Electronic beam steering for keyhole avoidance

• US 7,324,046 B1
• Filed: 03/25/2005
• Issued: 01/29/2008
• Est. Priority Date: 03/25/2005
• Status: Active Grant

• Current Assignee: The Boeing Co.
• Original Assignee: The Boeing Co.
• Inventors: Wu, Yeong-wei Andy
• Primary Examiner(s): PHAN, DAO LINDA
• Application Number:
  US11/090,410
• Publication Number:
  US 20080018534A1
• Time in Patent Office: 1,040 Days
• Field of Search: 342 74- 76, 342/81, 342/154, 342/359, 343/754, 343/757
• US Class Current: 342/359
• CPC Class Codes: H01Q 1/28 Adaptation for use in or on aircraft, missiles, satellites, or balloons
• CPC Class Codes: H01Q 3/16 for varying relative position of primary active element and a reflecting device
• CPC Class Codes: H01Q 3/26 varying the relative phase or relative amplitude of energisation between two or more active radiating elements; varying the distribution of energy across a radiating aperture H01Q3/12, H01Q3/22, H01Q3/24 take precedence

Abstract

An airborne radio frequency (RF) antenna terminal system includes a two-axis gimbals control system and a phased array antenna. The phased array antenna electronically steers the receive and transmit beams using phase shifters. The electronically steered beams provide a virtual third-axis for the two-axis gimbals control system. The combination of the electronically steered beams and the two-axis gimbaled system provides accurate beam steering for the keyhole region of the two-axis gimbals control system so that the RF communication link is prevented from being lost in the keyhole region.

Claims

1. A communication system comprising:

a two-axis gimbals control system adapted to adjust an antenna pointing direction relative to a gimbals azimuth axis and a gimbals elevation axis; and

an antenna mounted to the two-axis gimbals control system along the gimbals elevation axis, wherein the antenna is adapted to provide a third axis of control of the antenna pointing direction by generating an electronically steered beam, at electronically steered angles that are calculated based on azimuth angles and elevation angles commanded to the two-axis gimbals control system, and to adjust the antenna pointing direction relative to a cross-elevation axis that is perpendicular to the gimbals elevation axis, and

wherein the antenna is adapted to adjust the antenna pointing direction using the two-axis gimbals control system when the antenna pointing direction is outside of a keyhole regions and wherein the antenna is adapted to perform electronic beam steering to adjust the antenna pointing direction when an elevation angle is within a keyhole region.

2. The communication system of claim 1, wherein the two-axis gimbals control system provides measured values for azimuth angle and elevation angle from which is computed an LOS pointing error vector and cross-elevation and cross-azimuth electronically steered angles for canceling the LOS pointing error vector.

3. The communication system of claim 1, further comprising a moving platform that carries the two-axis gimbals control system.

4. The communication system of claim 1, further comprising a satellite wherein the antenna pointing direction is steered toward a satellite.

5. A communication system comprising:

a two-axis gimbals control system having a gimbals azimuth axis and a gimbals elevation axis;

an antenna mounted to the two-axis gimbals control system along the elevation axis, wherein the antenna generates an electronically steered beam that adjusts the antenna pointing direction relative to a cross-elevation axis that is perpendicular to the gimbals elevation axis; and

a satellite wherein measured values for azimuth angle and elevation angle from the two-axis gimbals control system and a satellite range pointing vector relative to an Earth-centered, Earth-fixed frame are used to compute an LOS pointing error vector,

the LOS pointing error vector is used to compute cross-elevation and cross-azimuth electronically steered angles for canceling the LOS pointing error vector, and

cross-elevation and cross-azimuth electronically steered angles are used to adjust the antenna pointing direction to align an antenna LOS pointing vector with the satellite range pointing vector.

6. A communication system comprising:

a two-axis gimbals control system having a gimbals azimuth axis and a gimbals elevation axis; and

an antenna mounted to the two-axis gimbals control system along the elevation axis, wherein the antenna generates an electronically steered beam that adjusts the antenna pointing direction relative to a cross-elevation axis that is perpendicular to the gimbals elevation axis, wherein

a range pointing vector has coordinates r₁, r₂, r₃,

the two-axis gimbals control system provides a measured value AZ_m for azimuth angle and a measured value EL_m for elevation angle, and

the two-axis gimbals system is commanded with an azimuth angle AZ and elevation angle EL, wherein

