Antenna array pointing direction estimation and control

Systems for maintain pointing of a phased array antenna in a direction maximizes to an extent possible the effective antenna gain is provided. The system includes a gyroscope and a Kalman Filter. The Kalman filter estimates an antenna pointing direction for each successive time step based on gyroscope readings and based on the results of a local search for maximum gain performed in the neighborhood of a previous antenna pointing direction.

RELATED APPLICATION DATA

This application is based on provisional application 61/982,287 filed Apr. 21, 2014.

FIELD OF THE INVENTION

The present invention relates generally to wireless communications with phased array antennas.

BACKGROUND

In many cases it would be desirable to conduct wireless communications with and/or from a moving vehicle or other moving object. For example, as far as vehicles, it would be desirable to conduct communications from a moving boat, airplane or truck and it is desirable to conduct communications from such vehicles through a communications satellite which may be in geosynchronous orbit but may also be in another orbit such that the satellite is moving relative to the earth and relative to a vehicle on the earth (even if the vehicle's position is stationary with respect to the earth).

Wireless communication systems use antennas to couple radio waves to and from the “free space” medium. Antennas come in very wide variety of types. For our present purpose, one relevant way in which antennas can be classified is whether they are high gain, directional antennas or low gain non-directional antennas. A high gain antenna has an advantage in that it increases link budget and thus can be used to sustain communication at a relatively higher information rate with a given allowance of energy per information symbol. This may enable types of communications such as audio, and video that require higher data rates than would simple text messages, for example. However to realize the aforementioned advantage of directional antennas, such antennas must be correctly pointed such that the high gain direction is aimed in the direction of another communication terminal with which communication is being conducted. This is problematic if the directional antenna is mounted on a moving vehicle and the difficulty is further compounded if the other communication terminal is moving such as would be the case if the other communication terminal is a non-geosynchronous satellite.

Mechanically steered directional antennas are available, however these are bulky, require extra clearance for movement, require expensive precision rotation mechanisms, servo motors and associated servo motor drive electronics. Such requirements limit the application of mechanically steered directional antennas.

DETAILED DESCRIPTION

FIG. 1is a schematic representation of a satellite communication system100according to an embodiment of the invention. The earth102is represented by a doubled line circle and a vehicle in particular a truck104is located on the surface of the earth102. In lieu of the truck104a sea going object (e.g., vehicle), a flying object (e.g., vehicle), or another type of apparatus that is mobile over land may be used. An Earth Centered Coordinate System (ECCS)102is represented by a latitude angular coordinate ⊖ and a longitude angular coordinate ϕ and radial coordinate R which is related to altitude.

It is also useful to consider a Local Earth Coordinate System (LECS) that has its origin at the position of the truck104and has its axes oriented at certain orientations relative to the earth. In particular, one form of LECS can have: a first axis (“North”) oriented in a plane that includes the axis of rotation of the Earth facing up (in the compass North direction); a second axis (“East”) oriented perpendicular to the plane that includes the axis of rotation of the earth, facing in the compass East direction; and a third axis “Up” oriented radially with respect to the center of the earth. Alternatively, the third axis may face oppositely, i.e., down toward the center of the earth.

The truck104(or other object) can, in general have any arbitrary orientation. Orientation of an object has three degrees of freedom. (Translation adds an additional three degrees of freedom) By way of example, one manner of specifying the orientation of an object is to specify the yaw, pitch and roll angles of the object. Another way is to use a quaternion that specifies a rotation.FIG. 2schematically illustrates the truck104having an associated vehicle coordinate system that includes X, Y and Z axes that can be arbitrarily oriented in the LECS (North-East-Up coordinate system). The yaw, pitch and roll rotations are respectively rotations about the Z, Y, and X axes of the vehicle coordinate systems. By way of example the truck104can be driving on a road through a banked curve descending a mountain so that all three of the yaw, pitch and roll rotations occur as the truck104travels along its route.

The system100includes a number of satellites106,108,110including a first satellite106, a second satellite108and a third satellite110. The satellites106,108,110can, in general, include satellites that are in geostationary orbit and hence fixed in the ECCS and satellites that are not in geostationary orbit and hence not fixed in the ECCS.

