Access Constraints

When you are determining access times for a given pair of STK objects, you can apply access constraints to model requirements and limitations (placed on each object) to meet your mission objectives. For more insights into access computations in STK, see Access in the Technical Notes section of the Help.

Selecting and applying constraints

Many STK objects have available access constraints. The constraints available for a specific object are a subset of all possible STK access constraints.

To see and select access constraints for an object, open the object's properties and go to the Constraints - Active page. At the top you will see Active Constraints, which lists the constraints you have already selected. A selected constraint is active if the Enabled check box is selected. For most objects, the Line Of Sight constraint appears active in this list by default.

Click () to see other available constraints in the Select Constraints to Add dialog box. Under Filter by Category, the default is that all categories are selected, which causes all possible constraints to appear on the right. To find a specific constraint, start typing the name in the Search field. You can clear the All Categories check box to clear all individual category check boxes. Then you can select a category check box to see available constraints for just that category.

To add a specific constraint, select it and click Add or just double-click it. You must click Apply or OK in the object's Properties Browser to activate a newly selected constraint for STK access computations. To see information about modifying a constraint, click () next to Constraint Properties. If you modify a constraint property or select/clear the Enable check box for a constraint, Apply will activate, and you must click Apply or OK to make your latest changes effective in access computations.

You can learn more about a particular constraint category in one of the following ways:

  • Highlight a constraint from that category in the Active Constraints list and click Help at the bottom of the Properties Browser.
  • In the Select Constraints to Add dialog box, select a single category on the left and click Help at the bottom of the dialog box.

These methods will bring you to the specific STK Help page for that constraint category. If you do not select a category in the Select Constraints to Add dialog box, clicking Help will bring you to this Help page.

For each constraint you select to enable, make sure to set the parameters under Constraint Properties, to model the problem of interest at the fidelity you require. Many constraints have the following properties:

Property Description
Min

Set the minimum value for the constraint parameter.
Max Set the maximum value for the constraint parameter.
Exclude Time Intervals Select this check box to have STK only include access time intervals that do NOT meet the constraint.
Use Max Time Step Select this check box to apply a maximum time step for this specific constraint. You can specify a time step in the Max Time Step field. This value will only be effective if it is smaller than the Max Time Step settings for both the Access Tool and the object to which you are applying this constraint. STK will apply this Max Time Step only for access computations involving this individual constraint. This setting is particularly useful for constraints that could have sudden changes, such as Az-El Mask.

Active Constraint List

When multiple objects of the same class are selected and the Contraints - Active page is opened, the Active Constraints list presents the intersection of the Active Constraint lists of the selected objects. Constraints that can be added multiple times to the same object are excluded from the list and not available to add.

For example, consider three satellites that are multi-selected. If all three have the Line of Sight constraint in their Active constraint list, then the Line of Sight constraint appears in the multi-select Active Constraint list.

If any of the satellites do not have the Line of Sight constraint in their Active Constraint list, then Line of Sight does not appear in the Active Constraint list.

If all three of the satellites have the same Analysis Workbench (AWB) Vector constraint in their Active Constraint list, the constraint does not appear in the multi-select Active Constraint list because AWB Vector constraints can be added multiple times to the same object.

Adding a constraint

The process for adding a constraint is the same as described in Selecting and applying constraints.

Consider three satellites (A, B, C) that are multi-selected. If Satellite A has the Elevation Angle constraint in its Active Constraint list, but Satellite B and C do not, then the Elevation Angle constraint will not appear in the multi-select Active Constraint list.

However, the Elevation Angle constraint can be added to Satellites B and C by adding the constraint through the process described in section Selecting and applying constraints. The Elevation Angle constraint will be added to Satellites B and C with default properties, and the Elevation Angle constraint of Satellite A will not be modified by the add.

Modifying a constraint

Changes applied to a constraint in the multi-select Active constraint list are applied to all multi-selected objects.

