Propagator Defaults
The Propagator default options enable you to set preferences for the propagators in STK.
SGP4 propagator
For a comprehensive overview, see SGP4 Propagator.
Update mode
This 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. You must then manually set TLEs. 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 tells STK whether to pull only the first available TLE or all available TLEs for the time interval of interest when using an automatic update mode. STK will find the following:
- An appropriate set of TLEs during the propagation interval for the specified SSC
- 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 setting, STK will only use the earliest TLE in the set, which is usually the first in the set defined by (1) above.
Switch method
This defines when to switch between TLE sets, when multiple TLEs are used. Set the following parameters:
- Epoch: This enables you to switch between TLE sets based on the epoch of the TLE.
- Mid-Point: This designates a switch between TLE sets based on the midpoint between two TLE epochs.
- TCA: This sets the switch between TLE sets based on the time of closest approach (TCA). STK calculates this time 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 enables you to select the SGP4 routine used for propagating ephemeris. Normally, only STK's internal routine (called simply SGP4) is available. If you have installed additional SGP4 routines, you will have other choices. Contact customer support for an FAQ article containing installation instructions.
Source file
You can select the TLE file. Click 
to browse for a TCE orTLE 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 | This enables SGP4 one-point interpolation. |
| Use SGP4 1-Point Validation | This triggers a validation check 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 | If you select this, , STK does not display a warning when an error occurs when using one-point interpolation. 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 is available at the US Coast Guard Navigation Center website: ICD-GPS-200 (PDF).
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
This 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: you must then manually set almanacs. 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
- 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, STK uses only the earliest almanac in the set, which is usually the first in the set defined by (1) above.
Switch method
This defines when to switch between almanacs, when multiple almanacs are used. Set the following parameters:
- Epoch: This enables you to switch between almanacs based on the epoch of the almanac.
- Mid-Point: Enable this to switch between almanacs based on the midpoint between two almanac epochs.
- TCA: Set this to switch between almanacs based on the time of closest approach. STK calculates this time 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. Click to browse for an 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.
Week reference
This defines the reference epoch for the week number in the specified Source File if the specified file is a GPS almanac file. The reference epoch requires that the week numbers given in SEM and YUMA almanac files roll over every 1024 weeks.