GPS not accurate

[Firetiger]

This morning before I left for work I plotted a point in my Lowrance Globalmap 100 to see how accurate it is now that S.A. has been "turned off". When I returned to the waypoint tonight, I was still 85 ft off which isn't a lot but is more than the new approximate deviance (3-10 m) that was reported the past few days on W.C. If someone could explain why I haven't achieved that accuracy it would be appreciated.

[buzbait9]

Unfortunatly the latest news of GPS has been greatly exaggerated by the press. Most press were reporting 'Military Accuracy'. The truth is that you will still be 10-20 meters or more off because that's as good as the non-military signal gets. DGPS will be about 7.5 meters and the WAAS system from Raytheon will be less than 2 Meters off. Let's face it 85 feet isn't bad but there is better!

[deputydan]

I have a Garmin GPS III and over the past couple of days have found it to be very accurate since the S/A was shut off usually being 30 feet or under, more often under. I did notice that when I have been off, my satelite coverage (ie. the number I am receiving) is down. I have repeatedly been under 10 feet on several of the waypoints I have entered. Perhaps your GPS hadn't settled in yet when you entered the waypoint.

[Silver/MI]

I'm with Deputy, my Garmin12 has seems to be about 10 feet since the SA removal. It puts me in the same parking space now, where previously it would not in put me in the parking lot. But then again my units go to 100th of a mile (.01 miles) when returning to a waypoint, so how can it ever be more accurate than 50 feet? My position averaging feature used to get to about 35 ft., but now it goes to about 12. Could be you're not getting enough sattelites.

[brad/nd]

Silver,
Are you using the "go to" function or a trac back function when checking your waypoint accuracy? I am experimenting with my garmin some, but haven't used all the functions yet. According to the manual, there are a few different options to use to get back to your waypoint. Just wondering what works best for you.
Thanks,

Brad

[GreggB]

I checked my Magellen 315 last night. It reported and estimated position error (EPE) of 3ft! I marked the end of my driveway went to town and back and it was within 5 ft actuall. I checked another waypoint and was within 15ft actuall. Both times I had at least 7 satelites locked in. The fewer the satelites I had locked on it the greater EPE became. For instance, with only three satelites my EPE was 45ft. Before SA was turned off the best reading of EPE I ever got was 34ft. Also the amount of error can flucuate considerably from day to day or hour to hour.

Check how many satelites you are locking on too, maybe thats why you were off by so much.

[bob oh]

Where did you read all these "exaggerations"? Actually the press reports I have been reading have mentioned going from the accuracy of a "football" field to the accuracy of a "tennis court", about what you are reporting. That's exaggeration??? News reports have been reporting 10 to 20 meters, but it is possible that it is better than 10 to 20 meters amd DGPS is better than that. Most of the really accurate claims I have seen have come from "users" of GPS. Try looking at the sci.geo.satellite-nav newsgroup where GPS users hang out - they are reporting a lot better results, and they aren't the "press."

[DHD]

Gentleman:

I have always been told that even if you have more then 3 Sat lock on it doesn't matter,about accuate, what matters is that if you are in a area ,like woods you would loose contact for awhile,but as long as you have 3 lock ons your accuate should be just as accuate with 5or6.

[Firetiger(MN)]

I believe that the GlobalMap 100 won't lock on unless it has at least 8 satellites. I'm not positive on this, I'll have to check my manual again. It is a 12 channel GPS, so I would think that it would be pretty accurate. I'm going to try it again tonight.

[Greggb]

If theory serves me correctly the more satelites you lock onto the better triangulation you'll get. Thus better accuracy. I have seen on my own unit that, the more satellites I acquire the better EPE becomes. Just my own experience.

[REW]

I believe that you are both right.
My understanding is that the position is calculated by 3 satellites. However, if some satellites are quite close to each other, or if one or more are quite close to the horizon -- you don't get optimum trangulation.

The advantage of having more satellites, is that the unit can calculate the best accuracy, and it has the ability to discard the readings from the Satellites having the least accuracy.

Take care

REW

[Silver/MI]

I always just use the goto.

[Jim]

Before the government was saying accuracy within 100 meters. Now they are offical saying accuracy within 20 meters. Your 85 feet is still within that range.

[Jeremy]

My EPE has been about 15ft with good satellite signal, no where near the drifting of the altitude, speed and coords like it used to be. I have the Garmin 12

[Matt]

I agree with REW. Its not just as simple as more or less satellites. There are many variables at work.