Patent Description:
Multilateration systems for accurately determining the position of aircrafts and ground based vehicles on an airfield are known. By way of example, document EUROCAE (European Organisation for Civil Aviation Equipment) ED-117A Minimum Operational Performance Specification for Mode S Multilateration Systems for Use in Advanced Surface Movement Guidance and Control Systems (A-SMGCS) sets out minimal requirements for an airfield multilateration system. These systems typically are configured to listen to transponder signals that are broadcast from aircrafts and ground based vehicles equipped with such devices. These transponder signals are typically used in existing airfield traffic control systems, such as Automatic Dependent Surveillance - Broadcast (ADS-B) where the conventional radar systems interrogate the aircraft transponders to obtain identification, position, velocity and additional data of the aircraft or vehicle. Hence, multilateration systems can provide for an additional safety level when they are integrated in an advanced airfield surface movement guidance and control system (A-SGMCS).

To be able to determine aircraft and vehicle position, multilateration systems comprise distributed ground stations, referred to as remote surveillance units, which are arranged across the airfield. The disposition of the remote surveillance units must be such that, at each relevant location on the airfield, a transponder signal can be intercepted by multiple remote surveillance units simultaneously, allowing to determine position based on a time difference of arrival (TDOA) principle. Furthermore, the airfield area can be challenging in terms of multipath effects, wherein signals may arrive at the antenna of a remote surveillance unit by more than one route due to signal reflection by many objects of sufficient size, possibly imposing an additional number of remote surveillance units on the airfield to obtain a predetermined position accuracy level. As a result, quite a number of remote surveillance units typically would need to be installed on the airfield, and these would all need to be connected to power supply and data communication lines.

It follows that the roll-out of a multilateration system can be costly in terms of installation costs when power supply or data communication lines are unavailable at the desired locations. The high installation cost also constitutes an impediment for further expanding the multilateration system to cover a wider area of the airfield, e.g. in order to obtain Wide Area Multilateration (WAM).

<CIT>, describes a ground-based airport vehicle tracking service, including ground-based aircraft, which collects positional data from a plurality of remote data acquisition sites, including a plurality of different types, and processes and filters the data to identify various ground-based events and subsequently transmit notifications of such events to specified users. A number of remotely located servers are located on the airfield. Each location includes an ADS-B transceiver, a positional data server and an antenna. In each instance, the transceiver of that location is connected to both the positional data server and antenna of the same location via a series of connections and/or interfaces. Each location is connected to a network via a wired or wireless communication link. The locations report altitude, latitude and longitude for each position report for ADS-B equipped aircraft and ground vehicles.

Hence, the system described in <CIT> is not a multilateration system since no position data is determined through multilateration of signals received by a number of distributed antennas. To the contrary, in <CIT>, the positional data of the ADS-B transmitted signal of the aircraft or vehicle is read. As a result, <CIT> does not require a large number of antenna locations.

<CIT> discloses an aircraft multilateration system integrating Aircraft Communications Addressing and Reporting System (ACARS) and Secondary Surveillance Radar (SSR) data. Ground stations send time-stamped SSR data received from an aircraft to a central workstation and a processor of the central workstation calculates the position of the aircraft. At the central workstation ACARS data is also received and modulated, linking registration number and aircraft assigned flight number. <CIT> discloses a flight verification system based on <NUM>, including an airborne sensor subsystem, a ground reference subsystem for providing position reference datum to the aircraft, a <NUM> network configuration subsystem and a ground computing and display subsystem. A virtual private channel is configured for wireless access and network slicing in the <NUM> network for information exchange between the airborne sensor subsystem, the ground reference subsystem and the ground computing and display subsystem.

There is therefore a need in the art to provide for multilateration systems which are more economical in terms of installation and/or operational cost.

It is further an aim of the present invention to provide multilateration systems allowing for additional functionality.

According to the invention, there is provided an airfield multilateration system as set out in the appended claims. The airfield multilateration system comprises a plurality of airfield surveillance units (or remote surveillance units) for detecting an airfield moving object.

