Special Constraints

A number of special access constraints, described below, are available. You can also opt to exclude time intervals that satisfy a given constraint.

Geostationary Belt Exclusion Angle

If the Geostationary Belt Exclusion Angle is ON, access to an object is constrained when it falls within the specified angle. This constraint is useful when calculating access from a facility or target to a LEO satellite (see the illustration below).

Note: The Geostationary Belt Exclusion Angle, Height Above Horizon, and Terrain Grazing Angle constraints are available for facilities and targets only.

Height Above Horizon

If the Height Above Horizon constraint is ON, enter the minimum and/or maximum height of the facility or target above the horizon.

Terrain Grazing Angle

If the Terrain Grazing Angle constraint is ON, 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 properties. If the slope of the terrain is zero, this constraint is equivalent to the Elevation Angle constraint.

Object Exclusion Angle

Use the Object Exclusion Angle area to select and remove objects that are to be excluded from access computations if they are at the indicated angle from the original object.

If the line between the source object and the special body is obstructed (for example, the earth is between a satellite source object and the special body), then the exclusion constraint is not applied.

Warning: The Object Exclusion Angle constraint is not applied in the determination of access to an excluded object.

Assign Objects

Use the following steps to select objects that are to be excluded from access computations if they are at the indicated angle from the original object:

1. Turn ON the Use option.
2. Use the Exclusion Angle field to indicate the angle from the original object at which an object should be to be excluded from access computations.
3. Select the objects to which this constraint should be applied in the Available Objects list.
4. Use the Insert button to move objects to which this constraint should be applied from the Available Objects list to the Assigned Objects list.

Remove Objects

Use the following steps to remove objects from the list of Assigned Objects:

1. Select the object you wish to remove in the Assigned Objects list.
2. Click Remove.

Elevation Rise-Set

The Rise-Set Elevation angle constraint allows you to specify different elevation angle limits for the rise-stage and setting-stage of a satellite. For example, a receiver (attached to a sensor and/or a facility) will acquire a rising satellite only when it is 15 degrees above horizon, and will track it 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 the rising and/or setting phase of a satellite.