Sensor Pointing Definitions

This topic provides technical notes on sensor pointing definitions that are installed with STK. Regardless of where your sensor is pointing, its point of origin will always be the center of your model, unless modified by Vertex Offset.

Sensor pointing, represented in the Vector Geometry Tool as sensor Body axes, is defined using one of the following types:

You can choose the type by going to Basic > Pointing in the sensor's properties.

To expand or collapse all sections in this topic at the same time, click in the Help toolbar.

You can only use the 3D Model pointing type with objects represented by 3D models containing pointable elements. With this type, the sensor will always be aligned to the designed vector of that element. When that element is articulated, either through a script or by pointing it to another object in the scenario, the sensor follows.

This setting only affects where the sensor is pointing. The sensor's origin remains at the center of the model (image 1), unless you change it through the Vertex Offset panel (image 2).

A list of available elements for the model appears after you have selected 3D Model as the pointing type.


This type points the sensor along any defined vector of any object in the scenario.

You must select an alignment vector and a constraint vector.

The Clock angle offset value can be between -360 and 360 degrees. The default is 0 degrees.


For this option, you specify th orientation of the sensor using an external sensor pointing file, which has a (*.sp) format.

If you select the External pointing type, you can import your own sensor pointing file. Failure to specify a file results in an error message.

  1. Enter a sensor pointing file path or use the ellipsis () button to browse to a file.
  2. Click OK or Apply to load the new sensor orientation information.

The Fixed pointing type enables you to specify the orientation of the sensor with respect to the body frame of your parent object. While the sensor remains fixed relative to the parent object (See note below), motion of the parent object changes the direction in which the sensor is pointing.

You can define the sensor's orientation in terms of Azimuth/Elevation, Euler Angles, Quaternion, or YPR Angles. For more details on these options, see Orientation Methods.

Fixed subobject pointing for a facility, place, or target

Facility, place and target objects have body-fixed coordinate axes that align the X axis along local horizontal north direction, the Y axis along local horizontal east direction, and the Z axis along local nadir direction, which is opposite to the local surface normal. In such axes, pointing of subobjects, such as sensors, transmitters, receivers, radars, and antennas, using azimuth and elevation angles is defined differently compared to subobjects on vehicles.

The vehicle-based subobjects define elevation as an angle measured from the XY plane of the vehicle body-fixed coordinate axes toward its Z axis.

Subobjects not attached to vehicles define elevation as an angle measured from the XY plane toward the negative Z axis, i.e., in a manner that makes positive elevation look up from the local horizontal plane.

For sensors only, STK also uses this special transformation of coordinates when you select the Fixed in Axes type. This makes Fixed and Fixed-in-Axes pointing, using body axes as a reference, produce the same orientation. However, this also means that you need to be aware that pointing for vehicle- and nonvehicle-based subobjects using Fixed-in-Axes type with equivalent reference axes produce different orientations because the latter includes this additional coordinate transformation designed to make elevation be measured toward the negative Z axis of the reference instead of the positive Z axis.

When the sensor is attached to a Facility, Target, or Place object, the parameters specifying the orientation of the sensor body frame (i.e. az-el, Euler angles, YPR angles, quaternion) actually measure the orientation relative to the set of axes resulting from flipping the Facility/Target/Place body frame 180 deg about its X-axis (i.e. NorthWestUp axes with the X-axis along local North, Y-axis along local West, and the Z-axis along zenith). The NorthWestUp frame is a more natural reference than the body frame for a Facility/Target/Place (i.e. NorthEastDown axes) for expressing sensor orientation since its Z-axes points upward.

When the sensor is attached to a vehicle, the parameters specifying the orientation of the sensor body frame measure orientation relative to the vehicle's body frame as is.

The Fixed in Axes pointing type specifies the orientation of the sensor with respect to a specified set of reference axes that do not have to be the body frame of the parent object. Click Select... and choose a set of reference axes in the selection window that appears.

Fixed subobject pointing for a facility, place, or target

When the sensor is attached to a Facility, Target, or Place object, the parameters specifying the orientation of the sensor body frame (i.e. az-el, Euler angles, YPR angles, quaternion) can measure the orientation in two ways:

  • When the "Flip reference axes about X-axis" checkbox is enabled, the parameters measure orientation relative to the set of axes resulting from flipping the specified Reference Axes 180 deg about its X-axis. This behavior is similiar to the Fixed pointing when the Reference Axes are chosen as the parent's body axes.
  • When the "Flip reference axes about X-axis" checkbox is not enabled, the parameters measure orientation relative to the specified Reference Axes as is. This is the same behavior that occurs when the sensor parent is a vehicle.

