[time-nuts] GPS receiver stuck at South Pole :)

Hal Murray hmurray at megapathdsl.net
Tue Jun 8 13:22:18 EDT 2010


[From last April.  Garmin GPS-USB-18 (no x).]

I think I've figured out what's going on.  This is reverse engineering 
(guessing) by looking at the output.

When the signal fades out, it remembers the most recent position and velocity.

It does mark the following info packets as no-good.  Yes, I could ignore the 
data, but I happened to plot it.

It seems to update the position.  Since this is my house, the velocity should 
be 0.  Often it is.  That's a boring case.  It just keeps repeating the last 
position.

But since the signal is fading out, the position is often quite a ways off.  
If the position on the previous slot was good, that turns into a large 
velocity.  (When I started typing this in, it was off by more than 1 degree.)

If the velocity has a N/S component and it doesn't recover the signal soon 
enough, it will eventually get to one of the poles.  For me, that's happening 
several times a week.  (I just noticed one that recovered at 88.464659.)

It seems to be flying on a Mercator cylinder rather than a sphere.  As you 
get closer to the poles, it still goes X meters per second E/W, but that gets 
translated into degrees which get smaller.  If you plot latitude and 
longitude vs time, one is a straight line, the other turns into a curve.

I've forgotten my spherical trig.  What's the term for the change in 
direction as you fly a great circle route on a Mercator projection?  This 
unit doesn't seem to be making that correction.

It doesn't know how to fly over a pole.  When it gets within a step of the 
pole, the N/S motion stops.  It does know how to cross the international date 
line.

I'm plotting the position every 64 seconds.  As it gets near the pole, you 
get interesting aliasing effects as it wraps around the pole multiple times 
per step.


-- 
These are my opinions, not necessarily my employer's.  I hate spam.







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