Re: [Paddlewise] GPS errors

I have been following this but I'm not sure I digested all of the details so I
apologize if I am repeating.  Datum was my first suspect but you are right.
Datums show up usually as a fraction of a NM difference (assuming the common
100 or so datums). However, some GPS units allow you to enter a custom or USER
datum which could have any amount of error.  So, you still want to double
check this.

Multipath usually shows up as a jump.  For those that don't know what
multipath is, it is a reflected signal to your GPS rather than a direct signal
from the satellite to your GPS.  Since the basis of your position is a
calculation of distance from the satellite to your GPS antenna, a reflection
is an additional distance traveled, hence an induced error.  However, at the
frequency of the GPS, there is rarely a sustained reflection path, especially
on the water.  Therefore you will see a temporary jump of position for a few
seconds.  A high quality GPS can do a lot to discriminate between direct and
incident signals.  Less expensive ones cant.  Question, were you docked when
you did these tests?  Was there a lot of stuff around like buildings, bridges
etc?

Jamming.  GPS by its very nature is very resistant to jamming and spoofing.
Remember this system was designed for military use first and jamming is an A#1
tactic.  The code and frequency, even in the civilian C/A section is pretty
stable.  Interference from other things will shut most GPS units down rather
than give false readings.  Again, I'm not up on the really inexpensive
architectures of systems out there.  Possibly low end units use analog
circuits that are susceptable to outside noise.  This seems unlikely though.

The incident out here of the grounding of the "Royal Majesty" was due to a GPS
error on the order of a couple of miles.  It turned out that the GPS was not
locked into the satellite system.  The antenna system had failed and the unit
was dead reakoning.  The officer on watch failed to pick up this subtle
problem.  Everything "seemed" to be working.  Are you SURE this unit was
actually LOCKED onto the constellation?

Finally,  how was altitude being determined?  Was it entered manually and you
were navigating in 2D mode or were you resolving lat/long and altitude?
If a false antenna height were entered ...say of 30,000 feet, and the unit
were forced into 2 dimensional resolution, you will see an error because it is
trying to use 30K feet as a part of an assumed position in its calculation.
Worth checking.

Finally, finally, some units have an oscillator offset check in the service
screen.  There is a tolerance for offset on some units....maybe this unit is
.....broken.  

          
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