TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of wireless communication technology, and more specifically to a wireless communication system and method particularly suitable for aircrafts, such as helicopters and airplanes.

BACKGROUND

It is not an understatement that the last few decades have introduced vast improvements and advancements in the field of communication technology. In fact, the advent of the internet, cellular phones and more recently smart phones and tablets has greatly changed the way we

communicate and quite possibly accelerated the technological field

surrounding these devices. As an inevitable consequence, there is an ever increasing demand for bandwidth in order to satisfy the market need for online connectivity which results in an increased focus on constantly developing and improving the underlying technology and systems in order to accommodate this demand.

Further, there is a rapidly increasing demand from consumers to be able to communicate through mobile phones and other handheld terminals at all times, even while traveling on trains, busses, ships and even aircrafts. This is partially embodied in the increasing availability of in-flight entertainment systems and wireless communication (Wi-Fi, GSM, 3G, LTE, 5G) capability onboard aircrafts.

Wireless communication capability onboard aircrafts is not a new concept, even the earliest commercial aircrafts had rather primitive voice communication capability with ground personnel over shortwave radio, which improved flight safety and enabled accelerated commercialization of air transport. Since then, airborne communication systems have been further improved with advent of radar, computers and data links, which serve to improve in-flight safety as well as the overall traveling experience for passengers. Still further, in the effort of providing connectivity to high bandwidth communication networks, such as the Internet, for aircrafts, it is known that existing terrestrial cellular networks have potential for cost effective operation. The terrestrial cellular networks are however designed for use by terrestrial equipment (e.g. handheld cell phones) and not for aircrafts. Therefore, successful use of terrestrial networks from aircrafts depends on the ability to handle and work around the assumptions built in to such networks, most predominantly, the assumption that the client device is terrestrial. One of the more prominent consequences of the assumption that the client device is terrestrial is the geographical cell size employed in these networks. The assumptions are accordingly that the client devices will have very limited range, enforced by the fact that the radio propagation path between client devices and base stations is limited by obstructions (buildings, mountains, trees, etc.) and the Earth's horizon. These assumptions are simply no longer valid from an airborne vantage point where the distance to the horizon is much larger, which causes performance degradation in terms of interference between neighboring cells. Therefore, regardless of recent developments of communication platforms for aircrafts, it has proven to be difficult to provide robust, broadband data communication for aircrafts such as airplanes, helicopters and the like.

Thus, in view of the above, there is a need for an improved wireless aircraft communication system which provides better capacity, improved reliability while still being cost effective. SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a wireless communication system and method for an aircraft, such as a helicopter or an airplane, which alleviates all or at least some of the drawbacks of presently known systems. Another object of the invention is to provide a means for robust and stable wireless connectivity in aircrafts.

This object is achieved by means of a wireless communication system and method for wireless data communication between a wireless communication system in an aircraft and a stationary communication server outside the aircraft, as defined in the appended claims.

According to a first aspect of the present invention, there is provided a wireless communication system for an aircraft, the wireless communication system comprising:

a router connected to an antenna, wherein the router is configured to transmit and receive wireless data communication to and from a stationary communication server outside the aircraft through at least one ground base station via the antenna;

wherein the antenna is a directional antenna;

a control unit configured to determine an attitude change of the aircraft by determining a change in at least one of a roll angle, pitch angle and/or yaw angle of the aircraft;

an antenna steering unit connected to the control unit and being configured to steer an antenna beam of the directional antenna based on the determined attitude change of the aircraft.

The inventive aircraft communication system is accordingly capable of mitigating the negative impacts of rapid aircraft movements, thereby improving network connection stability.

The present invention is at least partly based on the realization that due to distance to the horizon being relatively large form an airborne vantage point, there is a large number of network cells simultaneously "visible" for the communication system. Thus, a radio device in the aircraft, which tries to communicate with one network cell, will interfere with other network cells operating on the same frequency, and receive interference from the network cells with which it is not trying to communicate. One way to mitigate this would be to use directional antennas, i.e. antennas which a relatively sharply focused beams in order to limit the geographical area "seen" by each antenna, and therefore by the radio.

