satellites lead us on a wild goose chase

[BlackBeard Pirate]

Here is something I noticed while caching with a friend this week.

 

We each have our own GPSr and are hunting the same cache, so we are walking in circles in the woods under moderate tree cover. I use a Vista with compass; he uses a pda with better reception.

 

So, I let my unit lead me to a distance of zero, do a bit of walking around to confirm, see that my buddy is getting the same result. A bit more walking around and my unit is now leading me to a different spot maybe 20 or 30 feet away from the first spot. I figure this is expected as i am under tree cover and my accuracy is about 45 feet.

 

Here is the interesting part: when my unit changes its mind and tells me that the cache is "over there" my buddy gets the same reading on his.... this goes on for 30 minutes or so... we are on a wild goose chase, but the SAME chase... when my unit changes its mind and leads me to a new location, my buddy's unit is now also pointing to this same new location. At this point we think that it's not a fluctuation in the recieving end, but a fluctuation in the transmit end (seems logical) because the fluctuations we observe are in unison.

 

Now it is my understanding that the GPS satellites recieve ground signals, and determine their own relative locations. They send their location based on their calculation, and our reciever recieves this signal.

 

So I'm thinking that this strange phenomenom is due to the satellites trying to auto-correct themselves.

 

Anybody observe this? what are your thoughts?

 

Rob

St. Augustine Fl.

 

P.S. Any geocaching societies in NE Florida?

[+NightPilot]

This is normal. The satellites don't try to auto-correct themselves, they just send the time, all the time. Your GPS receives the time from several satellites, and calculates the position based on the differences in the time received from the satellites. As the atmosphere moves, and the satellites move, the received time differences changes, so a new position is calculated. Keep in mind that a difference of one microsecond (a millionth of a second) results in a position difference of many yards. The effect of relativity caused by the speed of the satellites and the lack of gravity in orbit has to be accounted for just to get within 50 feet or so of the correct position. WAAS mainly corrects for the effects of changes in the atmosphere, since radio waves travel at a slightly different velocity in different air densities, not much but enough to make a difference. It's still not perfect, but it gets closer to a correct solution.

 

What you should do is watch the GPS, and when you start getting close to the cache, start looking for it. You have to search about a 60-foot circle, often more. The GPS will never lead you right to the cache, except by lucky chance. It's just not that accurate.

[+Prime Suspect]

Here is something I noticed while caching with a friend this week.

 

We each have our own GPSr and are hunting the same cache, so we are walking in circles in the woods under moderate tree cover. I use a Vista with compass; he uses a pda with better reception.

 

So, I let my unit lead me to a distance of zero, do a bit of walking around to confirm, see that my buddy is getting the same result. A bit more walking around and my unit is now leading me to a different spot maybe 20 or 30 feet away from the first spot. I figure this is expected as i am under tree cover and my accuracy is about 45 feet.

 

Here is the interesting part: when my unit changes its mind and tells me that the cache is "over there" my buddy gets the same reading on his.... this goes on for 30 minutes or so... we are on a wild goose chase, but the SAME chase... when my unit changes its mind and leads me to a new location, my buddy's unit is now also pointing to this same new location. At this point we think that it's not a fluctuation in the recieving end, but a fluctuation in the transmit end (seems logical) because the fluctuations we observe are in unison.

 

Now it is my understanding that the GPS satellites recieve ground signals, and determine their own relative locations. They send their location based on their calculation, and our reciever recieves this signal.

 

So I'm thinking that this strange phenomenom is due to the satellites trying to auto-correct themselves.

 

Anybody observe this? what are your thoughts?

 

Rob

St. Augustine Fl.

 

P.S. Any geocaching societies in NE Florida?

 

Except for the WAAS birds, GPS satellites are not in geosynchronous orbit. They're always moving relative to your position. So one might move above a hill on the horizon, as another is obscured by heavy tree cover.

 

Or maybe god is just screwing with you.

Edited March 24, 2007 by Prime Suspect

[+NightPilot]

Yeah, he seems to spend a lot of time screwing with geocachers. Especially me.

[+MN Hunter]

I have a similar problem with my Legend CX. I was looking for a cache in a wide open area and when I reached ground zero with accuracy of 8 ft and when I moved my GPSr a little it would say that the cache position was 35 ft back the way I came from. When I went to the new position it would say the cache position was 30+ back the way I just came from even though the cache was still another 35 feet from the first zero reading I got and in the original direction of travel. After I found the cache I set the Legend down next to the cache and got a reading of 9 ft back the way I came from with a accuracy of 8 ft, I went back to see if I could repeat this "problem" and the same thing happened getting me no closer than 35 feet from the cache. Later that day I made sure I had the latest upgrades (I did) and did a hard reset. Yesterday i repeated the same exercise and got the same results. I then called Gamin and they couldn't explain why this was happening and suggested that i exchange it for a different one. Will be taking it back this weekend and maybe looking at upgrading to a GSPMAP 60cx.