Coverage Assets

Assets properties enable you to specify the STK objects used to provide coverage.

STK Attitude Coverage: The following discussion applies, in general, to STK Attitude Coverage as well as STK Coverage. However, in applying the following sections to STK Attitude Coverage, bear in mind that the relevant accesses and constraints are defined not for grid points as such, but for the associated object that you select on the Grid page. Accesses are computed from that object pointed via each grid point to the selected assets, and constraints on that object affect those accesses.

Available asset classes are facilities, targets, all types of vehicles, sensors, chains, constellations, and satellite collection subsets. If you have other STK capabilities, such as Communications/Radar, additional objects may be available as assets.

STK Attitude Coverage: In addition to the above objects, Stars, Planets, and Area Targets are available as assets for STK Attitude Coverage.

The Assets list contains a list of assets that can be assigned to the coverage definition. If an object in the list appears in bold with an asterisk (*) to the left of its name, it is currently assigned to the coverage area.

To assign an asset to the coverage area, highlight the object in the Assets list, then click Assign. To remove an item from the Assets list, select it, then click De-Assign.

You can do the following for an asset in the Asset list:

Set the status to either Active to Inactive. This enables you to remove selected assets from consideration without removing access information from the coverage calculations, and is typically useful after accesses have been computed. You can use the Inactive option to simulate the effect of an asset failure on the coverage provided by a set of satellites.
Designate whether or not access to a particular asset is required. Valid accesses for coverage must simultaneously have access to all required assets. See below for special considerations related to constellations.
Designate whether a constellation is viewed as a collection of independent objects or as a single entity via the Grouping setting, described in detail below.
Specify whether or not Coverage applies a constraints set on a constellation. To view the constraints on a constellation, open the properties of the constellation object and to the Constraints > Basic panel. The setting is valid for Constellations and Chains that end in a constellation.

Recomputing accesses

If access computations have already been performed for a coverage definition, the following actions will result in the computation of access data if you enabled the Auto Recompute option and set the Data Retention setting on the Advanced properties to “All Data”:

A modification to the definition or access constraint for an assigned asset will result in recomputation of the accesses for that asset.
A change to the Required status of an asset will result in the recomputation of the accesses for all assets.
A change in the grouping status for a constellation from Grouped to Separate will result in recomputation of the accesses for that asset.
The addition of an asset will result in the computation of the accesses for that asset.

A modification to the definition or access constraint for an assigned asset will result in recomputation of the accesses for all assets.
A change to the Required status of an asset will result in the recomputation of the accesses for all assets.
Any change in the grouping status for a constellation will result in recomputation of the accesses for all assets.
A change in the activation status of an asset will result in the recomputation of the accesses for all assets.
The addition or deletion of an asset will result in the recomputation of the accesses for all assets.

If you will be making a number of asset modifications, AGI recommends that you turn off the auto recompute access feature prior to making the changes to avoid recomputing multiple times. Turning the auto recompute feature back on after all changes have been applied will trigger the update of the access information.

Using sensors as coverage assets

While sensors may be used as coverage assets, more time is required to compute accesses to them due to the need for finer sampling and attitude information. It is often possible to substitute constraints on either the grid points or the parent object of a sensor for the sensor itself to improve the performance of access computations. For example, a conical sensor pointing to nadir on a satellite in a circular orbit can often be replaced with an equivalent ground elevation angle constraint if the effects of the oblateness of the Earth are not considered to be important.

Using a conical sensor as a coverage asset:

Using chains

When you identify a chain object as a coverage asset and that chain object is set to "Append Grid Object to End of Chain," STK adds each grid point to the end of the chain during access computations for that grid point.

The angle between the chain constraint is not enforced if the chain contains any strands with more than two nodes (which extends to three with the addition of the coverage grid point).

Calculating coverage with a chain object

When you identify a chain object as a coverage asset and that chain object is set to "Update Grid Object in Chain," STK updates the chain's intervals as each grid point is evaluated during access computations for that grid point. The coverage grid instance specified must be an object used in the chain, otherwise all grid points end up with the same intervals provided by the chain asset.