$\begin{array}{c}\mathrm{AZ}=-{\tan }^{-1}<br><br>\left(\frac{r_2}{r_1}\right)\\ \mathrm{EL}={\mathrm{cotan}}^{-1}<br><br>\left(\frac{r_1^{\mathrm{\prime <br><br>}}}{r_3^{\mathrm{\prime <br><br>}}}\right)\end{array}<br><br><br><br>\mathrm{and}<br><br>\left[\begin{array}{c}{r}_1^{\mathrm{\prime <br><br>}}\\ r_2^{\mathrm{\prime <br><br>}}\\ r_3^{\mathrm{\prime <br><br>}}\end{array}\right]=\left[\begin{array}{ccc}\cos <br><br>\left({\mathrm{AZ}}_m\right)& -\sin <br><br>\left({\mathrm{AZ}}_m\right)& 0\\ \sin <br><br>\left({\mathrm{AZ}}_m\right)& \cos <br><br>\left({\mathrm{AZ}}_m\right)& 0\\ 0& 0& 1\end{array}\right]<br><br>\left[\begin{array}{c}{r}_1\\ r_2\\ r_3\end{array}\right].$

7. The communication system of claim 6, wherein:

a cross-elevation electronically steered angle xEL and a cross-azimuth electronically steered angle xAZ are used to adjust the antenna pointing direction to align an antenna LOS pointing vector with the range pointing vector;

$\begin{array}{c}\mathrm{xEL}=-{\tan }^{-1}<br><br>\left(\frac{r_2^{\mathrm{″<br><br>}}}{r_1^{\mathrm{″<br><br>}}}\right)\\ \mathrm{xAZ}={\tan }^{-1}\left(\frac{r_3^{\mathrm{″<br><br>}}}{\sqrt{{\left(r_1^{\mathrm{″<br><br>}}\right)}^2+{\left(r_2^{\mathrm{″<br><br>}}\right)}^2}}\right)\end{array};<br><br><br><br>\mathrm{and}<br><br>\left[\begin{array}{c}{r}_1^{\mathrm{″<br><br>}}\\ r_2^{\mathrm{″<br><br>}}\\ r_3^{\mathrm{″<br><br>}}\end{array}\right]=\left[\begin{array}{ccc}\cos <br><br>\left({\mathrm{EL}}_m\right)& 0& \sin <br><br>\left({\mathrm{EL}}_m\right)\\ 0& 1& 0\\ -\sin <br><br>\left({\mathrm{EL}}_m\right)& 0& \cos <br><br>\left({\mathrm{EL}}_m\right)\end{array}\right]<br><br>\left[\begin{array}{c}{r}_1^{\mathrm{\prime <br><br>}}\\ r_2^{\mathrm{\prime <br><br>}}\\ r_3^{\mathrm{\prime <br><br>}}\end{array}\right].$

8. The communication system of claim 7, further comprising:

a moving platform that carries the two-axis gimbals control system and has a body reference frame; and

a satellite wherein the range pointing vector is the normalized range pointing vector of the satellite with respect to the body reference frame.

9. A method for antenna pointing comprising the steps of:

controlling antenna pointing using a two-axis gimbals control system when an antenna LOS pointing vector is outside a keyhole region;

controlling antenna pointing using the two-axis gimbals control system with additional electronic beam steering using electronically steered angles when the antenna LOS pointing vector is inside the keyhole region;

providing a measured value AZ_m for azimuth angle and a measured value EL_m for elevation angle from the two-axis gimbals control system; and

computing an electronically steered cross-azimuth angle xAZ and an electronically steered cross-elevation angle xEL wherein

$\begin{array}{c}\mathrm{xEL}=-{\tan }^{-1}<br><br>\left(\frac{r_2^{\mathrm{″<br><br>}}}{r_1^{\mathrm{″<br><br>}}}\right)\\ \mathrm{xAZ}={\tan }^{-1}\left(\frac{r_3^{\mathrm{″<br><br>}}}{\sqrt{{\left(r_1^{\mathrm{″<br><br>}}\right)}^2+{\left(r_2^{\mathrm{″<br><br>}}\right)}^2}}\right)\end{array};<br><br><br><br>\left[\begin{array}{c}{r}_1^{\mathrm{″<br><br>}}\\ r_2^{\mathrm{″<br><br>}}\\ r_3^{\mathrm{″<br><br>}}\end{array}\right]=\left[\begin{array}{ccc}\cos <br><br>\left({\mathrm{EL}}_m\right)& 0& \sin <br><br>\left({\mathrm{EL}}_m\right)\\ 0& 1& 0\\ -\sin <br><br>\left({\mathrm{EL}}_m\right)& 0& \cos <br><br>\left({\mathrm{EL}}_m\right)\end{array}\right]<br><br>\left[\begin{array}{c}{r}_1^{\mathrm{\prime <br><br>}}\\ r_2^{\mathrm{\prime <br><br>}}\\ r_3^{\mathrm{\prime <br><br>}}\end{array}\right];<br><br><br><br>\mathrm{and}$

$\left[\begin{array}{c}{r}_1^{\mathrm{\prime <br><br>}}\\ r_2^{\mathrm{\prime <br><br>}}\\ r_3^{\mathrm{\prime <br><br>}}\end{array}\right]=\left[\begin{array}{ccc}\cos <br><br>\left({\mathrm{AZ}}_m\right)& -\sin <br><br>\left({\mathrm{AZ}}_m\right)& 0\\ \sin <br><br>\left({\mathrm{AZ}}_m\right)& \cos <br><br>\left({\mathrm{AZ}}_m\right)& 0\\ 0& 0& 1\end{array}\right]<br><br>\left[\begin{array}{c}{r}_1\\ r_2\\ r_3\end{array}\right],$

wherein r₁, r₂, and r₃ are the coordinates of a range pointing vector for pointing the antenna.