For optimum performance the antenna radiation pattern should be focused as sharply as possible, however, the sharper the radiation pattern (i.e. the beam) the smaller a change in the aircraft's attitude (pitch, roll or yaw) is necessary to lose the connection to a particular base station. An analogous situation would be to try to target an object on a ground surface with a laser mounted to an aircraft, minimal changes in the aircrafts attitude will move the laser point a great distance, and at a very great speed, on the ground surface.

Accordingly, the present inventors realized that in order to allow as sharp an antenna beam as possible while reducing the negative effects of rapid attitude changes by the aircraft, it is suggested to use an antenna steering unit which can (electronically and/or mechanically) steer the direction of the antenna beam of some antennas in the system, based on information about the aircraft's attitude changes. The attitude changes of the aircraft may for example be detected by means of a rate-gyro and/or an accelerometer (which may be combined in a sensor-fusion algorithm such as a Kalman filter).

In more detail, the act of steering an antenna beam may also be known as a beam-steering operation, and is to be understood as changing the direction of the main lobe of a radiation pattern of an antenna, for example by switching the antenna elements or changing the relative phases of the RF signals driving the antenna elements. Accordingly, the directional antenna may comprise a plurality of antenna elements in the form of a phase array or the like, as known in the art.

The "router" is preferably a networking router, which is a machine that forwards data packets between computer networks, preferably on at least two data links in each direction. Stated differently, the networking router is capable of providing data communication between an internal local area network (arranged within the aircraft) and an external wide area network

(WAN) outside the aircraft. The router may be a mobile access router (MAR), and preferably a mobile access and applications router (MAAR). The router further comprises means (e.g. a control unit or controller) for determining attitude changes of the aircraft. More specifically, the router preferably comprises means for detecting or determining an attitude change of the aircraft, either by having a control unit connected to the aircraft's own sensory systems, and/or by arranging one or more accelerometers and/or gyroscopes (e.g. rate gyros or a three-axis gyroscopes) within the router connected to the control unit. In the latter case, retrofitting to existing aircrafts is facilitated as the need for configuring the router to comply and cooperate with various systems of the aircraft is reduced. Instead the whole wireless communication system may be a type of plug and play solution.

Moreover, the antenna steering unit and the control unit may in some embodiments of the invention be in the form of an integrated unit or as separate units depending on the intended application and product

specifications. For example, the antenna steering unit may comprise a mechanical steering unit which is configured to physically move the antenna or any antenna elements. However, in another example embodiment the antenna steering unit may be an electronic steering unit which can be partly or wholly integrated in the control unit of the router. Naturally, the antenna steering unit may be a combination of the two, i.e. based on combined electronic and mechanical steering.

In terms of general operation of the communication system, the router and the stationary (remote) communication server are preferably connected through a plurality of exterior mobile/cellular networks (provided by the ground base stations), which are simultaneously useable. Also, the router is preferably arranged to communicate with the stationary communication server on at least two different data links (communication routes) having different characteristics (e.g. on different frequency bands), and then to automatically separate the data traffic between the data links based on an evaluation of link quality. The evaluation of link quality may for example be executed as disclosed in WO 2015/169917, by the same applicant, said document incorporated herein by reference. The data streams are then forwarded on one or several links to and from a dedicated external server, which may be referred to as an aggregation server or gateway. The different links thereby form a single virtual link between the router and the gateway.

Further, in accordance with an embodiment of the present invention the antenna steering unit is further configured to steer the antenna beam of the directional antenna based on the determined attitude change of the aircraft in order to compensate for the determined attitude change. Stated differently, the antenna steering unit is configured to completely or at least partly counter-act the attitude changes of the aircraft in order to at least partly decouple the direction of the antenna beam from the aircraft's movements. The term compensate in the present context is to be understood as counter act or at least reduce (the unwanted antenna beam deviations caused by aircraft attitude changes) by exerting an opposite force or effect. Thus, the attitude change need not be entirely compensated for, but can instead be only partly compensated for such that the antenna beam maintains a general direction towards a predetermined sector of a ground surface below the aircraft.

