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Garmin GPS 72 - Unusual readings ?
I recently purchased a Garmin GPS 72. Since it's not really boating weather here - and the trailer isn't done yet - I've been experimenting with it in other situations. Last week I helped a friend of mine drive a Pensky box truck with a car carrier trailer (and car) from Boston to LA. We took turns driving and sleeping and made it in 71 hours. ANYWAY - I took the GPS along and got some strange readings. On the trip page it shows average speed while moving, overall average speed, trip odometer, etc. The other thing it shows is max speed. I've used this in my car and it seems to work OK, but in the truck it sometimes showed max-speeds as high as 115 mph. The truck *could not* go over 75. I figured this was probably some interference from being in the metal cab, from the engine (which is under the cab) or from the radio or our cell phones. So I moved the GPS to a spot away from the engine and cell phones and the problem seemed to go away. BUT - in CA we went on a 6 mile hike/walk. Again I brought the GPS and for a while it operated normally. But by the end of the trip it showed max-speed as 15 MPH. I know we were only walking at 3 or 4 mph. We never jogged or fell or anything - there is no way we got up to even 10 mph. And I didn't have any cell phone with me to create any interference. Though I did have a digital camera (off) in the same bag. So, questions: - Is this normal? - Why might it happen? (Is it innaccuracies of the GPS system, this particular model, or maybe interference?) - If I see that the max-speed is inaccurate, should I also assume that the other readings (odometer, average speed) are inaccurate? Gary |
#2
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Garmin GPS 72 - Unusual readings ?
So, questions: - Is this normal? Yes. - Why might it happen? (Is it innaccuracies of the GPS system, this particular model, or maybe interference?) Speed is calculated by using simple time/distance calculations, measuring the time it takes to get from one known fix to another. It is common for a GPS to get an errant fix from time to time. Sometimes, these errant fixes can be quite a ways off, but are usually only momentary. Nevertheless, they are included in speed calculations. The seemingly unbelievable max speed results from the very fast side trip your GPS thought you made while visiting the errant fix. I've seen max speeds in the many hundreds of MPH. Probably the closest I'll ever get to warp speed in my lifetime. - If I see that the max-speed is inaccurate, should I also assume that the other readings (odometer, average speed) are inaccurate? Inaccurate is a relative term. I would suggest that your GPS is likely to be more accurate and reliable for the above measurements than any other instrumentation that you have on board. |
#3
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Garmin GPS 72 - Unusual readings ?
Hi Gary,
I have found a number of things can interfear with a GPS's operation. Power lines, radar emissions, RF from your VHF or cell phone, and even strong magnets. Whne running south on the "Vita" we run underneath some "power towers" crossing the river. All of my NAV goes nuts. Compass spins, GPS resets, radar shows snow and and clutter that does not exist, and VHF simply does not receive. -- Capt. Frank __c \ _ | \_ __\_| oooo \_____ ~~~~|______________/ ~~~~~ www.home.earthlink.net/~aartworks "Gary Warner" wrote in message ... I recently purchased a Garmin GPS 72. Since it's not really boating weather here - and the trailer isn't done yet - I've been experimenting with it in other situations. Last week I helped a friend of mine drive a Pensky box truck with a car carrier trailer (and car) from Boston to LA. We took turns driving and sleeping and made it in 71 hours. ANYWAY - I took the GPS along and got some strange readings. On the trip page it shows average speed while moving, overall average speed, trip odometer, etc. The other thing it shows is max speed. I've used this in my car and it seems to work OK, but in the truck it sometimes showed max-speeds as high as 115 mph. The truck *could not* go over 75. I figured this was probably some interference from being in the metal cab, from the engine (which is under the cab) or from the radio or our cell phones. So I moved the GPS to a spot away from the engine and cell phones and the problem seemed to go away. BUT - in CA we went on a 6 mile hike/walk. Again I brought the GPS and for a while it operated normally. But by the end of the trip it showed max-speed as 15 MPH. I know we were only walking at 3 or 4 mph. We never jogged or fell or anything - there is no way we got up to even 10 mph. And I didn't have any cell phone with me to create any interference. Though I did have a digital camera (off) in the same bag. So, questions: - Is this normal? - Why might it happen? (Is it innaccuracies of the GPS system, this particular model, or maybe interference?) - If I see that the max-speed is inaccurate, should I also assume that the other readings (odometer, average speed) are inaccurate? Gary |
#4
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Garmin GPS 72 - Unusual readings ?
"RG" wrote ... Speed is calculated by using simple time/distance calculations, measuring the time it takes to get from one known fix to another. It is common for a GPS to get an errant fix from time to time. Sometimes, these errant fixes can be quite a ways off, but are usually only momentary. Nevertheless, they are included in speed calculations. The seemingly unbelievable max speed results from the very fast side trip your GPS thought you made while visiting the errant fix. I've seen max speeds in the many hundreds of MPH. That makes complete sense but for what it's worth my Garmin GPSMAP 168 (external antenna) has never done this. The max speed recorded is 43.something mph and I've watched the speed go over 42 steady state. Different software perhaps. -rick- |
#5
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Garmin GPS 72 - Unusual readings ?
I have a Garmin Street pilot; a Street Pilot Color; a 176color and all three
do that sometimes. "Gary Warner" wrote in message ... I recently purchased a Garmin GPS 72. Since it's not really boating weather here - and the trailer isn't done yet - I've been experimenting with it in other situations. Last week I helped a friend of mine drive a Pensky box truck with a car carrier trailer (and car) from Boston to LA. We took turns driving and sleeping and made it in 71 hours. ANYWAY - I took the GPS along and got some strange readings. On the trip page it shows average speed while moving, overall average speed, trip odometer, etc. The other thing it shows is max speed. I've used this in my car and it seems to work OK, but in the truck it sometimes showed max-speeds as high as 115 mph. The truck *could not* go over 75. I figured this was probably some interference from being in the metal cab, from the engine (which is under the cab) or from the radio or our cell phones. So I moved the GPS to a spot away from the engine and cell phones and the problem seemed to go away. BUT - in CA we went on a 6 mile hike/walk. Again I brought the GPS and for a while it operated normally. But by the end of the trip it showed max-speed as 15 MPH. I know we were only walking at 3 or 4 mph. We never jogged or fell or anything - there is no way we got up to even 10 mph. And I didn't have any cell phone with me to create any interference. Though I did have a digital camera (off) in the same bag. So, questions: - Is this normal? - Why might it happen? (Is it innaccuracies of the GPS system, this particular model, or maybe interference?) - If I see that the max-speed is inaccurate, should I also assume that the other readings (odometer, average speed) are inaccurate? Gary |
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