Access Options

You can use Access Options to set parameters related to event detection, light time delay, step size control, and signal path to be used in access computations. The values specified in this window will override Access Defaults set at the application level.

Event Detection

Option Description
Find Precise Event Times

The visibility algorithm first takes samples, then subsamples, to determine visibility intervals to within tolerances. Use this option unless you are satisfied with the resolution using a fixed time step. The Find Precise Event Times option takes more time for the same sampling step size than the Use Samples Only option. However, for larger time steps, Find Precise Event Times is better for achieving answers within your tolerance.

Time Convergence This parameter specifies the allowable error in the computed start and end times of visibility periods during subsampling. Making this value larger should reduce the number of subsample iterations and decrease the amount of time required for coverage computations. For most applications, a value of one second is sufficient.
Value Tolerance

These parameters specify the allowable error in the constraint values when determining the crossing of the threshold that defines visibility. When values near zero, STK instead uses the absolute tolerance requirement.

  • Relative: An event is said to be detected when a constraint value's relative difference compared to the previous sample is within a tolerance per the equation:

    (value - previousValue)/value < RelativeTolerance
  • Absolute: The criterion used for convergence in value for values near 0. An event is considered "detected" when a constraint value's difference compared to the previous sample is within a tolerance defined by the equation:
    (value - previousValue) < absoluteTolerance

    The default values provide accurate event detection over a wide variety of problems.

Use Samples Only If you choose this option, the visibility intervals are determined based on the taken samples. In other words, visibility starts at the first sample time where visibility is achieved.

Step Size Control

Option Description

This method allows the access algorithm to determine the sampling step size based on the constraints being processed. It is typically the safest mode to use, especially when the motion of the assets is complex. However, this method does require some additional computation time to determine the next step size. If the conditions of the scenario are slowly changing, you can increase the Max step size to improve the performance of the coverage computations.

Max This sets the maximum step size allowed for the Adaptive method.
Min This sets the minimum step size allowed for the Adaptive method.
Fixed Step This method forces the sampling performed by the access algorithm to run at a fixed step. This outcome is desirable for cases where no sudden changes exist in the motion or behavior of objects involved in the coverage analysis. It's also desirable when the ephemeris of a moving object is stored at a fixed step size. For these cases, ephemeris interpolation can be avoided during the access sampling phase. You must make sure that the Use Light Time Delay option is not selected, and you must also set the start time of the coverage analysis to coincide with the start of the stored ephemeris.

Ephemeris interpolation would still be performed during subsampling if Find Precise Event Times is selected.

Step Size This sets the fixed step size for the fixed step control.
Time Bound Use this parameter to control the alignment of samples with a UTC time grid. The use of a proper Time Bound value can improve computational performance if the ephemeris involved in the sampling happens to lie on a fixed UTC time grid. The Time Bound parameter determines the reference time for taking fixed step samples. Fixed steps are taken starting with this time.

For instance, a value of 60 seconds indicates that the reference time is the closest time value before the interval start time that lies on the whole minute in UTC.

A value of 600 indicates that the reference time is the closest time value before the interval start time having a UTC time on a multiple of 10 minutes.

The Time Bound value must be nonnegative and less than or equal to 86,400 seconds. A value of 0 indicates that no UTC alignment is used (i.e., the interval start time is the reference time).

Light Time Delay

Select Use Light Time Delay to have STK consider light time delay in access computations. When light time delay is not considered, aberration is also ignored. Some Light Time Delay parameters are not valid for Chains and Coverage. For a discussion of light time computation, including terminology and equations, see Light Time Delay (PDF). For targeted sensor pointing, the Use Light Time Delay option is set by the Track Mode setting. Below are descriptions of the parameters for the Light Time Delay window.

Star objects are always modeled with total (stellar) aberration, regardless of the Aberration Type and Use Light Time Delay settings.

Option Description
Light Time Delay Convergence This parameter sets the tolerance used when iterating to determine the light time delay. The iteration stops when the improvement in the value is less than this tolerance.
Aberration Type This option sets the model of aberration to be used in access computations. The values are:
  • None: STK will ignore aberration.
  • Annual: STK will consider annual aberration only. Thus, aberration is applied only in cases when the light time delay is computed using the solar system barycenter frame.
  • Total: STK will apply the full aberration correction when light time delay is computed.

Signal Path

Below are descriptions for Signal Path parameters. These parameters are not valid for Chains, Coverage, or targeted sensor pointing. The Use Default Clock Host and Signal Sense and Clock Host parameters are not available for vehicle targeted attitude.

Use Default Clock Host and Signal Sense

Select this option to use the default settings. Clear this option if you want to specify parameters that will override the default settings.

Clock Host Time values are reported with a clock colocated with the clock host object.
  • Base refers to the object that opened the Access Tool.
  • Target refers to an object from the Associated Objects list.
Signal Sense of Clock Host Enables you to set the direction of the signal.
  • Transmit: The signal is being sent from the object you selected as the Clock Host.
  • Receive: The signal is being received by the object you selected as the Clock Host.

If Use Default Clock Host and Signal Sense is not selected and a Bistatic Receiver Radar or Bistatic Transmitter Radar is selected as the Clock Host, this option is automatically set to the direction of the radar and cannot be changed.

The associated objects will remember their computational settings while the Access window is open, but the settings will be discarded when the window is dismissed.

Save computed data

If this option is selected (which is the default state), access computations and their derived data (e.g., satisfaction intervals) are saved when you save the scenario. The computed data is stored in an *.sca file. When loading, STK compares the loaded data and the recomputed data. If they match, STK accepts the access computed data and loads the derived data without recomputing.

If the objects that are involved in an access were saved separately, apart from the scenario, then the derived data may be out of sync with the object's data. In this situation, clear this option so that access does not save its derived data. Access will be recomputed when you load the scenario.

You can control the default behavior of saving computed data using the settings on the Access Defaults preferences page.