When you define a constellation or satellite collection subset as a coverage asset, STK computes accesses between the members of the constellation or subset and the grid point either independently or as a group of assets.

Select Constellation Use Constraints to use the constellation criterion (Any of, All of, At Least N, Exactly N, None) to specify the number of constellation members needed to complete the access. Clear Constellation Use Constraints to ignore the constraint on the constellation object.

When a Constellation is set to inactive, its subassets become inactive regardless of their individual states. If the Constellation is Active, then the individual states of the subassets are taken into account. You can declare objects within a constellation Inactive by opening the selected constellation tree to view, selecting individual objects in the constellation and setting Status to Inactive. However, if you select to use constellation constraints, the constellation constraints are applied before deactivating the asset. Therefore, making the asset inactive does not remove it from consideration when a constellation constraint is being computed.

If an active chain includes a satellite collection subset, coverage considers all the satellites in that subset active. If the subset is the end object in the chain, then you cannot activate that subset as an individual coverage asset. You can activate other subsets of the satellite collection as individual coverage assets.

You can use any of the following ways to apply group criteria to the coverage definition: the Grouping parameter, constellation constraints, the Number of Assets option on the Advanced panel, and the N Asset Coverage option when defining a related Figure Of Merit.

Only transmitters or receivers that are a part of simplex links can be used as grid objects. For example, you can use the following as grid objects:

A transmitter that links to a receiver.
A receiver that links to a transmitter.

Some STK Communications objects that are part of a multihop chain cannot be used as grid objects for a STK Coverage Figure Of Merit (FOM). For example, in a multihop chain that contains transmitter1 – receiver1 – transmitter2 – receiver2:

Only objects that are part of an independent link within the chain are used.
If transmitter1 or receiver1 is used as a grid object, then the FOM will compute link parameters only for the transmitter1-receiver1 link.
If transmitter2 or receiver2 is used as a grid object, then the FOM will compute link parameters only for the transmitter2-receiver2 link.
When receiver2 is used, FOM will not compute all the way back to transmitter1, even if it is defined in a Chains object.

Also, STK Communications links with retransmitter objects cannot be used with the STK Coverage Figure of Merit.

When using a Chain object as a Coverage asset, if the chain accesses need to be computed, accesses for the Chain will not be computed in parallel if the coverage is set to compute accesses in parallel. This is to ensure STK does not start too many compute nodes. When the Coverage compute is started, it will not need to compute Chain accesses if the Chain has already been computed. For best performance, compute the Chain accesses first in parallel and then compute the Coverage asset in pallel.

Using grouped assets

A group of assets can be any of the following:

a constellation
a satellite collection subset
a chain with a constellation at the end
a chain with a satellite collection subset at the end

When you assign one of these as a coverage asset, you have the option to consider the members of the constellation or satellite collection subset as a group or as separate entities. If you choose to consider the assets as a group, overlaps in intervals of coverage from the members of the constellation or satellite collection subset are removed and the assets are considered to be a single object for the purpose of counting the number of assets providing simultaneous coverage.

This feature is useful if you are interested in grouping like types of assets for the purpose of determining when coverage is simultaneously available from different types of collectors. One example is a situation where you have two generations of remote sensing satellites in orbit and want to know when you have simultaneous visibility between a ground location and a satellite from each generation to facilitate quality comparison.

The grouping option constellation may be switched from separate to grouped after the computation of accesses while maintaining a computed state in the coverage definition. If, however, the grouping option is switched from grouped to separate anytime after accesses have been computed, the coverage definition will no longer be in a computed state. If the option to automatically recompute is enabled, accesses will be recomputed at this time. If not, you will need to manually perform select compute accesses tool option to update the access information.

The use of grouped assets with the Dilution Of Precision, Navigation Accuracy, or Access Constraint Figure Of Merit can lead to undefined figure or merit values. This is due to the need for asset location information to support the computation of these figures of merit. When the grouping operation is performed, the identity of the specific asset providing coverage is lost and the notion of position of the asset becomes ill defined. A supported exception to this limitation is when all elements of the grouping are parented to the same object, such as a grouping of sensors on a single satellite.