Czech Mates Posted January 27, 2003 Share Posted January 27, 2003 Out on the cache we created, Dogtown Commons, there are at least 48 coordinates. We have gone back out to the site to confirm the coordinates on different occasions and have found a big problem. Some of the coordinates remain the same, but some change everytime we go out there. And not by just a few degrees. Dramatic changes that would put you in a different location. Does anyone have any clue as to why some coords stay the same and some change so dramatically? Is it the satillite or could it be something interferring with the signal in these areas? The variations are on the same coords each time. Quote Link to comment
+chaosmanor Posted January 27, 2003 Share Posted January 27, 2003 We have a multi-cache in Thousand Oaks where everyone has found the first micro exactly where we posted it, but eveyone gets radically different numbers at the second one. I've been out there twice and can't get a steady reading, and others have had similar problems. I had a good discussion with Big Eagle about this, and he told me of a military GPS unit (much more accurate than our "toys") that went haywire for about an hour, then settled back down; they never did figure out why. There is an error-factor programmed into the system that prevents our GPS units from getting a reading any more accurate than about 30 feet off, from any one satellite. Averaging can reduce this, by "jawboning" the differences, but it can also increase the error, for the same reason. Walls, trees, hills, buildings, etc. can all obstruct a satellite, and the fewer satellites that a unit can read, the less info it has to work with. It has been suspected (but not proven that I've ever seen) that high-tension lines can affect GPS units, but on a recent cache hunt that ran under some lines of this type, my unit (a Magellan 315) appeared to be unaffected. So, why do these things go whacky sometimes? Wish I knew! I just know that they do. My hassle is that far too many users don't believe me when I tell them that our "toys" are not pin-point accurate. They go to the co-ordinates I reported, don't see the cache right there, look under a bush and two rocks, can't find it and say that my numbers are wrong! People! Quote Link to comment
+Smitherington Posted January 27, 2003 Share Posted January 27, 2003 Power lines make it hard to get a good reading. Are any nearby? Quote Link to comment
Czech Mates Posted January 28, 2003 Author Share Posted January 28, 2003 No power lines. That was our first thought, though there is a comm. tower. But woulsn't that affect all coords??? Quote Link to comment
n1niq Posted February 3, 2003 Share Posted February 3, 2003 The toer could definitely affect your signal if it is a noisy RF site. There could be harmonics from frquencies affecting GPS channels. The odd readings that I have experienced seem to be caused by GPS signals reflecting off of cliffsides and sttep hills. Chris Quote Link to comment
+Captain Morgan Posted February 5, 2003 Share Posted February 5, 2003 Yup, if there's a comm. tower or radar near it may spoil your coordinates or even disable you to receive satellite signals totally. Regards, Quote Link to comment
Czech Mates Posted February 5, 2003 Author Share Posted February 5, 2003 We have now discovered, after reading a few things on line about the GPS, is that you need to leave your GPS on for a little while, sometimes overnight, to be sure you have all the satilites in line. Otherwise, that can throw you off. We are going to leave the GPS on overnight and go back out and check the coords again. Will let you kow if this helps! To all that have put their 2 cents in, thank you so very much! I just love cachers! WHat great people! Quote Link to comment
+parkrrrr Posted February 5, 2003 Share Posted February 5, 2003 quote:Originally posted by chaosmanor:There is an error-factor programmed into the system that prevents our GPS units from getting a reading any more accurate than about 30 feet off, from any one satellite. Actually, you can't get any position reading from any one satellite, though you can figure out what time it is within a second or so with just one satellite. The error on consumer (L1) GPS is due to the speed and period of the bitstream being sent more than due to any "programming." According to the NAVSTAR documents, "[t]he coarse/acquisition (C/A) code has a 1.023 MHz chip rate...." A chip rate of 1.023 MHz means that each bit in the stream is about 978ns long. Keeping your local bitstream correlated with that bitstream to an accuracy of a few nanoseconds each way is actually a pretty amazing feat, with the relatively long time it has to drift between transitions, but it's something they've managed to get done pretty well. The programming you're probably thinking of was the "Selective Availability" feature of the NAVSTAR service, but that was turned off some time ago (It led to errors on the order of 100 meters, not 10 meters, but its effects could be eliminated completely with differential GPS or WAAS.) That said, the worst position errors you'll see are usually a result of satellite geometry rather than any timing or other problems with the signals themselves. Consider the two situations shown here: If you know that you're on the black line, and you know your distance from two satellites within the precision represented by the colored rings, then you know you're somewhere within one of the purple regions where it touches the black line. In the case on the left, you can determine your position much more precisely than you can in the case on the right, even though you know your distance from the satellites with exactly the same accuracy. (There are worse geometries than these, too. Consider the case where the satellites are practically on top of each other.) That's most of the reason that some areas get worse position fixes than others: something nearby - possibly a building, a cliff, a grove of trees, or potentially even power lines - is blocking your view of one or more satellites, forcing you to use the satellites you can see in the rest of the sky. That constrains the geometry to a greater extent, so you're more likely to have a bad arrangement of satellites. Quote Link to comment
Micqn Posted February 20, 2003 Share Posted February 20, 2003 I always wondered about how it all worked and the accuracy issue. Is this why I have to tell my Magellan 315 what state/country I am in? Any way great representation! Happy. Hunting. Quote Link to comment
white_owl Posted March 13, 2003 Share Posted March 13, 2003 quote: Is it the satillite or could it be something interferring with the signal in these areas? The variations are on the same coords each time. Sometimes the simplest explaination may be the best. I'm intrigued by your comment that the variations are always the same. Did you maybe change the map datum on your unit? I was once looking for a co-ordinate which I thought was in WGS84 but was actually in NAD27. The location was 84 meters off. Once I changed my unit to NAD27 and re-entered the co-ordinates, my reading was bang on. Hope this helps. White Owl Quote Link to comment
Leahi Posted March 28, 2003 Share Posted March 28, 2003 We have a cache in a Heiau, or ancient Hawaiian religious area where my Meridian goes lolo. Dee dee dee dee, dee dee dee dee (Twilight Zone) Quote Link to comment
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