However, in another embodiment of the present invention, the antenna steering unit is configured to steer the antenna beam of said antenna, when said aircraft makes a change in attitude, such that said antenna beam maintains a direction towards a predetermined sector of a ground surface below the aircraft by compensating for said determined attitude change.

Accordingly, loss of connection between the router and one or more specific base stations located within a predetermined sector can be prevented to some extent. Thus, during for example a pitch or roll maneuver, the beam steering angle (i.e. the angle between the main direction of the antenna beam or radiation pattern and the antenna's bore-sight direction in the aircraft's vertical transverse and vertical longitudinal planes) may be adjusted to deviate by a corresponding amount as the aircraft's pitch and roll angles.

The directional antenna may in accordance with an embodiment of the invention be a phased array antenna and the antenna steering unit may be configured to electronically steer said antenna beam. However, additionally, or alternatively, the antenna steering unit comprises a mechanical steering element and wherein the antenna steering unit is configured to mechanically steer the antenna beam.

Further, in accordance with yet another embodiment of the present invention, the control unit is configured to determine a pitch angle and/or a roll angle of the aircraft, and wherein the antenna steering unit is configured to steer the antenna beam such that it deviates from a nominal bore-sight direction by a deviation angle based on the determined pitch angle and/or roll angle. Since pitch and roll maneuvers may be considered to be temporary maneuvers, i.e. maneuvers that are performed only for relatively short durations of time before reverting back to a stable horizontal position, the compensation made by the antenna steering unit can be based directly on the pitch or roll angle. For example, the antenna steering unit can be configured to steer the antenna beam angle (i.e. the angle between the radiation pattern and the bore-sight direction in the aircrafts vertical transverse and vertical longitudinal planes) to be at least approximately equal to the aircraft's pitch and roll angles at all times. The control unit is preferably configured to determine a pitch angle and/or a roll angle in reference to a state when the aircraft is in a horizontal, stable flight.

Yet further, in accordance with another embodiment of the present invention, the control unit is configured to determine a change in yaw angle of the aircraft, and wherein the antenna steering unit is configured to steer the antenna beam such that it deviates from a nominal bore-sight direction by a deviation angle based on the determined change in in yaw angle of the aircraft. While pitch and roll changes are at most times temporary, yaw changes may often be intentional and more permanent. For antenna beam adjustments related to yaw changes, a mathematical decay function may be used, to the effect that antenna beam angle is merely dampened with respect to the yaw changes of the aircraft, not directly to the actual yaw angle. This may be useful since a yaw change generally be considered to be more of a permanent attitude adjustment, and the maximum beam steering angle may be limited (e.g. ± 20°, ±30°, ±35° or ±40°). Therefore, the antenna steering unit may be configured such that a rapid yaw change of the aircraft results in an approximately equal rapid and opposite, initial, change in antenna beam angle. The antenna beam angle is subsequently, slowly, returned to the bore- sight direction (nominal pointing direction).

Furthermore, more advanced algorithms may be used in order to take into account for angular moment of the aircraft and the yaw-rate of change. Such algorithms may either allow a higher initial ground-referenced angular velocity of the antenna beam in order to avoid a "bump" in angular velocity when the maximum beam-steering is reached, or lock the beam to one ground-referenced angle for as long as possible and when the maximum beam-steering angle is reached, move the beam instantaneously to an angle that anticipates or pre-empts the continued yaw change of the aircraft. The aircraft will not be able to stop its yaw change instantaneously due to its angular momentum, so the instantaneous beam-steering angle change might as well be large enough such that, when the change in yaw angle has stopped, the beam-steering angle is within the angular range of the beam- steering.

