Jul 28

Utilities and tools

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As an engineer and a developer, I tend to be pretty needy.  I need access to arcane bits of information that sometimes can be hard to find.  Usually when I find that information, I go on my merry way and continue my analysis.  Then I happen to need it again.  Frustrated that I didn't save a link to the info last time (and even if I did save a link my bookmarks get so unorganized that I can't find it), I go searching again.  That was then.  I've wised up a little bit, and started creating utilities that provide the information I use regularly in one (or more) handy place.  Let's look at a couple of examples.

First, before I can do any analysis with GPS, I need some type of ephemeris element set from which I can propagate the GPS satellites.  Usually I use an almanac, but we'll look at SP3 files as well.  So, first thing I need is a location for these types of files.  Luckily, AGI has an FTP repository that contains SEM almanacs.  So, I go to this year's directory and see about 200 files and they all have names with lots of numbers in them, like this: almanac_sem_week0465_589824.al3.  Which one do I pick for the analysis I want to do?  It's little things like this that frustrate us analysts and engineers.  Turns out that the almanacs are named according to the almanac element epoch, in the number of weeks since the latest GPS week rollover and the time of week in seconds since Sunday midnight - were you expecting something else?  Discerning which almanac file is the best for my analysis brought me to my knees over and over again.  At first, I created intricate spreadsheets that calculated these numbers, but they had to be updated regularly and then I lost the spreadsheet.

Take another example, the precise ephemeris for GPS satellites that the National Geospatial-Intelligence Agency (NGA) produces.  These files have equally cryptic names: nga14000.sp3.  The NGA description says that the file naming scheme is

ngawwwwd.[sp3], where wwww is the gps week and d is the day (Sunday being 0 through Saturday being 6)

The catch here is that the GPS week they are referencing is the GPS week since the beginning of GPS time on January 6, 1980.  So the file I found above is for Sunday (d=0) of GPS week 1400.  Is week 1400 what I'm looking for?  More back of the envelope calculation time.

I needed a better way.  I decided that if I could keep one set of data organized - my browser bookmarks - then maybe I had half a chance of finding what I needed when I needed it again.  So, I created a perpetual GPS Calendar that displays all the days and dates that were of interest to me, GPS weeks, days and dates of interest.  Here's the Calendar: GPS Calendar. This calendar lists Julian days and Julian dates as well (yes, they are different!) - you never know when you have to know what Julian day it is -

PAF Files: 2008_203_v02.paf

PSF files: 2008_203_234500_v01.psf

SOF files: 2008_205_204839_v01.sof

Oh right!  So take a look at this Calendar, especially if you are doing any type of navigation analysis.  You'll run across these files types eventually, and need to know how to relate them to 'real' dates.  I've put a link to it on the Nogroll on the right side of the page under the Utilities category.

Well, nothing monumental in this blog, but taking care of the details can be a big hassle when all you really want to do is finish the analysis - like trying to bail all of the water out of a sinking boat so you can get to the hole to fix it!  Whenever I run across or develop a new tool or utility, I'll post a blog on it - so you can keep things moving along too - without being brought to your knees.


2 Comments so far

  1. John Lavrakas August 13th, 2008 18:15

    This is very helpful info. Thanks for posting it. Your links and descriptions really make it easy to see the data you are talking about. It looks like the NGA has gone through several evolutions. I see no data before 2006, then EPH data in sets of 7 days shows up wrapped in exe file, then finally the set of files stored as the sp3 you mention.

    I’m interested in obtaining broadcast ephemeris. Do you know where this is available on line?


  2. Ted Driver August 15th, 2008 18:21

    Hi John,
    The NGA Broadcast ephemeris is located here: It looks like they don’t store much data there, so you may want to copy off the new files each day. I’ve been using their data since day 151 of this year, but they currently have broadcast ephemeris only back to day 200 of this year.

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