Insert Satellite from GPS Almanac

You can insert Satellite objects using orbital elements from GPS almanac files. From the Insert STK Objects tool, select Satellite on the left and From GPS Almanac as the method. The Insert from GPS Almanac tool will appear; use this to specify almanac information, scenario information, and GPS satellite selection choices.

You can store the Almanac files in local directories or pull them from AGI Servers (internet connection required).

Catalog Source

In the tool, specify the location of your external GPS almanac by choosing one of the options in the following subsections. The 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 - AGI Server

Using this option, you can have STK query the AGI Server to find the appropriate set of almanacs for the Analysis Time Period. STK determines which almanacs are selected and how they are used by reading the GPS Propagator parameters on the Propagator Defaults Preference panel.

If you are updating satellites from an online server, you should disable the storing of access intervals. Stored access intervals can become outdated when the latest satellite ephemerides are applied to the scenario.

Automatic - File

After selecting this option, click to browse for and select an almanac. When you select Automatic - File as the Catalog Source, STK will automatically use the selected almanac for the proper time interval.

This option will direct STK to reload the specified almanac file every time the satellite is propagated. For example, if your scenario has a start time of "Today" and an end time of "+ 3 days," when you load that scenario, it will repropagate the satellites based on the date.

Specify Catalog

Select this option and then click to browse to and select a locally stored almanac. This option does not enable the automatic update function. Therefore, the inserted GPS Satellites will only use the orbital elements initially retrieved from the specified catalog.

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 website for definition and catalog of files. These data include only ephemeris and clock data, and do 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 website for definition and catalog of files. These data include only ephemeris and clock data, and do not include covariance nor the high-accuracy GPS-to-UTC time correlation.

Almanac Information

When you select an almanac file, the following options will be available:

Option	Description
Almanac Reference Week	Some almanacs use a week number reference relative 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 STK needs the week reference to specify the almanac date. Although not conformant with almanac specifications, the almanac files may contain an unambiguous week number. When STK reads a week number greater than 1023, it will ignore the user-specified Week Reference Epoch and assume the date of the almanac information to be relative to the original GPS epoch of 06 Jan 1980.
Week Reference Epoch	Choose the reference epoch for the week number specified in the almanac file. STK determines the absolute date reference for the almanac by adding the almanac reference week and the number of seconds into the week (Time of Almanac) to this epoch.
Time of Almanac (secs)	This is the almanac reference time (Time of Applicability) for the file.
Date of Almanac (GPSG)	This is the almanac reference date for the file per ICD-GPS-200 (PDF).

Analysis Time Period

Specify the interval or start and stop times for the analysis time period for the GPS satellites. For more information on the options available for time intervals and start and stop times, see Time Options.

GPS satellite list

All GPS satellites found in the almanac will appear on the left of the Insert from GPS Almanac dialog box. Click anywhere in the row to select a satellite. To select more than one satellite, hold the Ctrl key and click rows for the additional satellites. To select all the satellites in the list, click Select All. To remove all your selections and start over, click Unselect All.

STK provides predefined attitude options for Block IIR and Block IIA GPS satellites. STK will automatically assign these attitudes to the GPS satellites as they are created. STK applies the IIR attitude option to Block IIR and IIR-M satellites and the IIA attitude option to all other types.

Option	Description
PRN	This is the satellite PRN number per ICD-GPS-200 (PDF). It is a required data item as it is the GPS user's primary means of identifying GPS satellites. The PRN is a transmission identifier in the GPS message. At certain times, the transmitted GPS message will not contain information for a particular PRN. This is particularly relevant during update of the GPS constellation, when a PRN is updated from one spacecraft to another. Typically, the PRN will be unavailable during an interval in which it switches assignment from one spacecraft (with a certain SVID) to another (with a different SVID). 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.
SVN	This is the space vehicle number as defined in the ICD-GPS-200 (PDF). Each SVN corresponds to a different spacecraft. During its operational time interval, it will transmit on one assigned PRN. When a spacecraft becomes decommissioned, it no longer transmits its PRN; instead a new spacecraft is assigned to transmit for that PRN.
SSC	This is the SSC number of the satellite.
Health	This is the satellite health code as defined in the ICD-GPS-200 (PDF) expressed in integer form, with 0=healthy. This option is only available when you select Specify Catalog and Automatic - File.
Avg URA	This is the satellite "average" user range accuracy (URA)number, with the URA number as defined in the ICD-GPS-200 (PDF). This option is only available when you select Specify Catalog and Automatic - File.
Span	Span will be blank if the PRN covers the entire scenario time. If the PRN is not available for some of the scenario time, it will have one of the following values: Early - The PRN disappears earlier than the scenario's end time. Late - The PRN appears later than the scenario start time. This parameter only appears if you select Automatic - AGI Server.

Graphics

Select the color choice or select Auto Select Color to allow STK to choose colors sequentially for the selected satellites.

Constellation

If you select the Create Constellation from Selected Satellites check box, STK creates a constellation that includes all of the satellites you selected. Enter the constellation's name in the text box.