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Old 04-15-2009, 06:58 PM   #41
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One 'flaw' in the Garmin is that it 'thinks' you are at the location you last turned it off... so if you turn it off in Austin and back on in Houston, it thinks you are still in Austin until it gets the sat signals... but I guess that might be the same for the Tom Tom, not sure...
I look at it as a feature more than a 'flaw'. By remembering the last location, the GPS can focus on the satellites it last saw so it can get back on line quicker. My Garmin 60Cx and V have the option allowing you to tell the GPS to forget its last location (Satellite page, Menu, New Location.) It might be available on your Garmin model too.

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Old 04-15-2009, 07:25 PM   #42
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Good to know, thanks.

Next question: Burglary. Is someone likely to break into my car if they see a GPS mount or even the GPS device? (I've stopped worrying about someone breaking in to get my crappy tracfone.)
As a resident of the New Orleans metro area, with our huge crime problem, I think I am highly qualified to answer this question.

Someone is likely to break into your car if they see ANYTHING in it, much less a GPS mount or device. I don't even leave my lunch in my car. People who break into cars are often high as a kite and looking for anything they can sell for drugs.
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Old 04-15-2009, 07:31 PM   #43
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Good to know, thanks.

Next question: Burglary. Is someone likely to break into my car if they see a GPS mount or even the GPS device? (I've stopped worrying about someone breaking in to get my crappy tracfone.)
I leave the mount in place and just snap the Garmin unit out of the mount and stick it in my purse or hide it out of sight.

I generally use the suction cup mount on the windshield. It works well except I have noted that if you put it on a below zero windshield it might release as things warm up, even if you take care to make sure all is clean and you press it down hard. Otherwise, I have had it on from Minnesota to Texas with no problems. Minnesota and a few other states don't allow windshield mounts so I have also mounted it on the dash, with the little circle mount. That mount works as well as the windshield. The disc seems unobtrusive.

I have found it accurate but it doesn't always pick the best route. One thing that is helpful is that you can put in a number of GPS units what type of vehicle you are driving or if you are walking. If you put in RV or truck you avoid those low bridges.
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Old 04-15-2009, 08:06 PM   #44
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Old 04-15-2009, 08:42 PM   #45
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Amazing how far satellite navigation GPS has come. Early seventies on board the research ship RV Vema, we had an antenna mounted on top of the mast, sucker weighed about 80 lb, had radials and a vertical sticking out of it for 3 feet. The receiver was 19" rackmount classified device with seals on it. With all sorts of rules on who can see it.

The pre-rocessor was another rackmount with about 40 plug in cards with discreet transistor flip flops, outputting data to a Freiden tape punch. Which produced typically an 10 foot long tape with holes in it representing the satellite orbit, ID, time etc.

To actually get the data, the receiver's speaker was always on. THe watch standers in the electronics lab had trained ears for the faint whistle, of the satellite coming up the horizon. When heard the nearest tech would leap to the control head and press the oscillator tune until the local oscillator matched the satellite doppler, and lock onto the signal. Then the receiver would track the doppler shift until the satellite disappeared over the far horizon.

During the satellite pass the Frieden punch would be clankin out the the punched tape with the satellie data.

At this point all we knew was that we had sat. data.

Now came the processing part. The good old DEC PDP8 with 2 Kilobyte of memory was standing by, or running some other program. We set the toggle switches on front panel to tell computer that it is to expect a program. Now, feed the program tape int the reader, hit the go paddle (switch), and the machine would read a punch tape with the proper holes, which had the satellite navigation program.

This completed, the PDP 8 would beep,indicating it was ready to accept data. Now we loaded the previously punched data tape ftrom the last satellite pass, hit the go toggle, and for 20 ro so minutes be rewarded with an entertaining sequence of flashing lights on the front panel as the computer was processing the data.

At the end of twenty minutes or so the winking blinking lights would stop for a moment, then the computer started to output the fix (our location) on a teletype machine. with a great amount of clanking.

THus we received four lines of data with latitude, longitude, confidence level and estimated time of next satellite pass. The fix was typically good for a 10 to 12 mile radius.

After all this we walked over to the chart table and plotted our location. And got on the intercom to the bridge and read them the fix.