10. A method for communication system antenna pointing from a moving platform, comprising the steps of:

commanding an azimuth angle and an elevation angle to a two-axis gimbals control system on the moving platform and having a gimbals azimuth axis and a gimbals elevation axis;

computing a cross-azimuth angle and cross-elevation angle for an antenna mounted to the two-axis gimbals control system along the elevation axis; and

adjusting the antenna pointing direction electronically relative to a cross-elevation axis that is perpendicular to the gimbals elevation axis, using the cross-azimuth angle and cross-elevation angle.

11. The method of claim 10, further comprising steps of:

defining a keyhole region for the two-axis gimbals control system based on a threshold elevation angle;

adjusting antenna pointing using the two-axis gimbals control system when the antenna pointing direction is outside the keyhole region; and

adjusting antenna pointing using electronic beam steering when the antenna pointing direction is inside the keyhole region.

12. The method of claim 10, wherein the commanding step further comprises steps of:

computing coordinates r₁, r₂, r₃ in a body reference frame of the moving platform for a normalized range pointing vector of a satellite in an Earth-centered, Earth-fixed frame;

providing a measured value AZ_m for azimuth angle and a measured value EL_m for elevation angle from the two-axis gimbals control system; and

commanding the two-axis gimbals system with the azimuth angle AZ and the elevation angle EL, wherein;

$\begin{array}{c}\mathrm{AZ}=-{\tan }^{-1}<br><br>\left(\frac{r_2}{r_1}\right)\\ \mathrm{EL}={\mathrm{cotan}}^{-1}<br><br>\left(\frac{r_1^{\mathrm{\prime <br><br>}}}{r_3^{\mathrm{\prime <br><br>}}}\right)\end{array}<br><br><br><br>\mathrm{and}<br><br>\left[\begin{array}{c}{r}_1^{\mathrm{\prime <br><br>}}\\ r_2^{\mathrm{\prime <br><br>}}\\ r_3^{\mathrm{\prime <br><br>}}\end{array}\right]=\left[\begin{array}{ccc}\cos <br><br>\left({\mathrm{AZ}}_m\right)& -\sin <br><br>\left({\mathrm{AZ}}_m\right)& 0\\ \sin <br><br>\left({\mathrm{AZ}}_m\right)& \cos <br><br>\left({\mathrm{AZ}}_m\right)& 0\\ 0& 0& 1\end{array}\right]<br><br>\left[\begin{array}{c}{r}_1\\ r_2\\ r_3\end{array}\right].$

13. The method of claim 12, wherein the computing step of claim 10 further comprises steps of:

$\mathrm{computing}<br><br>\left[\begin{array}{c}{r}_1^{\mathrm{″<br><br>}}\\ r_2^{\mathrm{″<br><br>}}\\ r_3^{\mathrm{″<br><br>}}\end{array}\right]=\left[\begin{array}{ccc}\cos <br><br>\left({\mathrm{EL}}_m\right)& 0& \sin <br><br>\left({\mathrm{EL}}_m\right)\\ 0& 1& 0\\ -\sin <br><br>\left({\mathrm{EL}}_m\right)& 0& \cos <br><br>\left({\mathrm{EL}}_m\right)\end{array}\right]<br><br>\left[\begin{array}{c}{r}_1^{\mathrm{\prime <br><br>}}\\ r_2^{\mathrm{\prime <br><br>}}\\ r_3^{\mathrm{\prime <br><br>}}\end{array}\right];<br><br><br><br>\mathrm{and}$

computing the cross-azimuth angle as cross-azimuth electronically steered angle xAZ and cross-elevation angle as cross-elevation electronically steered angle xEL, wherein;

$\begin{array}{c}\mathrm{xEL}=-{\tan }^{-1}<br><br>\left(\frac{r_2^{\mathrm{″<br><br>}}}{r_1^{\mathrm{″<br><br>}}}\right)\\ \mathrm{xAZ}={\tan }^{-1}\left(\frac{r_3^{\mathrm{″<br><br>}}}{\sqrt{{\left(r_1^{\mathrm{″<br><br>}}\right)}^2+{\left(r_2^{\mathrm{″<br><br>}}\right)}^2}}\right)\end{array}.$

14. The method of claim 13, wherein the adjusting step of claim 10 further comprises:

adjusting the antenna pointing direction using the cross-elevation electronically steered angle xEL and the cross-azimuth electronically steered angle xAZ to align an antenna LOS pointing vector with the normalized range pointing vector having coordinates r₁, r₂, r₃ in the body reference frame of the moving platform.