WO2012033413A1

WO2012033413A1 - Arrangement for stabilizing a communication antenna.

Info

Publication number: WO2012033413A1
Application number: PCT/NO2011/000241
Authority: WO
Inventors: Terje RØSTE; Börje FORSELL; Kjell Gunnar Nohr; Bjarne A. RISLØV
Original assignee: Jotron Satcom As
Priority date: 2010-09-08
Filing date: 2011-09-08
Publication date: 2012-03-15
Also published as: NO20101252A1; NO333559B1

Abstract

A pedestal arrangement for stabilization of a RF communication antenna that comprises an antenna group of at least three spatially separated GNSS antennas (Al, A2, A3) for receiving a navigation signal from a satellite transmitter a global navigation satellite system (GNSS). The GNSS antennas have respective outputs for providing GNSS navigation signals to an attitude determining device. A drive means for driving a movable pedestal part of said arrangement about at least one axis, has a drive control input for controlling the drive means for orientation of the communication antenna according to said control input produced by a control device operating on basis of an attitude output received from said attitude determining device. The GNSS antennas are attached in respective locations on or proximal to a circumference of a communication antenna for which said movable pedestal part is adapted to carry, and such that the GNSS antennas stay fixed relative to said movable pedestal part. The present invention also relates to a pedestal arrangement for stabilization of a radio frequency (RF) communication antenna, wherein the arrangement comprises an antenna group of at least two spatially separated GNSS antennas (Al, A2).

Description

Arrangement for stabilizing a communication antenna.

The invention relates to the field of stabilisation of antennas for one way or two way radio communications to and/or from satellites or terrestrial mast mounted equipment or the like, mounted on a moving body, such as e.g. a land vehicle or a vessel at sea.

Embodiments of the invention are particularly suitable for stabilisation of antennas for radio frequencies from 100 MHz to 60 GHz.

Background.

A stabilizing and motion compensating antenna pedestals has been described in the U.S. patent publication no. 5,419,521. Other antenna pedestals are disclosed in patent publications referred to in the aforementioned patent publication. Well known solutions for stabilizing and motion compensating antenna pedestals either do not address or only to some degree address one or more of the following problems:

1. A dependency on ships compass performance for reliable and fast satellite

acquisition after blockage or loss of satellite signal.

2. The antenna will drift off during communication signal blockage or rain- fade

resulting in poor service availability

3. The tracking accuracy depends on the signal to noise ratio of the received

communication signal.

4. In situations of severe dynamics, it may be very difficult to obtain fast re-acquisition of a communication satellite signal after blocking, such as e.g. when a ship carrying an antenna pedestal is subjected to heavy sea motion.

Typically, present state of the art solutions use inertial sensors and/or tilt sensors in combination with calculations based on the power level of the communication signal received by the antenna itself. The use of the received signal power level is typically required by the well-known solutions because of inherent drift in inertial sensors.

Stabilized antenna solutions are stabilized along the 3 (three) so-called Euler angles, namely the azimuth (Az), elevation (El) and cross-elevation (X-el) angles. The antenna is generally required to cover all azimuth directions, and also elevation angles from near zero to about 90 degrees. Furthermore, the antenna is generally required to be able to cover different angles of polarisation because the longitude of a ship or vehicle typically on or near the surface of the earth and the longitude of the zenith position of the satellite, typically a geostationary orbit satellite, transmitting the communication signal to be received by the stabilized antenna, are normally different.

In present art designs, the received signal power level is calculated along a deliberately introduced small elliptic or circular error of the communication antenna axis pointing direction. Hereinafter, the communication antenna axis pointing direction is denoted the bore-sight axis and the plane perpendicular to the bore-sight is denoted the aperture plane. This small elliptic or circular error is hereafter denoted the conical scan. The receiver of the communication signal that calculates the received power level during conical scan is denoted the Tracking Receiver. The conical scan can either be performed mechanically or electrically. The variation of the received power level along the circle or ellipse of the conical scan is the base for determining both the direction of the antenna pointing error and the magnitude of the error. There are several problems related to conical scan solutions that need to be addressed, some of which are:

5. The conical scan always introduces an antenna pointing direction error with respect to the actual direction towards the transmitter of the signal to be received.

6. The performance of the conical scan is significantly reduced with lower SNRs

(Signal to Noise Ratio) of the received signal, presently at levels provided by efficient system standards introduced like the ETSI EN 302 307 DVB-S2 which can operate down to SNR= -2 dB.

7. In present art designs the polarisation error is difficult to detect and control at

elevation angles in the vicinity of 90°

8. The conical scan or similar methods depend heavily on the quality of the received signal level. If the communication signal from a satellite transmitter is blocked, such as e.g. by the moving body's own structure, the tracking capability in practice is typically lost, and generally, the antenna will drift off.

9. Acquisition of the right communication satellite may take a long time, and as a consequence, after a blocking situation as described above, it may take a long time until communication is restored, which in turn typically leads to reduced availability of the communication link.

The global navigation satellite system (GNSS) based stabilised platform antenna solution according to the present invention significantly reduces or removes the deficiencies listed above. Summary of the invention.

The present invention is defined in the appended independent claims. Embodiments are set forth in the appended dependent claims.

In one aspect the invention concerns:

A pedestal arrangement for stabilization of a RF communication antenna, comprising

- a movable pedestal having means for affixing the RF communication antenna thereto,

- a drive means for driving the movable pedestal part of said arrangement about at least one axis, said drive means having a drive control input for controlling the drive means for orientation of the communication antenna according to said control input produced by a control device operating on basis of an attitude output received from an attitude determining device, and

- an antenna group of at least three spatially separated GNSS antennas (Al, A2, A3) for receiving navigation signals from satellite transmitters of a global navigation satellite system (GNSS), said GNSS antennas having respective outputs for providing GNSS navigation signals to an attitude determining device.

In other aspects the invention concerns:

The arrangement as described above, wherein- said GNSS antennas are attached in respective locations on or proximal to a circumference of a communication antenna for which said movable pedestal part is adapted to carry, and such that the GNSS antennas stay fixed relative to said movable pedestal part.

The arrangement as described above, wherein said locations are such that said at least three GNSS antennas are located substantially at the corners of an equilateral triangle.

The arrangement as described above, wherein said antenna group comprises a fourth GNSS antenna spatially separated from said at least three GNSS antennas.

The arrangement as described above, wherein said at least three GNSS antennas are attached in a respective locations on or proximal to said circumference of said RF communication antenna for which the pedestal is adapted.

The arrangement as described above, wherein the first and the second ones of said GNSS antennas define two points on a first line and third and fourth ones of said GNSS antennas define two points on a second line, and said GNSS antenna locations are such that the second line is substantially perpendicular to said first line.

The arrangement as described above, wherein said locations are such that the second line intersects said first line substantially at a location corresponding to a location of centre of said communication antenna for which the pedestal is adapted.

The arrangement as described above, wherein the arrangement comprises said control device, and said control device is arranged to produce said control input by optimal combination of said attitude output and angular position outputs provided by at least one or more inertial sensor systems.

The arrangement as described above, comprising said communication antenna for which said pedestal part is adapted to carry, attached to said pedestal part, and wherein a rim of said communication antenna provides means for the attachment of said at least three GNSS antennas.

