Archive for August, 2008

How long can you use an almanac?

August 21st, 2008 | Category: General Navigation

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When you turn on your GPS receiver - what happens? Well, lots of stuff.  But primarily, GPS receivers work in a two stage process:

  1. Look for available satellites to track
  2. Do everything else

In this article, I want to focus on step 1, we'll get to step 2 later.  This is probably common knowledge, but for the record I want to state it.  The GPS receiver uses an almanac downloaded from a single GPS satellite to help it determine what satellites are above the horizon as it searches for signals.  Makes sense to not look for satellite signals that aren't even visible - it decreases your time to first fix (TTFF).  Since the receiver is simply determining whether a satellite is above the horizon, the almanacs don't have to be very accurate.  They are typically a coarser version of the precise ephemeris broadcast by each individual satellite.  In a recent Nog I discussed how well an almanac compares to the precise ephemeris - so I won't discuss that here.

What I do want to discuss is how long you can use an almanac for analysis.  Your receiver will usually download a new almanac when it sees that a new one is available, so it always has the freshest data.  When you do analysis though, sometimes you may not have the latest almanac (or the one correct for the time of analysis - a related problem).  So, is it ok to use any old almanac for analysis?  Since they are not the most accurate ephemeris representations, should I be using them for analysis anyway?  I'll answer these questions and show some interesting graphics that bring the point home.

For analysis sake, let's say we're interested in Position Dilution of Precision (PDOP) prediction.  PDOP is calculated from standard formulas and requires a source of ephemeris for the satellites.  You can calculate PDOP from precise, actual ephemeris, or almanac generated ephemeris, or whatever quality ephemeris you have.  I'll compare the PDOP calculated using the almanac to 'true' PDOP - that calculated from the precise ephemeris.

For our first analysis, let's look at 24 hours of PDOP values - using a precise ephemeris for the actual PDOP and a three week old almanac to generate the ephemeris for the predicted PDOP.

Three Week PDOP prediction

The actual PDOP is in yellow, with the almanac predicted PDOP in pink.

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