I see the 660 does dead reckoning

We had a discussion about that in a thread here a few weeks ago. Most current units do it to some degree.

I believe, however, that calling it "dead reckoning" is not a good thing to get started.

Dead reckoning depends on an actual measurement of speed and direction; what you describe is an estimate and not a measurement and is not really "dead reckoning" at all.

The industry has a term for the feature but it escapes me at the moment....something like "Signal loss continuation", I think.

If there's a brief interruption - it's plotting based on speed and direction from the last known location until it gets another fix - if the interruption is too long, you get the 'lost satellite reception' notice.

Oh, I still think dead reckoning is an accfeptable term for what they do. Before GPS & LORAN, we did dead reckoning all the time in boats & I would guess aircraft. It's an estimate of when you should arrive at a location, or where you project to be at any given time. Dead reckoning has always been an estimate, just like what Garmin is projecting.

https://buy.garmin.com/shop/shop.do?cID=164&pID=31946#specsT...

I'll take their word on it.

According to Wikipedia,
"Dead reckoning (DR) is the process of estimating one's current position based upon a previously determined position, or fix, and advancing that position based upon known speed, elapsed time, and course."

I would say that's exactly what my 660 did while I was in the tunnel.

I see Garmin says dead reckoning: no for the 660. I remember a few years ago Garmin had a unit that was wired into the speedometer & I think it had it's own compass. That is probably their definition of DR. What they're saying is the 660 does not interface to the speedometer.

All the sailing books I have read use Wikipedia's definition, not Garmin's. Another difference w/Garmin, is they only do DR for about 1 minute. While sailing, we use DR for hours as the speeds are much different.

Garmin's definition of "dead reckoning" is: "Dead Reckoning is a feature on select Garmin® units that will continue to navigate you to your destination even when GPS signal is lost. For the Dead Reckoning capability to be available, the Garmin unit must be installed by an authorized Garmin dealer." per thier FAQ web site. And the link to the Nuvi660's specification site I posted above says the 660 is not capable of DR. I'll take tier word for it.

All I'm suggesting is Garmin's definition is different than the definition of DR for YEARS, by lots of other authorities. Google dead reckoning, I believe you will see many more references like estimating one's current position... than when GPS signal is lost.

It could be that your 660 is still picking up Sats...most of the way through the Tunnel it works inside some buildings for me that I have no coue how it is getting a signal. Unless it is using Cell towers or something, I am very impressed at my 660 for keeping signals.

No, it didn't do any such thing.....because the 660 did not KNOW enough to make a true dead reckoning calculation.

It knows elapsed time because it has an internal clock; it does NOT "know" speed and it does not "know" direction. There could be a traffic jam and/or you could have made a U turn and it still would show you coming out the other end of the tunnel.

By the definition YOU provided, it is NOT doing dead reckoning and repeating it over and over will not make it true.

Your 660 is "guessing" at your position based on speed and direction estimates. Dead reckoning does not rely on guesses and estimates but is a calculation based on actual measurements.

I think the definitions are effectively the same, the difference is when the 660 loses signal, it has no reasonable estimate of speed, and thus is not dead reckoning. Dead reckoning for vehicles involves a speed input so speed is known when gps signal is lost.

Regarding:
It could be that your 660 is still picking up Sats...most of the way through the Tunnel it works inside some buildings for me that I have no coue how it is getting a signal. Unless it is using Cell towers or something, I am very impressed at my 660 for keeping signals.
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I thought about that, so during another pass, watched the birds disappear. This is under a very heavy steel/cement tunnel which is under the LAX runway. Definitely did not see any satellites. That got reinforced when I went slow thru the tunnel to hear the prompt "lost satellite reception".

Pretty interesting, though XM radio still works! Noticed that when I 1st got XM, to learn they have terrestrial repeaters for covered areas. Would be nice if GPS could do that, but, I know it couldn't--need to triangulate actual birds.

I am also impressed on the 660 (and expect many other nuvi's) reception. Works very well in my house that has quite a bit of attenuation of the approximate 25 watt signal about 11,000 nautical miles above.