Advanced CAT Advanced Options

Advanced CAT Filters

The Advanced Page allows you to select one or more pre-filters to use to speed up the close approach processing and to define a pad for each filter selected. The use of the Out of Date TLE, Apogee/Perigee, Orbit Path and Time Pre-Filters, as well as filter Pads , is like the use of these devices in the Close Approach Tool.

Log File

Keeping a log file allows you to understand how filtering is affecting your computations. Information about how filters are affecting secondaries selected before (i.e., a list of secondaries removed by each filter) is written to a log file as computations progress. To use this option, select the Use log file option and enter a path and file name in the File name field or click the ellipsis (...) button to browse for a file.

Advanced Ellipsoid Properties

The Ellipsoid Databases frame enables you specify which database file to use for each of the dimension definition types that requires use of a database. The following databases are available:

Dimension Definition Type	Applicable Database
Fixed	Does not use database.
Orbit Class	Fixed Orbit Class Database
Quadratic	Quadratic in Time Database
Quadratic Orbit Class	Quad / Orbit Class Database

Ellipsoid Scaling Factor

This option allows you to enter a scaling factor comprising any real number equal to or greater than zero, which is applied to the dimensions you entered, if the Fixed definition type is being used, or to those supplied by the selected database. In the case of the two quadratic databases, the scaling factor is applied to the calculated dimensions, not to the coefficients used in deriving them.

The ellipsoid scaling factor applies only when determining close approach event times, when determining the minimum separation between ellipsoids. The computation of uncertainties and probabilities do not incorporate the ellipsoid scaling factor—the 1-sigma values are obtained from the ellipsoid size as configured for each object without any scaling being applied.

Cross Reference Databases

This feature allows a cross-reference lookup of a TLE SSC number to retrieve the common name of the corresponding satellite. For example, if the SSC number of the satellite is 16609, the cross-reference feature gives you the common name 'Mir'. To use this option, select the Use Cross Reference Database option and enter a path and file name in the Database field or click the ellipsis (...) button to browse for a database.

Other Data Files

Related Objects File

This option allows you to load a text file in which a primary object and a list of related secondary objects are outlined. When a related objects file is loaded and the 'Ignore events involving related objects' option is active, the secondary objects listed in the file are excluded from consideration when determining close approach events with the specified primary object.

As illustrated in the sample file, a Related Objects (*.rel) file consists of one or more blocks of the form:

BEGIN RelationGroup
   Member ...
   ...
END RelationGroup

BEGIN RelationObject
   Name ...
   RelatedObj ...
   ...
END RelationObject

where Member is the ssc number or file identifier of a member of the group, Name is the file name (e.g., an *.e or *.sa file) of the primary object, and RelatedObj is a related SSC number or file that should be ignored by Advanced CAT processing.

The primary object in a RelationObject must be listed first or all secondary objects listed above it will be ignored.

SSC Hard Body Radius File

This option allows you to load a text file containing hard body radius sizes for individual SSC numbers used to represent the size of the object when determining the likelihood of potential collisions. As illustrated in the sample file, an SSC Hard Body Radius file (*.rad) consists of a series of lines, each containing two numbers: the SSC number of the spacecraft, followed by its HardBodyRadius in meters. The value in the file will override any other value for that object that has been specified in the user interface.

Sample Step Options

Starting with STK7, AdvCAT uses a sampling algorithm that is based on how much the relative position vector is allowed to rotate between samples, subject to maximum and minimum step sizes, while using at least 10 samples. You specify the minimum and maximum sample step sizes using the Max Sample Step and Min Sample Step fields. The resulting sampling uses much fewer samples but achieves the same accuracy. Also, the tolerances associated with event detection have been tightened.

The propagation step or strategy that is used for the primary or secondary selection will depend on the type object or file type you select (see below).