The arrangement as described above, comprising direction means defining a

communication antenna bore-sight axis, said GNSS antennas comprising GNSS direction means defining a GNSS antenna bore-sight axis of each of said GNSS antennas, and said GNSS direction means being oriented with respect to said direction means such that the bore-sight axes of the GNSS antennas are substantially parallel, and at an elevation angle above an elevation angle of the bore-sight axis of the

communication antenna that is substantially greater than zero and substantially less than ninety degrees .

The arrangement as described above, wherein the bore-sight axes of the GNSS antennas are at an elevation angle above an elevation angle of the bore-sight axis of the communication antenna that is about forty- five degrees.

The arrangement as described above, wherein said GNSS navigation antennas have a 6 dB beamwidth of about 135 degrees.

The arrangement as described above, wherein the bore-sight axes of the GNSS antennas are aligned substantially in parallel, allowing a maximum deviation of about 10 degrees from each other. Detailed description of the invention.

In the following, the present invention will be described by way of example, and with reference to the accompanying drawings, wherein

Figure 1 is a perspective view drawing, illustrating a typical 3-axis antenna system allowing the antenna to move about an elevation axis, a cross-elevation axis and an azimuth axis;

Figure 2 is a schematic drawing, illustrating exemplary frame systems, wherein x, y, z define the navigation frame (NF), x', y' and z' define the antenna frame (AF), the Z- axis is pointing towards the centre of the earth, the z'-axis is the bore-sight of the antenna, and the unit vector in this direction is denoted k';

Figure 3 is a schematic drawing, illustrating GNSS attitude determination, wherein and x denote vectors;

Figure 4 is a block schematic drawing, illustrating an exemplary block diagram of the GNSS-based stabilized platform for satellite communications; and

Figures 5a and 5b are schematic front view and side views drawings, respectively, illustrating an embodiment example of the invention employing a parabolic

communication antenna, with 4 navigation antennas onto the communication antenna, wherein the navigation antenna set is made up by the antennas Al, A2, A3 and A4; and

Figures 6a and 6b are schematic front view and side views drawings, respectively, illustrating an embodiment example of the invention employing the parabolic communication antenna, with 3 navigation antennas onto the communication antenna, wherein the navigation antenna set is made up by the antennas Al, A2 and A3.

The invention applies to stabilized antenna platforms with one, two and three mechanical axes, providing corresponding degrees of freedom for pointing the antenna. The following detailed description of the invention is given as an example with a 3-axis antenna platform.

According to a first aspect of the present invention, a two or three axes pedestal for stabilization of a communication antenna on a moving body for satellite communication is stabilized by using attitude determination based on GNSS receivers and with the navigation antennas integrated onto the communication antenna, is provided. The three- axis attitude determination from the GNSS system is combined with inertial sensors according to well-known optimum estimation techniques and principles. These estimates give an input to a control loop system for stabilizing the antenna platform. The invention covers any type of communication antenna e.g. parabolic dish, patch antenna of different shapes and Yagi antennas.

According to a second aspect of the present invention, to obtain three axis GNSS attitude determination, a minimum of three spatially separated navigation antennas is provided. As an example, a solution with 4 navigation antennas is described. However, the invention covers other numbers of GNSS antennas as well. With the four antenna solution there are four GNSS antennas consisting of two pairs. These two pairs form a cross as with Figures 5a and b that is close to perpendicular to each other. The embodiment of the navigation antennas and the communication antenna that is as shown in Figures 5 a and b, is such that when the antenna is stabilized, one pair of the navigation antennas, that is the A1-A2 pairs of antennas according to Figures 5a and b, is close to being parallel to the xy-plane of the NF. This is made possible because the angles of the horizontal and the vertical polarization of the transceiver system in the direction of the satellite with which the system is supposed to communicate, is motor controlled by rotating the transceiver OrthoMode Transducer (OMT) to the appropriate angle a between X' and X" axes of Figures 5a and b. The embodiment with 4 antennas maximizes the distance between two navigation antennas which results in the maximum accuracy for the GNSS attitude estimation.

With reference to Figures 5a and b, a third aspect of the present invention is explained, wherein the navigation antennas are attached to the communication antenna in such a way that it will not obstruct reception of the communications radio signals while the communication antenna gives the least possible obstruction of the GNSS signals. In the design according to Figures 5a and b the navigation antenna are placed on the rim of the communications antenna. However, the construction also includes designs where the navigation antennas are moved away but still rigidly attached to the communication antenna, thus decreasing obstruction of the GNSS signals.

According to a fourth aspect of the present invention, the bore-sight of the navigation antennas is 45 degrees (but not restricted to this angle) with respect to the aperture plane of the communications antenna. This embodiment together with the axis between the A1-A2 pair of navigation antennas being parallel to the xy-plane of the NF keeps the elevation angle of the navigation antennas bore-sight with respect to the xy-plane of the NF always higher than 45 degrees when the antenna is stabilized. This high elevation angle (angle between the bore-sight of any navigation antenna and the xy-plane of the NF) will make the system less vulnerable to multipath of the GNSS signal caused by reflection from the sea and structures on the ship. To obtain this an appropriate antenna diagram of the navigation antennas is prescribed such that reflection from the sea or ship is minimized (see 6th aspect).

According to a fifth aspect of the present invention, the invention is such that the above chosen bore-sight of the navigation antennas together with the prescribed navigation antenna diagram is a trade-off between GNSS satellite availability and multipath reflection. Thus the error of GNSS-based attitude measurement caused by multipath is reduced.

According to a sixth aspect of the present invention, the 6 dB beam-width of the navigation antennas is between, but not restricted to, 135 degrees and 90 degrees.

According to a seventh aspect of the present invention, the inertial sensors are rigidly attached to the communication antenna. The invention will then allow the tightest possible connection between the movement of the communication antenna and the movement of the inertial sensors.

According to an eighth aspect of the present invention, the use of GNSS attitude measurements as with the present invention, establishes a measurement system that is totally independent of frequency bands used and propagation conditions for the communications signals. Thus there is no need for, but does not exclude, the use of the communications systems own signal to control and stabilize the antenna.

According to a ninth aspect of the present invention, the above described application of the invention is in satellite communications using a stabilized communications antenna and a 3-axis mechanical system. The present invention also covers and includes applications with other configurations of the mechanical axis, like two or one axis systems optimised for communications to terrestrial systems.

With the present invention, the tracking receiver and conical scan are omitted and replaced by a GNSS-based three axis attitude determination system. The GNSS system could be any system, such as e.g. GPS, Galileo, Glonass, Compass etc. In the following, we use GNSS in the meaning that utilization of any satellite based navigation system performing as any of the aforementioned example systems is covered in the description. In a GNSS-based attitude determination system, there is a need for at least 3 navigation antennas attached to the communication antenna or platform to be stabilized according to the present invention. However, more than three antennas for receiving the navigation satellite signals may be used, and the invention encompasses any number of navigation antennas that is equal to three or larger. Based on phase measurements of the received carrier signals from the navigation satellites, it is possible to determine the attitude of the communication antenna that is attached to the antenna stabilizing pedestal.

