Satellite Collection Definition

The Satellite Collection object models a group of satellites as a single object in the Object Browser, but no individual satellites appear in the Object Browser. A subset is a group of some or all of the satellites in the collection; members of a subset all meet the same criteria. You can use subsets in any analysis tool that can operate on a group of satellite objects. Examples of such tools are the Ansys Systems Tool Kit^® (STK^®) application's Coverage capability, CommSystem, DeckAccess, and AdvCAT.

Use the Basic - Definitions page of the satellite collection properties to define its members and subsets. The STK software constructs the group of satellites based on the Type that you choose:

• Walker
• Custom
• Database
• SP3
• CR3BP

The sections below provide details on how to specify these types.

In the 3D Graphics window, if you point to a satellite in a collection, right-click, and choose Promote to Satellite, a new satellite object will appear in the Object Browser. It will have a name and orbital elements based on the satellite you pointed to. For the satellite propagator, the default is J2Perturbation except for type CR3BP, which defaults to Astrogator for the propagator.

Common panels

The following panels appear for all the Type choices listed above.

Parameter	Description
<time span>	Directly below "Type" is a display box with a selection arrow. Select the satellite collection time span from one of the following options: Use Scenario Interval Interval Component... - Select a scenario object; the satellite collection will exist during the interval of this object. Replace With Times - Specify start and stop times. The selection arrow also enables you to select different ways to specify Start Time and Stop Time. All selections use UTCG time for the start time and stop time of the existence of the satellite collection.
Supplemental Metadata	This panel gives you the option to include a supplemental data file for members of the satellite collection. This file can also provide definitions of additional subsets. On the panel, type or browse to a file path. If you change the contents of that same file, click the refresh icon () to update your data in the STK application. For format information, click here.
Default Subset Reference Object	This panel gives you the option to include a reference object that specifies the properties (attitude, access constraints, etc.) and child objects (sensors, transmitters, receivers, etc.) that the STK application will associate with each member of a subset when it is used by an analysis tool. If you do not specify an object, then STK software will use the default satellite. On the panel, type the object name or click to specify the reference object.
Subsets	See the Subsets subsection.

Subsets

A subset is a group of some or all of the satellites in the collection; members of a subset all meet the same criteria. For all Satellite Collection objects, the STK application automatically generates a subset named AllSatellites that contains all members of the collection. Members of the Satellite Collection can belong to two or more subsets if they meet the criteria for more than one subset.

In the Walker type, you can choose to have STK software create subsets from satellites belonging to (1) the same shell and (2) the same plane in a shell.

In the Custom type, you define the subsets through the orbit definition file.

Regardless of the definition type, the Supplemental Metadata file may define additional subsets.

Subsets with entries only associated with the default reference object (not an explicit one) have the name Default as the name of the reference object in the pane. In this case, you can edit the reference object. If all the entries of a subset have the same reference object, the name of that object will appear in the pane. You can edit the reference object if you explicitly assigned it to the subset. For subsets containing entries with different reference objects, the reference object name is "Not Equal" in the pane and you cannot edit this.

For each subset, the pane displays the following information about the subset:

Column	Description
Name	Name of the subset
Type	Type of objects
Reference Object	The name of the reference object for that subset
Objects	The number of objects in the subset
Entries	The number of entries in the subset

Use the buttons above the Subsets panel to view and change the subsets.