An airfield surveillance unit as described herein comprises a power supply module, a first radio antenna configured to receive an aircraft transponder signal, a receiver module coupled to the power supply module and configured to be coupled to the first radio antenna for receiving the aircraft transponder signal. The receiver module comprises a first data communication port. The receiver module is operable to convert or process the aircraft transponder signal received by the first radio antenna for transmission via the first data communication port. By way of example, the receiver module is configured to convert the received aircraft transponder signal to a digital signal that is transmitted via the first data communication port. Advantageously, the receiver module is configured to convert or process the aircraft transponder signal and the aircraft transponder signal as converted or processed can be utilized for determining a position of a corresponding aircraft via multilateration. Advantageously, the receiver module is configured to convert or process the aircraft transponder signal such that it is usable for determining a position of a corresponding aircraft via multilateration, i.e. the aircraft transponder signal is converted to a multilateration-purpose signal. By way of example, the receiver module can be operable to add a time-stamp to the converted signal and to communicate the time-stamped signal via the first data communication port. Alternatively, a time-stamp can be added after transmission through the first data communication port, e.g. at a central processing unit of an airfield multilateration system.

According to an aspect of the present invention, the airfield surveillance unit further comprises a second radio antenna configured to receive and/or transmit <NUM> radio signals, and a <NUM> communication module coupled to the power supply module and configured to be coupled to the second radio antenna. The <NUM> communication module is operable to process <NUM> radio signals received or transmitted by the second radio antenna. Advantageously, the <NUM> communication module along with the second radio antenna are configured to operate as a transceiver for <NUM> radio communications. Advantageously, the <NUM> communication module along with the second radio antenna are configured to operate as a transceiver for a private <NUM> cellular network.

One advantage of equipping the airfield surveillance unit with a <NUM> communication module is that existing ground infrastructure of a multilateration system can be used for setting up a <NUM> cellular network providing optimal coverage of the airfield area. This reduces total installation costs, providing a low-cost solution to roll out a <NUM> local, private network, specifically for mission critical communication on airfield premises. One example of a mission critical communication on the airfield is detection and identification of aircraft movement, such as performed by the airfield surveillance units and systems described herein. Furthermore, power supply circuitry can be shared between the receiver module and the <NUM> communication module of the airfield surveillance unit.

Advantageously, the receiver module and the <NUM> communication module are operably coupled for data communication. In particular, the <NUM> communication module comprises a data communication port connected to the first data communication port of the receiver module for receiving the digital representation of the aircraft transponder signal. The <NUM> communication module is advantageously configured to process the digital representation for transmission via the second radio antenna.

By allowing data communication between the <NUM> communication module and the receiver module of the airfield surveillance unit, a cellular (<NUM>) and mission critical communication channel can be created for communicating multilateration critical data from the remote airfield surveillance unit to a central processing unit. This cellular (wireless) communication channel can be provided in addition to, or in alternative to existing and possibly wired communication channels between the remote airfield surveillance unit and the central processing unit. In particular, the <NUM> communication channel can provide for communication redundancy, and/or can aid in decongesting existing critical data communication channels, such as the <NUM>/<NUM> channel. This improves safety and security of the multilateration system and any system dependent thereupon, such as an A-SMGCS. Furthermore, by integrating the <NUM> communication channels into the data communication lines of the multilateration system, additional communication bandwidth can be created to allow more multilateration critical data to be communicated from the remote surveillance units to the central processing unit allowing to further increase positional accuracy of the multilateration system.

An airfield multilateration system as described herein comprises a plurality of remote surveillance units as described herein and a central processing unit operably coupled to the plurality of remote surveillance units for data communication, e.g. through one or a plurality of data communication lines. The central processing unit comprises a processing module operable to determine an aircraft location based on the aircraft transponder signal received by multiple ones of the plurality of remote surveillance units. Advantageously, one of the data communication lines can be a <NUM> network operated by the <NUM> communication modules of the remote surveillance units.