This principle overcomes most weaknesses with the present conical scan principles and has the following advantages:

• The pointing error due to the conical scan movement of the antenna is eliminated

• The method is independent of the received communication signal and is a) not

susceptible to degradation due to low SNR of the received communication signal b) blocking of the received communication signal is not affecting the attitude measurement c) the redundancy of the GNSS satellites makes the solution less vulnerable to blocking of GNSS satellites d) the method will work even at low elevation angles to the communication satellite e) is not susceptible to fading of the communication signal as rain fading may be significant in the Ku and Ka bands, whereas rain fading is insignificant at GNSS carrier frequencies (L-band)

• Acquisition is made fast and reliable because the GNSS-attitude determination

works directly towards an absolute reference in the navigation frame

GNSS attitude determination is a known technology and has been applied to stabilized platforms for e.g. spacecrafts and precise positioning of ships or oil drilling platforms. The present invention is describing a completely new method for stabilization of radio communications antennas. The description also includes embodiments of the navigation antennas onto the communication antenna that overcome some weaknesses of the GNSS attitude determination for this application like i.e. multipath of the GNSS signals.

Before the description of the system some definitions of coordinate systems are given. It is referred to Figure 2. We define the navigation frame (NF) as the frame, or co-ordinate system, with z-axis pointing towards the centre of the earth, the x-axis pointing north and the y-axis pointing east. This is defined by the {x, y, z} coordinate system. Further the

communication antenna frame (AF) is defined with the x-axis (χ') aligned with the horizontal polarization axis of the communication antenna RF front end, the y-axis (y') aligned with the direction of vertical polarization and the z-axis (ζ') along the bore-sight axis of the communication antenna. This is defined by the {x',y',z'} coordinate system. The direction of the axes is according to the right hand rule. The 3 Euler angles are then the angles of the antenna frame (AF) with respect to the navigation frame, and these 3 angles then determine the attitude of the communication antenna. In a control or tracking loop for antenna tracking, the attitude of the communication antenna, is compared with the reference attitude at the given position on earth of the moving body to which the communication antenna is attached.

The stabilized communication antenna rotates around the {x',y',z'} axes. The direction of rotation is according to the right-handed convention. Bore-sight is defined by a unit vector k' along the z'-axis. Further we introduce still another axis system denoted the Reference Antenna Frame (RAF). The RAF is the same as the AF if the antenna were perfectly stabilized. We denote this coordinate system {x_D,y_D,zo} and the unit vector of the desired bore-sight as k_D . The Euler angles {φϋ,θο,ψο} of {XD^ZD} with respect to the NF may be calculated based on the position (latitude, longitude) of the stabilized antenna, and the position of the geostationary communication satellite.

The GNSS attitude determination principle is now briefly described with reference to Figure 3.

Two antennas Al and A2 with a distance |x| = L_x apart may be used to calculate two angles a) the elevation angle of the vector x and the azimuth of 3c with respect to the navigation frame (NF). If we have another pair of antennas A3 and A4 with a distance |y| = L_Y apart, the azimuth and elevation of the vector y is determined. The two pairs of systems, i.e. four GNSS antennas, are most favourable as to accuracy if the vectors 3c and y are perpendicular. This is not a necessity; the system may work well with only three navigation antennas. However, the description is focused on using 4 navigation antennas due to the enhanced performance compared to a system with 3 navigation antennas. Based on the directions of the x and y vectors respectively referred to the NF, the attitude of the communication antenna is denoted the GNSS attitude output vector g_G(k) at time index k. The Euler angles of the vector q_G(k) at time k are found from the

3c and y vectors by known equations from analytical geometry. We denote q_G(k) the attitude vector meaning any representation of attitude (Euler angles or quaternions).

In the following, further details of the invention are described with reference to the functional block diagram of Figure 4.

In the example illustrated in Figure 4, there are four GNSS antennas that are attached to the communication antenna, as will be discussed in further detail for an embodiment described later in this text with reference to Figures 5a and b. The GNSS antennas labelled Antenna #1 and Antenna #2, denote the above defined A1-A2 pair of navigation antennas and form the basis for the x- vector. The GNSS antennas labelled antenna #3 and antenna #4, denote the above defined A3-A4 pair of antennas, and form the basis for the y-vector. The GNSS-signals from the GNSS antennas are delivered to the block "GNSS attitude determination unit". Based on measurements of the relative phases of the input carriers of the two pairs of navigation antennas, use may be made of well known technology, such as described in e.g. in publication Cohen, C.E. (1992) Attitude Determination Using GPS, Ph.D. Thesis, Stanford University, Dec. 1992., for determining the attitude of the communication antenna. The attitude of the

communication antenna thus determined is denoted the GNSS attitude output vector

^^G^ at time k. An attitude vector may have different representations, such as e.g. by Euler angles or Quaternions.

In addition to the GNSS attitude determination unit, inertial sensors (see the block "Inertial sensors" of Figure 4) could also be attached to the communication antenna (see Figures 5 a and b), to provide inputs to an inertial sensor block unit that records the movements of the communication antenna. The output from the inertial sensors block unit is the sensor vector ^ω ^ , which together with the GNSS attitude output vector j_{s m}p_Ut to _an "Optimal attitude estimator" unit. The optimal estimator unit may be based on known technology, such as e.g., the most commonly employed estimator known as the Extended Kalman Filter. The use of inputs from many sensors to make an estimate is denoted "sensor fusion", and is widely used to estimate parameters in control systems, discussed in more detail in e.g. publication B. Vik. Integrated Satellite and Inertial Navigation Systems. Dept. of Engineering Cybernetics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway.

In a stabilized antenna system according to the present invention, the system includes a Position Determination Unit (PDU) that is based on GNSS. This PDU has knowledge of the position in space of the geostationary communication satellite to which it is supposed to communicate, and the position on earth of the communication antenna system given in longitude and latitude. Based on this information the PDU calculates the reference or desired attitude vector q_D(k) of the communication antenna bore-sight axis. Based on the reference attitude vector and the estimated attitude vector, the attitude error vector Aq (k) is calculated. This is done by the block "Attitude Error

Determination" unit shown in Figure 4.

The control system including the motors moving the antenna around its 3 axes use

Aq (k) as input to give a new observable updated attitude vector q_G(k +l) at time k+1.

The above mentioned reference attitude vector q_D (k) needs to be updated because the mobile stabilized antenna changes its position as it moves on the earth, and the update rate is adapted according to the speed and direction of the movement

It is known that the GNSS attitude determination principle may be vulnerable to:

• Blockage of GNSS satellites due to signal obstruction from e.g. a ship's or vehicle's structure, from nature such as e.g. mountains in narrow valleys or fjords that may reduce the number of satellites seen, etc.

• Multipath caused by reflection from the sea, from the ship or vehicle itself, or from ground.

• Limited number of GNSS satellites seen by the GNSS antennas

The invention includes measures against blocking, multipath and limited observed GNSS satellites, and is described below

In the following is described an example solution according to the present invention, applied on a three axis stabilized platform design supporting e.g. a 1.25m parabola dish communication antenna for satellite communication in the Ku-band. This is merely an example of an application of the invention. The invention can be used with other types of communication antenna, such as e.g. patch, helical, and Yagi type antennas, where the antenna attitude vector q_G(k) of the above defined antenna coordinate system

{x',y',z'}, herein denoted the antenna frame (AF), is to be controlled to be as close as possible to a predefined reference attitude vector q_D(k) .

