30) of the claimed HF system, interconnected via HF radio channels (29) and connected via interfaces (6) to the ground communication subsystem (34), which in turn, it is connected through the interfaces (7) to the control center (3), and through the interfaces (8) to the control centers of the air traffic control and the UAL (4).

The structural diagram of the HF airborne station (1) is presented in figure 2, where it is indicated:

10 - on-board HFDL RF transceiver;

11 - on-board antenna matching device (ACS);

12 - onboard HF antenna;

13 - control panel of the transceiver (PU);

14 - onboard router (BM);

15 - control device RF data exchange;

16 - RF transceiver ARINC 719.

Moreover, the on-board HFDL RF transceiver (10) consists of the ARINC 719 RF transceiver (16) and the RF data exchange control device (15), which contains a modem and an HFDL data exchange protocol controller.

In this case, the RF transceiver (16) is connected on the one hand to the ACS (11), and on the other hand to the RF communication control device (15). The RF communication control device (15) is connected on the one hand to the RF transceiver (16), and on the other hand to the control panel (PU) (13) of the radio station and to the on-board router (BM) (14). ACS (11) is connected on one side to the onboard RF transceiver (16), and on the other hand to the RF onboard antenna (12).

Structurally, the RF data exchange control device (15) can be made both in the form of an independent unit connected externally to the RF transceiver (16), and in the form of a subunit or boards built into the transceiver (16).

The RF airborne station (1) is connected to the airborne router (BM) (14) (a communication management unit that complies with the ARINC 724 or 758 specifications), which sends (receives) packet messages to (from) the airborne sources / receivers of information (avionics) such as a multifunctional control panel, display, printer, computer, maintenance system, airborne navigation system, electronic indication and alarm system, navigation system, etc. The on-board router (14) is connected not only with the HF on-board station (1), but also with the on-board stations of other frequency ranges.

The structural diagram of the HF ground station (30) of the inventive HF packet data exchange system is presented in Fig.6, where it is indicated:

17 - HF radio transmitter;

18 - RF transmitting antenna;

19 - HF radio receiver "Air-to-Earth";

20 - RF receiving antenna;

21 - RF modulator single-tone signal of multi-position phase shift keying;

22 - RF air-to-ground demodulator of a single-tone signal of multiposition phase shift keying;

24 - receiver of signals of a single time;

25 is an interface device with a terrestrial communication network (34);

26 - a receiving antenna of signals of uniform time;

31 - RF radio receiver "Earth-Earth";

32 - RF demodulator "Earth-Earth" single-tone signal of multi-position phase-shift keying;

33 - controller HF ground station (30).

The HF ground station (30) of the claimed HF packet data exchange system comprises:

- N RF transmitters (17) associated with N transmitting RF antennas (18), a controller (33) of the RF ground station (30), and N RF modulators (21) of a single-tone multiposition phase shift keying signal;

- N RF receivers “V-3” (19) associated with a common RF receiving antenna (20), a controller (33) of the RF ground station (30) and N RF demodulators “V-3” (22) of a single-tone multiposition phase shift keying signal ;

- N RF modulators (21) of a single-tone signal of multi-position phase shift keying associated with N RF transmitters (17) and a controller (33) of the RF ground station (30);

- N RF V-3 demodulators (22) single-tone multi-phase phase shift keying signals associated with N RF V-3 receivers (19) and RF ground station controller (33) (30);

- at least one Earth-to-Earth RF receiver (31) connected to a common RF receiving antenna (20), the controller (33) of the RF ground station (30), and the RF signal demodulator ZZ (32) of a single-tone signal multiposition phase manipulation;

- at least one RF demodulator “ZZ” (32) of a single-tone signal of multiposition phase shift keying associated with the RF receiver “3-3” (31) and the controller (33) of the RF ground station (30);

- a controller (33) of the HF ground station (30) associated with N RF receivers "V-3" (19), RF receiver "3-3" (31), N RF transmitters (17), N RF modulators (21) a single-tone signal of multi-position phase-shift keying, N HF demodulators "V-3" (22) a single-tone signal of multi-position phase-shift keying, HF demodulator "3-3" (32) a single-tone signal of multi-position phase shift keying, a receiver of single-time signals (24), an interface device with terrestrial communication network (25);