Button name (icon)	Description
Edit ()	Click to edit the reference object to use with a particular subset, thus overriding the default subset reference object. You cannot modify subsets within the user interface, except to assign a reference object. You can only assign a reference object to a subset containing no entries already associated with a reference object via another subset. For example, if Subset1 and Subset2 both contain a particular entry and Subset1 has an associated reference object, then it is not possible to define a reference object for Subset2. On the other hand, if the STK application assigns a default subset reference object and none of the subset entries are part of another subset having an explicit reference object, then you can specify a reference object for the subset.
View entries in subset ()	To view the entries in a subset, select a subset from the display and click . A box will appear with the name "Satellite Collection Entries for Subset X," where X is the name of the subset you selected. The list shows the available metadata, including any supplemental metadata, for the collection. You cannot edit this list.
Clear selected Reference Object ()	Select a subset with a reference object that you want to remove and click to clear that reference object from the subset.
Add a new subset ()	If you want to create a new subset, click and enter the following information in the New Subset dialog box: Subset Name (Required) Enter a unique name for the new subset; this is required. Reference Object (Optional) Click to choose to add one from the available reference objects, or click to delete the preassigned reference object. Attached Objects (Informational) Gives the number of subobjects for the selected reference object. Grouping Criteria (Required) You can create up to two group criteria conditions. Click in this panel to add a second group. If you add a second criteria group, you will see an operation selector (AND, OR) to choose how to relate the two groups. Inside each group box, specify the following: Include selector (All, Any) - This indicates how STK should apply the criterion. Criterion - Select from a drop-down list of parameters. Condition - Depending on the Criterion selected, one or more of these options will be available to you: "is", "is not", "contains", "does not contain", "matches", "does not match", "is greater than or equal to", and "is less than or equal to". String matching supports two wildcard characters, * and ?, where * means "match any sequence of characters" and ? means "match any single character." Examples are PRN-?? and NAVSTAR*. Value - Enter a value within the range of the selected Criterion parameter. Click at the end of the criterion line to add another criterion line to the group. You can delete an individual criterion using the button next to it. You can delete the whole group of criteria by clicking the button in the upper-right corner. Criteria building rules: You must specify at least one criteria group. Your criteria group(s) must have at least one criterion with Criterion, Condition, and Value specified, otherwise you will get an error message when attempting to add the subset. STK will not check your criteria for validity. You must check the resulting subset entries to see if your criteria worked correctly. When you are finished specifying the subset, click Add Subset.
Edit selected subset definition ()	This button is only active if you select a user-defined subset. Clicking this will bring up the Editing Subset dialog box, which is the same as the New Subset dialog box described above. When you are finished editing, click Update Subset to apply the changes.
Delete subset ()	This button is only active if you select a user-defined subset.

Collection subsets can overlap in content. For example, a Walker Shell subset contains all of the entries in each of the nonoverlapping Plane subsets. Due to such potential overlaps, the entries in a subset may have reference objects associated with them even though there is not an explicit association to that particular subset. In the previous example, if a Walker Shell subset has an associated reference object, then that object is also associated with all of the entries in the Plane subsets that are parts of that shell.

Walker

The Walker selection provides a constellation of satellites distributed in a series of altitude shells. Each shell contains evenly spaced orbital planes, and Walker populates each orbital plane with evenly spaced satellites. All the satellites in a shell have the same period and inclination. Walker evenly spaces the ascending nodes of the orbital planes over a range (angle spread) of right ascensions (RAAN). This capability is similar to the Walker Tool available from a Satellite object.

You can generate multiple shells within a single satellite collection object. However, if you access the Walker tool from a single satellite object, it only generates one shell (at that satellite's period and inclination).

Walker properties

The Walker Properties pane displays the current Walker constellation shells; the default has one shell populated. Each shell contains satellites with the same period and inclination. Select a row and click to edit that shell's properties or click to add a new shell. In either case, the same dialog box will appear. You can then specify the parameters listed in the table below.