Further, in accordance with another embodiment of the present invention, the router is connected to a plurality of directional antennas defining at least two groups of directional antennas, wherein each group comprises at least one directional antenna and each group is configured to radiate and/or receive radio waves towards/from a selected sector of a ground surface below the aircraft, the selected sectors being at least mostly non-overlapping, and preferably non-overlapping. By configuring the system such that the different groups of antennas cover different, preferably non- overlapping sectors of the ground surface below, problems related to signal interference can be reduced. However, a small amount of overlap may still be acceptable, such as an area overlap of less than 25%, less than 20%, less than 15%, less than 10% or less than 5%. Each group may comprise a plurality of directional antennas, for example each group may be configured to be compatible with a plurality of different operators, therefore having one or more antennas for each operator.

Moreover, in accordance with yet another embodiment of the present invention, the antenna steering unit is configured the antenna beam(s) of each group based on the determined attitude change of said aircraft in order to compensate for said determined attitude change. Thus, the system may have a plurality of different sectors that are to be targeted by separate antenna groups without interruptions in the communication path due to attitude changes of the aircraft, rendering the communication system more robust and reliable. Further, the antenna steering unit may be configured steer the antenna beam(s) of each group, when said aircraft makes a change in attitude, such that said antenna beam(s) in each group maintain(s) the direction towards the selected sector of a ground surface below the aircraft by compensating for said determined attitude change. Thereby the connection stability can be improved and the risk of the communication system losing contact with one or more ground base stations due to attitude changes of the aircraft can be reduced.

Yet further, in accordance with another embodiment of the present invention, the control unit is configured to evaluate a data link quality between each group and the at least one ground base station in the selected sector, and wherein the antenna steering unit is configured to steer the antenna beam(s) to radiate and/or receive radio waves towards/from a new sector of the ground surface when the data link quality is below a predefined quality threshold value. Hereby, the system may be arranged to be self-regulating in that it can maintain a minimum link quality level for each group of antennas. In more detail, the aircraft will be traveling rather rapidly above ground

wherefore a static antenna beam would result in relatively large number of handovers as the antenna beam "scans" the ground surface below a velocity corresponding the aircraft's "ground speed". Thus, by utilizing the antenna steering unit to maintain the connection to a selected sector (having one or more ground base stations) until the link quality is too low, a lower number of handovers are required as compared to the static situation where the antenna beam continuously "scans" the ground surface. Moreover, by having a plurality of groups of antennas targeting different sectors on the ground, the system can be arranged such that there is always at least one group of antennas maintaining a connection to a specific sector while one or more other antenna groups are scanning for a new sector.

The new sector may for example be located in a direction in front of the aircraft along a planned traveling route. Thus, in an illustrative example, the router may be connected to two different groups having one or more directional antennas. The antenna steering unit may then be configured to steer the antenna beam(s) of each group to target and maintain a direction towards two different, preferably non-overlapping sectors on the ground surface below the aircraft. However, as the aircraft travels the sectors will be located further and further behind the aircraft, wherefore the link quality will decrease over time. Thus, when a search for a new sector is to be performed, it is advantageous to ensure that it is located in front of the aircraft, since the same sectors (and the associated ground base stations) can be utilized for a longer duration of time, requiring less handovers and thereby less

interruptions in connectivity.

Furthermore, the antenna steering unit may be configured to select the new sector by steering the antenna beam(s) along a search pattern until the data link quality is above a predefined establishment threshold value. The antenna steering unit may for example to be configured to follow a specific search pattern (e.g. spiral) while searching for a new sector comprising the appropriate ground base stations. Further, the system may for example comprise a Global Navigation Satellite System, GNSS, provided within the router, such as e.g. GPS, GLONASS, Galileo system, BeiDou system, etc. The GNSS may be connected to the control unit and/or the antenna beam steering unit whereby the antenna beam(s) may be directed towards predefined sectors of the ground surface below the aircraft based on GNSS data. Moreover, the control unit or the GNSS may be provided with a planned route comprising a plurality of different sectors along the route and the selection of the new sector may be based on a predetermined series of preselected sectors along the route.