- an interface device with a terrestrial communication network (25) connected to a HF NS controller (33) on the one hand, and on the other hand, to a terrestrial communication network through an interface (6);

- a receiver (24) of signals of a single time associated with the controller (33) of the HF NS (30) and with a receiving antenna of signals of a single time (26);

- a receiving RF antenna for general use (20) connected to RF receivers "V-3" (19) and to RF receivers "3-3" (31),

- a receiving antenna (26) of a receiver (24) of signals of a single time, connected to a receiver (24) of signals of a single time;

- N transmitting RF antennas (18) connected to N RF transmitters (17).

The structural diagram of the ground communication subsystem (34) used by the inventive RF packet data exchange system is shown in Fig. 7, where it is indicated:

3 - control center (CC) of the claimed RF packet data exchange system;

4 - air traffic control and UAL control centers (users of the RF packet data exchange system);

6 - HF NS interface (30) with the ground communication subsystem (34);

7 - interface of the control center (3) with the ground communication subsystem (34);

8 - interface of air traffic control and air lines (4) with the ground communication subsystem (34);

27 - regional routers;

28 - interface between regional routers;

29 - HF radio channel "Earth-Earth" between HF NS (30);

30 - HF ground station (HF NS) of the claimed HF packet data exchange system.

The ground communication subsystem (34) used by the RF packet data exchange system contains regional routers (27), which are interconnected by interfaces (28), are also connected to the RF HF (30) interfaces (6), are connected to the control center (3) interfaces (7), connected to control centers (4) by interfaces (8), contains interfaces (6), (7), (8), connecting routers (27) with HF NS (30), control center (3), dispatchers ( 4), respectively, and contains interfaces (28) that connect regional routers (27) with each other.

The inventive RF packet data exchange system implements a method for exchanging packet data between on-board users of the mentioned system (multifunctional control panel, display, printer, computers of the maintenance system and airborne navigation system, electronic indication and signaling system, navigation system, etc.) and its terrestrial by users (air traffic control and UAL control centers (4)), as well as the control center (3) of the RF packet data exchange system, similar to how this is done in the prototype.

To ensure a given level of communication reliability in the area of responsibility of each HF ground station (30) from the general list of M HF frequencies allocated for the claimed HF packet data exchange system, in the control center (3) the said system is assigned for each HF ground station (30) to each time interval of a day with a duration of 1-2 hours at N active frequencies, optimal according to the conditions of propagation of radio waves and electromagnetic compatibility, bring the assigned set of frequencies along with the time interval of its activation to each RF station (30) through the ground subsystem (34), thus realizing the frequency division multiple access (FDMA) protocol, the usage time of each frequency channel is divided into time frames of 32 s duration, and each frame is divided into 13 time slots of duration 2.461538 s for implementing the time division multiple access (TDMA) protocol.

At the end of each frame, on each HF ground station (30), the access slots of the next frame are assigned for transmissions from the ground, or for transmissions from specific sides according to their preliminary requests for access slots, or for transmissions from any side in random access mode.

From each HF ground station (30), at all active frequencies in the first slot of each frame, marker signals are emitted that contain the slot assignments of the current frame, as well as receipts for messages received from the sides in the previous two frames.

At each HF airborne station (1), according to the results of evaluating the quality of the reception of marker signals, the best communication frequency is selected (HF radio channel Air-to-Earth (9)).

Each HF airborne station (1) is registered on the HF channel (9) chosen by it on the HF ground station (30) corresponding to this channel, packet data is exchanged in TDMA mode via the Air-to-Earth HF radio channel (9) between the HF ground station ( 30) and the HF airborne station (1), which is registered on it, as long as the quality of the HF radio channel "V-3" (9) corresponds to an acceptable level.

