I own a Nuvi67LM and I have recently updated to the 2020.10 mapping.
After this map update, I have noticed that the unit shows extremely inaccurate speed for roughly 10 seconds when first locking the GPS signal.
I am going roughly 35 mph on the local road while the GPS suddenly showed I was going 113 mph then it corrected to the speed I see in my speedometer. When I contacted support, they weren't sure what was causing it and wasn't able to provide much help as the unit is old and discontinued.
It's not a big deal for me but has anyone else experience this?
More than likely, you have moved a considerable distance since it's last fix when you cut it off. As soon as the GPS lock is finalized, it has to move the position symbol from that previous location to the new location. The software will present that relocation as movement and report the speed of that movement. It thinks it just traveled at 113 MPH to your present location.
"Beam me up Scotty"
Thanks for the information
While multipath is always a potential source of GPS inaccuracy, in my experience it is especially likely during initial signal acquisition.
When this happens, the signal from one of the satellites gets to your receiver after bouncing off a nearby surface. That means it took extra time to get from the satellite to your receiver, and since the fundamentals are deeply time-based, that causes an error.
I live right next to a mountain, and multipath error during start up sometimes gives me very interesting initial positions and paths.
I have also seen multipath error long after start up but that has been far less common. The example that comes to mind had my GPS receiver peeking out of a porthole of a sailing ship as that ship came alongside a giant cruise ship in a Caribbean port. The limited fraction of the sky available through the porthole meant that the receiver could only see four satellites for a while and accepted one satellite signal with multipath error from bouncing off the cruise ship. That put my reported position a few hundred feet away, on land.
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