Ok, you've had some time to digest the first two parts of this series on assessed navigation accuracy.  First I discussed what ephemeris and clock errors were, then I went over how those errors are created.  Now, in this final installment, I'll show how these errors are are turned into accuracy measurements for your GPS receiver.  For now, let's assume that our only errors are those we've discussed - ephemeris and clock errors.  There are other errors that affect your receiver and those are covered in the GPS Error Budget nog.

Now that we know what our typical ranging errors are, we need to understand how those are translated into a receiver error. The other piece of the error puzzle (and there are only two pieces) is Dilution of Precision (DOP).  DOP is the effect that arises from measuring signals from spatially separated sources - be they lighthouses, GPS satellites, semaphore technicians or pulsars.  DOP typically has the affect of diluting the accuracy of your ranging measurements.  I'll cover DOP much more thoroughly in a future nog as well - because we want to get to the good stuff now!

Notionally, your receiver's accuracy can be modeled as:

receiver error ~ DOP x URE

This is not a mathematically robust description, but it does show the general behavior of your errors.   The higher your DOP or URE, the higher your receiver's error.  Lower either, and you're likely to find that geocache quicker.  The mathematically correct construct for DOP is a matrix, and the UREs are a vector.  The elements of the URE vector are the total root sum square (RSS) errors along the line-of-sight from your receiver's antenna to each GPS satellite.  So far we've only discussed the ephemeris and clock errors, but any error can be RSS'd into the URE and be used in the receiver error calculation.  We'll see this when we look at the GPS error budget.

So, there it is, on the table - now you know how accurate your receiver's position estimates are.  As long as you know all of the satellites ephemeris and clock errors at a given time (and all the other errors we glossed over) and you can do the matrix multiplications on the fly.  Piece a cake!  Right - we don't think so either.  That's why we automated all the data collection and math for you in our tools.

Here is a graph of the assessed receiver position error over a day, using just ephemeris and clock errors.  This graph is from the GPS Basics page.

 

So now, when you see or hear about assessed navigation accuracy, you know that means the actual errors from a time in the past were used to calculate the position errors.

It's always nice to see where you've been, to see the wonderful position errors you had in the past - (like at 08:00 UTC in the graph!).  But what good does that do you for tomorrow's buried treasure diving expedition?  You need to be able to predict what these errors will be... and I will cover that topic - predicting GPS accuracy - in a future nog as well.

In the mean time, grab your receiver, turn on your track log and go express yourself (thanks Frank!)

-Ted

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