Even further, in accordance with yet another embodiment of the present invention, the steering unit is configured to the antenna beam(s) of each group sequentially such that the direction of the antenna beam(s) of at least one group is maintained towards the selected sector(s) while steering the antenna beam(s) of at least one different group to radiate and/or receive radio waves towards/from the new sector. During travel the aircraft will pass through a large number of cells, therefore, by sequentially switching to new cells (sectors) along the travel route a certain level of connectivity can be maintained during the travel. For example, if the system would comprise four groups of antennas targeting four different sectors (cells), the system may be configured such that at any given time three or at least two groups are arranged to maintain the antenna beam(s) to receive/transmit from/to a selected cell while the remaining one or two groups are steered towards a new cell, c.f. a horse trotting where two legs are on the ground and the other two are moving forward.

According to another aspect of the invention, there is provided a method for wireless data communication between a wireless communication system in an aircraft and a stationary communication server outside the aircraft, the method comprising:

providing a router within the aircraft, the router being connected to a directional antenna and configured to transmit and receive wireless data communication to and from the stationary communication server outside the aircraft through at least one ground base station via the directional antenna; determining an attitude change of the aircraft by determining a change in at least one of a roll angle, pitch angle and/or yaw angle of the aircraft; steering an antenna beam of the directional antenna based on the determined attitude change of the aircraft.

With this aspect of the invention, similar advantages and preferred features are present as in the previously discussed first aspect of the invention, and vice versa. For example, the step of steering the antenna beam may comprise maintaining a direction towards a predetermined sector of a ground surface below the aircraft by compensating for said determined attitude change.

Further, in accordance with an embodiment of the present invention, where the router is connected to a plurality of directional antennas defining at least two groups of directional antennas, wherein each group comprises at least one directional antenna and each group is configured to radiate and/or receive radio waves towards/from a selected sector of a ground surface below the aircraft, the individual sectors being at least mostly non- overlapping, and preferably non-overlapping, the method further comprising: evaluating a data link quality between each group and the at least one ground base station in the selected sector; and

steering said antenna beam(s) such that said at least one antenna in the evaluated group radiates and/or receives radio waves towards/from a new sector of the ground surface when said data link quality is below a predefined quality threshold value. These and other features and advantages of the present invention will in the following be further clarified with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

For exemplifying purposes, the invention will be described in closer detail in the following with reference to embodiments thereof illustrated in the attached drawings, wherein:

Fig. 1 is a partly exploded perspective view illustration of an aircraft having a wireless communication system in accordance with an embodiment of the present invention;

Fig. 2A is a top view illustration of an aircraft having a wireless communication system in accordance with an embodiment of the present invention;

Fig. 2B is a perspective view illustration of the aircraft in Fig. 2A making a roll maneuver;

Fig. 2C is a perspective view illustration of the aircraft in Fig. 2A making a pitch maneuver;

Figs. 3A - 3E are top view illustrations of an aircraft having a wireless communication system in accordance with another embodiment of the present invention.

Fig. 4 is a schematic flowchart representation of a method in accordance with an embodiment of the present invention. DETAILED DESCRIPTION

In the following detailed description, preferred embodiments of the present invention will be described. However, it is to be understood that features of the different embodiments are exchangeable between the embodiments and may be combined in different ways, unless anything else is specifically indicated. Even though in the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well known constructions or functions are not described in detail, so as not to obscure the present invention. In the detailed embodiments described in the following are related to airplanes. However, it is to be acknowledged by the skilled reader that the method and system are correspondingly useable on other aircrafts, such as helicopters and the like.

Fig. 1 illustrates a schematic perspective view illustration of an aircraft 10, here in the form of an airplane, having a wireless communication system 1 in accordance with an embodiment of the present invention. The system 1 has a router 3 connected to a plurality of (external) antennas 2a - 2c. The router 3 is configured to transmit and receive wireless data communication to and from a stationary communication server (not shown) outside the aircraft 10 through at least one ground base station 6a - 6c via the antennas 2a - 2c. The antennas 2a - 2c are directional antennas, which may for example be passive beam forming arrays having various polarizations. Moreover, each antenna 2a - 2c may be realized as an antenna orthogonal pair by e.g. using a dual polarized antenna setup with a 90-degree angle between two linear polarizations or using circular left- and right handed polarizations. However, in alternative embodiments spatial diversity may be utilized to achieve

orthogonal antenna diversity.

