GPS Propagator

The STK GPS Propagator is the propagator used by GPS receivers. The propagation scheme in the GPS Propagator is defined in the Interface Specification ICD-GPS-200 (PDF), but the algorithm amounts to Two-Body propagation in an Earth-fixed frame. This is the propagator to use if you want to propagate GPS satellite positions in the manner that a GPS receiver would.

You can create GPS Satellites using element sets from GPS almanac files. 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. By default, STK picks the correct almanac(s) for your specified scenario time.

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.

Analysis Time Period

You can specify the analysis time period for the GPS satellites by setting the vehicle's Start and Stop Times. You can use the current scenario time period by selecting Use Scenario Interval from the drop-down menu.

Option	Description
Step Size	The Step Size is the interval between computed ephemeris output points.
PRN	The satellite PRN number per ICD-GPS-200 (PDF). This is a required data item as it is you r primary means of identifying GPS satellites. At a given time, it corresponds to a SVID that indicates a particular spacecraft transmitting this PRN identifier in the GPS message. PRNs may be only valid for a period of time; then they are re-assigned to different satellites. Sometimes PRNs are invalid over a period of time or valid over only parts of the requested interval. If you are operating in online mode, STK will automatically account for the assignment intervals and prevent propagation over an interval in which the PRN is unassigned or will only create ephemeris for the satellite over this reduced interval. You can input the PRN number directly if you know it. You can also search for the PRN number by clicking Search. This will list the PRN for all GPS satellites that are active during the specified analysis time period.

Option

Description

Step Size

The Step Size is the interval between computed ephemeris output points.

PRN

The satellite PRN number per ICD-GPS-200 (PDF). This is a required data item as it is you r primary means of identifying GPS satellites. At a given time, it corresponds to a SVID that indicates a particular spacecraft transmitting this PRN identifier in the GPS message.

PRNs may be only valid for a period of time; then they are re-assigned to different satellites. Sometimes PRNs are invalid over a period of time or valid over only parts of the requested interval.

If you are operating in online mode, STK will automatically account for the assignment intervals and prevent propagation over an interval in which the PRN is unassigned or will only create ephemeris for the satellite over this reduced interval.

You can input the PRN number directly if you know it. You can also search for the PRN number by clicking Search. This will list the PRN for all GPS satellites that are active during the specified analysis time period.

Catalog Source

The method for obtaining element sets from GPS almanacs. A GPS almanac is a set of data that every GPS satellite transmits, and it includes information about the state (health) of the entire GPS satellite constellation, and coarse data on every satellite's orbit.

Automatic Update

Allows the GPS Satellite's orbital elements to be automatically updated from the selected almanac 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 you to configure the satellite to find Almanac 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 almanacs during the propagation interval that includes the specified PRN. Which Almanacs are selected and how they are used is determined by the parameters on the Setup panel.

Click Preview to view the almanac information that will be used to propagate the GPS satellite.

File

Select this option and browse to a locally stored almanac. STK will automatically reload the specified almanac file every time the satellite is propagated.

Specify Catalog

Click to browse for an almanac. When loading a GPS Almanac there are three additional file types that can be loaded into STK besides the standard *.al3 and *.alm.

Almanac Information

When you select an almanac file, via the Automatic Update from a File option or the Specify Catalog option, the following options will be available:

Option	Description
Health	The satellite health code as defined in the ICD-GPS-200 (PDF) expressed in integer form. 0=healthy.
Avg URA	The satellite "average" URA number, with the URA number as defined in the ICD-GPS-200 (PDF).
Type	SEM or YUMA. See Below.
Week	The almanac week. This counts the weeks since the week reference date.
Week Reference	The Almanac Reference week. Some almanacs use a week number reference to the original GPS start epoch (1980); others report the actual week number as transmitted by the GPS message which has a rollover every 1024 weeks. Almanacs such as SEM and YUMA use the rollover so the week reference is needed to specify the almanac date.
Time of Almanac (secs)	The almanac reference time (Time of Applicability) for the file.
Date of Almanac (GPSG)	The almanac reference date for the file per ICD-GPS-200 (PDF).

GPS Source File Types
Type	Description
SEM	The SEM Almanac is an ASCII message containing the almanac information in SEM format. Refer to the U.S. Coast Guard Navigation Center web site for definition and catalog of files. Note that this data includes only ephemeris and clock data, and does not include covariance, nor the high accuracy GPS to UTC time correlation.
YUMA	The YUMA Almanac is an ASCII message containing the almanac information in YUMA format. Refer to the U.S. Coast Guard Navigation Center web site for definition and catalog of files. Note that this data includes only ephemeris and clock data, and does not include covariance, nor the high accuracy GPS to UTC time correlation.

STK provides predefined attitude options for Block IIR and Block IIA GPS satellites. These attitudes will be automatically assigned to the GPS satellites as they are created. The IIR attitude option is set for Block IIR and IIR-M satellites and IIA attitude option is set for all other types.