[Paddlewise] Fwd: GPS errors

Hmmm, can you stand a few more thoughts 

>From the sourroundings you mention, I would rule out multipath.  If you had
said you were in an area near shore where structures could provide reflective
elements for the GPS path, I would say ...possible.  You weren't so lets just
rule that as highly unlikely.

Regarding the grounding of "Royal Majesty".  Most GPS units work in this
order: (assuming they are set up to do so)  If 4 or more satellites are in
view and give proper crossing angles for fixing (PDOP or position dilution of
precision is low) your system will operate in 3 dimensions...lat/long and
altitude.  If you drop to 3 satellites,  your unit will revert to 2
dimensional fixing of lat/long.  If you drop another sat the unit will go into
"DR".  Some units sound an alarm, some don't, some have a setting that you can
turn the alarm off.  In any event if you go into DR without noticing, the unit
will grab the last position fix and apply a course and speed.  You typically
enter course and speed manually so if they are default to 0.0 knots at 000
degrees the position simply won't change.  As you sail away you will just show
further and further away...possibly 3 miles?  In any event, there will
eventually be a horrible error that if left unnoticed will cause a horrible
problem at sea.  That was the basis for my question.....was the unit actually
tracking at the time?  The watch officer of the Royal Majesty never noticed
that the GPS had automatically reverted to DR and locked on an old position. 
Your question about integrating other gear to a GPS answers yes.  Many console
mounted units have the ability to interface to other instrumentation for speed
and heading input.  Those sensors allow a unit to DR with an automatic
speed/hdg input and they also allow a calculation of SET & DRIFT.  Most high
end sail boats have the interfacing done via the B&G or Ockam instrumentation.

Here's the reason for my altitude question.  Remember I mentioned you want 4
satellites but you can get by with 3?  In a 3 satellite fix the center of the
earth is assumed for the replacement of the 4th satellite.  Now if your on the
surface of the earth and you enter your altitude as 0...close enough.
However, many units allow you to enter a manual antenna height.  Well, if you
entered (manually) 30000 feet there would be a (substantial - but I didn't
calculate how much) error.  I see lots of strange operational setups by owners
that didn't have all the information to make a good decision.  This type of
error is ....believe it or not....believable.

Finally, another thought on jamming.  You said there were a bunch of handhelds
being used.  I was involved in an incident where a ship experienced
intermittant GPS failure while in port.  Turned out a GPS on another ship
docked nearby was noisy and, yes, acted as a noise transmitter.  It did jam
the unit....made it go to DR.  I would be interested to know if that console
unit was actually tracking and if not could it have been jammed by one of your
handhelds?  I suspect this to be a very very rare problem.

If you ever see this problem again, give a look to speed and heading
information that the console unit is showing.  Look at the scatter plot of the
satellites it is tracking and finally, look at the PDOP (should be under 8 or
so...4 is great).  These will give the clues necessary to answer the question.
Sorry to say, until then this is all speculation.  

Tom  

attached mail follows:

Great input everyone.  Especially Dan and ?WILAX? (who is that masked poster
who posts knowledgeably and drives too fast?).

I have little more input to help with this conundrum, but I'll try to at least
guess.

<Multipath usually shows up as a jump.  Question, were you docked when you did
these tests?>
No, the ones involved (a friend and others in a class) were actively sailing
about, practicing navigational and sailing skills.

<Was there a lot of stuff around like buildings, bridges etc?>
I assume no closer than 1/2 to 1 nM.  But even if, they would be moving away
and toward.

<The antenna system had failed and the unit was dead reckoning.>
Wouldn't the officer on watch (or previous watch) have needed to manually
initialize the unit to start DR?  I never heard of a GPS doing DR, so I assume
you are referring to some fancy nav system which integrates a GPS receiver for
obtaining fixes?
 
<Are you SURE this unit was actually LOCKED onto the constellation?>
Nope.

<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?>
Huh?  Sorry, I'm the dummy who asked the question.  Actually, the handhelds
were confirmed (supposedly) as being accurate.  Since the console unit was
mounted in a sailboat rather than an RV or helicopter, I would be surprised if
anyone ever dinked with the altitude setting after initial installation.  Of
course, its precisely because it should always be set for sea level + antenna
height that nobody would ever think to check it, and since no instrument is
idiot proof. . .   

<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.>
Could be, but your analytical skills are functioning.  Thanks

Harold
So. Cal., so impressed - thanks guys.


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