A method of detecting airfield traffic through multilateration is described herein as well. The method detects airfield traffic through the airfield multilateration system as described herein, comprising setting up a private <NUM> cellular network. The <NUM> communication modules of the plurality of airfield surveillance units and possibly the central processing unit communicate mission critical data through the private <NUM> cellular network. The mission critical data may include multilateration related data provided by the receiver modules or central processing unit of the airfield multilateration system.

Referring to <FIG>, an airfield multilateration system <NUM> comprises a plurality of surveillance units <NUM> distributed on an area of interest of the airfield or airport <NUM>. The surveillance units <NUM>, also known as ground stations, will be referred to herein as remote surveillance units or airfield surveillance units. The area of interest can typically comprise one or more of the runway area, taxiway area, Apron, and any other area of the airfield as desired. The remote surveillance units <NUM> are all connected to a central processing unit <NUM> through an advantageously wired data communication link <NUM>. The central processing unit <NUM> can further be connected to the Surface Movement Guidance and Control System (SMGCS) of the airport, schematically represented in <FIG> by the control tower <NUM>, to feed calculated position data of aircrafts and airfield vehicles to the SMGCS via a possibly wired data communication link <NUM>.

The remote surveillance units <NUM> are advantageously configured for receiving <NUM> signals <NUM>, such as ACAS (Airborne Collision Avoidance System) transmissions, ADS-B broadcasts and Mode S / SSR replies originating from on-board transmitting devices. Possible message formats are defined in Annex <NUM> to the Convention on International Civil Aviation, Aeronautical Communications, Volume IV, Surface and Collision Avoidance Systems, International Civil Aviation Organisation (ICAO). Additionally, the remote surveillance units can be configured to interrogating vehicles or aircrafts according to the same standard as well.

To this end, and referring to <FIG>, each remote surveillance unit <NUM> comprises a receive radio antenna <NUM> capable of receiving/intercepting transponder messages, a receiver module <NUM>, and a power supply module <NUM>. Receiver module <NUM> comprises a signal input port <NUM> coupled to antenna <NUM> and a data output port <NUM> which in use is connected to data communication link <NUM> connecting the remote surveillance unit <NUM> to the central processing unit <NUM>.

Optionally, the remote surveillance unit <NUM> comprises a transmitter module <NUM> coupled to a transmit radio antenna <NUM>, which may or may not be integrated with receive antenna <NUM> into a single antenna. The transmitter module <NUM> is typically configured for transmitting transponder interrogation signals.

Optionally, the remote surveillance unit <NUM> comprises a control module <NUM> operably coupled to receiver module <NUM> and/or transmitter module <NUM> for controlling operation thereof. Power supply module <NUM> is coupled to any of the above modules to supply the modules with electrical power. Power supply module <NUM> comprises a power input terminal <NUM> for connection to an external power supply line <NUM>, suitable power conversion circuitry <NUM>-<NUM> for converting the external power to operating voltages of the respective modules and components, and additionally or alternatively an internal energy storage device, such as a battery <NUM>, e.g. to ensure operation in case of external power failure.

All modules <NUM>, <NUM>, <NUM>, <NUM> can be accommodated in a housing of the remote surveillance unit, such as a cabinet.

In an ADS-B transponder communication system, interrogation messages are transmitted at <NUM> and the transponder device on board the aircraft communicates its response at <NUM>. To this end, the receive antenna <NUM> and/or transmit antenna <NUM> is operable in a frequency range of <NUM> - <NUM>. The antennas may be configured to transmit in any appropriate power range. The antenna may be configured as a half-wave dipole, and advantageously having a vertical polarization direction as the antenna is typically mounted vertically. The antenna can be provided with a reflector for reducing the azimuth range, e.g. from <NUM>° to <NUM>°, which may aid in multi-path effect reduction.

Referring to <FIG>, the receiver module <NUM> advantageously comprises an oscillator, a downconverter for downconverting the <NUM> signals, such as a dual channel <NUM> to <NUM> downconverter chain, a digital modulator, and any required decoding and modulating circuitry. All these functional blocks can be provided in one or more circuits. Further, the receiver module <NUM> is advantageously capable of communicating with every other module of the remote surveillance unit and may control operation of the remote surveillance unit modules, i.e. the control module <NUM> can be integrated in the receiver module <NUM>.

