Applying Grid Constraints

When using an Aviator aircraft as a grid point (or a child object of an Aviator aircraft as a grid point), the attitude of the Aviator object will be topocentric.

To assign additional access constraints to the grid points, select Use Object Instance and select the object you wish to use in the Instance list. By selecting a specific object, all of the properties and constraints that apply to that one object, excluding positional information, are also applied to every point in the grid. Before you select the object in the Instance list, make sure you have set all of the attributes you want to apply.

If you choose a sensor as the object type, you must choose an object instance. This ensures that the desired sensor field of view and pointing are used during access computations.

If you choose a receiver or transmitter as the object type, you must choose an object instance. This ensures that STK uses the desired operating frequency and antenna pattern definitions when calculating accesses. It makes sense to define the grid elements as receivers or transmitters only if your assets are members of the complementary class.

The Instance list will contain only the objects that are on the same central body as the grid.

If you make a change to the selected instance after computing access information, STK automatically recomputes accesses if the Auto Recompute option is enabled.

If you select Use Actual Object on the Grid Points, the actual object in the scenario that you selected will be used in coverage computations. Otherwise, a copy is used that inherits nothing but basic properties, access constraints, and the shape of the obstruction surface from the selected object.

The Use Actual Object on the Grid Points option can only be enabled when the grid instance is not one of the objects in the coverage assets list.

During a coverage computation, when evaluating a specific grid point characterized by a unique position, the basic properties, access constraints, and shape of the obstruction surface of the object are applied to the grid point as before. What the Use Actual Object on the Grid Points option provides in addition, is that the location of the selected object is updated with the positional information of the grid point. This can be thought of as temporarily placing the actual object on each grid point for evaluation. In many simple coverage problems, this has no effect on the result. However, as soon as the coverage computation depends on the object's position, rather than simply the grid location, this setting becomes important. Typically, this is the case if a property, component, or child object associated with the selected object varies with position, and at the same time factors into the coverage computation. Examples of such situations are:

• The coverage asset is a targeted sensor, pointing at the selected object.

  As the Use Actual Object on the Grid Points option ensures that the position of the selected object is updated for each grid point, the sensor is re-targeted as each grid point is evaluated.

• The selected object is used in a CommSystem as a transmitter, receiver, or interferer, and the coverage computation depends on the CommSystem.

  As the Use Actual Object on the Grid Points option ensures that the position of the selected object is updated for each grid point, the CommSystem is recomputed with varying positional information as each grid point is evaluated.

• The selected object is a sensor that relies on a Sensor Attach Point, 3D Model Pointing, Model Articulation, or otherwise relies on the 3D model of the sensor parent object.

  The Use Actual Object on the Grid Points option ensures that all properties relevant to the positioning and pointing of the selected sensor will be honored in the coverage computation as each grid point is evaluated.

• The Figure of Merit is a Scalar Calculation value based on a component that varies with the location of the selected object. See also Scalar Calculation Dependencies.

  As the Use Actual Object on the Grid Points option ensures that the position of the selected object is updated for each grid point, any Analysis Workbench component that depends on the location of the selected object will be recomputed as each grid point is evaluated. This also applies to Analysis Workbench components which belong to other objects but depend on the selected object, such as Access or Chain computations involving the selected object.

• The asset or grid object is a child object of an object described above.

Under the typical use case, Use Actual Object on the Grid Points is a safe option, and has a positive or neutral effect on the desired outcome.

If you seek to optimize large Coverage computations, you should be aware that Use Actual Object on the Grid Points may impact performance.

Specify the shape of the ellipsoidal obstruction surface used by the Line Of Sight constraint. Valid only for grid instances that are Targets, Facilities, or sub-objects parented to a Target or Facility. The ellipsoid obstruction surface, for each grid point P, is determined as the ellipsoid with the same surface normal as P that passes through a height you specified. Those grid points whose altitude is higher than the line of sight obstruction surface are permitted to look downward from their height to the surface without being obstructed by the line of sight constraint. This is useful, for example, when modeling grid points above the ground, such as cell phone towers that need to communicate to objects near the ground. This feature can be combined with the use of terrain to produce a grid of points with a known height above the local terrain. Values are:

Altitude above MSL. The height is a constant offset from the mean sea level (MSL), which varies as the latitude and longitude of each grid point. This option is available on Earth only.

Altitude at Terrain. The height is defined by terrain, as specified on the scenario's Basic terrain properties page, and varies as the latitude and longitude of each grid point. If terrain is selected, the grid points consider terrain during the coverage analyses.

Altitude above WGS84. The height is a constant offset from the WGS84 ellipsoid surface measured along the surface normal, where the surface normal varies as the latitude and longitude of each grid point. This option is available on Earth only.

Altitude above Ellipsoid. The height is a constant offset from the central body's ellipsoid surface measured along the surface normal, where the surface normal varies as the latitude and longitude of each grid point. This option is available on central bodies other than Earth only.

Use Point Altitude Type. Uses the Point Altitude setting on the Grid properties page, except for Depth below MSL and Radius in which case the setting is interpreted as Altitude above WGS84.