The antennas 2a - 2c may be mounted to an external surface of the aircraft 10, such as e.g. to the aircraft's 10 fuselage. However, the antennas 2a - 2c may also be integrated in the external surface of the aircraft 10. A combination of these two is also feasible.

The system 1 further has a control unit 4, e.g. a microprocessor, which is configured to determine an attitude change of the aircraft 10 by determining a change in at least one of a roll angle, pitch angle and/or yaw angle of said aircraft. The control unit 4 is preferably realized as a software controlled processor. However, the control unit 4 may alternatively be realized wholly or partly in hardware. Moreover, the system 1 may further comprise at least one of a gyroscope and an accelerometer (not shown) so that the attitude change can be determined by means of the gyroscope(s) and/or accelerometer(s). A change is roll angle is to be understood as the aircraft 10 making a rotation about its longitudinal axis 101 , also commonly referred to as a roll axis. Similarly, a change in pitch angle is to be understood as the aircraft 10 making a rotation about its lateral/transverse axis 102, also commonly referred to as a pitch axis, and a change in yaw angle is to be understood as the aircraft making a rotation about its vertical axis 103, also commonly referred to as a yaw axis.

The system 1 further comprises an antenna steering unit 5 connected to the control unit 4. The antenna steering unit 5 is configured to steer the antenna beams 20 of each directional antenna based on the determined attitude change of the aircraft 10. Even though the control unit 4 and antenna steering unit 5 are here illustrated as two separate entities, the skilled person readily realizes that these may be integrated into one single unit. The antenna steering unit may for example be in the form of an electronic antenna steering unit which steers the antenna beam(s) 20 by e.g. by switching the antenna elements or changing the relative phases of the RF signals driving the antenna elements. The antennas may as mentioned be in the form of passive beam forming arrays comprising a plurality of antenna elements (not shown). However, additionally or alternatively the antenna steering unit 4 may comprise a mechanical steering element in order to mechanically/physically steer the antennas 2a - 2c, such as e.g. an actuator mounted to the antennas (not shown).

The router 3 further has a plurality of modems 9, where each antenna 2a - 2c, or each antenna orthogonal pair, preferably is assigned and connected to a separate modem 9. In case of the latter each modem 9 is preferably provided with 2 antenna ports for connection to each orthogonal antenna pair. However, each modem may also be provided four or more ports for compliance with MIMO (Multiple Input Multiple Output) systems. Moreover, the router 3 preferably comprises a subscriber identity module pool (SIM pool) 13 which includes a plurality of SIMs 14, and the control unit 8 is accordingly configured to periodically assign SIMs 14 within the SIM pool 13 to any one of the plurality of modems 9 provided within the router 3. In other words, the SIMs 14 form a common SIM pool 13, accessible for all the modems 9. The SIMs 14 are preferably SIM cards, and the SIM pool 13 is realized as a SIM card holder, comprising a plurality of slots for receiving a plurality of SIM cards 14.

The assignment of SIMs to modems at every specific time is preferably determined based on a set of rules in the controller. The set of rules may e.g. be used to assign SIMs to the modems based on information such as in, the current altitude of the aircraft 10, which country the aircraft is currently travelling, the amount of data that has been conveyed by use of the different SIMs, the current price related to conveying data through the different SIMs, the type of data being conveyed, etc.

Furthermore, the router 3 is preferably configured for receiving and transmitting data between an internal local area network (LAN) and a plurality of external wide ware networks (WANs). The LAN is preferably a wireless network, using one or several internal antennas to communicate with clients within the aircraft 10. To this end, it is e.g. feasible to use a distributed antenna, such as a leaky feeder extending through the vehicle, but other types of antennas may also be used. The wireless network may be realized as a wireless local area network (WLAN), and may e.g. operate based on the IEEE 802.1 1 standard, ("Wi-Fi"), and wherein one or more access point(s) is provided in the aircraft. However, it is also possible to use a wired network within the vehicle.