The receiver module <NUM> can operate as follows. The signal received by antenna <NUM> is passed into a pre-selector bandpass filter and optionally a signal limiter for overdrive protection. This configuration protects the receiving chain from saturation by a jamming signal outside the operation frequency. From the limiter the signal is then fed to a splitter, feeding the signal to a dual-channel downconverter chain. By way of example, the signal is split through a <NUM> dB directional coupler in a direct signal chain (lower signal chain in <FIG>) and an attenuated signal chain (upper signal chain in <FIG>). The attenuated signal may be further attenuated by a digital step attenuator. Each of the direct signal chain and the attenuated signal chain can comprise a low noise amplifier (LNA), followed by a bandpass filter. Next, the mixer converts the <NUM> signal down to the intermediate frequency (IF) of <NUM>. The <NUM> signal can be filtered by a bandpass section, amplified and fed to the analog-to-digital converters (ADCs). The receiver can be construed as a direct intermediate-frequency (IF) sampling architecture. The <NUM> local oscillator coupled to the mixers is advantageously PLL stabilized by a <NUM> reference signal.

The receiver module <NUM> can further be configured to time-stamp the digitalized messages and to communicate the time-stamped messages to the data output port <NUM>, from where they are further sent to the central processing unit <NUM> via data communication link <NUM>, possibly through control module <NUM>.

The transmitter module <NUM> can be configured for interrogating transponders, e.g. on <NUM> frequency. The transponders are interrogated by sending Mode-S interrogations. The interrogations can be triggered by the central processing unit <NUM> which may define the type and content of the interrogations and send them to the remote surveillance units <NUM>, where the command messages from the central processing unit may be received wirelessly through antenna <NUM>, or through another data communication line, e.g. data communication link <NUM>. Transponder interrogation can be useful to obtain further information from the aircraft or vehicle which is not automatically broadcast. This may help in improving positional accuracy.

In addition, or alternatively, the transmitter module <NUM> can be configured as a reference transmitter for periodically transmitting synchronization messages, which are used by the receiver modules <NUM> for synchronization. In reference transmitting mode, the transmitter is advantageously configured to transmit at a same frequency as the one at which receiver modules receive signals, e.g. <NUM>. By way of example, when operating as a reference transmitter, the transmitter module can send <NUM> Mode-S reply messages at a configured rate (typically <NUM> reply per second). The Mode S reply signals can be compliant to the constraints defined in Annex <NUM> to the Convention on International Civil Aviation, Aeronautical Communications, Volume IV, Surface and Collision Avoidance Systems, International Civil Aviation Organisation (ICAO).

The data output from the remote surveillance units <NUM> can comprise surface surveillance radar (SSR) reply receptions, interrogation information and service messages. SSR receptions and interrogation information contain accurate time of arrival (TOA) or time of interrogation (TOI) timestamps, which can form an input for the Multilateration calculation performed by the central processing unit. Remote surveillance unit output data can furthermore contain payload data of the SSR replies such as Mode-A/C and/or Mode-S information. This payload information can be used as the input for the Mode-S and ADS-B decoding.

Referring to <FIG>, the central processing unit <NUM> is configured to receive data output by the remote surveillance units <NUM>, e.g. via data communication link <NUM>. The central processing unit further comprises a computing and control module <NUM> configured to process this data for determining an object position through multilateration techniques. The object position can form an output of the multilateration system and the central processing unit can be configured to provide the output to the user, which can be a SMGCS or any other airfield traffic control system, e.g. via data communication link <NUM>. The central processing unit can comprise a suitable user interface <NUM>. In addition, the central processing unit can be configured to control the remote surveillance units by sending them command messages. These commands can be used to trigger transponder interrogations by the transmitter modules <NUM> of the remote surveillance units and/or select a mode of operation of the remote surveillance units.