Name	Description
Shell Name	Give a name for the shell.
Shell Properties panel	You can choose or edit the following parameters for a shell: Pattern Type - Choose how you want to distribute the orbit planes around the central body. The options are: Delta: evenly distributed over 360 degrees Star: evenly distributed over 180 degrees Custom: distributed based on your explicit instructions, primarily through RAAN Spread or RAAN Spacing Planes - Choose an integer number of orbit planes for the shell. Satellites in Planes (Slots) - Choose the number of satellites for a single orbit plane. These are spaced evenly around the orbit plane. Inter Plane Phase Increment - Specify the number of "slots" between the first vehicles in adjacent planes. The value can be, at most, one less than the number of orbital planes in the shell. This parameter only applies to Delta and Star pattern types. Inter Plane Phase Angle - Specify the relative angular spacing between satellites in adjacent planes when using a Custom pattern type. The value must be in degrees. RAAN Spread or RAAN Spacing - For Delta and Star types, RAAN Spread is the only choice. For Custom type, choose one of these and specify the angle.
Plane 1 : Slot 1 panel	Use this panel to specify the orbital parameters for the seed satellite of the shell. STK software then generates all the other satellites in the shell based on this seed plus the Shell Properties data. You must provide the following: Semi-Major Axis Eccentricity Inclination RAAN Argument of Periapsis Mean Anomaly Epoch - This is the time at which the orbital elements apply; it must be within the Start Time and Stop Time specified above. Propagator - This is the propagator to use for all entries within a shell. Choose from TwoBody, J2Perturbation, and J4Perturbation. Propagation Frame - This is the coordinate frame of propagation. STK software will apply the analytic orbit equations to the orbit initial conditions and interpret the results in the propagation frame. Choose from Inertial, TrueOfDate, and TrueOfOrbitEpoch.

When you add or delete a shell, STK software deletes the list of subsets because the subsets may have changed during editing. STK software will regenerate new subsets when you apply the edits.

STK software automatically generates the names for the satellites in a Walker-type satellite collection based on the shell, plane, and slot that a member occupies. For more details on Walker parameter designation, click here.

From the Walker Properties panel, you can also specify Subsets to be Generated, which has the following choices: None, Shells, and Shells and Planes.

Custom

For this option, you provide an orbit definition file that defines the members, orbital parameters, and metadata.

Custom properties

In the Custom Properties panel, you can provide the following information:

Name	Description
Custom File	This is a Custom Orbit Definition File that defines the members of the satellite collection. Choose your file by typing or browsing to a file path. If you change the contents of that same file, click the refresh icon () to update your data in the STK application.
Trajectory Source	Choose one of the following options to describe the type of orbit definition file: Orbit Elements - This option means that your file supplies the six orbital elements for the satellites. Choose a Propagator from the selection box: Two-body, J2 Perturbation, or J4 Perturbation. Ephemeris - This option indicates that your satellites have an accompanying ephemeris file. Provide the directory where the ephemerides are located. SSCID - This option indicates that your file contains SSC identifiers for each of the satellite entries that STK software will use to extract TLE information from the satellite database. Select the configuration of satellite database usage settings.
Propagator	Select a propagation method for STK processing to use in the generation of ephemeris from orbit elements. STK processing applies this selection only if the Orbit Definition File contains orbit elements as the means of defining orbits in the collection.
Default Epoch	(Optional) You can specify a default epoch to use in defining the orbits in the collection. STK software will only apply this default if the Orbit Definition File contains orbit elements and there is no epoch in the file. Orbit Definition Files without epochs are easy to share among scenarios with different time frames. This approach is convenient when the structure of the collection is more important than the exact positioning of the individual elements of the collection.

Database

For this option, you provide search parameters for STK software to apply in extracting satellites for the collection from a satellite database.

To see which satellite database you are searching, open your scenario's properties and go to the Basic - Database page. For Database Type, select Satellite and see which Default or Auxiliary database is currently selected for the scenario. You can change the database here to change your satellite collection search.

When you click Define Search Parameters, a dialog box appears, with a panel on the left for parameter specification, and one on the right to display results. Proceed as per the the descriptions below.

Setup

At the top of the Define Search Parameters dialog box, click Setup. The Search Setup dialog box appears, and you can specify the parameters that are listed in the table below. When finished, click OK.

Parameter	Description
Propagation TLE Source	Select a source from which to extract TLE data for satellite(s) you get from the search. The choices are: AGI Online Server A TLE file that you provide Default Satellite Database (from the scenario properties setting)
TLE Selection	Specify which TLEs to use from the records of a selected satellite. The choices are: Use all relevant TLEs - Also choose from the following switching methods: epoch, midpoint, and TCA. Use first relevant TLE
Out-of-date Duration (Days)	Specify the maxumum number of days that a TLE file can be out of date and still be included in the selection process. The default is 30.