When the sensor is attached to a vehicle, the parameters specifying the orientation of the sensor body frame measure orientation relative to the specified Reference Axes as is. There is no option for using a flipped set of axes.

If you switch from the Euler Angles, Quaternion, or YPR Angles orientation methods to the Az-El method, the difference in the number of angles that define these representations can be problematic. STK will attempt to convert the attitude expression from three angles to the two of the Az-El method; however, the two angles used by the Az-El representation cannot represent any arbitrary attitude created using one of the other representations. There is no problem converting from Az-El to another representation, nor are there any problems converting between any of the other representations.

The sensor's boresight vector grazes the central body at a specified altitude.

Use Azimuth Offset to define the plane in the parent object's body-fixed frame in which the sensor boresight lies; it is the angle between the X axis and the azimuth vector in the XY plane (see the diagram below). An Azimuth Offset of 0 degrees means that the sensor boresight lies in the XZ plane, while an Azimuth Offset of 90 degrees means that the boresight lies in the YZ plane.

The sensor boresight's elevation angle is measured along a plane defined by the azimuth vector and the Z plane, and is measured from the XY plane. This elevation angle is solved for at each time of interest in such a way that the sensor boresight will graze the central body at the requested Grazing Altitude.

The Azimuth Offset can vary from -360 degrees to 360 degrees. The solved-for elevation angle is between -90 and 90 degrees. You can examine these values by generating a Boresight AzEl report.

There are conditions in which the requested grazing altitude will not be achieved. Two known cases are when the position of the sensor is below the requested grazing altitude, and when the parent object is oriented in such a way that the specified azimuth plane does not intersect the central body at the requested grazing altitude. For nadir-pointing attitudes, this latter case should never occur. During periods when the requested grazing altitude cannot be achieved, sensor pointing is undefined.

There are also cases when two solutions for sensor attitude meet the requirements. In this case, STK uses the solution with the greater positive elevation angle (the solution nearest the +Z axis).

The Targeted pointing type causes the sensor to point to other objects in the scenario. The sensor can point to different objects over time. If two objects are specified at the same time, the sensor points to the first object it sees.

Targeted sensors are considered to be on during its schedule of target times and off otherwise. In order for access to exist to a targeted sensor, the sensor must be on. When off, the sensor pattern is not displayed in graphics.

Initially, the schedule of target times is computed using access computations between the sensor and each selected target, where any constraints that depend on the attitude of the sensor are ignored. Once computed, the schedule may be manually edited and targets re-assigned.

The sensor's imposed constraints for an access involving the targeted sensor are the same as those when computing the target times (except for the attitude dependent constraints). If you wish to point the sensor based upon one set of constraints and then do access from that sensor to other objects when using a different constraint set, you may need to either:

  • manually edit the scheduled target times so that the access times are not directly used
  • switch the sensor pointing type to be Along Vector pointing
  • switch the sensor pointing type to be Fixed in Axes pointing, choosing an Axes that aligns to targets. When using Along Vector or Fixed in Axes pointing, the sensor no longer has on-off times. However, these can be simulated using an Intervals constraint (for use in Access) and by choosing Display Times for the sensor (for graphical display).

To define a targeted sensor:

  1. Select one or more targets. If you select two or more targets that are in view of the parent object at the same time, and you do not specify compatible Target Times, STK locks to the first target the parent object sees and remains locked until it is no longer in view.
  2. Select a Boresight Type.
  3. Depending on the Boresight Type, select an Orientation Method or a Track Mode.
  4. In the case of an Orientation Method, enter the required values to define it.
  5. Define the About Boresight option. If you select UpVector, specify a constraint vector and Clock angle offset. The Clock angle offset value can be between -360 and 360 degrees. The default is zero.
  6. Click Target Times to schedule or view access times from the sensor to the selected targets. Using the Target Times dialog box, you can create explicit target pointing intervals and select target pointing deconfliction methods to adjudicate situations where two or more targets are simultaneously available.
  7. Click Advanced... to open the Target Access Options dialog box.
  8. Select the Save Targeted Pointing Times check box to save computed sensor-target pointing access times with the sensor.