The .e file uses the ephemeris steps in the file; you have no control over it.
A satellite uses the step size of the propagator or exact step size you see on the Satellite Properties > Basic > Orbit page.
The .tce file does not use a constant step size and there are no settings for step size. Instead, the algorithm chooses a good step for each TLE based on its orbit qualities and knowing how fast the satellite geometry changes. On the Advanced tab, there are settings for min and max sample step. While it does not set the ephemeris step size, it does help to control the algorithm. It helps to control sampling of ephemeris when looking to find conjunction events. We think the default values work well in providing accuracy with computational speed. When you select a TCE file, ephemeris is not generated and held. Rather, position and velocity information is generated on an as needed basis using the SGP4 algorithm. For example, ACAT filters need position/velocity information from one or both objects at particular times to support the filtering algorithms. If the objects are specified using TLEs, the position/velocity at the requested time is computed directly by passing the TLE to the SGP4 algorithm. This prevents interpolation effects from influencing the results. Regular sampling of the orbits does not occur until a candidate pair of objects makes it all the way to the range filter. At this point, if the time filter has been applied, you are usually taking a handful of samples to detect the trends in the range. The timing of the points will be determined from the length of the interval coming from the time filter and again the position/velocity will be computed by direct calls to SGP4. If a conjunction is detected, iterative sub-sampling is used to converge on the actual start and end times of the conjunction where, again, SGP4 is used to compute position/velocity information at the desired sub-sample times.

If Use Range Measured is set on the AdvCAT Basic Main properties page, range is used as the measure for determining event start and stop times. (The filtering process does not include either object's threat volume sizes as part of the threshold used by the filters.) Using the Range measure results in the same computations as if the ellipsoid sizes for all objects were 0.0 (that is, points not volumes). Because the ellipsoid size is not being used during filtering when using the Range measure, you may want to examine the selection of the filter pad values to determine whether they need to be increased to achieve the same accuracy as previous versions of STK.

Conjunction Type

Select the type of conjunction reporting that you want to use from the drop-down menu.

Global Only - A conjunction interval is reported if a secondary object crosses the threshold range within the interval time; only the conjunction time of closest approach (ConjTCA) that is the global range minimum over the interval will be reported.
Global + Local - A conjunction interval is reported if a secondary object crosses the threshold range within the interval time; each local range minimum (i.e., a range rate of zero) within the interval is reported as a separate event with its corresponding ConjTCA. In addition, an interval endpoint time will be reported as a separate event with its corresponding ConjTCA if it is the global range minimum over the interval.
Local + EndPoints - A conjunction interval is reported if a secondary object crosses the threshold range within the interval time; each local range minimum within the interval is reported as a separate event with its corresponding ConjTCA. Also, the interval start and stop times will be reported as separate events with their corresponding ConjTCAs if the range rate at the time is increasing or decreasing, respectively.
Local Only - A conjunction interval is reported if a secondary object crosses the threshold range within the interval time and if a local range minimum exists within it; each local range minimum within the interval is reported as a separate event with its corresponding ConjTCA.

An example of a 24 hour conjunction interval

Additional Options

The following options appear at the bottom of the Advanced page of Advanced CAT Basic properties:

Show Messages in Message Viewer - Detected close approach events are displayed in the message viewer for immediate notification during the computational process.
Force Repropagation on Load - Any ephemeris in any satellite file used by Advanced CAT is ignored and the ephemeris is instead created by repropagation (see note below).
Compute on Load- This option is automatically selected when creating a VDF file using the Authoring a VDF function in STK. It forces computation of the Advanced CAT in the VDF file.
Do not require ephemeris for entire analysis time period - An object with an ephemeris that does not span the entire analysis interval will still be processed if this option is selected.
Remove secondaries matching primary SSC number - Secondary objects that have the same SSC number as the primary (and presumably are the primary) will not be processed.

In previous versions of Advanced CAT, satellite ephemeris was always regenerated. The ephemeris file (if any) was ignored. Beginning with STK version 5.0.4, Advanced CAT loads the ephemeris for satellites from the *.sa file and uses it in the computations (i.e. does not regenerate it). The Force Repropagation on Load option can be used to recover the previous behavior.