Anyone for the good old days??

I now have a laptop in my suburban with Street Atlas, a surplus Rockwell GPS receiver, a puck antenna on the roof and can navigate on the go.

I like this newer version better.
What you described was the predecessor to GPS, called the Transit Doppler system. It dated back to the early 60s, hence was before my time. In the early 80s, I worked in a research group at a major aerospace company. We built one of the early GPS receivers, that had the form-factor of the 19" rack that you described. The full GPS constellation wasn't up, and we only had signal coverage for testing a few hours a day. By the way, the full satellite constellation was not up until the mid 90s, about the time of the first Gulf war if my memory is correct.

The technology was such that the GPS hardware could be built onto a couple of chips 10 years ago. However, without a cheap LCD display and memory for the map storage, GPS by itself is not too useful for civilians. What good is it to tell someone his geodetic position (latitude, longitude, and altitude)? You would still be lost without a paper map or a map display!
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Old 04-15-2009, 09:03 PM   #46
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I look at it as a feature more than a 'flaw'. By remembering the last location, the GPS can focus on the satellites it last saw so it can get back on line quicker. My Garmin 60Cx and V have the option allowing you to tell the GPS to forget its last location (Satellite page, Menu, New Location.) It might be available on your Garmin model too.

Sam
There are currently 32 GPS satellites orbiting the earth. To get a position fix, a GPS must lock onto signals of a minimum of 4 satellites to solve for 4 unknowns: 3-D position and time.

Note that although the GPS receiver has an internal clock, it is not accurate enough as each microsecond of time error is equivalent to 1000 ft of position error. If you carry your own atomic clock, you don't need 4, but only 3 satellites to do a position fix by trilateration.

Now, when first turned on, how does a GPS receiver know which satellites in the set are visible? I should note here that the satellites are not stationary but circling the earth about twice a day.

By assuming that the position has not changed since it was turned off, the GPS receiver would try to acquire the satellites that SHOULD be receivable at THAT time and at THAT position on earth. This default policy works 99.9% of the time. But when the receiver has been moved a sizeable distance, it would try to acquire satellites that may be simply on the other side of the earth.

A GPS receiver always has a backup mode called "cold start" or "cold search" that is entered if the "assumed visible" satellite set cannot be received after a certain timeout. This timeout period can be avoided if the user helps it along by forcing the cold start manually, or to give it a rough initial position. The initial position does not have to be very accurate, as it is only used to decide which satellites should be above the horizon for the GPS receiver to tune to.

And then, depending on the hardware/software sophistication, each GPS receiver has a different "cold start" acquisition time.
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Old 04-15-2009, 09:21 PM   #47
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I've been using GPS units now for around ten years or so. Rarely even look at paper maps anymore, except when I want the "big picture." Accuracy is remarkable (most of the time). Been using Garmins, no opinion regarding other brands.

From what I've observed the sweat spot (most bang for the buck) seems to be around the $200 dollar range these days.

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Old 04-16-2009, 09:57 AM   #48
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I leave the mount in place and just snap the Garmin unit out of the mount and stick it in my purse or hide it out of sight.
In the old pull-out radio days, I heard a crook say that if he saw a pull-out radio slot in a car, he knew that the radio was hidden in the car somewhere -- most people get tired of taking it with them after a few days. However, a GPS is a lot smaller so maybe this strategy is reasonable.

Thanks for the tech info, NW. Here's a question:

The unit calculates distance from a satellite based on the time it takes the signal to arrive. The signals travel at the speed of light, so a distance change of 50 feet involves a change in signal delay of less than 53 nanoseconds (1/186,000 = 5.3 microseconds/mile. 50 ft = 1/100 of a mile). Am I understanding it correctly?
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Old 04-16-2009, 06:39 PM   #49
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The other useful feature I've discovered is that it estimates when you'll arrive at your destination. That's good to know if you have to be somewhere at a specific time.

Anyone figured out the algorithm it uses?