For example, if you modify the Minimum property of an Elevation Angle constraint, the Minimum property of the Elevation Angle constraint for each multi-selected object is modified.

Deleting a constraint

To delete a constraint, follow the same process described in section Selecting and applying constraints.

If you would like to delete a constraint that is only Active on a subset of multi-selected objects, you must first add that constraint so that it is active on all of the multi-selected objects and then delete that constraint.

General constraint settings for objects

You can set a maximum time step for an object that applies generally to access constraint computations involving that object. To do this, go to the Constraints - Settings page in the object's Properties Browser and select the Use This Maximum Time Step in Access Computations check box.

Selecting the Use Max Time Step option for an individual constraint will override this object-level maximum time step setting, assuming it is a smaller value, but only for access computations involving that individual constraint.

Access constraints categories

All possible STK access constraints appear in categories. The category designation does not affect any access constraint's computation. Some constraints appear in two or more categories, such as Elevation Angle.

The categories are:

  • Basic: The most commonly used constraints
  • Angle: Constraints that involve an angular measure
  • Geometry: Constraints that use a geometric measure (other than angle), such as range, altitude, and grazing altitude
  • Analysis Workbench (AWB): Constraints that involve an Analysis Workbench (AWB) component (Angle, Calculation Scalar, Condition, or Vector)
  • Central Body: Constraints involving the location of the Sun, planets, and moons
  • Communications: Constraints used to measure radio-frequency link performance
  • EOIR: Constraints pertaining to an electro-optical and infrared receptor (EOIR) sensor
  • Interference: Constraints used to measure radio-frequency link performance in the presence of interfering transmissions
  • Multifunction Radar: Constraints pertaining to a multifunction radar
  • Multifunction Radar Jamming: Constraints pertaining to a multifunction radar in the presence of jamming transmissions
  • Plugins: Your own plugin constraints, which compute a plugin-defined metric that you define
  • Radar Bistatic: Constraints pertaining to a bistatic radar
  • Radar SAR: Constraints pertaining to a SAR radar
  • Radar SAR Jamming: Constraints pertaining to a SAR radar in the presence of jamming transmissions
  • Radar SearchTrack: Constraints pertaining to a search/track radar
  • Radar SearchTrack Jamming: Constraints pertaining to a search/track radar in the presence of jamming transmissions
  • RF Loss: Constraints used to measure radio-frequency losses
  • SEET: Constraints that leverage the Space Environmental and Effects Tool (SEET)
  • Temporal: Constraints related to specified time-of-day or time intervals
  • Zones: Constraints using longitude or latitude as the measure

Example use case

Typically, you choose a set of constraints that models a desired objective to answer a question, for example " When can a facility see a satellite? In other words, during which time intervals is the direction to the satellite not blocked by the ground?"

The answer could come in the form of imposing the one or more constraints to implement increasing model fidelity:

  • Simple model: You would just enable the Line Of Sight constraint. The ground is modeled as being locally flat at the facility's ground altitude, so having a positive elevation angle indicates access. This permits the facility to see a satellite even at the horizon. In most cases, however, a ground location cannot see all the way to the horizon, given the local topography, vegetation, and near-by buildings. To prevent having access too close to the horizon, you can apply the Elevation Angle constraint with a minimum angle (Min) of 10 degs.
  • Medium-fidelity model: You would also enable the Az-El Mask constraint, where an az-el mask for the facility was loaded or computed from terrain data. The mask prevents access when the elevation is too low for a given azimuth. In some directions, a 10 deg minimum elevation is adequate to avoid local topography, while in other azimuthal directions a larger value is required. The Az-El Mask data contains this information.
  • High-fidelity model: You would enable the Terrain Mask constraint to model the ground. You would need to enable terrain in the scenario to use this constraint. In this case, the terrain data models the ground. While this model is the most accurate for determining ground blockage, it is also computationally expensive and so results take longer to compute.