In Figures 5a and b an example is shown of how the integration of the GNSS navigation antennas onto the communication antenna is made. The GNSS navigation antenna system comprises 4 (four) GNSS navigation antennas, denoted Al, A2, A3 and A4. For an application in Ku-band satellite communications, the distance between the GNSS navigation antennas is in the order of 0.6m to 1.5m. However, for an application in Ku- band satellite communications according to the present invention, distances between the GNSS navigation antennas could be in a range from about 0.2 meters to about 3 meters. The main issue is the accuracy of the GNSS attitude determination that increases with the distance between the navigation antennas, details of which are discussed in more detail in e.g. the publication Chaochao Wang: Development of a Low-cost GPS-based Attitude Determination System. University of Calgary. Master Thesis June 2003

The A1-A2 and A3-A4 pairs of antennas define a new coordinate system

{x",y",z"}={x",y",z'}, where the x"-axis is formed by the A1-A2 line, and the y"- axis is formed by the A4-A3 line. The {x",y",z'} axis system is merely a rotation of the {x',y',z'} axis system around the z'-axis with a rotation angle a that is depending on the horizontal polarization angle with respect to the horizontal plane of the NF. We denote the {x",y",z'} coordinate system the Navigation Antenna Frame (NAF). This rotation angle is calculated based on the longitudinal zenith position of the geostationary communication satellite and the longitude and latitude of the position on earth of the stabilized antenna. The angle a to the horizontal plane is given by the expression

where v is the latitude of the position of the stabilized antenna and λ is the difference in longitude between the stabilized antenna position and the satellite position. In the case where the stabilized antenna and the satellite are at the same longitude, the two axis systems {x",y",z'} and {x',y',z"} are identical

Surprisingly, the inventors have found that for a stabilized antenna in which is adapted to operate within an elevation angle range between zero and ninety degrees, as shown in Figures 5a and b, good performance is obtained over this range when the GNSS navigation antennas have an orientation with a bore-sight that is 45 degrees off and above the bore-sight elevation angle of the communication antenna. It has been found that this offset angle provides a number of advantages, such as:

• When the communication antenna has a low elevation angle, it is important to avoid multipath from the GNSS satellites caused by reflections from the sea or obstacles near the GNSS antenna.

• With an elevation angle of the communication antenna ranging from 5 degrees to 90 degrees, the GNSS antennas should see as many GNSS satellites as possible.

Surprisingly also, the inventors of the present invention have found that with the configuration discussed above, a very good compromise between multipath suppression and availability of GNSS satellites is obtained with GNSS navigation antennas having a 6 dB beam- width in the order of 135°. The system disclosed herein may however be operated with GNSS navigation antennas having other antenna pattern characteristics.

Multipath will reduce the GNSS attitude measurement accuracy, and, as mentioned above, the accuracy will also be reduced if the number of GNSS satellites visible from GNSS antenna is reduced. Accordingly, the solution to those problems may be to find a good compromise between GNSS satellite availability and minimization of multipath. However, the solutions disclosed herein may, when properly adapted, find use at other angles of the GNSS bore-sight axis with respect to the communication antenna bore- sight than angles that are particularly specified herein, and other beam-widths of the GNSS antenna than beam-widths that are particularly specified herein.

The present invention also encompasses an embodiment wherein the antenna A1-A4 contain switchable elements or patches that will make possible different bore-sight directions with respect to the communication antenna and changing beam- widths as the elevation of the communication antenna changes, and further, the shape of the antenna may have any suitable shape different from the flat square shape depicted in Figures 5a and b.

Further, in Figure 5b, there is a box containing the inertial sensors and the GNSS attitude determination unit. Coaxial cables connect the RF signal from GNSS antenna to the GNSS attitude determination unit. Possible switching control signals are also transmitted over these cables. The DSP functions of block diagrams denoted "Optimal attitude estimator", "Position determination unit i.e. GNSS" and "Control Loop & Motors" are implemented in an electronic unit that contain DSP processors. Referring to Figure 1 this unit may be placed on the Azimuth Motor Drive Housing or another suitable place. This electronic unit is connected to the inertial sensors and the GNSS attitude determination unit box via cables using a standard digital interface (e.g. SPI, I2C, Ethernet) or a standard digital radio interface (e.g. Bluetooth).

Referring again to Figures 5 a and b, the navigation antennas are advantageously fastened to the rim of the parabolic dish in such a way that the communication signal is not obstructed. On the other hand, the placement of the navigation antenna is such that obstruction of the GNSS signals is made low.

Yet another aspect and embodiment of the invention is described in the following:

Antennas for radio communications with e.g. a satellite may receive and transmit different types of polarizations e.g. linear polarization with a vertical component and/or a horizontal component and circular polarization with a Right Hand (RHCP) component and/or a Left hand (LHCP) component. With circular polarization the transmission and reception of radio signals are indifferent to rotation about the antenna bore sight axis as defined by the Z'-axis of e.g. Figure 9b. However, with linear polarization the polarization angle of the antenna must be kept within a given tolerance for proper radio communication to take place. In radio standards like e.g. EN302340 requirements on the polarization alignment error for linearly polarized systems are set forth.

In the case of linear polarisation, attitude determination of the antenna along all three Euler angles is a necessity. With circular polarised antenna systems the problem is simplified as a rotation around the boresight axis of the antenna is indifferent with respect to polarization discrimination. In this case only two angles are needed that is elevation and azimuth. The US patent 5,347,286 to Babitch describes a solution with GPS-based attitude determination of a communication antenna. However, the solution disclosed by Babitch does not describe an embodiment or technical solution for stabilizing a RF communication antenna to the correct polarization angle of a linearly polarized systems.

Well known solutions for stabilizing and motion compensating antenna pedestals either do not address or only to some degree address one or more of the following problems: 10. A dependency on ships compass performance for reliable and fast satellite acquisition after blockage or loss of satellite signal and for tracking of polarization angle for linearly polarised systems for elevation angles in the vicinity of 90°

11. The RF communication antenna will drift off over time, such as e.g. during

communication signal blockage or rain- fade, resulting in poor service availability

12. The tracking accuracy depends on the signal to noise ratio of the communication signal received by the RF communication antenna.

13. In situations of severe dynamics, such as e.g. when a ship carrying an antenna

pedestal is subjected to such heavy sea motion that it leads to blocking, it is difficult to obtain fast re-acquisition of a communication satellite signal after blocking,.

Typically, present state of the art solutions use inertial sensors and/or tilt sensors in combination with tracking based on the power level of the communication signal received by the antenna itself. The use of the received signal power level for tracking of the communications satellite is required largely because of the inherent drift in other sensors of present art solutions, such as e.g. tilt sensors and rate sensors

Stabilized antenna solutions are stabilized along the 3 (three) so-called Euler angles, namely the azimuth (Az), elevation (El) and cross-elevation (X-el) angles. The antenna is generally required to cover all azimuth directions, and also elevation angles from near zero to about 90 degrees. Furthermore, the antenna is generally required to be able to cover different angles of polarisation because the longitude of a ship or vehicle typically on or near the surface of the earth and the longitude of the zenith position of the satellite, typically a geostationary orbit satellite, transmitting the communication signal to be received by the stabilized antenna, are normally different. The stabilization is usually implemented by having rate sensors (gyroscopes) attached to the RF

communication antenna and used as input to a stabilization control loop. The rate sensors can detect angular movement around any axis of the antenna (azimuth, elevation, cross-elevation) and the stabilisation control loop will seek to regulate towards zero movement of the antenna (stabilised antenna). The rate sensors will be susceptible to angular drift due to the integration of the rate sensor noise and

temperature drift of the rate sensor null reference. Typically present systems will then use tilt sensors to maintain the long term pointing direction in elevation and cross- elevation. However, tilt sensors are also vulnerable to temperature changes and linear acceleration, and are generally not sufficient to ensure a very accurate pointing accuracy in situations of high dynamics. There are different ways to place the tilt and rate sensors. The sensors can be attached directly to the RF communication antenna dish (according to the strap-down principle shown in Figure 7), or as commonly used, in a sensor box mounted on the dish and connected to an elevation motor to ensure that the sensor box is in level irrespective of antenna elevation, or even mounted on the antenna base and using motor axis encoders to determine the actual antenna position. The different placements of the sensors have pros and cons with respect to cost, performance and processing requirements, but are not considered to be of particular significance for the solution proposed by the present invention. In one embodiment of the present invention, we have chosen to place the sensor box mechanically directly affixed to the antenna dish. This is illustrated in Figure 7. This solution is according to the principle of "Strapdown", as described in e.g. David Titterton , John Weston: Strapdown Inertial Navigation Technology (IEE Radar, Sonar, Navigation and Avionics Series). In this case, use of a separate motor to turn the sensor box to a predefined attitude has been eliminated, and the sensors are rigidly and directly attached to the antenna with no moving axels or drives between the sensor box and the body (antenna) that may cause wrong sensor attitude readings. Accordingly, a simpler solution is achieved.

