Applicable Constraints
When computing access, STK only uses constraints that are applicable to a pair of objects. STK combines the active constraints for each object together and then individually assesses whether the models provided by the object pair support the constraint’s computation. STK ignores the unsupported constraints, leaving only the set of constraints that it can compute for the pair (i.e., the applicable constraints).
An example scenario illustrates the set of applicable constraints when computing access for different object pairs and their active constraints.
- Satellite with active constraints: line of sight (LOS) enabled, Range < 1000 km
- Sensor on Satellite with active constraints: LOS enabled, field of view (FOV) enabled, Sun Exclusion angle = 10 deg, Elevation angle < -45 deg
- Receiver on Sensor with active constraints: LOS enabled, Range < 500 km, Link Margin > 3 db
- Facility with active constraints: LOS enabled, Elevation angle > 10 deg
- Transmitter on Facility with active constraints: LOS enabled, BER < 1.e-6
| Object A | Object B | Applicable Constraints |
|---|---|---|
| Facility | Satellite |
(A): LOS enabled (A): Elevation angle > 10 deg (B): Range < 1000 km |
| Facility | Sensor |
(A): LOS enabled (A): Elevation angle > 10 deg (B): FOV enabled (B): Sun Exclusion angle = 10 deg (B): Elevation angle < -45 deg |
| Facility | Receiver |
(A): LOS enabled (A): Elevation angle > 10 deg (B): Range < 500 km |
| Transmitter | Satellite |
(A): LOS enabled (B): Range < 1000 km |
| Transmitter | Sensor |
(A): LOS enabled (B): FOV enabled (B): Sun Exclusion angle = 10 deg (B): Elevation angle < -45 deg |
| Transmitter | Receiver |
(A): LOS enabled (A): BER < 1.e-6 (B): Range < 500 km (B): Link Margin > 3 db |
Observations to be made by looking at the table:
- In general, STK combines the constraints from objects A and B , except:
- There’s a special rule for LOS to only use A’s LOS (see below).
- STK ignores constraints that can’t be modeled properly for an object.
- Receiver Link Margin only applies when the access involves a Transmitter.
- Each applicable constraint remains associated with the object that owns the constraint, either (A) or (B).
- For the Facility-Sensor access, STK evaluates both the Elevation angle constraint on the Facility and the Elevation angle on the Sensor.
- The definition of elevation angle depends on the object type; see the constraints help page.
- Child objects do not inherit the constraints of their parent.
- The Facility-Sensor access has no Range constraint, whereas the Facility-Satellite access does.
- The Facility-Receiver access has a Range constraint using the setting from the Receiver, not from its grandparent Satellite.
- Accesses involving the Receiver do not include FOV, though accesses to its parent Sensor do.
- The access constraints for each object apply to all accesses involving that object.
- Each object has only one set of active constraints.
- STK ignores an active constraint only if it is not applicable for the access pair.
- An exception is that LOS uses a special processing rule; see below.
- If you modify the constraints for an object, STK will recompute all accesses involving that object.
Special LOS behavior
It is very common to model situations where the ground restricts access, meaning the ground blocks the line of sight between the objects. However, there is no global model of the ground used by all objects. Instead, each object is allowed to model the ground appropriately for its behavior. Since those models vary in fidelity, STK may choose to ignore a lower fidelity LOS model , such as the model used by the Satellite, in preference to a higher fidelity LOS model, such as the model used by a ground location. See LOS Constraints.