Satellite Properties

SGP4 Propagator

The Simplified General Perturbations (SGP4) propagator is used with two-line mean element (TLE) sets. It 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 TLE. The information contained in a TLE is a set of mean orbital elements that are specific to the SGP4 propagator.

The Air Force makes available TLEs for a subset of space objects being monitored (about 12,000 as of April 2009). These TLEs are then provided to customers using AGI online server.

A TLE contains an epoch (in YYDDD.ddddd format); ephemeris produced by SGP4 using this TLE is generally valid for a few days about the epoch [No official validity interval is provided by the Air Force]. New TLEs are produced over time for each space object because of this limitation. In using TLEs, then, it is important to use the correct set of TLEs for the time interval of interest.

You can input the initial conditions for the SGP4 propagator manually, but it is much more common to input TLE sets from a TLE file. TLE files must be in the TLE File format (*.tce/*.tle). STK provides several options for managing element sets, thus avoiding the need for manual input.


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.


Click Advanced to set the 1-point propagation details for the SGP4 satellite.


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 TLEs.

Automatic Update

Allows the Satellite's TLEs to be automatically updated 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 allows the user 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.


Select this option and then click Setup to browse for a locally stored TLE file. STK will automatically reload 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.


Click Preview to view the tle information that will be used 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.

Related Topics:

History of the SGP4 Propagator