In the sensor box as shown in Figure 7, the sensors are aligned such that one tilt sensors measure the tilt (elevation with respect to the horizontal plane defined by the {X,Y}- plane of Figure 2) of the X"-axis (see Figure 9a) of the RF communication antenna and the other measure the tilt of the Z'-axis (see Figure 9b) i.e. the elevation of the RF communication antenna with respect to the horizontal plane . Further, and also with reference to Figure 7, Figure 9a and Figure 9b, the 3 rate sensors measure the angular rate around the X"-axis, Y"-axis and Z' axis respectively

In present art designs, power level of the communication signal received by the RF communication antenna is acquired along a deliberately introduced small elliptic or circular deviation of the pointing direction of the axis of the RF communication antenna, the pointing being in the direction of the communications satellite. Hereinafter, the pointing direction of the axis of the RF communication antenna is denoted the bore- sight axis (Z' axis of Figure 9b), and a plane perpendicular to the bore-sight axis is denoted the aperture plane. This small elliptic or circular error is hereafter denoted the conical scan, There are various ways to implement conical scan, which is considered well-known from descriptions of radar systems, like e.g. in Merrill Ivan Skolnik, Introduction to Radar Systems , McGraw-Hill 1981. The receiver of the communication signal that calculates the received power level during conical scan is denoted the Tracking Receiver. The conical scan can be performed mechanically or electrically. The variation of the power level of the received signal along the circle or ellipse of the conical scan is the base for determining both the direction of the antenna pointing error and the magnitude of the error.

In present art designs where an elevation motor is used to bring the sensor box in level, or alternatively, a sensor box fixed to the antenna according to the strapdown principle, the reading of tilt sensors are used to control that the X"-axis of Figure 9a is close to being in level, enabling the polarization angle (the angle between the X' axis and the X"-axis of Figure 9a) to be kept at the correct angle. The conical scan principle, however, does not give any information of the polarization angle. Thus the stabilization of the polarization angle is relying on outputs from the tilt sensors. However, at high elevation angles of the communication antenna (from about 80° to 90°) the tilt sensors will not provide reliable inputs to polarisation control, and they are in practice substantially useless for that purpose with the RF communication antenna pointing near or at 90° elevation. The invention described in the following overcomes this deficiency.

In this aspect of the invention, the above mentioned conical scan method is applied, and the above mentioned deficiency at high elevation angles is eliminated as described in the following.

In this aspect there is an antenna group of two GNSS antennas, Al and A2, as illustrated in the Figure 9a and Figure 9b. The placement of the two antennas Al and A2 is such that the straight line between them defined by the X"-axis of Figure 9a is in the horizontal plane when the RF communication antenna is stabilized to the correct polarisation angle. In the following the description is given with the X"-axis being close to horizontal when the antenna is stabilised. However, the invention does not exclude the use of any angle in the boresight plane between the horizontal polarisation axis X' (See Figure 9a) and the X"-axis. With only two GNSS antennas, a 3- dimensional attitude solution is not available from a GNSS-based solution alone, thus the conical scan principle is applied to obtain a 3 -dimensional antenna attitude stabilisation. According to the present invention, the conical scan attitude determination principle defines an estimate of a two-dimensional error vector for the pointing direction of the boresight axis (Z'-axis in Figure 9b). Further the vector defined by the straight line from GNSS -antenna Al to GNSS antenna A2, the latter vector (along the direction of the X"-axis in Figure 9a) being perpendicular to the direction of the Z'- axis (boresight axis), determines a two-dimensional vector for the X"-axis. The above defined conical scan error vector together with the X" attitude vector, and, together with a position determination unit (shown in Figure 8) defines a unique 3-dimensional estimate of the attitude error vector of the RF communication antenna. Any attitude movement of the X"-axis, measured by the GNSS-based attitude determination unit in Figure 8, may be decomposed into an angular movement around the Z'-axis. The decomposition is based on well known algebra from analytical geometry. Measurement of the above mentioned angular movement (rotation) around the Z'-axis is input to the control system for the stabilisation of the polarisation angle. The antennas Al and A2 are stabilised to be in level by a tilt sensor in a sensor box attached and staying fixed to the antenna main dish, or by way of the GNSS attitude determination itself which is able to make a GNSS-based estimate of the deviation of the direction of the Al - A2 line from level (or the horizontal plane). In particular the GNSS-based estimate of the azimuth direction of the A1-A2 line will regulate the communication antenna towards the desired polarization angle a at high elevation angles (near 90°) solving the mentioned problem at these elevation angles.

A detail description of the invention comprising two GNSS antennas Al and A2 is given in the following:

Figure 7 is a perspective view drawing, illustrating a typical 3-axis antenna system allowing the antenna to move about an elevation axis, a cross-elevation axis and an azimuth axis, a sensor box fixed to the antenna wherein there are at least 3 rate sensors, one for each of the above mentioned axes axis, and none, one or more tilt sensors;

Figure 2 is a schematic drawing, illustrating exemplary frame systems, wherein X, Y, Z define the navigation frame (NF), X', Y' and Z' define the antenna frame (AF), the Z- axis is pointing towards the centre of the earth, the Z'-axis is the bore-sight axis of the antenna, the X'-axis is at an angle a with the X" axis (X' and X"-axes shown in Figure 9a), the X' axis being in the horizontal polarization plane of the OrthoMode Transducer (OMT) of the communication antenna system. The angle a is denoted polarisation angle hereafter, and the polarization angle is depending on the position of the communication satellite and the position of the antenna on earth and is calculated based on the position of the antenna on earth, position determined by one of the earlier defined GNSS- receivers, and knowledge of the position of the communication satellite. A polarization motor is used for the positioning of the OMT to the correct angle a with respect to the X" axis of the antenna, such that when the X"-axis is in the {X,Y}-plane (the horizontal plane) the OMT has the correct polarisation angle for communication with the satellite. Further the Y'-axis is perpendicular to the {X',Z'}-plane and is along the vertical polarization plane of the OMT;

Figure 8 is a block schematic drawing, illustrating an exemplary block diagram of the GNSS-based stabilized platform for satellite communications; and

Figures 9a and 9b are schematic front view and side views drawings, respectively, illustrating an embodiment example of the invention employing a parabolic

communication antenna, with 2 navigation antennas fixed to the communication antenna, wherein the navigation antenna set is made up by the antennas Aland A2 (see Figure 9a and 9b).