One specific task of the computing and control module <NUM> is to calculate aircraft and other ground vehicle position via multilateration based on the time-stamped messages received by the remote surveillance units <NUM>, advantageously according to the TDOA principle. Before the calculation can be performed, the computing and control module must determine which timestamps/measurements belong to one and the same signal emission. This is performed in a tuple mapping step where payload data and timestamps are used to identify measurements which belong together. Tuples of measurements are generated as output and are forwarded to the calculation step.

The central processing unit calculates the object positions from the time difference of signal arrival measurements of the tuples as explained above. Two types of calculations are possible: Hyperbolical Multilateration and Elliptical Multilateration and the central processing unit may be configured to use any one of these for determining the position of an object. In addition, the position calculation can be performed in 3D or also in 2D in case the height determination is neglected or if the height is already known. If 2D calculations are performed, the measured time differences of signal arrival (TDOA) lead to hyperbolas in the horizontal plane. If at least two linear independent TDOA measurements are available, for which at least <NUM> time of signal arrival (TOA) measurements from different stations must be available, the hyperbolas intersect at the point the signal was emitted. In the 3D calculation hyperboloids are used instead of hyperbolas, so the height coordinate can also be determined. However, one more TDOA is necessary for a solution, so at least <NUM> receiving stations are needed for the 3D solution. The <NUM> or <NUM> stations are the minimum needed in order to determine a position. As a MLAT system consists of several receivers, mostly there are more measurements available. These additional measurements are also involved in the calculation to increase the solution accuracy. Hyperbolical Multilateration is highly accurate in the area enclosed by the RSUs, e.g. the airport/runway/taxiway/apron area. It only uses the time of signal arrival (TOA) measurements of the transponder replies.

Elliptical Multilateration can be used to determine the position of objects outside the area enclosed by the RSUs, e.g. the approach area. In the latter case, the objects need to be interrogated by the system as the TOA as well as time of interrogation (TOI) measurements are required to determine the object position. The time of interrogation (TOI) refers to the signals travel time from the interrogating (transmitter) module up to the transponder and back down to the receiver module. Having this knowledge, in addition to the hyperbolas, an ellipse is obtained which leads to a better intersection geometry. However, due to the inaccurate reply delay of the SSR transponders (±<NUM> in Mode-A/C and ±<NUM> in Mode-S), the elliptical calculation can be less accurate than the hyperbolical calculation, and positional accuracy is generally only improved in the outside areas, which are not enclosed by RSUs.

Both calculation types can be performed to obtain a two-dimensional (2D) or three-dimensional (3D - including height information) calculation type. Data from three receivers permits the system to determine a 2D position estimate. Data from a fourth receiver is required to determine a 3D position estimate. Alternatively, a 3D positional estimate can be calculated using only three receivers when altitude can be ascertained from an outside source, such as Mode C code or 'on ground' bit from the aircraft transponder. Downlinked altitude information can be used as additional information in the multilateration calculation in order to increase the height accuracy. The downlinked altitude information is mostly available in the form of air pressure dependent flight levels and can be converted into a height. This can be performed via QNH (barometric altimeter setting) information, if provided to the central processing unit, or via fixed transponders with known height.

Multilateration systems as described herein can be passive multilateration systems or active multilateration systems. A passive multilateration system uses transponder broadcasts and/or acquisition squitters. The passive multilateration system is not configured to interrogate transponder devices. In particular, the remote surveillance units of a passive multilateration system may not comprise transmitter modules <NUM> configured to interrogate the transponder devices. Passive multilateration systems can perform hyperbolic multilateration, but not elliptic multilateration. Since the broadcast transponder messages only contain limited information, the availability of transponder data is however limited.

An active multilateration system comprises remote surveillance units incorporating transmitter modules <NUM> configured to send transponder interrogation messages as described hereinabove. Transponder interrogation messages can be used for gathering Time of Interrogation (TOI) timestamps which are needed in elliptic multilateration and/or gathering transponder data such as Altitude, Identification, etc. which is sometimes not automatically broadcast by transponders.