The truck104is equipped with a phased array antenna112that can be configured to produce high gain patterns centered at any one of a set of directions. When the phased array antenna112is used to communicate with one of the satellites106,108,110it is beneficial to configure the phase array antenna112such that the high gain direction is as close as possible to the direction of the satellite. A benefit of doing so is to increase the link budget and allow for higher data rate communications at a given energy per information symbol. However, the movement of the satellites106,108,110, movement of the truck104and the yaw, pitch and roll of the truck104tends to quickly render any previously established pointing direction obsolete.

FIG. 3is a block diagram of a radio system300comprising a satellite radio302coupled to the phased array antenna112according to an embodiment of the invention. The phased array antenna112includes a controller306coupled to a digital phase shifter array308which is coupled to antenna element array310. The satellite radio302is coupled to the controller306of the phased array antenna112through a control bus312. An indication of the strength of a received signal can be coupled from the satellite radio302to the controller306through the control bus312. Alternatively, a subsystem for measuring received signal strength is incorporated in the phased array antenna112itself. The satellite radio302is also coupled to an input/output port314of the digital phase shifter array308through a communication signal link316. The digital phase shifter array308is coupled through a set of N (where N>1) antenna element channels318to the antenna element array310. Each of the antenna element channels318serves one antenna element (seeFIG. 4) of the antenna element array310. Under the control of the controller306, the digital phase shifter array308applies a requisite phase shift between the input/output port314and each of the antenna element channels318in order to steer the peak gain of the phased array antenna112in one of a set of directions. The controller306can monitor the strength of the received signal through information received via the control bus312while trying at least a subset of the set of the possible directions. In this way the controller can “search” for the best direction, i.e., the direction in which the peak gain direction is closest to the direction from which signal from one of the satellites106,108,110is received. However, blindly running through each direction will in many cases be ineffective because the amount of time required to check a given direction compared to the shorter time scale characterizing the changes in orientation of the truck104(or other vehicle or object).

FIG. 4is a perspective view of the antenna element array310of the phased array antenna112according to an embodiment of the invention. The antenna element array310is an array of Quadrifilar Helical Antenna (QHA) elements402(a limited number of which are labeled to avoid crowding the drawing). Alternatively, in lieu of QHA elements another type of element, such as for example a patch antenna element, dipole, or slot can be used. As shown the antenna element array310is a four by four square array of QHA elements402. Alternatively, a different number and/or a different pattern (i.e., non-square array) of elements may be used. Examples of QHA elements that may be used are disclosed in co-pending patent application Ser. No. 13/297,854 filed Nov. 16, 2011, entitled “Co-axial Quadrifilar Antenna”. In certain embodiments of the present invention only one of the co-axial elements, i.e., either the inner or outer set of elements, disclosed in the '854 application may be used in the antenna element array310.FIG. 4shows a set of coordinate axes and a polar angle ⊖ and an azimuth angle ϕ superimposed on the drawing. The polar angle ⊖ and the azimuth angle ϕ can be used to describe the direction of maximum gain for a given configuration of the phased array antenna112.

FIG. 5is a spherical coordinate system graph500schematically representing a set of possible gain patterns502(a limited number of which are labeled to avoid crowding the drawing) that can be produced with a phased array antenna, with each gain pattern centered on a particular azimuth ϕ and polar angle ⊖. The gain patterns502are represented schematically, and in practice the shape of the gain patterns502may be different from what is depicted inFIG. 5. The same set of X, Y and Z coordinate axes presented inFIG. 4establish the (ϕ,⊖) domain of the spherical graph500shown inFIG. 5. Also, because the phased array antenna112is fixed to the truck104(or other moving vehicle or moving object) the X, Y, Z axes of the coordinate system of the phased array antenna112either correspond to, or a have a fixed orientation with respect to the X, Y, Z axes of the vehicle coordinate system shown inFIG. 2. In the former case where the coordinate systems directly correspond, a yaw rotation of the vehicle will call for change in the azimuth of the antenna pointing direction, while pitch and roll rotations of the vehicle will call for a change in azimuth and/or polar angle of the antenna pointing direction.

