Chain Definition

This window lets you assign objects to the chain and define the order in which the objects are accessed. Use the following buttons to define the chain:

Button	Purpose
	Assign the objects that are selected in the Available Objects list to the chain.
	Remove the objects that are selected in the Assigned Objects list from the chain.
	Move the objects that are selected in the Assigned Objects list to a higher position in the list, so that access calculations are performed for those objects earlier.
	Move the objects that are selected in the Assigned Objects list to a lower position in the list, so that access calculations are performed for those objects later.

You can also double-click an object to move it. Use the Selection Filter to move one or more groups of objects by category.

When multiple paths through Chains and Constellations exist, Chains will try to compute links through all the paths, even through paths that are not valid as Comm links. To help ensure that only valid paths are computed, enable the Disable Cross Parent Access constraint on the constellation of transmitters when adding constellations of transmitters and receivers to a Chain. If an invalid link is encountered when computing the Comm link budget, computation will be halted and an error message will display in the message viewer.

A child (sub-object) of an object is not automatically moved when its parent is moved. For example, if you want to assign a sensor that is attached to a satellite to the chain, you must move the sensor to the Assigned Objects list; it is not sufficient to move the parent satellite.

The order of objects in the chain 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 a planet is placed in the chain incorrectly.

Constellation constraints can affect access calculations in a chain.

The following figure illustrates the impact of light time delay on the first access calculation in a Sun-satellite-facility chain:

Impact of Reversing Constellation Order on Constraints

Reversing the constellation order 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.