In an active multilateration system, the central processing unit comprises interrogation logic configured to determine whether an object needs to be interrogated and which data shall be requested. Therefore, the logic needs knowledge of the received Mode-S data. By knowing the received data it determines which transponder needs to be interrogated, and which data is needed from it.

The central processing unit can comprise a Mode-S and/or Mode A/C decoding module <NUM> configured to extract and decode respective Mode-S and/or Mode A/C data available in the transponder reply messages received by the RSU. Possible Mode-S downlink formats (DF) that can be selected to be decoded by the decoding module are one or more of the following:.

Another task of the central processing unit can be time synchronization of the remote surveillance units. The object positions are advantageously calculated via the Time Difference of Signal Arrival (TDOA), i.e. the time difference between the "arrivals" of one transponder's signal received at the receiver modules of several remote surveillance units. This TDOA should be highly accurate in the range of <NUM> to <NUM> nanoseconds which is equivalent to a range measurement accuracy of <NUM> to <NUM> meters. This accuracy can be achieved by the use of highly stable oscillators at the remote surveillance units and by an appropriate synchronization algorithm in the computing and control module <NUM>.

One possible synchronization algorithm makes use of <NUM> SSR reply signals continuously emitted by remote surveillance units (RSU) equipped with transmitting modules with reference transmitter functionality as described hereinabove. These replies are received at the RSU which have direct line of sight to the transmitter RSU. As the positions of the RSU are accurately known to the central processing unit, a common time base for all RSU can be calculated as known in the art.

Due to the system topology not all RSU must necessarily have direct line of sight between each other. In the latter case, more than one transmitter can be used for synchronization. The RSUs can be grouped into several synchronization clusters, each formed of a reference transmitter RSU and the RSUs that are seen by it. Some RSUs that are in line of sight of more than one reference transmitter RSU can form part of more than one synchronization cluster. To maintain the common time base the clusters have to be synchronized with each other. This can be realized over specific RSU which are seen by multiple reference transmitter RSU.

An alternative possible synchronization method makes use of a central clock. With this method, the transponder signals received by the receiver modules are time-stamped centrally at the central processing unit <NUM>. This method removes the need to compensate for time differences between receivers. Yet alternatively, use can be made of Global Navigation Satellite System (GNSS) signals, wherein a GNSS signal, such as the <NUM> PPS GNSS receiver signal can be used as common signal and received by all receiver modules which can time-stamp the received transponder messages based on the GNSS signal. Additional precision can be achieved through synchronisation to a common transmitting satellite (known as the Common View method).

Referring back to <FIG> and <FIG>, according to an aspect of the present invention, the remote surveillance unit <NUM> further comprises a <NUM> communication module <NUM> coupled to a <NUM> antenna <NUM>. The <NUM> communication module <NUM> is configured to process <NUM> radio signals which are received by antenna <NUM>. In particular, <NUM> communication module <NUM> advantageously is configured to operate as a base station of a <NUM> cellular network <NUM>.

In the present disclosure, the term '<NUM>' can refer to any radio access technology operating according to the International Mobile Telecommunications-<NUM> (IMT-<NUM>) standard issued by the Radiocommunication Sector (ITU-R) of the International Telecommunication Union (ITU), and/or according to any one of the <NUM> radio access technologies set out by the 3GPP (<NUM>rd Generation Partnership Project) consortium, such as but not limited to <NUM> NR (New Radio), LTE-M (Machine Type communication) and NB-loT (Narrowband Internet-of-Things). In the context of the present disclosure, the term `<NUM>' can additionally refer to any further evolving cellular communication technology, such as <NUM>, and which advantageously allows setting up private, i.e. non-public cellular radio communication networks, specifically for mission critical communication.

Advantageously, <NUM> communication module <NUM> along with antenna <NUM> are configured to operate as a transceiver for <NUM> radio communications. <NUM> communication module <NUM> is connected to power supply module <NUM> for drawing electrical power for operation.

One advantage of equipping the remote surveillance units with a <NUM> communication module is that existing ground infrastructure of the multilateration system can be used for rolling out a <NUM> cellular network providing optimal coverage of the airfield area. This reduces total installation costs. Furthermore, power supply circuitry can be shared between the receiver/control/transmitter modules of the multilateration system and the <NUM> communication module.

