Access
The simplest definition of "access" is the ability of one object to see another object during a period of time. For example, if a satellite is passing over a ground facility, unblocked by the surrounding landscape, it has line-of-sight access between the primary object (the satellite) and the associated object (the facility) during the defined time interval.
The first condition for access is geometric line of sight, meaning the ability to draw a straight line between the positions of two objects. STK can determine whether this initial condition has been met for any two STK objects that have position data, such as a vehicle and a facility. STK cannot calculate access between STK objects that have no positions, such as a Figure of Merit object. Finally, special cases exist for objects like an Area Target, where position is defined as the centroid of the target.
Additionally, there may also exist other conditions for access, called constraints. Constraints are imposed by you. For example, you may want to know if access from a facility to a satellite only exists above a certain elevation angle. You may want to exclude accesses where the sun is directly behind the associated object. Furthermore, constraints are not limited to geometry. There are many access constraints associated with communication signal transmission analysis. For example, you can use comm link metrics, such as minimum signal-to-noise ratio, as a condition for access.
Each constraint is defined in STK as a property of the primary or associated object. For information on how to apply constraints and a list of the constraint categories, see Access Constraints.
Access exists between two objects only when all conditions have been met. When this occurs, STK reports access as a time interval, which identifies a start and stop time during which the conditions for access have been satisfied.
How is Access calculated?
Access is calculated using AGI's proprietary algorithms and user-defined settings. For further information about these settings, see Access Settings. You can modify the universal default setting values on the Access Defaults page of STK's Edit -Preferences, or you can modify the settings for an individual access by clicking in the Access tool.
In addition to basic algorithmic settings, such as step size, convergence, and tolerance, STK also addresses factors such as light time delay, aberration, refraction, and signal transmission directionality. STK usually considers light time delay between objects, but only considers aberration in certain circumstances.
For a full discussion of the light time delay issue, see Light Time Delay (PDF).
STK accounts for refraction after it computes the apparent relative direction between the objects and accounts for light time delay, but before applying aberration. You can control the use of refraction in Access computations through a setting in the Refraction properties for the object.
How is Access reported?
STK provides many preconfigured report and graph styles that enable you to visualize access results. A subset of this list is shown below. For a complete list, see Installed Styles.
Additionally, you may create your own custom report styles using the Access Data Providers.
| Name | Description |
|---|---|
| Access | Reports a list of the access intervals. |
| Gaps | Reports intervals during the computational interval where access does not exist. |
| AER | Reports azimuth, elevation, and range of the relative position vector between the objects over time during access intervals. The azimuth and elevation values are computed with respect to the default AER frame of the object selected in the Access Tool. See Access Coordinate Frames. |
| AER Reference Frame Info | Reports the name of the reference frame used when using the AER data provider to report in its default AER frame. Also reports the reference object used that defines the frame. |
| Access Configuration | Reports computational settings and the constraints used in the computation. |
| Access Intervals by Constraint | Reports the satisfaction intervals of each access constraint used in the access computation, where each subsection shows the time intervals satisfied by a constraint. |
| Access Violated Constraints | Reports each access interval and associates the start and stop time with the constraint that caused the access interval to start or stop. |
| Link Budget | Reports key link budget metrics over time during the access intervals. This is available only when the access involves a transmitter or receiver. |
| Link Budget - Detailed | Reports detailed metrics of the communication link over time during the access intervals. This is available only when the access involves a transmitter and receiver. |
| Radar SAR | Reports dynamic (spot) SAR mode radar performance data between a Radar and the associated object of an access. This is available only when one object of the access is a radar. |
| Radar Search Track | Reports the dynamic Search Track mode radar performance between a radar and the associated object. This is available only when one object of the access is a radar. |
Tools that use Access
The ability to calculate access between two objects is fundamental to many capabilities within STK, including the Access tool, the Close Approach tool, Coverage, Communications modeling, and more. There are two variations of access that enable you to calculate access between a single primary and multiple associated objects: Deck Access and Chains.
Chains enables you to compute accesses to a group of objects in the scenario in an order that you specify. The Deck Access tool enables you to compute access from a single object within the scenario to a set of objects not currently defined within the STK scenario.
Access and Analysis Workbench
STK's Analysis Workbench capability enables you to define access objects. These access objects have the following components, which you can select in the same way as for any other objects.
Vector Geometry components
Access generation provides Points, Axes, and Systems, built from each of the access’s objects, as well as the relative position and relative velocity vectors from each object’s perspective. The elevation angle of the relative position vector, as measured from the XY plane of an object’s body axes, is also available for each object involved in the access.
Time components
Calculating access provides several time components to AWB, including the input times over which STK computes an access (ConsideredIntervals), the computed times when access is available (AccessIntervals), and the satisfaction intervals for each constraint (ConstraintIntervals).
Calculation components
Access calculations provide parameter sets describing the relative position vector between the objects, in several different representations (Cartesian, Spherical, Cylindrical). In addition, an access between a transmitter and a receiver provides a CommLinkInformation parameter set from which you can obtain many communications metrics at a given point in the access interval.