If the quality of the HF radio channel (9) deteriorates below an acceptable level, a new HF radio channel (9) is selected for the airborne station (1) and it is registered on a new selected HF radio channel (9), the best frequency for receiving messages from each other HF ground station is selected (30) according to the results of evaluating the quality of reception of marker signals using additional RF receivers (31) Earth-Earth and demodulators Earth-Earth (32) of a single-tone multiposition phase-shifted signal, an audibility table is formed according to the results of choosing the best reception frequencies, in which They indicate the sign of their accessibility (inaccessibility) for the subsystem of terrestrial communication, identifiers of ground stations and the corresponding numbers of the best reception frequencies with codes of the recommended maximum allowable data rates.

On each frequency channel, one slot of the channel access frame for Earth-to-Earth transmissions is allocated, the audibility table is transmitted simultaneously using N RF transmitters in slots that are allocated for Earth-to-Earth transmissions, and audibility tables from other RF ground stations are received ( 30) at the preselected best reception frequencies using additional RF receivers (31) and Earth-to-Earth demodulators (32) of a single-tone multi-position phase-shifted signal, form the Earth-Earth network connectivity table based on the received sl determination to in which point ground stations identifiers from their accessibility features (unavailable) for terrestrial communication subsystem and the corresponding number of the best frequency reception and transmission codes recommended maximum allowable data rates.

The connectivity table of the Earth-Earth network is used to select the frequencies of communication (reception and transmission) with other HF NS (30).

The data packet received at an inaccessible HF ground station (30) from the HF airborne station (1) registered on it is transmitted simultaneously with the audibility table via the HF radio channel (29) in the Earth-to-Earth slot to another available HF NS (30), with which it is transmitted to the control center of the air traffic control unit or UAL or to the control center (3) through the ground communication subsystem (34).

The data packet from the air traffic control center or UAL control center (4) or from the control center (3), intended for the HF airborne station, which is registered on the inaccessible HF ground station (30), is transmitted through the ground communication subsystem (34) to the available HF NS ( 30), from which it is then transmitted via the Earth-to-Earth RF channel (29) to an inaccessible RF ground station (30) and from which it is then transmitted via the Air-to-Earth RF channel (9) to the RF airborne station (1) )

A data packet from the control center (3) by the HF data exchange system addressed to an inaccessible HF ground station (30) is transmitted via the ground subsystem (34) to an accessible HF ground station (30), from where it is transmitted via the Earth-to-Earth radio channel "(29) to an inaccessible HF ground station (30), moreover, a packet message for a ground user (air traffic control or UAL controller) containing the address of the receiver - air traffic control or UAL control center (4), as well as the sender address (ICAO board address), in the on-board communication control unit (on-board march teaser) (14), transmit to the HF airborne station (1), where it is packaged in a package designed for transmission via the HF radio channel (9), then transmit via the HF radio channel (9) to the HF ground station (30), on which it is registered The HF airborne station (1), where it is packaged in a package intended for transmission via the ground communication subsystem (34), and transmitted through the interface (6) to the ground communication subsystem (34), from where it is transmitted to the air traffic control center via the interface (8) or UAF (4).

If in this case the HF ground station (30), on which the HF airborne station (1) is registered, is inaccessible to the ground communication subsystem (34), then the packet message received by it on the HF radio channel (9) is packaged in a packet intended for transmission via the HF radio channel Earth-to-Earth (29), and transmitted via the HF channel (29) to another HF ground station (30), available for the ground subsystem (34), where the message is packaged in a packet designed for transmission over the ground subsystem (34) ) are transmitted via the interface (6) to the ground communication subsystem (34), where provide an interface (8) to the control tower ATC or Wal (4).

A packet message for the control unit (3), containing the address of the recipient - control unit (3), as well as the sender address (ICAO board address), is formed in the HF on-board station (1), where it is packaged in a package designed for transmission via the HF radio channel (9 ), transmitted via the HF radio channel (9) to the HF ground station (2), on which the HF airborne station (1) is registered, where it is packaged in a package intended for transmission via the ground communication subsystem (30), transmitted via the interface (6) to ground communication subsystem (30), from where it is transmitted through the interface (7) to the control center (3).