FIG. 6is a graph600including a schematic illustration of a 2-D cut through a set of adjacent gain patterns that can be produced with a phased array antenna (e.g.,112) according to an embodiment of the invention. Actual gain patterns may differ in shape from what is shown inFIG. 6. The 2-D cut represented inFIG. 6corresponds to a cut at constant azimuth angle ϕ with the polar angle ⊖ varying from 0 to π/2. (However a qualitatively similar plot would be obtained taking a cut at constant polar angle ⊖ with the azimuth angle ϕ varying or taking a cut through some arbitrary contour in the 2-D angle space, although in the latter case the peaks might vary substantially). With reference toFIG. 6we assume that the azimuth angle ϕ to which the phased array antenna112has been pointed is as close as possible (though not in general equal) to the actual azimuth angle describing the position of a satellite with which communication is being conducted. We assume further that the satellite is located at a polar angle ⊖ of 0.7 radians (40.1°). The graph600includes a vertical line602at the polar angle value of 0.7 radians.

To the extent that a digital phase shifter array308is used in the phase array antenna112the pointing angle cannot be adjusted infinitesimally Rather the phased array antenna112can have its direction of maximum gain set to one a certain finite set of directions each of which is described by an azimuth angle ϕ and a polar angle ⊖. In the example illustrated with reference toFIG. 6we assume that the polar angle can be adjusted to multiples of π/6 radians (30°). (In practice the achievable polar angles may not be equally spaced.) A first gain pattern604with its peak at π/2 radians is closest to the polar coordinate (0.7 radians) of the satellite, and therefore produces the highest gain indicated by an upper dot606on the vertical line602. A second gain pattern608with its peak at π/3 (to the left of the first gain pattern604) exhibits a lower gain at 0.7 radians represented by middle dot610. A third gain pattern612with its peak at (⅔)π (to the right of the first gain pattern604) exhibits an even lower gain at 0.7 radians represented by lower dot614.

In certain embodiments of the invention the controller306(or other controller in a system) will perform a “local search” around an estimated pointing angle (gain pattern) for a gain pattern that in fact produces the highest gain. For example if the first gain pattern604were estimated (by other subsystems to be described hereinbelow) to be the gain pattern that would maximize gain, a 1-D version of a local search could check the gain for the first gain pattern604and the two gain patterns adjacent to it which are the second gain pattern608and the third gain pattern612. Through such a local search the controller306could determine which gain pattern actually produces the highest gain (highest dot on vertical line602). The estimate made prior to the local search may be correct or it may be in error (e.g., off by one or more steps in angle).

In practice, in certain embodiments, rather than performing a 1-D local search, a 2-D local search is performed around an estimated pointing angle. For example we can identify an estimated best pointing angle as (⊖i, ϕk) where the index i identifies an ithpolar angle to which the phased array antenna112can be steered and the index k identifies a kthazimuth angle to which the phased array antenna112can be steered, and there is a monotonic relation between the index k and the azimuth angle and there is a monotonic relation between the index l and the polar angle. In this case a local search can initially check all of the following pointing angles: (⊖i, ϕk) itself and all neighboring angles: (⊖i+1, ϕk), (⊖i−1, ϕk), (⊖i, ϕk+1), (⊖i, ϕk−1), (⊖i+1, ϕk+1), (⊖i−1, ϕk−1), (⊖i+1, ϕk−1), (⊖i+1, ϕk−1). In certain embodiments if the initial local search finds that one of the neighboring angles does in fact produce higher gain initially estimated pointing angle (⊖i, ϕk) a further search of angles adjacent to the pointing angle that produced the largest gain (but excluding angles already checked) can be performed. Alternatively, the local search can initially check just two angles in addition to estimated best pointing angle as (⊖i, ϕk). One of the angles will differ in respect to the azimuth angle ϕ and the second of the angles will differ in respect to the polar angle ⊖. From the gain values attained with these three directions, a discrete gradient estimate can be computed and then a fourth angle that is in the direction of the direction of the gradient can be checked, and the local search can proceed to follow the locally computed gradient.

