Archive for the 'Satellite Tool Kit' Category

GPS gets its due

February 28th, 2009 | Category: AGI Products,General Navigation,Satellite Tool Kit

We get no respect!  That's what the AGI Product Managers used to hear me say - no respect - just like Rodney Dangerfield.  That's the way it felt using STK for GPS analysis.  Use a TLE for the orbits?  What's a TLE?  How do I bring in an almanac?  Really, there's a separate tool to do that?

Well, GPS's time has arrived.  With the release of STK 9 coming soon, GPS gets the respect its due, and then some.  Those of us in the GPS world know there are other satellites in the sky - especially since they can't seem to stay away from one another.  But - we really only care about one very special set of satellites.  Prior to STK 9, GPS was just another wallflower at the satellite party - but now, everyone wants to talk to her.

I'll give you some examples - if you're familiar with STK already, you'll feel the love in the next set of pictures.  If not, now you have to try it, you'll see why - and it's not just about GPS either.

Here goes, after starting STK 9, and creating a new scenario for today (with one-click!), you get the brand-spanking new Insert STK Objects dialog:


For the Satellite scenario object, you can either select Load GPS Constellation or Select GPS Satellite From Catalog. 25% !! We have 25% of the options to load a satellite into STK!  25% is fine - we'll let the other dozen thousand or so satellites have the other 75%.  (Though technically we could add GPS from most of the other options as well).  When you select the Load GPS Constellation option and click the Insert... button, STK will go to the continuously updated AGI data servers and grab the latest GPS almanac, create a new GPS satellite object for each item in the almanac, naming them appropriately, and also create a GPS constellation object.  So far, after starting STK we've used three, count them, THREE clicks of the mouse to load the complete, up-to-date GPS constellation for today in a new scenario.  Now that's service.GPSConstellation

The picture at right shows the objects STK 9 created, note the SVN and PRN number in the satellite name.

Suppose you want to use your own almanac - or one from a different date?  Just select the Select GPS Satellite From Catalog Method and you'll get a new dialog that allows you to specify further details for your constellation:


Here, you get detailed information about the current almanac and the ability to load your own almanac.  The Catalog source options at the top allow three options:

  • Automatic - AGI Server: Allows STK to pull the the required almanacs from the AGI server. When you change the scenario time, STK will go to the server and update all the satellite's almanac data for you.  Respect!
  • Automatic - File: Will load the same file from your local storage anytime the satellites are told to propagate.  This is useful if you update the files automatically by some other method.  If you choose the GPSAlmanac.al3 file for example, then use the Data Update Utility in STK, the GPS almanac in your local storage will be updated regularly, and the GPS satellites in your scenario will propagate accordingly.
  • Specify Catalog: A specific almanac you choose, that will always remain with the satellite - no updating is done at all.

The table lists the PRN, SVN, SSC (NORAD Catalog identifier), Health and Average URA information contained in the almanac for each satellite.  The SSC number is not contained in the almanac, but read from a cross-reference list maintained by AGI. There is an additional column that specifies the Span: specifying when a given PRN was not available.  For example, currently PRN 1 is not assigned to a GPS satellite, in the past it was though.  If your scenario contains a long time span, there may be times when PRNs are unavailable because of PRN reassignment among satellites.  The Span column will tell you whether a PRN is early or late, meaning that it disappears earlier than the scenario end time, or appears later than the scenario start time.  In either case, the PRN is not available for the entire scenario interval.


Looking at the properties of a single GPS satellite above, you can see that there are a lot more details to drool over. Not only is the propagator the correct IS-GPS-200D GPS propagator, but you can change the almanac source for this single satellite - accepted types of ephemeris information are SEM and YUMA almanacs as well as SP3 precise ephemeris files.  I didn't mention it above, but you also load the entire constellation from a single SP3 file as well.  Additional information regarding the GPS week number, the epoch and PRN can be viewed and changed here.

In coming Nogs, I'll go further into what you can accomplish with STK for navigation analysis.  STK 9 has significant advances not only with respect to navigation analysis, but in usability and functionality over all of its feature sets.  We just happen to care more about one of these feature sets than the others... :) .