Search parameter specification

In the Define Parameters (left) panel, you can select and specify one or more search parameters from the list in the table below. This list includes default selections (if a parameter has one) that you can keep, modify, or clear. At the bottom of the Define Search Parameters dialog box, click Apply Parameters to save your parameter settings. At the bottom of the Define Parameters panel, you can click Clear All Fields to clear all parameter selections and start again.

Name	Description
Name or ID	Provide a name or ID number associated with one or more satellites.
Owner	Select from a list of country acronyms.
Mission	Select from a list of mission types.
Launch Site	Select from a list of three-letter site acronyms.
Launch Date	Select and specify a Begin date, an End date, or both.
Periapsis Altitude	Specify a minimum, maximum, or both. You can also choose from a list of units. The default is the selection of a minimum at 34750 km.
Apoapsis Altitude	Specify a minimum, maximum, or both. You can also choose from a list of units.
Period	Specify a minimum, maximum, or both. You can also choose from a list of units. The default is the selection of both a minimum (1425 minutes) and a maximum (1455 minutes).
Inclination	Specify a minimum, maximum, or both. You can also choose from a list of units. The default is the selection of a maximum at 25 degrees.
Operational Status	Select from Active, Inactive, or Unknown. The default is the selection of Active.

Naming convention

You can specify a method for creating satellite collection entry names. At the bottom of the Results panel in Database Search dialog box is the parameter Entry Naming Convention. Select from the following parameter options:

• None (to avoid using a naming convention)
• Common Name
• Common Name & SSC Number (the default)
• SSC Number
• SSC Number and Common Name
• Prefix & SSC Number
• SSC Number & Suffix

Use the text-entry box Name Prefix/Suffix to specify the prefix or suffix for the final two options.

As an example, selecting "Common Name & SSC Number" will direct STK software to apply names in the form CommonName_SSC, such as ISS_25544.

Results

After selecting and specifying your search parameters, click Search at the bottom of the Define Parameters panel. This will populate the Results panel (on the right) with all satellites matching the search criteria. To retain these results, click Apply Parameters, which closes the Database Search dialog box. Click Apply in the Properties window to build the resulting database satellite subsets into the satellite collection.

If you perform a search using a Propagation TLE Source other than AGI Online Server, the Results panel for the search will only show satellite entries that have a TLE in the specified Propagation TLE Source.

SP3

For this option, you provide an ephemeris directory containing SP3 files. STK processing uses these files to provide precise orbits of satellite navigational systems (like GPS and GLONASS). Each file contains ephemerides for a set of satellites, and each satellite ephemeris has a file ID comprised of one letter identifier (the system ID) and a two-digit number (the satellite ID), e.g., G23 or R06. Often the satellite ID is related to a PRN (Pseudo Random Noise) number; for GPS, the satellite ID is the PRN. The name style setting sets the convention for creating satellite entry names used by the collection. All entries of the SP3 collection use the same interpolation settings.

When the SP3 Satellite Collection computes its entries, it reads all of the SP3 files in the ephemeris directory and collects together the ephemerides associated with each file ID. The expectation is that each file ID will contain one continuous time interval of data without gaps. For any file ID with an ephemeris that has gaps in time, STK software will treat it as invalid and will not created an entry for that satellite; a message will appear in the Message Viewer when this occurs. AGI recommends not using files that overlap, as it will not be clear which file's ephemeris STK processing is using during the overlapping time periods.

Within one SP3 file, the file ID uniquely identifies each satellite. However, when it is associated with a PRN (like it is for GPS) that is assigned to different satellites over time, then the same file ID may correspond to different satellites for different dates. For example, PRN 23 for the GPS constellation has been assigned to three different spacecraft: 20959 (Apr 1991 - Feb 2004), 28361 (Jul 2004 - Mar 2020), and 45854 (Jul 2020 - present, as of Oct 2022). The Satellite Collection object identifies ephemerides by file ID, treating the ephemerides as if the same file ID specifies the same satellite. If a file ID maps to different satellites within a set of SP3 files, then you should create different Satellite Collection objects using the requisite files that keep the file ID mapping to just one satellite, so that the resulting trajectory does not contain large positional jumps.

