Propagator Defaults

The Propagator default options allow you to set preferences for the propagators in STK.

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 simple drag model. By default, STK utilizes the CSSI SGP4 routine, Version 2008-11-03.

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.

Update Mode

Allows the satellite's designated TLEs to be automatically updated from the selected source prior to each propagation. You can update TLE sets directly from the AGI Server or from a file stored locally.

Automatic update mode is an effective mechanism for obtaining the correct set of TLEs to use for the propagation interval of interest.

Setting the Mode to None disables the automatic update: TLEs must then be set manually by the user. The Insert Satellite from Database Tool will use the default Satellite database (set on the Scenario Basic Properites Database page) as the TLE source.

Retrieve Method

This option will tell STK whether to pull only the first available TLE or All available TLE's for the time interval of interest when using an automatic update mode. STK will find the following:

An appropriate set of TLE's during the propagation interval for the specified SSC.
Find the closest TLE previous to the start time for the SSC
The closest TLE past the stop time for the SSC [when in All Mode].

When using the first available, only the earliest TLE in the set is used (usually the first in the set defined by (a) above).

Switch Method

Defines when to switch between TLE sets, when multiple TLEs are used.

Epoch - Switch between TLE sets based on the epoch of the TLE.
Mid-Point - Switch between TLE sets based on the mid-point between two TLE epochs.
TCA - Switch between TLE sets based on the time of closest approach. Calculated by propagating each TLE of a pair over the time period between their respective epochs, and then determining the point at which the two paths are nearest each other. This option provides the smoothest possible fit between two TLE sets but may not represent the best estimated state of the vehicle.

Routine

This option allows you to select the SGP4 routine used for propagating ephemeris. Normally, only STK's internal routine (called simply SGP4) is available. Users who have installed other routines (e.g. the one available from Air Force) will have other choices.

Source File

You can select the TLE file. Use the button to browse for a *.tce or *.tle file. This file is the default file to use when inserting from a file using the Insert from Satellite Database Tool. It is also the default file for automatic update from file mode.

Field	Description
Use SGP4 1-Point Interpolation	Uses SGP4 1Pt interpolation.
Use SGP4 1-Point Validation	Validates that the SGP4 routine computes a good state, before accepting the ephemeris. If you wish to properly model decaying TLEs, you should turn this on.
Suppress SGP4 1-Point Interpolation Warning	When using 1-point interpolation STK does not display a warning when an error occurs. SGP4 errors can occur for decaying TLEs.

GPS Propagator

The propagation scheme in the GPS Propagator is defined in the Interface Specification ICD-GPS-200. The ICD can be found at the US Coast Guard Navigation Center website: ICD-GPS-200

The Coast Guard makes available GPS almanacs in SEM format on a routine basis (currently one per day). AGI provides these almanacs through its AGI Online Server.

Each SEM almanac contains orbit elements for the then-active GPS constellation for a specified almanac date. The almanac date and elements are used by the GPS propagator to produce ephemerides. It is important when using almanacs to use the correct set of almanacs for the time intervals of interest.

Update Mode

Allows the satellite's designated almanacs to be automatically updated from the selected source prior to each propagation. You can update almanacs directly from the AGI Server or from a file stored locally.

Automatic update mode is an effective mechanism for obtaining the correct GPS Almanac to use for the propagation interval of interest.

Setting the Mode to None disables the automatic update: almanacs must then be set manually by the user. The Insert Satellite from GPS Almanac Tool will use the default GPS Almanac (set using the Source File option).

Retrieve Method

This option will tell STK whether to pull only the first available almanac or All available almanacs. When using multiple element sets obtained using automatic online update, STK will find the following:

An appropriate set of almanacs during the propagation interval for the specified PRN.
Find the closest almanac previous to the start time for the PRN
The closest almanac past the stop time for the PRN [when in All Mode].

When using the first available, only the earliest almanac in the set is used (usually the first in the set defined by (a) above).

Switch Method

Defines when to switch between almanacs, when multiple almanacs are used.

Epoch - Switch between almanacs based on the epoch of the almanac.
Mid-Point - Switch between almanacs based on the mid-point between two almanac epochs.
TCA - Switch between almanacs based on the time of closest approach. Calculated by propagating each almanac of a pair over the time period between their respective epochs, and then determining the point at which the two paths are nearest each other. This option provides the smoothest possible fit between two almanacs but may not represent the best estimated state of the vehicle.

Source File

You can select the almanac file. Use the button to browse for a *.alm, *.al3, *.sem, *.sp3, or *.yuma file. This file is the default file to use when inserting from a file using the Insert Satellite from GPS Almanac Tool. It is also the default file for automatic update from file mode.