GPS Accuracy

[rabbithutch]

Just discovered geocaching and thought it might be something I'd like to try. I'm retired and it would be nice to have some goal oriented activity that will take me out to interesting pursuits. I found geocaching.com and have been reading various web pages. I have an iPhone and downloaded the geocaching app. From investigating the app, I discovered that there are lots of caches in my area.

 

I have an old Magellan hand held GPS (don't remember the model). I bought it new sometime between '97 and '00. It is a brick! About the same size and weight and burns batteries pretty quickly; however, I wondered if I could use it (instead of my Garmin auto unit ) for Long/Lat readings. IIRC, when I got this receiver, the accuracy was within about 50 yards. What I don't recall is whether this was built into the receiver (for military/national defense, I think) or the signal.

 

Can anyone help me here? I realize one might need the Magellan model number but I'll have to find it before I can tell you. I thought maybe this question might save me the effort of looking for it if the offset is built into the receiver.

 

TIA

[+dfx]

there's two points here. first is the deliberate inaccuracy that the US used to introduce into the GPS signal, which was turned off may 2nd 2000. before that, the signal was consistently off by up to 100 meters/yards (unless you had a military receiver which could filter this error out). after they turned it off, the signal was and still is as accurate as it can get.

 

the other point is the receiver electronics. the GPS signal itself didn't get any more accurate over time (apart from turning off the mentioned deliberate position error), so under optimal conditions a modern receiver is as good as an old one. however, improvements have been made in terms of sensitivity of the receivers (helping you under tree cover), faster response times, faster first fix, number of satellites they can track simultaneously, WAAS (not sure if that was around before 2000), etc etc.

 

AFAIK there never were any GPS receivers with a built-in offset, so that shouldn't be a problem.

 

a modern GPS receiver will be accurate to about 3 meters under optimum conditions and about 5-6 meters under average conditions (ballpark figures here). you can just take your old receiver and see how well it works, it may be just as good as a modern one, or it may turn out to be considerably worse. YMMV.

Edited June 3, 2010 by dfx

[+sduck]

You quite likely have a magellan 4000 (and some combo of letters afterwards), which was pretty much all that was available back then. I've got an old 4000xl that my dad gave me in 1998. Even though I managed to find my first 2 caches with it, it's fairly terrible compared to newer units. The selective offset was a government thing, that President Clinton turned off soon after taking office. Even without it, this gps still has terrible accuracy, that is when you can even maintain any kind of satellite lock for any length of time. http://www.geocaching.com/reviews/gps_magellan_gps-4000-xl

 

Definitely consider upgrading to something better. Your garmin auto unit is better.

[+PDOP's]

Some other issues with receivers of that vintage:

 

- the early receivers looked at satellites sequentially where it would get data from a few (3?) then switch to another few and so on. This resulted in terrible reception under any kind of cover. Modern units are are all parallel receivers having 12 or more channels.

 

- some early Magellan receivers did not display coordinates to as many decimal places as modern receivers. I don't remember the models but I believe some only worked with minutes to two decimal places instead of three.

[+kwcahart]

Speaking of GPS accuracy, is it just me or does my Garmin GPSMAP 76CX lose accuracy when I enable WAAS? It sure seems to, and the accuracy indicator on the sat page goes from 12-13 to 20 or so?

[+dfx]

Speaking of GPS accuracy, is it just me or does my Garmin GPSMAP 76CX lose accuracy when I enable WAAS? It sure seems to, and the accuracy indicator on the sat page goes from 12-13 to 20 or so?

probably just the way the unit calculates the accuracy. it's not much more than a wild guess really. the GPS chip itself will very likely not report the accuracy to be any different with a WAAS lock than without one - what the software/firmware (i.e. the user interface) does with the data is a completely different story.

[+splashy]

My accuracy on a Oregon 550 today was between 7 and 10 Feet.

[rabbithutch]

Thanks for the responses, folks!

 

I think I'll dig the old Magellan out and see if it gives me anything close. It'll be a lot easier than trying to get the funds for an Oregon out of the Finance Department. I remember that the old receiver tracked 3 sats; so that signal acquisition will not be like a modern unit. Then again, we don't have the tree cover here in Texas that will hamper it all that much either. But, it doesn't matter much because I might discover that I won't spend much time at the hunt, anyway.

 

Thanks again!

[+Entropy512]

Any accuracy estimate other than DOP numbers are just that - a wild estimate that varies from manufacturer to manufacturer, and sometimes from receiver to receiver.

[+kwcahart]

Hey DFX, thanks for the answer. I tried it again and you are right on. I let it settle down both ways, WAAS enabled and disabled, and the accuracy was about the same. I just didn't wait long enough the first time.