SP3 file properties

In the SP3 Properties panel, you can provide the following information:

Name	Description
EphemerisDirectory	Select a directory containing SP3 files of ephemerides.
Interpolator	By default, the SP3 propagator uses the standard Lagrange interpolation scheme, interpolating position and velocity separately. Some files may be configured to enable you to select the standard Hermitian interpolation scheme, which uses the position and velocity ephemeris to interpolate position and velocity together (i.e., using a polynomial and its derivative). Use the shortcut menu to select a method.
Interpolation Order	Enter an integer from 1 to 13. An interpolation order of 1 specifies linear interpolation and is appropriate for closely spaced data or data with significant jitter. You can use higher interpolation orders to yield more accurate interpolation of the position and velocity of the object when the data is smooth and continuous.
Interpolate across file boundaries	If you select this check box and the ephemeris for an entry uses two or more SP3 files, then interpolation samples are allowed to span different SP3 files. If you do not select this option, then interpolation samples are restricted to be in the same SP3 file as the requested time interval.
Extrapolate 1 step	If you select this check box, STK processing will calculate an additional ephemeris step beyond the last data point provided by the SP3 file(s) assigned to the satellite.
NameStyle	Sets the convention for assigning names to satellite entries. Select one of the following options: SysIDSatID - Uses the file ID composed of concantentating System ID and Satellite ID (e.g., G03). PrefixSysIDSatID - Prepends a specified prefix to the file ID (e.g., prefixG03). SysIDSatIDSuffix - Appends a specified suffix to the file ID (e.g., G03suffix). SatID - Uses the Satellite ID (e.g., 03). PrefixSatID - Prepends a specified prefix to the Satellite ID (e.g., prefix03). SatIDSuffix - Appends a specified suffix to the Satellite ID (e.g., 03suffix). When you choose SatID or PrefixSatID or SatIDSuffix, STK processing may append an integer to the name so that the name of each satellite collection entry remains unique.
Prefix/Suffix	Sets the prefix or suffix to use with the specified NameStyle.

CR3BP

You can create a collection of satellite orbits corresponding to constituent members of a family of circular restricted three-body problem (CR3BP) orbits. The STK installation currently comes pre-populated with a substantial selection of orbit families associated with the Earth-Moon CR3BP. Additional families for the Earth-Moon system as well as other systems are intended for inclusion later.

Each family provides 325 representative orbits that generally span its associated domain. The number to include in the collection from this set is adjustable with a variety of options supporting a balance between performance and granularity. STK processing defaults to propagating for one orbital period with a 60-second time step, but various options exist for adjusting the orbital timing.

For subsets, in addition to creating AllSatellites, you also get subsets for both stable and unstable orbits in the family. By applying interpolation and differential corrections for an in-range value of Jacobi constant or orbital period, you may also compute and populate subsets for Sourced (original) and Interpolated (added) orbits.

Use the CR3BP Properties area to provide the following information:

