Angle Constraints
In this category, you can choose to apply access constraints concerning geometric angles in your scenario. You can also opt to exclude time intervals that satisfy a given constraint.
When an access involves a child object (i.e., sensor, transmitter, receiver, antenna, or radar), use the child object's parent or grandparent in the table below to determine whether you can use the constraint for access to that child object. For example, if a constraint can be used for accesses to all vehicles, then that constraint can also be used for any child object of a vehicle.
In the following table, abbreviations in the Constraints used for accesses to... column are:
| A = aircraft | AT = area target | F = facility | G = ground vehicle | L = launch vehicle | LT = line target | M = missile | P = planet | Pl = place |
| Rd = radar | Rx = receiver | S = satellite | Sh = ship | Sn = sensor | St = star | T = target | Tx = transmitter | V = all vehicles |
| Constraint | Description | Constraint used for accesses to... |
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| Angle Off Boresight | This is the angle between the direction to the target object and the boresight direction. The boresight is the Z axis of the object's Body axes. | Rd Rx Sn Tx |
| Angle Off Boresight Rate | This is the rate of change of the angle between the direction to the target object and the boresight direction. The boresight is the Z axis of the object's Body axes | Rd Rx Sn Tx |
| Angular Rate |
The angular rate is defined as the angular rate of the direction of the apparent relative position between the two objects, as observed by the owner of the constraint. Let r represent the apparent relative position and v represent the apparent relative velocity. Then the angular rate ω is h⁄r2, where r=||r||, h=||h||, and h = r ⨯ v. Facility, Target, and Place objects observe and measure v in central body fixed coordinates; vehicles observe and measure v in central body inertial coordinates.
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F Pl T V P St |
| Azimuth Angle | For facilities and targets, STK measures azimuth in the local horizontal plane, tangent to the surface of the central body. This angle is measured from the local north direction toward the local east direction. An azimuth of 0 degrees specifies a location directly to the north, and an azimuth of 180 degrees specifies a location directly to the south. For ships, ground vehicles, and aircraft, STK measures the azimuth from the projection of the earth-fixed velocity vector. In the case where the vehicle is stationary, STK measures azimuth from local north in the topocentric frame. For other objects, STK measures azimuth in the plane perpendicular to nadir from the projection of the inertial velocity vector to the projection of the relative position vector. This angle is measured in a positive sense according to the right-hand rule about the nadir vector. An azimuth of 0 degrees specifies a location directly in front of the object, and an azimuth of 180 degrees specifies a location directly behind the object. |
F Pl T P St V |
| Azimuth Rate | The azimuth rate is the rate of change of the azimuth angle. | F Pl T P St V |
| Boresight Grazing Angle | Describes how high the target object appears above the edge (or limb) of the Earth relative to the sensor boresight direction. It is measured as the angle between the Earth limb and the target object. You can use this constraint to prevent a sensitive optical sensor, such as those on a telescope, from aiming too close to the Earth, which might blind the sensor due to reflections off the surface. | Sn |
| Central Angle | Central Angle is a constraint for vehicles and ground locations. It provide a simple measure of physical separation using the angle between the position vectors relative to the central body of the object owning the constraint. | F, T, Pl, V, P, St |
| Doppler Cone Angle | This is measured with respect to the +/- Earth-fixed velocity vector directions. Access within the defined cones is precluded. Typically used to model the collection area for radars. | F Pl T P St V |
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| Elevation Angle | For objects other than facilities and targets, elevation is measured as the angle between the nadir vector and the relative position vector minus 90 degrees. The elevation angle is positive for objects above the plane perpendicular to nadir. For facilities and targets, elevation is measured as the angle above the local horizontal plane, where the local horizontal plane is tangent to the surface of the central body. The elevation angle is positive for objects above the local horizontal plane. |
F Pl T P St V |
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Elevation Angle constraint as applied to a satellite
Elevation Angle as defined for a facility or target |
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| Elevation Rate | The elevation rate is the rate of change of the elevation angle. | F Pl T P St V |
| Elevation Rise-Set |
The Rise-Set Elevation angle constraint enables you to specify different elevation angle limits for the rise-stage and setting-stage of a satellite. For example, you can set a receiver (attached to a sensor and/or a facility) to acquire a satellite only when it rises to 15 degrees above horizon, but have the receiver track the satellite down to a 5-degree elevation angle when the satellite is setting. This constraint provides additional flexibility as compared to the normal Elevation angle constraint, which applies the same elevation angle limits on both the rising and setting phases of a satellite pass. |
F, T, Pl, V, P, St |
| Geostationary Belt Exclusion |
Use the Geostationary Belt Exclusion Angle constraint to disallow access when the angle between the direction to the target object and the direction to any point of the geo belt is within the specified value. As of STK 11.x, Earth obstruction is modeled so that only those points on the geo belt that are visible from the site's ground location are considered. This constraint is useful when calculating access from a facility, place, or target to a LEO satellite (see the illustration below). The Geostationary Belt Exclusion Angle, Height Above Horizon, and Terrain Grazing Angle constraints are available for facilities, places, and targets only.
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F, T, Pl |
| Grazing Angle | Describes how high one object appears above the edge of the Earth (or limb) relative to the satellite. Measured relative to the satellite, as the angle between the Earth limb and the other object. This constraint can be used to prevent a sensitive optical sensor, such as those on the Hubble Space Telescope, from aiming too close to the Earth, which might blind the sensor due to reflections off the surface. | P St V |
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| Ground Elevation Angle | For a target, place or facility, the elevation angle is measured as the angle between the local horizontal and the direction from the ground-based object to the satellite. For an area target, the elevation angle is measured as the maximum elevation from any point within an area target. | F Pl T AT |
| Object Exclusion Angle |
Use the Object Exclusion Angle constraint to select objects that are to be excluded from access computations if they are at the indicated angle from the original object. See Selecting objects for exclusion. If the line between the source object and the special body is obstructed, such as when the earth is between a satellite source object and the special body, then the exclusion constraint is not applied. The Object Exclusion Angle constraint is not applied in the determination of access to an excluded object. |
F, T, Pl, V, P, St |
| Squint Angle | The squint angle is the absolute value of the dihedral angle about the vehicle's position, from the in-track direction to the relative position of the target object. This is the angle between the X axis and the projection into the X-Y plane of the relative position vector to the target object. It provides a measure of angle between the target object and the ground track. See the Squint Angle figure below this table. | F, T, Pl, V |
| Terrain Grazing Angle |
For the Terrain Grazing Angle constraint, enter the minimum and/or maximum angle between the relative position vector and the local terrain. This constraint is dependent upon the terrain normal direction specified in the object's Basic - Terrain Slope properties. If the slope of the terrain is zero, this constraint is equivalent to the Elevation Angle constraint.
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F, T, Pl, V, P, St |
The following diagram illustrates the Squint Angle constraint option described above.
Selecting objects for exclusion
Use the following steps to select one or more objects that are to be excluded from access computations if they are at the indicated angle from the original object.
- In the Select Constraints to Add dialog box, select the Object Exclusion Angle constraint and click .
- Use the Exclusion Angle field to enter the angle from the original object at which an object should be to be excluded from access computations.
- Click Select Object, choose an object from the list of available objects, and click in the Select Object dialog box.
- Repeat steps 1-3 to apply the Object Exclusion Angle constraint to another object.