Exercising the Save Targeted Pointing Times option can greatly speed up load times for targeted sensors with complex constraints because the sensor does not recompute the access time when it is loaded.

Target assignment

To move selected objects and groups of objects from the Available Targets list to the Assigned Targets list and back, or you can click the right- and left-pointing arrows or double-click an object. Use the Selection Filter to move one or more groups of objects by category.

A child (subobject) of an object does not automatically moved when you move its parent. For example, if you want to assign a sensor that is attached to a satellite as a target, you must move the sensor to the Assigned Targets list; it is not sufficient to move the parent satellite.

Boresight Type

Type Description
Tracking Modeling a targeted sensor with a Tracking Boresight enables you to aim a sensor at one or more targets (such as vehicles and facilities). The sensor's footprint appears in the graphics window only when a given target is in view,as determined by the tracking schedule. A targeted tracking sensor slews or tracks an assigned target as soon as it appears over the horizon. No relative pointing is defined for this type of sensor.

For this option, you must select a Track Mode.
Fixed A targeted sensor with a Fixed Boresight models a sensor that always points in a fixed direction with respect to the parent body. Targets do not affect the pointing of the sensor. Since this is a targeted sensor, its footprint displays in the 2D Graphics window only when a target is in view. The sensor appears during animation only if one of the assigned targets is visible within the defined view.

For this option, you must select and define an orientation method.

The body-fixed negative Z axis for a facility, place, or target points toward zenith and for a vehicle is attitude dependent. For the STK default attitude, with body-fixed +Z toward nadir, positive elevation angles point sensors away from the Earth.


Defines the sensor as spinning on its axis or sweeping in a defined pattern. Control the behavior of the sensor boresight regardless of the shape of the sensor itself.

If you are calculating accesses and coverage using spinning sensors, or animating a scenario containing a spinning sensor, click these links to learn about certain factors you must take into consideration.

Scan Mode

STK provides the following Scan Mode options for spinning sensors.

Scan Mode Description
Continuous Indicates uninterrupted motion about the spin axis.
Unidirectional Scan from a specified start angle to a specified stop angle, and then begin again at the start angle.
Bidirectional Scan in both directions between a specified start angle and stop angle.

Start and Stop Angles

Both the unidirectional and the bidirectional scan mode require that you define start and stop angles.

Angle Definition
Start Specify the angle about the sensor's spin axis at which scanning begins. Zero is relative to the X axis in the spin axis coordinate frame.
Stop Specify the angle about the sensor's spin axis at which scanning ends and/or reverses direction.

Both angles are always measured in a positive, right-handed sense about the spin axis. In the following diagram, the viewer is looking down the spin axis:

Spin Rate

The spin rate is the rate at which the boresight spins about the spin axis, measured in revolutions per minute. The spin is positive in a right-handed sense about the spin axis. You can use negative rate to create a spin in the opposite direction.

Initial Offset Angle

The initial offset angle for a spinning sensor is the angle about the spin axis where the sensor boresight is at time zero.

Spin Axis

Defining the spin axis of a spinning sensor requires that you specify the location of the spin axis in the parent object's body frame. The location of the spin axis is defined by specifying the azimuth, elevation, and cone angle.

Option Description
Azimuth Specify the angle from the parent object's +X axis about the +Z axis in a right-handed sense.
Elevation Specify the angle between the spin axis and the parent object's body-fixed XY plane, measured as positive in the direction of the parent object's body-fixed +Z axis.
Cone Angle Specify the angle between the spin axis and the sensor boresight. As the boresight spins about the spin axis, it maintains this angular distance away from the spin axis.

If the cone angle is 0 degrees, the boresight aligns with the spin axis and the sensor looks in one direction all the time but spins about that direction. If the cone angle is 90 degrees, the boresight spins about in a plane perpendicular to the spin axis, typical when creating a sweeping or scanning sensor.

The following diagram illustrates the spin-axis geometry:


This option enables you to control scheduled sensor pointing.

This option is editable through automation interfaces only and appears in the list if the sensor has been set to this option via Connect. For more information, see the Point Schedule Connect command.