Name	Description
Family Definition File	Click the folder icon to choose a CR3BP orbit family file from the directory <Install Dir>/STKData/Flight_Dynamics/CR3BPFamilies. This directory is currently populated with a substantial selection of orbit families for the Earth-Moon CR3BP. The first time you load a CR3BP orbit family, the STK application will add the CR3BP design tool for that primary-secondary central body pair to the Design Tools folder of the Component Browser. The epoch of this Design Tool is prescribed by the interval start time of the satellite collection. Any subsequent CR3BP satellite collection objects you create with the same primary-secondary pair of central bodies will use that same CR3BP Design Tool component and its epoch. If you change the initial epoch of any of these satellite collections, it will change the initial epoch of all the collections.
Time manipulation	These controls provide options for adjusting the timing of the orbits in the satellite collection in various ways without necessarily changing the interval of the satellite collection. This allows for a mutual interval to be shared by multiple satellite collections.
Offset by rev. fraction	If you want the propagation state (initial state start time) to be offset by a fraction of a revolution from the usual initial location, select this option and then enter the value in the text field. This option is not available if the Offset by time option is selected.
Offset by time	If the Offset by rev. fraction option is not selected (the default), then you can enter a duration in this field to add to the analysis interval start time to create a new start time for the initial state propagation. The default value is zero (0) days. To shift the start position of the orbit forward into the analysis period, use a negative time offset.
Use interval stop time	The satellite collection processing will default to propagating for one orbit period. If you select this option, then propagation will go to the end of the analysis interval, which may be shorter or longer in duration.
Number to include in collection	The default number of orbits to include in the collection is 65, meaning that STK will process 20% of the orbits in the family upon apply, including propagation and display. Depending upon the geometry and period of each orbit, among other factors, this processing time will vary. You may include more or fewer orbits or adjust the Graphics – Attributes settings to tune the processing time for performance vs. granularity of display.
Compute additional orbit(s) by	Use these controls to add orbits to the collection that fall between the representative cases selected to populate the family. Use the drop-down selector to choose a specific quantity and provide its value in the adjacent field, setting up STK processing to generate additional family orbits with that exact value for the quantity. This may result in one or more additions to the collection (occasionally, the interpolation and differential corrections process will fail resulting in no addition to the collection). An indicator (, ) is provided next the input value field along with parameter ranges on the panel to help guide selection of values. Choose from the following quantity options: (Blank) - This is the default, resulting in no additional orbit computation upon apply of the collection. Jacobi - Enter a Jacobi constant value within the range reflected above on the panel. If the input is valid, an attempt to find one or more orbits with the given Jacobi constant value will be made upon apply of the collection. Period - Enter an orbital period value within the range reflected above on the panel. If the input is valid, an attempt to find one or more orbits with the given period will be made upon apply of the collection. When you apply the collection, any additional orbits are captured in the "Interpolated" subset.

Viewing the CR3BP collection orbits

You can view the orbits in the 3D Graphics window by following these steps:

1. Go to Graphics - Attributes in the Satellite Collection Properties Browser.
2. In the 3D Orbit Lines area, you need to change away from the default Earth Inertial frame, so click Select next to Reference System. This opens the Select Orbit Lines System dialog box. On the left, choose IdealMoon, and on the right, choose EarthIdealMoonBarycenterCenteredRotating. Click OK in the dialog box.
3. For Interval to Show, select All Ephemeris.

   On the panel under Interval to Show, "One Period," unfortunately, does not mean the three-body orbital period, but is associated with the central body of the three-body system

4. At the top, select the 3D Orbits check boxes for the subset(s) you want to appear in the 3D Graphics window.
5. You may wish to change the time step to 600 or 3600 seconds vs the default of 60 seconds.
6. Click Apply or OK in the Properties Browser.
7. Go to the 3D Graphics window.
8. Click the View From/To icon () to open that dialog box. On the left, select Moon.
9. While you are here, you may wish to select "Unconstrained Rotation" and turn off "Show 'Track Ball'" near the bottom left.
10. Again click OK to save and close the View From/To dialog box.
11. You should now see the orbits. You may have to zoom out to see the entirety of the orbit family.

Note on appropriately viewing promoted CR3BP collection entries as Astrogator satellites. After promotion to an Astrogator satellite, you should select “Add VGT System…” from the 3D Graphics – Orbit System properties of the new satellite. Filter by “Primary Central Bodies”, select “IdealMoon” on the left and then “EarthIdealMoonBarycenterCenteredRotating” on the right. Click OK. You may deselect “Show” under “Inertial by Window” if you’d like. Upon apply of the satellite properties, you should see the rotating frame view of the Astrogator CR3BP orbit in the 3D graphics window coinciding with the satellite collection entry.