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Folks--

After spending some time during a very hot Florida day disoriented in the national forest down here, I got a GPS for Christmas, a Garmin etrex. I'm very happy with it; it's survived some serious throwing about while in my Camelbak, complete submersion, and just general heavy abuse, but seems indestructible.

There's one weird thing...on the info page, where the mileage, speed, moving time, etc, are listed, my max speed will occasionally register anything from 40 to 163 miles an hour. While I consider myself to be a fairly capable mountain biker, I have a hard time believing I've been doing 150mph on the Cadillac Trail down here.

I suspect it has something to do with briefly losing the signal, and picking up another satellite and miscalculating something in the transition, but I was curious if anyone knew what was going on? It's not a big deal, since I don't really care that much about my max speed right now, but I'm also a geek and would love to know if there's an explanation out there.

Thanks folks!

-jms
 

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I just returned from a week in Tallahassee! I enjoyed riding Tom Brown park. Fun stuff!
Has you GPS alway done this?
I think the government jams GPS signals for a few seconds every so often for security reasons.
 

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xcdude said:
I think the government jams GPS signals for a few seconds every so often for security reasons.
No, the Guberment won't occasionally "block" satellite signals either in one specific area, or the entire continent from over 20 satellites. In fact it was WAY back in '94 (not sure of the year) I think that they (the Guberment) allowed private GPS devices to have military accuracy. Survey GPS's are good to 1/2 centimeter. I would suspect your GPS is going south. Give Garmin an email or a call, they are very good to work with. You might also try downloading the lastest version of the firmware from the Gamin website, if it is a know/semi-common problem they may have fixed it with updated firmware.

As far the the "geek" aspect, your GPS need to receive reception from 3 or more satellites at a time to extrapolate distance, elevation, speed, etc. If you lose one, you typically have contact with several (like up to 15) so it shouldn't make a difference. If you lose them all, the GPS will have to re-aquire the signal and calculate it's positon from the start again. That's pretty common in heavy forest or when your body sheilds the reception.
 

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Easy answer...

Mackey said:
Folks--

After spending some time during a very hot Florida day disoriented in the national forest down here, I got a GPS for Christmas, a Garmin etrex. I'm very happy with it; it's survived some serious throwing about while in my Camelbak, complete submersion, and just general heavy abuse, but seems indestructible.

There's one weird thing...on the info page, where the mileage, speed, moving time, etc, are listed, my max speed will occasionally register anything from 40 to 163 miles an hour. While I consider myself to be a fairly capable mountain biker, I have a hard time believing I've been doing 150mph on the Cadillac Trail down here.

I suspect it has something to do with briefly losing the signal, and picking up another satellite and miscalculating something in the transition, but I was curious if anyone knew what was going on? It's not a big deal, since I don't really care that much about my max speed right now, but I'm also a geek and would love to know if there's an explanation out there.

Thanks folks!

-jms
Before I forget, the DoD turned off the SA (Selective Availability) in May of 2000 I believe. The net effect is that if you're particular gps receiver has a stated accuracy of 10 meters, you'll actually get 10 meters (or a lot better) when you have good satellite coverage. The stated accuracy is actually measured by saying the unit will be within 10 meters of a point 90% of the time. I've actually seen the WAAS enabled Geko measure down to 1 meter.

Here's what happens sometimes, especially when in you're trees, cities, canyons or anywhere with a limited or obstructed view of the sky. Gps signals are really low power fm signals so you may have the 3 satellites you need for a position fix BUT if the signals are obstructed, let's say in a broad leaf forest, the signals get degraded and cause bad geometry. If you had GIS or survey grade gps software, you could actually edit the file point by point and pick out the bad points which would give you super tight data afterwards. Not that I've ever done that... Oh yea that funky error in cities is called 'multipath' because the signals bounce off of buildings and screw up the timing.

Anyway, that's probably why you were riding at 300 miles an hour, it's a pretty simple answer and just one of those minor annoyances with really cool technology.

That's probably way more information than you wanted!

Next weeks class: elevation masks and PDOP!
 
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