Few weeks ago I took a ferry and GPS kept showing my car stationary, regardless of the clear sky with perfect reception and car actually sailing few miles an hour until I started driving off the ferry on the other side. I guess that's why completely disconnecting speed signal wouldn't work and you need to send the signal that car is traveling albeit at very low speed, close to 1 mile an hour. Also, dead reckoning won't work with this device so it's a good idea to have some bypass switch just in case you don't have enough sat. visible, for example driving in the city with tall buildings all around.
Thanks for the info and links