Fig. 2A is a schematic top view illustration of an aircraft 10 having a wireless communication system in accordance with an embodiment of the invention. Here, the aircraft is provided with one directional antenna 2 with an antenna beam or radiation pattern schematically indicated by the "dotdashed" lines 30. More specifically, the dotdashed lines 30 represent the antenna's "bore-sight" direction, i.e. its nominal pointing direction. The antenna beam 30 in Fig. 2A targets a predetermined sector on the ground surface below the aircraft, the sector comprising a ground base station 6, whereby a router within the aircraft 10 which is connected to the antenna 2 can transmit and receive wireless data communication to and from an external stationary communication server.

Fig. 2B is a schematic perspective view illustration of an aircraft from Fig. 2A during a roll maneuver. In other words, the aircraft 10 has changed its roll angle as compared to the horizontal stable orientation illustrated in Fig. 2A. Accordingly, the control unit of the router (not shown in this figure) determined the attitude change of the aircraft, e.g. by means of a gyroscope, whereby the antenna steering unit steered the antenna beam 20 of the antenna 2 in order to compensate for the determined attitude change. The dotdashed lines 30 serve to emphasize that the antenna beam 20 has deviated from its bore-sight direction 30. The steering unit steered the antenna beam 20 of the antenna 2 such that the antenna beam 20 maintains a direction towards the predetermined sector of a ground surface below the aircraft by compensating for the determined roll angle (attitude change).

Similarly, Fig. 2C a schematic perspective view illustration of an aircraft from Fig. 2A during a pitch maneuver, or stated differently, the aircraft 10 has changed its pitch angle as compared to the horizontal stable orientation illustrated in Fig. 2A. Analogously, the control unit of the router (not shown in this figure) determined the attitude change of the aircraft, e.g. by means of a gyroscope, whereby the antenna steering unit steered the antenna beam 20 of the antenna 2 in order to compensate for the determined attitude change. The dotdashed lines 30 serve to emphasize that the antenna beam 20 has deviated from its bore-sight direction 30. The steering unit steered the antenna beam 20 of the antenna 2 such that the antenna beam 20 maintains a direction towards the predetermined sector of a ground surface below the aircraft by compensating for the determined pitch angle (attitude change).

Further, Fig. 3A is a schematic top view illustration of an aircraft 10 having a wireless communication system in accordance with another embodiment of the present invention. The system has a router (not shown) connected to a plurality of directional antennas 2 which define four groups of directional antennas. Here the four groups are spatially separate along the aircraft's 10 fuselage to primarily target four separate quadrants of the ground surface below the aircraft. Each group has a directional antenna 2 and each group is configured to radiate and/or receive radio waves towards/from a selected sector 21 -24 of a ground surface below the aircraft 10. The sectors 21 -24 are preferably non-overlapping, but may be at least mostly non- overlapping as some overlap may be acceptable. The series of Figures 3A - 3E serve to show how the antenna steering unit may be utilized to steer the antenna beams so to select new sectors to be targeted during the duration of the flight in order to minimize the otherwise "continuous sweeping" of the ground surface by the antenna beam which may require a large number of handovers per unit of time and therefore reduced network performance. Instead, a control unit of the router may be configured to evaluate a data link quality between each group and at least one ground base station in the selected sector 21 -24, and based on this data, the antenna steering unit may be configured to steer the antenna beam(s) towards a new sector 31 -34 when the data link quality falls below some threshold value (e.g. due to a too long of a distance between the antenna and the base station(s)). Furthermore, the steering of the antenna beam(s) of each individual group may be performed sequentially one at a time, i.e. one antenna beam is steered towards a new sector while the remaining ones maintain their selected "old" sector, or other configurations are feasible e.g. 2 at a time, 3 at a time and so on depending on the number of groups and other

specifications.