FIG. 7is a block diagram of a system700for maintaining an antenna (e.g., phased array antenna112) pointed at a second device (e.g., one of the three satellites106,108,110) with which communications are conducted according to an embodiment of the invention. The system700includes inertial navigation sensors702which include a gyroscope set704and optionally includes accelerometers705which can provide additional orientation information by measuring the direction of gravity. The gyroscope set704suitably includes three gyroscopes that indicate rotation rates about three mutually orthogonal axes which, by way of non-limiting example, may be the yaw, pitch and roll axes identified inFIG. 2. Outputs of the gyroscope set704are coupled to inputs of a Kalman filter706tracking the maximum gain antenna pointing direction. The Kalman filter706evolves a best estimate of the maximum gain pointing angle forward in time based on readings of the rate of rotation of the truck104(or other vehicle or object) that are output by the gyroscope set704. The controller306is also coupled to the Kalman filter706. The result of the “local search” described above which is also an estimate of the maximum gain antenna pointing direction and that is found by the controller306is output by the controller306and is input into the Kalman filter706as a sensor reading of the maximum gain antenna pointing direction. The Kalman filter706produces a new estimate for each new time step of a series of time steps based, in part, on the result of the local search and based, in part, on adjustments of a previous estimate based on the gyroscope readings. The rates of rotation of the truck104(or other vehicle or object) read from the gyroscope set704can be used to predict how the azimuth ϕ and elevation angle ⊖ of the maximum gain pattern, which approximately corresponds to the direction to the second terminal (e.g., satellite106,108,110) will change with each time increment. However the local search provides a second piece of information concerning the direction of the gain pattern that exhibits maximum gain.

If the satellite signal is lost, for example because the truck104has driven behind an obstruction (e.g., building), the Kalman filter706can continue to update antenna pointing direction so that when the truck passes the obstruction, the satellite radio302and phased array antenna302can begin to try to reacquire the satellite signal and the phased array antenna112will be more likely to be pointed in the correct direction or close to the correct direction, even if the orientation of the truck104has changed (e.g., because the truck turned a corner).

FIG. 8is a flowchart800of a method of setting and maintaining an antenna (e.g.,112) pointed at a second device (e.g., satellite) with which communications are conducted according to an embodiment of the invention. In block802an initial global search for a satellite (or other second terminal communication device) is made. The global search can start a random or predetermined angle and search for the antenna pointing direction that maximizes gain. In block804communications with the second device (e.g., satellite) are commenced. It should be noted however that the global search will entail receiving signals from the second device, and can include decoding those signals. Moreover, it is also possible to commence communications when an antenna pointing direction that gives adequate signal strength is found and to continue the global search of block802while conducting communications.

In block806the Kalman filter706is initialized with the azimuth ϕ and polar ⊖ corresponding to the antenna pointing direction that was found in block802.

In block808the rotation rates are read from the gyroscope set704and in block810the rotation rates are input into the Kalman filter.

According to certain embodiments which include accelerometers705in the inertial navigation sensors, the direction of gravity sensed by the accelerometers is also read and input into the Kalman filter as an additional sensor reading related to orientation.

In block812the Kalman filter is run in order to predict new values of the azimuth ϕ and polar ⊖ for the signal second terminal (e.g., satellite106,108,110).

In block814the phased array antenna112is pointed in the direction predicted by the Kalman filter in block812.

In block816a local search is performed to check if the new pointing angle or an adjacent pointing angle yields the maximum gain. The local search has been described hereinabove.

In block818azimuth and polar angles that were found in the local search, that is the azimuth ϕ and polar ⊖ angles describing the antenna pointing direction that resulted in maximum gain are input into the Kalman filter as sensor readings of the angles (e.g., satellite).

Thereafter, the flowchart800loops back to block808and proceeds as previously described.

In certain embodiments the local search that is performed in block816takes longer than an interval at which the inertial navigation sensors702update their readings. In such cases the system700can continue to update the antenna pointing direction based on estimates that are produced by the Kalman filter706as it continues to process new information from the inertial navigation sensors702, and then, at intervals, when new AOA information is available from the local search the Kalman filter can incorporate that information. This alternative is signified by the dashed line connecting blocks814and808inFIG. 8.