The invention applies to stabilized antenna platforms with a plurality of mechanical axes, providing corresponding degrees of freedom for pointing the antenna. The following detailed description of the invention is given with a 3 -axis antenna platform.

In this aspect of the invention we apply the conical scan principle in combination with two GNSS antennas and the sensors for attitude determination. The two-dimensional attitude vector q_G(k) in the exemplary block diagram Figure 8 defined by the line from

Al to A2 (along the X"-axis in Figure 9a) together with the output error vector Δ q_c(k) from the Conical Scan unit and the output measurement vector q_s (k) from the sensors are inputs to the full 3 -dimensional optimal attitude error estimator in the same exemplary block diagram. With this embodiment that make use of conical scan, weaknesses explained earlier with the antenna polarization angle control at high elevation angles are reduced or eliminated. Further the invention also keep tracking of the communication satellite under blocking of the communication antenna. Still another feature is the use of conical scan that keep tracking of the well known perturbations of the satellites position around its geostationary position (ca +/- 0.14°). The latter is not possible with a pure GNSS attitude determination system as described earlier in the text. However, the movement in ellipses (or circles), typically 0.1°, when applying conical scan is contributing to the pointing error.

The attitude error estimate q_EST (k) being illustrated in Figure 8, give an input to a control loop system for stabilising the antenna platform. The invention covers any type of communication antenna e.g. parabolic dish, patch antenna of different shapes and Yagi antennas.

The embodiment of the navigation antennas and the communication antenna that is as shown in Figures 9a and 9b, is such that when the communication antenna is stabilized, as the straight line between the A1-A2 pairs of navigation antennas is close to being parallel to the {X,Y} -plane of the Navigation Frame. This is made possible because the angles of the horizontal and the vertical polarization of the transceiver system in the direction of the satellite with which the system is supposed to communicate, is motor controlled by rotating the transceiver OrthoMode Transducer (OMT) to the appropriate angle a between X' and X" axes of Figure 9a. The embodiment with the Al and A2 antennas maximizes the distance between two navigation antennas which results in the maximum accuracy for the GNSS 2-dimensional attitude estimation.

With reference to Figures 9a and 9b, a third aspect of the present invention is explained, wherein the navigation antennas Al and A2 are attached to the communication antenna in such a way that it will not obstruct reception of the communications radio signals while the communication antenna gives the least possible obstruction of the GNSS signals. In the design according to Figures 9a and 9b the navigation antenna are placed on the rim of the communications antenna. However, the construction also includes designs where the navigation antennas are moved away but still rigidly attached to the communication antenna, thus decreasing obstruction of the GNSS signals.

According to a fourth aspect of the present invention, the bore-sight of the navigation antennas is 45 degrees (but not restricted to this angle) with respect to the aperture plane of the communications antenna. This embodiment together with the X"-axis formed by the straight line between the A1-A2 pair of navigation antennas being parallel to the {X,Y} -plane of the Navigation Frame keeps the elevation angle of the navigation antennas bore-sight with respect to the xy-plane of the Navigation Frame always higher than 45 degrees when the antenna is stabilized. This high elevation angle (angle between the bore-sight of any navigation antenna and the {X,Y} -plane of the

Navigation Frame) will make the system less vulnerable to multipath of the GNSS signal caused by reflection from the sea and structures on the ship. To obtain this an appropriate antenna diagram of the navigation antennas is prescribed such that reflection from the sea or ship is minimized (see 6th aspect). According to a fifth aspect of the present invention, the invention is such that the above chosen bore-sight of the navigation antennas together with the prescribed navigation antenna diagram is a trade-off between GNSS satellite availability and multipath reflection. Thus the error of GNSS-based attitude measurement caused by multipath is reduced.

According to an eighth aspect of the present invention, the use of the 2-dimensional GNSS attitude measurements together with the sensor measurements as with the present invention (See Figure 8), establishes a measurement system that is independent of frequency bands used and propagation conditions for the communications signals. This make possible fast acquisition of the desired communication satellite, and in tracking mode keep track of the communication satellite direction for some time during loss of the communication signal (i.e. during blocking or deep fading of the

communication signal)

According to a ninth aspect of the present invention, the above described application of the invention is in satellite communications using a stabilized communications antenna and a 3 -axis mechanical system. The present invention also covers and includes applications with other configurations of the mechanical axis, like two or one axis systems optimised for communications to terrestrial systems.

The GNSS system could be any system, such as e.g. GPS, Galileo, Glonass, Compass etc. In the following, we use GNSS in the meaning that utilization of any satellite based navigation system performing as any of the aforementioned example systems is covered in the description.

As stated earlier in the text, the GNSS attitude determination is based on the attitude of the vector (with respect to the NF) formed by the straight line between the two navigation antennas Al and A2 . The attitude of this vector is denoted q_G{k) at time k. The attitude in this embodiment has only two dimensions and is represented by the two angles elevation and azimuth for this vector.

In the following, further details of the invention are described with reference to the exemplary functional block diagram of Figure 8.

In the example block diagram illustrated in Figure 8, we apply two GNSS antennas Al and A2 that are attached to the communication antenna, as will be discussed in further detail for an embodiment described later in this text with reference to Figures 9a and 9b. The GNSS antennas Al and A2, denote the above defined A1-A2 pair of navigation antennas and form the basis for the X"-axis. Further the conical scan system of Figure 8 gives as output a two-dimensional conical scan error vector Δ q_c(k) at time k that act as an input to the optimal attitude error estimator. The GNSS-signals from the GNSS antennas Al and A2 are delivered to the block "GNSS attitude determination unit". In Figure 8 the GNSS attitude output vector q_G (k) defined by the line from the GNSS antenna Al to GNSS antenna A2 is input to the block denoted "Attitude error estimator". Based on measurements of the relative phases of the input carriers of the two navigation antennas Al and A2, we may use well known technology, such as described in e.g. publication Cohen, Clark E (see reference [7]) to calculate the vector q_G(k) . In addition to the attitude output vector q_G(k) , we use the conical scan error vector Δ q_c (k) , the sensor vector q_s (k) from the sensors (see the block "Rate and tilt sensors" of Figure 8) and the reference attitude vector q_D(k) as input to the block "Attitude error estimator". As shown in Figure 9b the sensors are attached to the communication antenna in a sensor box (see Figures 7 and 9b). The sensors records the movements of the communication antenna. As is seen from Figure 8, there are 4 input vectors q_G(k) , q_s (k) , Aq_c(k) , q_D(k) and one output vector lq_EST k) The core algorithm of the "Attitude error estimator" unit in the description to follow is based on the Extended Kalman Filter (EKF). The combination of many measurement parameters from sensors to make an estimate using EKF is denoted "sensor fusion", and is widely used in control systems. See publication by Bjornar Vik, Integrated Satellite and Inertial Navigation Systems. Dept. of Engineering Cybernetics, Norwegian University of Science and Technology, NO-7491 Trondheim, Norway. Alternatively a traditional PID (Proportional Integral Derivative) regulator can do the basic antenna stabilization.

The GNSS attitude determination principle is now briefly described with reference to Figure 3. In this case the problem is simplified as there are only 2 GNSS antennas.

Two antennas Al and A2 with a distance |x| = L_x apart is used to calculate two angles a) the elevation angle of the vector x and the azimuth of 3c with respect to the navigation frame (NF).