Advantageously, when multiple and preferably all remote surveillance units <NUM> of the multilateration system <NUM> are equipped with a <NUM> communication module <NUM> and <NUM> antenna <NUM>, a <NUM> cellular network <NUM> can be set up covering the operation area of the multilateration system. Advantageously, the <NUM> communication modules are configured to set up a private <NUM> cellular network. The private <NUM> cellular network advantageously covers an area corresponding at least to the airport movement area, in particular the airport manoeuvring areas (including taxiways and runways), which area may correspond to the area covered by the airfield multilateration system. Possibly, all the airfield surveillance units of the airfield multilateration system are equipped with the <NUM> communication modules and second radio antennas. Any of the <NUM> modules can be configured as a base station of the (mission critical) private <NUM> network. Alternatively, the base station of the private <NUM> network can be provided as a separate unit and communicating with the <NUM> communication modules.

A private cellular communications network refers to a non-public broadband radio access technology communications network where network infrastructure is deployed across an area utilized by a private organisation, such as though not limited to an airfield operator, e.g. for air traffic control and/or airfield ground operations, and where the network infrastructure is used exclusively by devices authorised by the organisation. Devices which are registered on a public cellular communications networks will not work on the private network unless where specifically authorised by the organisation. A private network may operate in a radio frequency spectrum allocated for private (non-public) use. A private network typically ensures high bandwidth coverage, predictable error and delay characteristics, reduced latency and/or device capacity to ensure efficiency and safety. In a <NUM> private network, network bandwidth can be delivered at scale to suit the needs of the end user organisation for optimal quality of service and network resiliency. Time critical applications can be enabled through Time Synchronization Network (TSN) features. <NUM> private cellular communication networks can be implemented with Ultra Reliable Low Latency Communications (URLLC), with radio network latency reduced to <NUM> or less, such as <NUM>, along with a network reliability higher than <NUM>%, and even <NUM>%, enabling high reliability real-time applications. A <NUM> private network can offer superior security due to strong authorisation, authentication and access control features, such as data encryption and integrity protection mechanisms, 'Time Sensitive Network' bridges into which the private network can be integrated to safeguard time-sensitive communications from network attacks.

In addition, or alternatively, the <NUM> communication modules can be configured to provide public networking capability, i.e. providing radio access through a public <NUM> network provider. The (private) <NUM> cellular network can be so configured to provide for mission critical communications, either for the multilateration system itself, for external providers, e.g. in case of emergency situations such as fire, rescue and security services, or for both.

Advantageously, the <NUM> communication module <NUM> comprises a data communication port <NUM> which is coupled to a data communication port of the receiver module <NUM>, e.g. data communication port <NUM>. The data communication link between port <NUM> and port <NUM> can be configured for unidirectional data communication, e.g. from the <NUM> communication module to the receiver module <NUM> or vice versa, or advantageously for bidirectional data communication. In addition or alternatively, <NUM> communication module <NUM> advantageously comprises a data communication port <NUM> coupled to a data communication port <NUM> of the transmitter module <NUM>, which can be for unidirectional or bidirectional data communication. In the example of <FIG>, the connections between ports <NUM> and <NUM> on the one hand, and ports <NUM> and <NUM> on the other hand are provided via the control module <NUM>, although this is not a requirement.

By connecting the receiver module <NUM> and/or the transmitter module <NUM> to the <NUM> communication module for data communication, it becomes possible to use the (private) <NUM> cellular network for data communication between the remote surveillance units and the central processing unit, e.g. for communicating the transponder messages received and processed by the receiver modules <NUM> to the central processing unit <NUM>. The (private) <NUM> network set up by the <NUM> communication modules <NUM> can be used as an additional communication link, in addition to (wired) data communication link <NUM>. Alternatively, in particular in case of newly installed remote surveillance units, or remote surveillance units arranged at remote locations in the airfield area where a wired data communication line is not available, the (private) <NUM> network set up by the <NUM> communication modules <NUM> can be used to replace the (wired) data communication link <NUM>. It will therefore be appreciated that a data communication port coupled to data communication link <NUM> is optional in remote surveillance units <NUM> as described herein.