If in this case the HF ground station (30), on which the HF airborne station (1) is registered, is inaccessible to the ground communication subsystem (34), then the packet message received by it on the HF radio channel (9) is packaged in a packet intended for transmission via the HF radio channel Earth-to-Earth (29), and transmitted via the HF channel (29) to another HF ground station (30), available for the ground subsystem (34), where the message is packaged in a packet designed for transmission over the ground subsystem (34) ) are transmitted via the interface (6) to the ground communication subsystem (34), where provide an interface (7) to CO (3).

A batch message from the air traffic control center or UAL control center containing the address of the receiver - (ICAO board address), as well as the address of the sender - air traffic control center or UAL control center (4), form at the air traffic control center or UAL control center (4), transmit it through the interface (8 ) to the ground communication subsystem (34), from where the packet is transmitted via the interface (6) to the HF ground station (30), on which the HF airborne station (1) is registered - the destination, where it is packaged in a package intended for transmission via the HF radio channel ( 9), and transmit via the HF radio channel (9) to the HF airborne station (one).

If the HF ground station (30) at which the HF airborne station (1) is registered is not available for the ground communication subsystem (34), then the message is transmitted by the ground communication subsystem (34) to another HF ground station (30) available for the ground communication subsystem (34), where it is packaged in a package designed for transmission via the HF radio channel (29), and transmitted via the HF radio channel (29) to an inaccessible HF NS (30), on which the HF airborne station (1) is registered, where it is packaged in a packet intended for transmission over the HF radio channel (9), and transmitted over HF for channel (9) to the RF board station (1).

A batch message from the control center (3) for the board (1), containing the recipient address - (ICAO board address), as well as the sender address - the control center (3), form in the control center (3), transmit it through the interface (7) to the ground subsystem communication (34), from where it is transmitted through the interface (6) to the HF ground station (30), on which the airborne station (1) is registered - the destination, where it is packaged in a package designed for transmission over the HF radio channel (9), and transmit on the HF radio channel (9) to the board (1) - to the addressee.

If the HF ground station (30) at which the HF airborne station (1) is registered is not available for the ground communication subsystem (34), then the message is transmitted by the ground communication subsystem (34) to another HF ground station (30) available for the ground communication subsystem (34), where it is packaged in a package designed for transmission via the HF radio channel (29), and transmitted via the HF radio channel (29) to an inaccessible HF NS (30), on which the HF airborne station (1) is registered, where it is packaged in a packet intended for transmission over the HF radio channel (9), and transmitted over HF for channel (9) to the RF board station (1), - the addressee.

Table 1 provides an example of the structure of the audibility table. In this example, the network consists of six HF NS (N = 6) and the audibility table is formed by the HF ground station with number 4, which is unavailable (has a faulty interface with the ground subsystem). The first line shows the numbers of the HF NS, and the second line shows the numbers of the best reception frequencies selected by the HF NS 4. The frequency numbering corresponds to the system table, which is developed by the control center and brings to all the VCHNs. In parentheses next to the frequency numbers are the numbers of the recommended maximum data transfer rates (1 - 300 bps, 2 - 600 bps, 3 - 1200 bps, 4 - 1800 bps). In this example, HF NS No. 4 did not receive a single signal from HF NS No. 6, therefore, it characterized the audibility from HF NS No. 6 as 0 (0). The length of the connectivity table is 8 (N) bits.

Table 1 HF NS No. 4 sign of availability HF NS No. 1 HF NS No. 2 HF NS No. 3 HF NS No. 5 HF NS No. 6 0 4 (1) 2 (4) 5 (3) 3 (4) 0 (0)

Table 2 presents an example of a connectivity matrix for six HF NS. Each column of the connectivity matrix characterizes the numbers of the best transmission frequencies of one station indicated in the column heading for other stations indicated in the row headers. Each row of the connectivity matrix is characterized by the numbers of the best reception frequencies by one station, indicated in the row header, of signals of other stations, indicated in the column headers. In this case, the frequency numbering corresponds to the system table, which is developed by the control center and brings to all HF NS. The recommended maximum data rates are shown in parentheses. The HF on-board station uses the connectivity matrix to determine the frequency of listening to the signal transmitted in the Earth-to-Earth slot containing the hearing matrix, and also, if necessary, a message from another HF NS. The connectivity matrix is also used to determine the transmission frequency of the message for another station in the Earth-to-Earth slot.

