Insert Satellite from GPS Almanac

Use the Insert from GPS Almanac tool to select and insert Satellites using orbital elements from GPS almanac files. The Almanac files can be stored in local directories or pulled from AGI Servers (internet connection required).

Catalog Source

You can specify the location of your external GPS almanac. 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. Which almanacs are selected and how they are used is determined by 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 Stored access intervals can become outdated when the latest satellite ephemerides are applied to the scenario.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

Select this option and then click Setup. Using the button, you can browse for an almanac. When you select the Automatic - File Catalog Source, STK will automatically use the selected almanac for the proper time interval.

This option will re-load the specified almanac file every time the satellite is propagated. An example might be if your scenario has a start time of "Today" and an end time of "+ 3 days," when you load that scenario it will re-propagate the satellites based on the date.

Specify Catalog

Select this option and then click to browse to 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 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.

Almanac Information

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

Option	Description
Almanac Reference Week	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.
Reference Week Epoch	Represents the beginning and end dates of the selected almanac.
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.

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.

Graphics

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

Constellation

If On, STK creates a constellation that includes all of the satellites you selected. Enter the constellation's name in the text box.

The option to create a constellation is only available with the STKProfessional or STKExpert license.

GPS Satellite List

All GPS Satellites found in the almanac will be located on the left of the Insert from GPS Almanac window.

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.

Option	Description
PRN	The satellite PRN number per ICD-GPS-200 PDF. This 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 will be updated from one spacecraft to another. Typically, the PRN will be unavailable during an interval in which it switches to assignment from one spacecraft (with it's SVID) to another (with a new 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	The space vehicle number as defined in the ICD-GPS-200 PDF. Each SVN corresponds to a different spacecraft. During it's 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	The SSC Number of the Satellite.
Health	The satellite health code as defined in the ICD-GPS-200 PDF expressed in integer form. 0=healthy. This option is only available when you select Specify Catalog and Automatic - File.
Avg URA	The satellite "average" 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	Early - The PRN disappears earlier than the scenario's end time. Late - The PRN appears later than the scenario start time. This option is only available if you select Automatic - AGI Server.