FIG. 9is a block diagram of a system900for maintaining a phased array antenna pointed at a second communication terminal (e.g., satellite,106,108,110) and maintaining orientation information for a first apparatus (e.g., truck104) that includes the phased array antenna according to an embodiment of the invention. The system is suitable for mounting on the truck104or other moving vehicle or object. The system900includes a GPS antenna902coupled to a GPS receiver904. The GPS receiver904is coupled to a Kalman filter906that is tracking the location, velocity, acceleration, orientation and angular velocity of the truck104(or other moving vehicle or object). The GPS receiver904provides location and velocity estimates to the Kalman filter906.

The gyroscope set704is also used in the system900. The gyroscope set704provides angular velocity information (in three components) to the Kalman filter906.

The system900also includes a 3-axis accelerometer908that is coupled to the Kalman filter906and provides acceleration information (in three components) to the Kalman filter906.

An optional electronic compass910and stored earth magnetic declination map912are provided. A location estimate output by the Kalman filter906(but which may initially be obtained directly from the GPS receiver904) provides location information to the earth magnetic declination map912. The local magnetic declination output by the earth magnetic declination map912for the current location and a reading of the electronic compass are input to a second orientation computer914(the first will be described below), which computes an orientation estimate for the truck104(or other moving vehicle or object), and this orientation estimate is input into the Kalman filter906.

Optional vehicle control input sensors916, such as sensors on the steering, accelerator and brake of the truck104(or analogous parts of another vehicle or object) are also input into the Kalman filter906.

A stored model of position of a satellite106,108,110(or more generally a second communication terminal) as a function of time918, (e.g., satellite ephemeris) outputs a satellite location920at a current time. The stored model of satellite location as a function of time is coupled to the GPS receiver from which the current time is received. The current location of the truck104(or other moving vehicle or object) in combination with the current location of the satellite106,108,110can be combined to deduce what the polar and azimuth angles of the satellite, and hence the Angle of Arrival (AOAcalc) are in the Local Earth Coordinate System (LECS) which is described above with reference toFIGS. 1 and 2. By definition (with a high degree of accuracy) the direction of gravity in the LECS is in the negative “Up” axis direction, i.e., straight down.

An estimate of the actual Angle of Arrival (AOAmeas)922of the signal from the satellite is provided by the local search conducted by the controller306of the phased array antenna112as described above. A measurement of the local gravity vector is obtained (e.g., by low pass filtering) from the accelerometer908.

The first orientation computer924is coupled to the 3-axis accelerometer908, the Kalman filter906, the phased array antenna112, and the stored model of satellite location as a function of time. The first orientation computer924receives: the AOA estimate produced by local search922from the phased array antenna122; an estimate of the gravity direction from the 3-axis accelerometer908; the satellite location920from the stored model of satellite position as a function of time918; and an estimate of the current location from the Kalman filter906(although initially or alternatively the current location estimate may be received from the GPS receiver904directly).

Based on knowledge of the directions of the AOA and gravity in both the vehicle coordinate system and the LECS, the orientation of the vehicle coordinate system in the LECS can be determined. This determination is done by the first orientation computer924.

The system900also includes an antenna pointing direction computer926. The antenna pointing direction computer is coupled to the Kalman filter906, and the stored model of satellite location as a function of time918. The antenna pointing direction computer926receives current estimates of the location and orientation of the truck104(or other moving vehicle or object) from the Kalman filter906; and receives the satellite location920from the stored model of satellite location as a function of time918. Based on the current estimate of the location truck (or other moving vehicle or object) that is provided by the Kalman filter906, and based on the satellite location920, the antenna pointing direction computer926computes the correct antenna pointing direction in the LECS (seeFIG. 2), and then subsequently translates the correct pointing direction into the vehicle coordinate system (seeFIG. 2) based on the orientation estimate for the truck (or other moving vehicle or object) maintained by the Kalman filter906.

The Kalman filter906aggregates various sources of information on the orientation, including the information from the first orientation computer924, information from the second orientation computer914, information from the gyroscope set704and accelerometers908. The first orientation computer924uses the maximum gain (AOA) information received from the phased array antenna112. Alternatively rather than using the Kalman filter906to aggregate orientation information a least squares estimator is used as an orientation information aggregator. All of the blocks112,302,704,902,904,906,908,922that feed into the first orientation computer924along with the first orientation computer itself can be considered a first subsystem that determines one orientation estimate. The second orientation computer914and the blocks which feed into it910,912can be considered a second subsystem that determines another orientation estimate.