In the multilateration system <NUM>, one remote surveillance unit <NUM> can be configured to operate as a reference remote surveillance unit gathering all multilateration-related data communicated by the <NUM> communication modules <NUM> via the (private) <NUM> network and feeding this data to the central processing unit <NUM>, e.g. via data communication link <NUM>. Alternatively, referring again to <FIG>, the central processing unit <NUM> can comprise a <NUM> communication module <NUM> and <NUM> antenna <NUM> which are coupled for data communication with the computing and control module <NUM>. By so doing, a direct communication link between the central processing unit <NUM> and the remote surveillance units <NUM> via the (private) <NUM> network can be provided. Advantageously, any multilateration-related communication between the central processing unit and the remote surveillance units and/or between different remote surveillance units that is performed via the <NUM> network can be performed as mission critical communications within the (private) <NUM> network.

The data communication between receiver module <NUM> and/or transmitter module <NUM> on the one hand and <NUM> communication module <NUM> on the other can be used for communicating any or all data between different remote surveillance units and any and all data between the remote surveillance unit and the central processing unit as described above.

By way of example, the data communication between receiver module <NUM> and <NUM> communication module <NUM> can comprise time information which can be processed by receiver module <NUM> to time-stamp the received transponder messages. The time information is advantageously received by the remote surveillance units allowing to synchronize their internal clocks for correct and synchronized timestamping purposes.

In addition or alternatively, the data communication between transmitter module <NUM> and <NUM> communication module <NUM> can comprise control or instruction messages sent by the central processing unit <NUM> to the transmitter module <NUM>, e.g. instruction messages for interrogating the transponder of an aircraft or vehicle.

The use of the <NUM> communication channel as alternative synchronization method advantageously helps in decongesting the <NUM>/<NUM> channel. Whilst transponder interrogations by the transmitter modules provide a degree of autonomy and reliable performance, the interrogations introduce a load upon the <NUM>/<NUM> channel capacity and contribute to transponder occupancy. Congestion of the <NUM>/<NUM> spectrum and excessive transponder occupancy are major issues facing European surveillance infrastructure. Therefore, avoiding non-interrogation messages by using the <NUM> channel can be beneficial.

Claim 1:
Airfield multilateration system (<NUM>) for detecting airfield traffic, comprising a plurality of airfield surveillance units (<NUM>), each of the plurality of airfield surveillance units comprising:
a power supply module (<NUM>),
a first radio antenna (<NUM>) configured to receive an aircraft transponder signal,
a receiver module (<NUM>) coupled to the power supply module (<NUM>) and configured to be coupled to the first radio antenna (<NUM>) and comprising a first data communication port (<NUM>, <NUM>),
wherein the receiver module (<NUM>) is operable to convert the aircraft transponder signal received by the first radio antenna (<NUM>) for transmission via the first data communication port, wherein the converted aircraft transponder signal allows for determining a position of a corresponding aircraft via multilateration,
characterised in that each of the plurality of airfield surveillance units further comprises:
a second radio antenna (<NUM>) configured to receive and transmit <NUM> radio signals, and
a <NUM> communication module (<NUM>) coupled to the power supply module (<NUM>) and configured to be coupled to the second radio antenna and operable to process <NUM> radio signals received or transmitted by the second radio antenna,
wherein the <NUM> communication module and the second radio antenna are configured to operate as a transceiver for a private <NUM> cellular network, wherein the airfield multilateration system further comprises a central processing unit (<NUM>) operable for data communication with the plurality of airfield surveillance units (<NUM>), the central processing unit comprising a computing module (<NUM>) operable to determine an aircraft position based on the aircraft transponder signal received by multiple ones of the plurality of airfield surveillance units,
wherein the <NUM> communication modules (<NUM>) of the plurality of airfield surveillance units (<NUM>) are configured to form a private <NUM> cellular network.