  • Uses speed limits?
  • Takes into account your current speed?
  • Takes into account your current speed in relation to speed limits?
Since my Magellan unit makes the estimate before I've started moving, I'd guess speed limits.
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Old 04-16-2009, 09:04 PM   #50
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The unit calculates distance from a satellite based on the time it takes the signal to arrive. The signals travel at the speed of light, so a distance change of 50 feet involves a change in signal delay of less than 53 nanoseconds (1/186,000 = 5.3 microseconds/mile. 50 ft = 1/100 of a mile). Am I understanding it correctly?
Yes. Same as I wrote earlier, 1 microsecond=1000 nanosec=1000ft (roughly).
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Old 04-16-2009, 10:02 PM   #51
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I leave the mount in place and just snap the Garmin unit out of the mount and stick it in my purse or hide it out of sight.

I generally use the suction cup mount on the windshield. .

Martha, don't you live in a state that prohibits windshield mounts? Leaving the mount in place might be tempting fate. Have you considered a beanbag mount?

I had a GPS moment today in rural NJ. When I drove through a small town business district, several streets that I was supposed to turn on were closed for construction. I continued past the closed streets and let GPS recalculate my route. I ended up in the middle of NOWHERE when GPS announced "Destination ahead on the right" and sure enough my customer's business (the only business within 10 miles of unmarked roads) was just beyond a clump of trees right where GPS said it would be.
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Old 04-16-2009, 11:08 PM   #52
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I continued past the closed streets and let GPS recalculate my route. I ended up in the middle of NOWHERE when GPS announced "Destination ahead on the right" and sure enough my customer's business (the only business within 10 miles of unmarked roads) was just beyond a clump of trees right where GPS said it would be.
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Old 04-17-2009, 02:06 PM   #53
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We bought a Garmin Nuvi 350 last fall and now we can't live without it! My husband travels quite a bit with his job and it has proved invaluable. When we are feeling silly we will purposely go a different way just so we can hear "Jill" say "RECALCULATING"! She sounds more pissed off every time! Get one!!
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Old 04-17-2009, 04:21 PM   #54
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"Jill," huh? I call mine "Giz" -- for GIZMO.

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Old 04-17-2009, 05:12 PM   #55
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Ours is Gunilla Pia Svenson (GPS).

Wouldn't it be fun to produce one that says things like: "Sheesh -- you missed the turn again. Typical...."?
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Old 04-17-2009, 06:30 PM   #56
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What you described was the predecessor to GPS, called the Transit Doppler system. It dated back to the early 60s, hence was before my time.
Right, but if I called it by that name most would be clueless about it.

It was good then, a bit cumbersome, still better than sextant fixes. I still like the new versions better. You are right about the technology being useless for general public use without cheap/easy user interface.
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Old 04-17-2009, 06:33 PM   #57
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Yeah, Jill(?) sounds annoyed when she says "recalculating". I sometimes talk back at her when I miss my turn and she says recalculating (I am not the best driver in the world, so she really pushes my button with that one. )

I need an upgraded software with improved Jill, who is more patient with me.
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Old 04-18-2009, 09:10 AM   #58
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Martha, don't you live in a state that prohibits windshield mounts? Leaving the mount in place might be tempting fate. Have you considered a beanbag mount?
Sometimes I follow the law, sometimes I don't.

The disk mount came with the GPS so I attached it to the dash of the MH. In the car I use the windshield. In the car I usually drop the GPS into my purse when I leave the car. I don't in the MH. I thought about the beanbag mount but decided all was well with what I was already doing.


I just took a 4400 mile road trip in the MH over the past couple of months. Mrs. Garmin was invaluable. Oddly though, she choose a different route to Texas than she chose the last time. This one had more stops and goes as it wasn't all freeway. It was a bit shorter but took about the same amount of time. On the way back I forced a route using waypoints.
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Old 04-20-2009, 06:29 PM   #59
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Sometimes I follow the law, sometimes I don't.
Hope I did not offend. I only meant to point out that the windshield mounts are not legal in some places. It's all fairly new to me since I had no experience with GPS before Xmas and now we have four in the household. I bought one on Ebay that came without the mount which is how I found out about the bean bag mounts. Some folks claim even leaving the suction cup mounts on the windshield glass is an invitation to thieves, but that seems extreme to me.
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Old 04-21-2009, 08:07 AM   #60
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No offense taken. The beanbag mount sounds like a good idea, the only worry would be a quick stop.
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