Chain Definition
The Chain definition properties enable you to setup the Start and End objects and object connections used when computing access times from the Start to End objects. Objects in the Chain can be individual objects (Satellites, Sensors, Places, etc) or grouped objects (Constellations or Satellite Collections).
The following options can be used to define the chain configuration:
| Option | Description |
|---|---|
| Start Object | Specify the STK object (individual or grouped) from which the chain access starts. |
| End Object | Specify the STK object (individual or grouped) from which the chain access ends. |
| Max Strand Depth | Maximum number of objects allowed in a strand, used to limit number of strands computed when many Connections and/or many uses of links are specified. |
| Connections | List of all available connections used to create possible paths from the Start object to the End object when the Chain is computed. |
| Add | Add a new blank Connection to the Connection grid. |
| Remove | Remove the selected connection from the Connections grid. |
| Remove All | Remove all connections from the Connections grid. |
| Extend | Create a new Connection with the From Object set to the To Object from the selected Connection Grid. The new Connection's To Object must be selected. |
| Duplicate | Create a new Connection by copying the From and To objects and the min and max number of uses from the selected Connection in the Connection field. |
The following options can be used to configure a selected Connection in the Connection grid:
| Option | Description |
|---|---|
| From Object | Specify the From STK object for a connection |
| To Object | Specify the To STK object for a connection |
| MinNumUses | Specify the minimum number of times this connection must be used in a strand for it to be valid |
| MaxNumUses | Specify the maximum number of times this connection may be used in a strand |
| Parent Platform Restriction | Option available when using grouped objects (Satellite Collection subsets or Constellations) in this connection to limit the consideration access pairs generated from the connection. If "No Restriction" is used, all possible access pairs between the From and To objects are considered when generating strands that include the connection. If the "Same Parents" or "Different Parents" restriction is used, each access pair between the From and To objects are evaluated for validity based on their parent platforms and only valid pairs (pairs with the same or different parent platform, respectively) are used when generating strands that include this connection. For the option to be available when using Satellite Collection subsets, both the From and To objects must be a Satellite Collection subset. This option is available for Constellations as either the From or To object unless the other object is a Satellite Collection subset. |
If an invalid connection is encountered when computing the chain, STK halts the computation and displays an error message in the Message Viewer. By default, strands with no access times available are removed depending on the Chains Advanced setting to retain empty strands for reporting.
Impact of Connection Order
The order of objects in a connection can be especially important if the chain includes planets. Access calculations in a chain take light time delays into consideration. This can lead to unexpected results if you place a planet in the Chain incorrectly.
Reversing the order of two constellations in a chain changes the way in which some constellation constraints are applied. For example, calculating access between a chain with a constellation containing two facility sensors and a constellation containing GPS satellites will differ depending on which constellation is first in the chain.
In both cases (sensor constellation to GPS satellite constellation and GPS satellite constellation to sensor constellation), STK first determines individual accesses (i.e., all pairs of GPS satellites and sensors). STK then applies the constellation constraints. In this example, the constraints are AnyOf for the GPS satellites and AllOf for the sensors.
In the case of GPS satellites to two sensors, STK computes the time intervals for each sensor when it has access to any GPS satellite. STK generates an interval list for each sensor. STK then applies the AllOf constraint by intersecting both interval lists. This results in the valid intervals satisfying the constraints. For each access pair, STK then intersects the pair's access times with the valid intervals. Thus, at every valid time, both sensors must be able to see at least one (i.e., AnyOf) Satellite in the GPS constellation. However, any GPS Satellite that can be seen by either sensor at such a time is considered to be included in the Chain's complete Access intervals.
In the case of two sensors to GPS satellites, STK computes the time intervals for each sensor when it has access to any GPS satellite. STK generates an interval list for each sensor. STK then applies the AllOf constraint to each GPS satellite individually. STK computes the intersection of each sensor's access to each GPS Satellite (i.e., AllOf) producing valid intervals for each GPS satellite. STK then compares the intersection of each Sensor's access time with these valid intervals. Since each GPS satellite is considered by itself, the complete Chain access includes times in which both sensors can see the same GPS satellite (i.e., All Of). Times at which one sensor can see a GPS satellite and the other sensor cannot are not taken into account.
Thus, when the GPS satellites constellation appears first, with the AnyOf constraint, the GPS satellites are considered as one entity; when the constellation appears second, it is treated as a collection of individual GPS satellites.