SGP4 Propagator
For the Simplified General Perturbations (SGP4) propagator, a standard US Space Force (USSF) propagator, use GP (General Perturbation) data. Typically, the data is formatted in two-line mean element (TLE) format, though you can use other formats. SGP4 considers secular and periodic variations due to Earth oblateness, solar and lunar gravitational effects, gravitational resonance effects, and orbital decay using a drag model. By default, STK utilizes the CSSI SGP4 routine, Version 2008-11-03.
The SGP4 propagator generates ephemeris in the True Equator Mean Equinox coordinate system based on the epoch of the specified GP data. In addition to certain metadata, the information contained in the GP data is a set of mean orbital elements that are specific to the SGP4 propagator.
USSF makes available GP data to the public for a subset of space objects being monitored, about 46,000 cataloged objects as of August 2020. They provide TLEs to customers using the AGI online server.
The GP data for an object contains an epoch, in YYDDD.ddddd format. Ephemeris produced by SGP4 using this data is generally valid for a few days about the epoch, though USSF does not provide an official validity interval. USSF produces updated GP data over time for each space object because of this limitation. In using SGP4, then, it is important to use the correct set of data for the time interval of interest.
CelesTrak, working closely with Space Track, makes the GP data available via standard queries using the Orbit Mean-Elements Message (OMM) that is part of the Orbit Data Messages (ODM). This is the recommended standard CCSDS 502.0-B-2 developed by The Consultative Committee for Space Data Systems (CCSDS) in November 2009.
You can enter the initial conditions for the SGP4 propagator manually, but it is much more common to import GP data sets from a file. Files must be in the TLE File format (*.tce/*.tle/*.txt), a GZIP file containing TLE-formatted data, or must contain CCSDS Obit Mean-Elements Message (OMM) content formatted as XML, KVN (Keyword Value Notation), or CSV files. STK provides several options for managing element sets, thus avoiding the need for manual input.
Historically, GP data was only provided in TLE format and so the terms "TLE" and "GP Data" are often treated as synonyms. The TLE format, however, only supports five digit SSC number identifiers. GP data for space objects uses nine digit numbers. File formats like OMM (XML, KVN, CSV) are available as an alternative to TLE.
Interval
Set the satellite's analysis time period by choosing an interval component, manually defining the start and stop time, or by selecting Use Scenario Interval to use the current scenario time period. For more information on the options available for setting interval times, see Time Options.
The Step Size is the interval between computed ephemeris output points.
Advanced
Click Advanced to set the 1-point propagation details for the SGP4 satellite.
Identification
The satellite SSC number, which is the catalog number of the spacecraft.
You can input the SSC number directly if you know it. You can also search for the SSC number by clicking Search Database.
TLE Source
Select the method for obtaining GP data.
Automatic Update
Select this radio button to have STK update the satellite's TLEs automatically from the selected TLE Source prior to each propagation. This guarantees the use of the most up-to-date orbital information whenever the scenario is loaded. Automatic Update also enables you to configure the satellite to use the correct TLE data that corresponds to any time interval updates.
AGI Server
Using this option, you can have STK query the AGI Server to find the appropriate set of TLEs during the propagation interval for the specified SSC. Which TLEs are selected and how they are used is determined by the parameters on the Setup panel.
The AGI Server is restricted to providing data for objects with five-digit SSC numbers only in the form of TLEs.
File
Select this option and then click to browse for a locally stored GP data file. STK automatically reloads the specified file every time the satellite is propagated.
This feature is most useful when you cannot use the AGI Server but can manually update a file with the correct data.
Preview
Click Preview to view the TLE information that STK will use to propagate the satellite.
Define/Import Elements
You can import SGP4 orbital elements from a file or enter them manually.
Click Add to enter or import SGP4 elements.