Antenna Orientation Methods
STK provides methods to specify the orientation of the antenna coordinate frame relative to the coordinate frame of the parent object. The antenna boresight corresponds to the +Z axis of the antenna coordinate frame. The default orientation for an antenna is alignment with the local coordinate frame of the parent object. For example, an antenna on a vehicle has a default alignment in which the antenna boresight looks down the +Z axis of the vehicle. The default vehicle attitude ("Nadir aligned with ECI velocity vector constraint" for satellites, and "Nadir aligned with ECF velocity vector constraint" for other vehicles) results in the antenna boresight pointing straight down toward the center of the Earth. The same antenna attached to a facility, place, or target is aligned looking straight up along the -Z axis of the parent.
To set the orientation:
- For an antenna object, go to the Antenna's Basic Orientation properties page.
- For an embedded antenna, go to the Receiver's, Radar's, or Transmitter's Basic Definition properties page, click the Antenna tab, and then click the Orientation tab.
Select the desired Orientation Method and specify the parameters appropriate to that method.
If you are not familiar with coordinate frame rotations, the Az-El orientation type is the most intuitive to use. For example, if you want the antenna to point along the vehicle's +X axis, enter an Azimuth of 0 deg and an Elevation of 0 deg.
Az-El Orientation Method
This method specifies the location of the antenna boresight in relation to the azimuth and elevation in the local coordinate frame of the parent object. A 323 Euler rotation sequence is performed, where the first rotation is the Azimuth value, the second is 90 degrees minus the Elevation value and the third is a 0 degree rotation. Ground-based antennas rotate by the negative of the Azimuth to account for the clockwise definition of azimuth for ground sites.
If you select the Az-El method, the following parameters are available to be specified:
Parameter | Description |
---|---|
Azimuth | This angle is measured in a right-handed sense about the +Z axis with zero azimuth on the +X axis. |
Elevation | This angle is measured from the X-Y plane toward the +Z axis. |
About Boresight |
Select one of the following options:
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Quaternion Orientation Method
The quaternion represents the transformation from the inertial coordinate system to the satellite body-fixed coordinates. The quaternion must be normalized to unit length. Quaternion input is accessible only when all three axes are being specified.
The following table describes the components of the quaternion:
Parameter | Description |
---|---|
qx | The vector component of the quaternion in the X direction. |
qy | The vector component of the quaternion in the Y direction. |
qz | The vector component of the quaternion in the Z direction. |
qs | The scalar component of the quaternion. |
Euler Angles Orientation Method
This method specifies antenna orientation as Euler A, B, and C angles. After you specify the Euler angles, you also need to specify the rotation sequence in the Sequence field. The most common Euler sequence consists of a rotation about the Z axis, followed by a rotation about the new X axis, followed by a rotation about the new Z axis, known as a 313 sequence.
The following table summarizes the parameters for this method:
Parameter | Description |
---|---|
Euler A | The angle of rotation about the first axis specified in the sequence of rotation. |
Euler B | The angle of rotation about the second axis specified in the sequence of rotation. |
Euler C | The angle of rotation about the third axis specified in the sequence of rotation. |
Sequence | Sequence of three numbers specifying the order of rotation, where 1, 2 and 3 correspond to rotations about the X, Y and Z axes, respectively. The default Euler sequence is 313. |
YPR Angles Orientation Method
Attitude can be defined:
- In terms of yaw, pitch, and roll angles,
- With reference to a local coordinate system moving with the object,
- A body-fixed coordinate system
- An inertial coordinate system.
The following parameters are available for this method:
Parameter | Description |
---|---|
Yaw | Angle of rotation about the Z axis of the reference coordinate system. |
Pitch | Angle of rotation about the Y axis of the reference coordinate system. |
Roll | Angle of rotation about the X axis of the reference coordinate system. |
Sequence | Sequence of three letters specifying the order of rotation, where R(oll), P(itch) and Y(aw) correspond to rotations about the X, Y and Z axes, respectively. The default sequence is YPR. |