table 2 HF NS Number Sign of serviceability of the interface Network connectivity matrix "3-3". The best transmission and reception frequencies and recommended speeds in the "3-3" network HF NS No. 1 HF NS No. 2 HF NS No. 3 HF NS No. 4 HF NS No. 5 HF NS No. 6 one one - 5 (3) 1 (4) 2 (3) 0 (0) 0 (0) 2 one 1 (4) - 2 (4) 1 (4) 5 (2) 0 (0) 3 one 5 (1) 3 (1) - 0 (0) 4 (3) 0 (0) four 0 4 (1) 2 (4) 5 (3) - 3 (4) 0 (0) 5 one 4 (2) 5 (2) 3 (3) 4 (2) - 0 (0) 6 one 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) -

On Fig presents an example of the structure of the access frame of the channel with time division in the inventive RF packet data exchange system, which differs from the structure of the channel access frame in the RF HFDL data exchange system only by the presence of one access slot to the Earth-to-Earth channel (" 3-3 ").

Figures 9 and 10 show timing diagrams of the emissions of marker signals and 3-3 packets for the topologies of the Earth-to-Earth network of the type "each with each" of 6 HF NS (A, B, C, D, E, F) .

11, 12 are timing diagrams of the emissions of marker signals and Z-Z packets for topologies of the Earth-Earth network of the type "network of M stars over K HF ground stations".

Thus, due to the implementation of the technical solutions proposed in the invention,

- elimination of disconnection from the RF packet data exchange system of technically sound HF NS (30), which became inaccessible to the ground communications subsystem (34) for various reasons (for example, due to the occurrence of a technical malfunction of the communication line (6) "HF NS (30) - router (27) ", a technical malfunction of the interface device (25) with the terrestrial communication subsystem that is part of the HF NS (30), a technical malfunction of the router (27) of the terrestrial communication subsystem), and

- it is possible to connect to the HF packet data exchange system HF ground stations (30) that do not have an interface with the terrestrial communications subsystem (34) located in remote areas of Siberia and the Far North with no terrestrial communications infrastructure, but which are advantageous from the point of view of information security flights on distant polar and oceanic air routes (such HF NS (30) can be mobile),

- and as a result, the task of improving the quality of service provided by the RF packet data exchange system (survivability, controllability, scalability and reliability of the Air-Earth communication system) is solved by eliminating the effects of the interface (6) of the RF ground station (30) with the ground subsystem communications (34) on the exchange of packet data between aircraft and the HF ground station (30).

Literature

1. Appendix 10 to the ICAO Agreements (Volume 3, Part 1, Chapter 11). Geneva. ICAO. 2000.

2. ARINC 634. Specification. HF Data Link System Design Guidance Material. 8/96.

3. ARINC 635-3. Specification. HF Data Link Protocols. 12/2000.

4. Guidance on the RF data line. Geneva. ICAO. 2001. The prototype.

5. Report from the AD HOC Working Group on HF Data Link. Draft Version 1.0. September 29, 1995.

6. ARINC Characteristics 753-3. HF Data Link System. 2001.

7. GLOBALLink / HF. HF DATA Link. Technical Experts Meeting. Moscow. May 16-17. 1996.

8. Dr. D. Yaviz. Cost of Truly Mobile Beyond Line-Of-Sight Communications or "How Much Does it Cost to Get a Bit From A to B?". HARRIS, 1994.

9. MIL-STD-188-141 B. Interoperability and performance standards for medium and high frequency radio systems. Department of defense interface standard. 1 March 1999. Superseding MIL-STD-188-141A 15 September 1988.