In a stabilized antenna system according to the present invention, the system includes a Position Determination Unit (PDU) shown in Figure 8 that is based on GNSS. The PDU utilize one of the GNSS receivers attached to the GNSS antennas Al or A2 or another separate GNSS receiver. The PDU has knowledge of the position in space of the geostationary communication satellite to which it is supposed to communicate, and knowledge of the position on earth of the communication antenna system given in longitude and latitude. Based on this information the PDU calculates the reference or desired attitude vector q_D{k) that uniquely defines the communication antenna bore- sight axis pointing direction (elevation and azimuth) and polarization angle. Based on the reference attitude vector and other inputs as described to the "Attitude error estimator" block in Figure 8, the attitude error vector Aq_EST (k) is calculated.

The control system including the motors moving the antenna around its 3 axes use Aq_EST (k) as input to give a new observable updated antenna attitude vector q {k + 1) at time k+1.

The above mentioned reference attitude vector q_D (k) needs to be updated for a mobile system because the mobile stabilized antenna changes its position as it moves on the earth, and the update rate is adapted according to the speed and direction of the movement

• Blockage of GNSS satellites due to signal obstruction from e.g. a ship's or vehicle's structure, from nature such as e.g. mountains in narrow valleys or fjords that may reduce the number of satellites seen, etc. • Multipath caused by reflection from the sea, from the ship or vehicle itself, or from ground.

• Limited number of GNSS satellites seen by the GNSS antennas

The invention includes measures against blocking, multipath and limited number of observed GNSS satellites, and is described below

In the following is described an example solution according to the present invention, applied on a three axis stabilized platform design supporting e.g. a 1.25m parabola dish communication antenna for satellite communication in the Ku-band. This is merely an example of an application of the invention. The invention can be used with other types of communication antenna, such as e.g. patch, helical, and Yagi type antennas, where the estimated antenna attitude error vector Aq_EST (k) of the above defined antenna coordinate system {Χ',Υ',Υ'}, herein denoted the antenna frame (AF), is to be controlled to be as small as possible.

In Figures 9a and 9b an example is shown of how the integration of the GNSS navigation antennas onto the communication antenna is made. The GNSS navigation antenna system comprises 2 (two) GNSS navigation antennas, denoted Al and A2. For an application in Ku-band satellite communications, the distance between the GNSS navigation antennas is in the order of 0.6m to 1.5m. However, for an application in Ku- band satellite communications according to the present invention, distances between the GNSS navigation antennas could be in a range from about 0.2 meters to about 3 meters. The main issue is the accuracy of the GNSS attitude determination that increases with the distance between the navigation antennas, details of which are discussed in more detail in e.g. the publication by Chaochao Wang, : Development of a Low-cost GPS- based Attitude Determination System. University of Calgary. Master Thesis June 2003 The A1-A2 pairs of antennas defines the X" axis that is at an angle a to the horizontal polarization along the X'-axis. The angle a is calculated based on the longitudinal zenith position of the geostationary communication satellite and the longitude and latitude of the position on earth of the stabilized antenna. The angle a to the horizontal plane is given by the expression a = arctan where v is the latitude of the position of the stabilized antenna and λ is the difference in longitude between the stabilized antenna position on earth and the satellite position. Surprisingly, the inventors have found that for a stabilized antenna in which is adapted to operate within an elevation angle range between zero and ninety degrees, as shown in Figures 9a and 9b, good performance is obtained over this range when the GNSS navigation antennas have an orientation with a bore-sight that is 45 degrees off and above the bore-sight elevation angle of the communication antenna. It has been found that this offset angle provides a number of advantages, such as:

Surprisingly also, the inventors of the present invention have found that with the configuration discussed above, a very good compromise between multipath suppression and availability of GNSS satellites is obtained with GNSS navigation antennas having a 6 dB beam-width in the order of 135°. The system disclosed herein may however be operated with GNSS navigation antennas having other antenna pattern characteristics.

Multipath will reduce the GNSS attitude measurement accuracy, and, as mentioned above, the accuracy will also be reduced if the number of GNSS satellites visible from GNSS antenna is reduced. Accordingly, the solution to those problems may be to find a good compromise between GNSS satellite availability and minimization of multipath. However, the solutions disclosed herein may, when properly adapted, find use at other angles of the GNSS bore-sight axis with respect to the communication antenna bore- sight than angles that are particularly specified herein, and other beam- widths of the GNSS antenna than beam- widths that are particularly specified herein.

When the communication antenna is mounted on a moving object like e.g. a ship, the multipath caused by e.g. objects on the ship may change over time, and may be averaged out over a time corresponding to typical cycling time of movements.

The present invention also encompasses an embodiment wherein the antennas A1-A2 contain switchable elements or patches that will make possible different bore-sight directions with respect to the communication antenna and changing beam-widths as the elevation of the communication antenna changes, and further, the shape of the antenna may have any suitable shape different from the flat square shape depicted in Figures 9a and 9b.

Further, in Figure 9b, there is a box containing the sensors and the GNSS attitude determination unit. Coaxial cables connect the RF signal from GNSS antenna to the GNSS attitude determination unit. Possible switching control signals are also

transmitted over these cables. The DSP functions of block diagrams denoted "Attitude error estimator", "Position determination unit i.e. GNSS" and "Control Loop & Motors" are implemented in an electronic unit that contain DSP processors. Referring to Figure 1 this unit may be placed on the Azimuth Motor Drive Housing or another suitable place. This electronic unit is connected to the sensors and the GNSS attitude

determination unit box via cables using a standard digital interface (e.g. SPI, I2C, Ethernet) or a standard digital radio interface (e.g. Bluetooth).

Referring again to Figures 5a, 6 a and 9a, the navigation antennas are advantageously fastened to the rim of the parabolic dish in such a way that the communication signal is not obstructed. On the other hand, the placement of the navigation antenna is such that obstruction of the GNSS signals is made low.

The present invention describes a system where according to Figure 8 there are four input vectors to the unit denoted "Attitude error estimator" 1) Conical Scan principle giving a Conical Scan error vector Aq_c(k) 2) a sensor vector q_s(k) , 3) a GNSS attitude vector q_G(k) based on two GNSS antennas Al and A2 and 4) a reference attitude vector q_D(k) . According to the present invention the number of GNSS antennas in alternative embodiments can be extended to 3 or more GNSS antennas. The only difference in Figure 8 being the dimension of the vector q_G(k) .

The advantage of using conical scan in combination with to GNSS attitude

determination (two or more GNSS antennas) is the ability of the communication antenna to track the small perturbations of a geostationary communication satellite position over time. The block "Attitude error estimator" of Figure 8, according known principles e.g. the Extended Kalman Filter theory, will use the inputs (sensor fusion) q_G(k) , q_s(k) &q_c(k) and q_D(k) to this block in a way to be described in the following.

The EKF-based attitude error estimator algorithm is depending on the characteristics of the process (dynamics of the stabilized platform and its driving forces i.e. process noise) and the measurement characteristics including measurement noise. The system is considered non-linear.

The attitude vector q_G(k) from GNSS is typically available 1 to 10 times per second and accuracy (measurement noise) is depending on the quality of the GNSS

determination unit (Ref C.E. Cohen), number of available GNSS satellites, the distance between the two antennas Al and A2 and the degree of multipath.