Fig. 3B illustrates how the steering unit control the antenna beam of the first group (top left in reference to the illustrated aircraft 10) to select a new sector 31 which is located in front of the aircraft along a planned traveling route. The selection may be based on predefined data provided by a GNSS (e.g. a GPS) and/or by steering the antenna beam along a predefined search pattern (e.g. a spiral path). The antenna beams of the remaining antenna groups are instead steered so to maintain a direction towards their previously selected sectors 22-24. Analogously, Figs. 3C - 3E show a series of how the antenna steering unit sequentially controls the antenna beam of each group so to search for a new sector while the remaining groups' antenna beams are steered so to maintain their direction towards a current sector.

As mentioned other beam steering configurations are feasible and within the scope of the present invention. For example, in reference to the system configuration illustrated in in Figs. 3A - 3E, where the wireless communication system comprises four groups of antennas, the antenna steering unit may be configured to steer the antenna beams of the four groups in pairs. In more detail, the antenna steering unit may be configured to steer diagonal pairs separately, meaning that the top left and bottom right antenna groups are steered towards a new sector while the remaining two are steered so to maintain the antenna beam(s) aimed at their current sectors, and subsequently the top right and bottom left antenna groups are steered towards a new sector while the remaining two are steered so to maintain the antenna beam(s) aimed at their current sectors, c.f. a horse trotting. Left and right are in reference to the illustrated aircraft 10 in Figs. 3A - 3E. However, in an analogous manner the antenna steering unit may be configured to steer front two groups towards a new sector together while the back two are steered so to maintain the antenna beam(s) aimed at their current sectors (front and back being in reference to the nose and tail of the aircraft 10).

As yet another alternative, the four groups may be paired with respect to which side of the aircraft they are arranged, meaning that the top left and bottom left antenna groups are steered towards a new sector while the remaining two are steered so to maintain the antenna beam(s) aimed at their current sectors and subsequently the top right and bottom right antenna groups are steered towards a new sector while the remaining two are steered so to maintain the antenna beam(s) aimed at their current sectors. Left and right are in reference to the illustrated aircraft 10 in Figs. 3A - 3E.

Fig. 4 is a schematic flowchart representation of a method for wireless data communication between a wireless communication system in an aircraft and a stationary communication server outside the aircraft in accordance with an embodiment of the present invention. The method includes a step of providing S401 a router within the aircraft. The router is connected to at least one directional antenna and configured to transmit and receive wireless data communication to and from the stationary communication server outside the aircraft through at least one ground base station via the one or more directional antennas.

Further, an attitude change of the aircraft is determined S402 by determining a change in at least one of a roll angle, pitch angle and/or yaw angle of the aircraft. After which, an antenna beam (or radiation pattern) of the at least one directional antenna is steered S403, based on the determined S402 attitude change of the aircraft. More specifically, the steering S403 of the antenna beam may include maintaining a direction (of the antenna beam) towards a predefined sector of the ground surface below the aircraft by compensating for the determined S402 attitude change. Thereby, the antenna may be kept in communication with any ground base stations in that sector in spite of any attitude changes.

Also, the steering S403 of the antenna beam(s) may be based on an evaluation S404 of a data link quality between each antenna and the one or more base stations which are present in the sector of the ground surface which each antenna is arranged to target. Accordingly, the steering S403 of the antenna beams may be done such that the "evaluated" antennas radiate and/or receive radio waves towards/from a new sector of the ground surface when the data link quality is below a predefined quality threshold value.

The invention has now been described with reference to specific embodiments. However, several variations of the communication system are feasible. For example, the control unit may be considered to include the antenna steering unit, the number of modems may vary, and so on.

Moreover, more advanced mathematical algorithms for compensating for yaw changes may be employed in order to take into account the angular moment of the aircraft and the yaw rate-of-change, in order to account for a situation where the yaw changes so rapidly that the beam steering reaches its maximum possible angle, as already exemplified. Such and other obvious modifications must be considered to be within the scope of the present invention, as it is defined by the appended claims. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative

embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting to the claim. The word "comprising" does not exclude the presence of other elements or steps than those listed in the claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.