Claims (2)

1. An RF packet data exchange system comprising HF airborne stations connected via Air-to-Earth HF radio channels to HF ground stations, which, in turn, are connected to the control center of the said system and to air traffic and air traffic control centers via the subsystem terrestrial communication, in which each HF ground station contains an HF ground station controller that is coupled to control N RF transmitters connected to N RF transmit antennas, and N RF Air-to-Earth receivers connected to the common RF receiving antenna, is also connected to the information outputs of N modulators of a single-tone multiposition phase shift keying signal connected to N RF transmitters, is connected to the information outputs of N air-to-ground demodulators of a single-tone multiposition phase shift keying signal, connected to N RF receivers, in addition, connected to a single-time signal receiver connected to a single-time signal receiving antenna, and with an interface device with a terrestrial communication network, characterized in that each HF terrestrial The station contains at least one additional RF ground-to-ground communication receiver and at least one additional ground-to-earth demodulator of a single-tone multiposition phase-shift keyed signal, the output of which is connected to the additional information input of the RF ground station controller, and the input to the output of the additional RF receiver "Earth-Earth", the information input of which is connected to a common RF receiving antenna, and the control input is connected to the additional control output of the controller of the RF ground station ui. 2. A method for exchanging packet data in the system specified in claim 1, which means that marker signals are emitted from each HF ground station in the first slot of each TDMA frame of the channel access protocol at all frequencies, which are periodically assigned and activated in the control center HF data exchange system for implementing the FDMA protocol for access to communication channels, according to which different HF ground stations have different sets of active operating frequencies, each HF airborne station is registered at the best HF ground station at the best The HF communication frequency selected by the HF airborne station based on the results of its evaluation of the quality of receiving marker signals, between the HF ground station and the HF airborne station registered on it, exchange packet data until the quality of the Air-to-Earth HF channel allows, if the quality deteriorates The air-to-Earth RF channel below the permissible level on the RF airborne station selects a new channel and registers on this channel on the new or old RF ground station, through the ground-based subsystem, packet data is exchanged Between each HF ground station and air traffic control and airline control centers, as well as the HF communication center control center, characterized in that at each HF ground station, the best frequency of receiving messages from each other HF ground station is selected based on the results of evaluating the quality of reception of marker signals with using additional RF receivers "Earth-to-Earth" and demodulators "Earth-to-Earth" of a single-tone multiposition phase-shift keyed signal, an audibility table is formed according to the results of choosing the best their receiving frequencies, in which they indicate a sign of their availability (inaccessibility) for the terrestrial communication subsystem, identifiers of ground stations and the corresponding numbers of the best receiving frequencies with codes of the recommended maximum allowable data rates, on each frequency channel they assign one slot of the channel access frame for transmission Earth-to-Earth, transmit the audibility table at the same time using N RF transmitters in the slots that are allocated for Earth-to-Earth transmissions, receive audibility tables from other RF ground stations on pre-selected best reception frequencies using additional RF receivers and Earth-to-Earth demodulators of a single-tone multi-position phase-shifted signal, form the Earth-Earth network connectivity table based on the received audibility tables, which indicate the identifiers of ground stations with signs of their availability (inaccessibility) for the ground communication subsystem and the corresponding numbers of the best reception and transmission frequencies with the codes of the recommended maximum allowable data rates, table "Earth-to-Earth" network identities are used to select communication frequencies (reception and transmission) with other HF NS, the data packet received at an inaccessible HF ground station from the HF airborne station registered on it is transmitted simultaneously with the audibility table via the HF radio channel in the slot " Earth-to-Earth "to another available HF NS, from which it is transmitted to a control center for air traffic control (ATC) or for managing an airline (UAF) or to a control center through a ground subsystem, a data packet from a control center for control D or UAL or from the control center, intended for the HF airborne station, which is registered on the inaccessible HF ground station, is transmitted through the ground subsystem to the available HF NS, from which it is then transmitted via the Earth-to-HF radio channel to the inaccessible HF ground station , and with which it is then transmitted via the Air-to-Earth radio frequency channel to the HF airborne station, the data packet from the HF data exchange control center addressed to the inaccessible HF ground station is transmitted through the ground subsystem to access th RF ground station, from where he broadcast on HF radio "Earth-Earth" available to the RF ground station.

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