The sensors are at least 3 rate sensors. These sensors measure the angular rate around 3 orthogonal axes. Rate sensors used for commercial stabilization platforms typically have sensor noise density in the order of 0.01-0.03 sec/VHz. The estimated attitude (i. e. Euler angles) based on rate sensors will suffer from random walk noise and drift and the output has to be updated and reset using reliable measurement input from other sensors. The rate sensors will react quickly to antenna movements and give accurate correction of the attitude in the short term. The rate sensors are typically reset every 5- 30 second depending on the quality of the sensors. There are none, one or more tilt sensors. In our description we describe the use of two tilt sensors, one tilt sensor measure the tilt angle of the X"-line (Figure 9a) , being according to the present invention, substantially in level (horizontal plan), and, the other tilt sensor measure the tilt of the communication antenna boresight axis or the Z'-axis (Figure 9b). Thus, according to the invention, the two-dimensional attitude measurement q_G(k) in combination with the tilt sensors tilt measurement (elevation) of the Z'-axis, the Z'-axis being substantially perpendicular to the X"-axis, will make a full 3 -dimensional attitude determination system that according to the invention is used for fast acquisition of the desired communication satellite, based on the calculated q_D(k) from the PDU, and for pointing tracking during blocking giving fast reacquisition when out of blocking . The use of the tilt sensor along the X"-axis can, if implemented, serve as a correction input to the GNSS attitude determination. In the EKF-based algorithm the characteristics of the tilt sensors to be considered is their drift with temperature and impact of linear accelerations from the movement of the stabilized platform. Typically, and in heavy sea movements (application on a ship), the sensor reading need to be averaged 10 seconds to 60 seconds

The conical scan measurement error vector &q_c(k) is availably typically at a rate of 1-

2 Hz for a mechanical scan, but can be much faster for electronic scan. This will give a correction input to the "Attitude error estimator" algorithm typically in the azimuth and the elevation direction. The measurement noise of this vector is depending on the signal to noise ratio (SNR) of the received communication signal and is thus useless during blocking. The Tracking Receiver is including a SNR estimator that give input to the EKF-based algorithm about the reliability of this estimate. When blocking is detected, the conical scan can be shut off. The algorithm includes an averaging of subsequent conical scan measurements to improve the estimate, the averaging time being a tradeoff between averaging gain and rate of fading of the communication signal.

Claims

P a t e n t C l a i m s 1.

- a movable pedestal part having means for affixing thereto the RF communication antenna,

- a drive means for driving the movable pedestal part of said arrangement about at least one axis, said drive means having an input for receiving a drive control signal produced by a control device operating on basis of an attitude output produced by a GNSS attitude determining device, and said drive means being adapted to control boresight and polarisation angles of the communication antenna according to said drive control signal, and

- an antenna group of at least three spatially separated GNSS antennas (Al, A2, A3) adapted to be positioned in or close to an aperture plane of the RF communication antenna for receiving navigation signals from satellite transmitters of a global navigation satellite system (GNSS), said GNSS antennas having respective outputs for providing GNSS navigation signals to said GNSS attitude determining device.

2.

The arrangement of claim 1, wherein said GNSS antennas are attached in respective locations on or proximal to a circumference of the RF communication antenna for which said movable pedestal part is adapted to carry, and such that the GNSS antennas stay fixed relative to said movable pedestal part.

3.

The arrangement of claim 1, wherein said locations are such that said at least three GNSS antennas are located substantially at the corners of an equilateral triangle.

4.

The arrangement of of claim 3, further comprising said controlling device adapted to output the drive control signal for keeping the GNSS-antennas oriented such that a straight line between two of said three GNSS antennas are kept as close as possible to be in the horizontal plane.

5.

The arrangement of claim 1, wherein said antenna group comprises a fourth GNSS antenna spatially separated from said at least three GNSS antennas,

6.

The arrangement of claim 5, further comprising said controlling device adapted to output the drive control signal for keeping the GNSS antennas oriented such that a straight line between two of said at least three and said fourth GNSS antennas are kept as close as possible to be in the horizontal plane.

7.

The arrangement of claim 5 or 6, wherein first and second ones of said GNSS antennas define two points on a first line and third and fourth ones of said GNSS antennas define two points on a second line, and said GNSS antenna locations are such that the second line is substantially perpendicular to said first line.

8.

The arrangement of claim 5 or 6, wherein said locations are such that said second line intersects said first line substantially at a location corresponding to a location of centre of said RF communication antenna for which the pedestal is adapted.

9.

The arrangement of claim 5 or 6, wherein the arrangement comprises said control device, and said control device is arranged to produce said control input by optimal combination of said attitude output and angular position outputs provided by at least one or more inertial sensor systems.

10.

The arrangement of claim 1, comprising said communication antenna for which said pedestal part is adapted to carry, attached to said pedestal part, and wherein a rim of said communication antenna provides means for the attachment of said at least three GNSS antennas.

11.

The arrangement of any one of the preceding claims, comprising direction means defining a RF communication antenna bore-sight axis, said GNSS antennas comprising GNSS direction means defining a GNSS antenna bore-sight axis of each of said GNSS antennas, and said GNSS direction means being oriented with respect to said direction means such that the bore-sight axes of the GNSS antennas are substantially parallel, and at an elevation angle above an elevation angle of the bore-sight axis of the RF communication antenna that is substantially greater than zero and substantially less than ninety degrees .

12.

The arrangement of claiml 1, wherein the bore-sight axes of the GNSS antennas are at an elevation angle above an elevation angle of the bore-sight axis of the communication antenna that is about forty-five degrees.

13.

The arrangement of claim 11 or claim 12, wherein said GNSS navigation antennas have a 6 dB beamwidth of about 135 degrees.

14.

The arrangement of any one of the previous claims, wherein the bore-sight axes of the GNSS antennas are aligned substantially in parallel, allowing a maximum deviation of about 10 degrees from each other.

15.

A pedestal arrangement for stabilization of a radio frequency (RF) communication antenna to three Euler angles of a navigation reference, the arrangement comprising

- a drive means for driving the movable pedestal part of said arrangement about a plurality of axes, said drive means receiving a drive control signal produced by a control device, and said drive means being adapted to control boresight and polarisation angles of the communication antenna according to said drive control signal,

- an antenna group of at least two spatially separated GNSS antennas (Al, A2) adapted to be positioned in or close to an aperture plane of the RF communication antenna for receiving navigation signals from satellite transmitters of a global navigation satellite system (GNSS), said GNSS antennas having respective outputs for providing GNSS navigation signals to a GNSS attitude determining device,

- a conical scan device for the RF communication antenna, the conical scan device providing a conical scan error vector output,

- a sensor device being coupled rigidly to the movable pedestal part or to the RF communication antenna and providing a sensor vector output,

- a position determination unit adapted to provide a reference attitude vector output, and - an attitude error estimator means, the attitude error estimator means receiving said conical scan error vector output and said sensor vector output and said reference attitude vector output and a GNSS attitude vector output provided by said GNSS attitude determining device, and providing an estimated attitude error vector output to said control device for generating said drive control signal.

16.

The arrangement of claim 15, wherein the sensor device comprises three rate sensors.

17.

The arrangement of claim 16, wherein the sensor device further comprises at least one tilt sensor.

18.

The arrangement according to any claim 15-17, wherein the GNSS attitude vector output is defined by the straight line between the two GNSS antennas (Al, A2).

19.

The arrangement according to any claim 15-18, wherein the GNSS antennas (Al, A2) are placed at a rim of the RF communication antenna at a maximum distance from each other.

20.

The arrangement according to any claim 15-19, the attitude error estimator means is configured to calculate the estimated attitude error vector output using the extended Kalman filter.