According to certain embodiments, which are appropriate for vehicles for which the orientation of the vehicle or object is aligned with the direction of the travel (e.g., a truck that can only travel forward or is assumed to be traveling forward, but not in reverse), the velocity information obtained from the GPS receiver can be used as yet another orientation estimate (although the information is not complete in that the roll orientation is not necessarily determined by the velocity).

FIG. 10is a flowchart1000of a method of maintaining an antenna pointed at a second device (e.g., satellite) and for maintaining orientation information for an apparatus that includes the antenna according to an embodiment of the invention. In block1002an antenna (e.g.,112) is used to search an angular domain (e.g., domain over azimuth ϕ and polar ⊖ angle ranges) for a maximum gain direction. When block1002is initially executed, the search can be a global search described above, which can for example cover an entire discretized 2π steradian hemisphere. When block1002is subsequently executed the search that is executed can be a local search as described above.

In block1004a current location, velocity of the truck104(or other moving vehicle or object) and the current time provided by a GPS receiver (e.g.,904) are read.

In block1006an accelerometer's (e.g., 3 axis accelerometer908) output is read and in block1008a gyroscope's (e.g.,704) output is read.

In block1010the direction or gravity is sensed. Alternatively the direction of another know local vector quantity is sensed. An example of another known local vector quantity is the earth's magnetic field, which may, for example be known based on the known earth magnetic declination map912. Another example of a known local vector quantity is the direction to a known celestial object which can be sensed optically.

In block1012the position of the satellite which was searched for in block1002is looked up in stored information (e.g.,918).

In block1014the AOA of the signal from the satellite is computed in the LECS based on the current vehicle or object (e.g.,104) position and the stored information on the satellite.

In block1016the orientation of the vehicle or object (e.g.,104) within (with respect to) the LECS is calculated based on: the measured AOA (corresponding to the direction of maximum gain) found in block1002; the AOA calculated in block1014; measurements of local gravity direction sensed in block1010(or direction of alternative local vector quantity as discussed above), and separately available information of the gravity (or alternative local vector quantity) in the LECS. Note that gravity is trivially directly down in the LECS (to a high degree of approximation).

In optional block1018vehicle or object (e.g.,104) control inputs (e.g., steering, accelerator, brake) are read. In a “fly by wire” system there may be no need for separately provisioned sensors to take control input readings, and the controls themselves generate electrical signals that may be read. The control inputs may be read through a signal conduit (e.g., wire or fiber optic) or wirelessly.

In block1020the orientation estimate generated in block1016, the accelerometer readings, gyroscope readings, GPS coordinates, GPS velocities and optionally the vehicle control inputs are input into a Kalman filter that is tracking the vehicle position, vehicle velocity and vehicle orientation, and in block1022the Kalman filter is run to determine new estimates of the aforementioned parameters.

In block1024a best estimate antenna pointing direction (AOA) for best gain is computed based on: the vehicle location, vehicle orientation, and satellite location and the antenna is pointed accordingly. After block1024the method loops back to block1002and continues to operate as described above.

If the phased array antenna112and satellite radio302cease receiving the satellite signal, for example because the truck104has driven into a tunnel, the Kalman filter906can continue to update orientation information for the truck and the orientation information can be used by the antenna pointing direction computer926to continue to update the antenna pointing direction so that when the truck emerges from the tunnel, the satellite radio302and phased array antenna302can begin to try to reacquire the satellite signal and the phased array antenna will be more likely to be pointed in the correct direction or close to the correct direction, even if the orientation of the truck104has changed (e.g., because the road within the tunnel is curved).

In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. Any letter designations such as (a) or (b) etc. used to label steps of any of the claims herein are step headers applied for reading convenience and are not to be used in interpreting an order or process sequence of claimed method steps. Any claimed steps that recite a particular order or process sequence will do so using the words of their text, not the letter designations.