GPS accuracy- What is normal?

[mrsoup]

Went to Chicago over the weekend and used my Garmin 2610 for the first time. When driving in town it seemed to be very accurate, telling me to turn in 500 yards, 200 yards, etc. When headed home on the intersate it was off 4 tenths of a mile on notifying me of my exits. It would tell me to turn in 4 tenths and I would already be at the exit ramp, is this normal?

I am using v8 of Mapsource. If it matters, I was 'off course' and it was giving me a recalculated route to follow. Does this have an effect?

This is my first venture with GPS so forgive me if I am being STUPID!

[David]

Was WAAS accidentally turned off during the second experiment?

[tgrrdr]

I was going to say, based on my experience, it should be accurate to +/- 100 feet - 2,000 feet off is ridiculous. I wanted some "real" information though so I looked on Garmin's website.

 

GPS Accuracy:

* Position: < 15 meters (49 feet), 95% typical**

[KDeline]

We had a new ramp put in by our house and the GPS now shows it with V7 (?) update, it is off by a few tenths, shows me in the field when turning and of course is telling me I'm off route.

[tgrrdr]

Here's another Garmin webpage about position accuracy.

 

 

100 meters: Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program.

 

15 meters: Typical GPS position accuracy without SA.

 

3-5 meters: Typical differential GPS (DGPS) position accuracy.

 

< 3 meters: Typical WAAS position accuracy.

[Boffin]

I find that in the UK. It seems to me that the GPS uses the actual position of the cross-over of the road as it position, so you 'lose' the distance occupied by the off-ramp.

 

Andy

[Jerry Johnston]

The accuracy depents on how many satilites it has locked in on. When you look at the GPS status it will tell you how many feet it can be off by.

[boatzo]

Go into the MAIN Menue and find the "SPEED FILTER" setting. I believe there are 3, High /Medium / Low. It is probably defaulted to Medium. Change the setting to one end or the other and see if that makes a difference. In the wrong position and if you are at fairly high speed, the GPS will either show your arrival before or after the way point.

 

Let us know what you find.

[Bob Palin]

Just throw the GPS away and use your brain

 

Another friendly message from the Anti-GPS ludditium

[Aluminum_Butt]

Was WAAS accidentally turned off during the second experiment?

 

Do check your WAAS setting. I just got a 2730, and WAAS was *disabled* by default.

[Fergie]

The accuracy depents on how many satilites it has locked in on. When you look at the GPS status it will tell you how many feet it can be off by.

 

Jerry is right. When I'm in an area where I have 5 or more satilites the accuracy is 15 feet with my 2610.

[Sidmariner]

The accuracy of the GPS has a lot to do with the number of satellites being tracked, the altitude of the satellites (low angle on the horizon versus overhead), the condition of the satellites (new, decaying orbit, defective), and the chart datum selected (should match datum of map or chart). A GPS receiver can also be affected by reflective signals (same signal bouncing off a building arriving at a different angle/time/distance).

 

The GPS determines your ground speed by processing one fix after another and then calculating a direction and rate bases on comparison. In a straight line at low speed the information is very accurate. Unfortunately, at high speed the processing time is sometimes comparitively slow, and especially so if the unit is bogged down refreshing graphics, generating a voice report or doing other memory intensive work. You'll notice that your GPS will lag a bit when you make a sharp reduction in speed and will seldom be able to provide accurate headings when you are in the twisties. The amount of graphic information and text being generated within city limits is huge and I have found my Garmin (Quest 2) is never up to the task. I have missed several critical offramps while listening to the report "Recalculating". I now only rely on my GPS when I'm driving slowly in town, otherwise, I make sure I've got a good set of notes and a decent map on my tank bag.

[smiller]

I don't think it was the GPS data itself that was inaccurate, probably more likely the map database has a minor error.

[Joe Frickin' Friday]

The amount of graphic information and text being generated within city limits is huge and I have found my Garmin (Quest 2) is never up to the task. I have missed several critical offramps while listening to the report "Recalculating".

 

Get into the menus and tell it to ask you before it recalculates, instead of automatically recalculating every time you go off route.

[grizzly660fan]

I am a noob with GPS so pardon if question isn't exact or I am totally in the wrong park on this.

 

I thought WAAS is ground based stations, so the GPS uses the statalites and WAAS stations to compute info. out here in Utah I have not been able to find out info about where the WAAS stations are physically located. my thought is in most of my non SLC riding there is no WAAS to refer to for the GPS unit.

 

how do you get it to be more accurate if no WAAS around? or do I just misunderstand this? (highly possible)

 

Thanks in advance,

 

Garry

[Aluminum_Butt]

Here is a primer on WAAS from Garmin.

 

Ground stations do monitor the GPS satellites, and develop correctional info. But they broadcast that info back up to another satellite(s), and you get it from there - not from the ground stations directly. Hence, you can be about anywhere in the US, and still get WAAS-corrected data.

[boatzo]

I think you may be thinking of "Differential" Those are stations that operate near port cities and water ways. Totally different from WAAS. You should be able to get WAAS coverage where you are, but as has been mentioned previously, many GPS units require that you turn WAAS on and then only certain sats. provide WAAS coverage. Also, older units did not have WAAS capability.

[Sidmariner]

I believe the Wide Area Augmentation Signal (WAAS) was developed to improve the accuracy of commercial navigation, particularly air navigation, at a time when the US was still applying "artificial dithering" to make GPS inaccurate for potential enemy use. When president Reagan removed the dithering GPS improved for everyone, but the WAAS system remained to add additional accuracy. One thing I learned about WAAS is that the few ground stations which monitor satellite performance and condition relay their corrections to low earth orbiting communicaton satellites. Aircraft seldom have difficulty picking up the signal because they are above obstructions, however, if you're driving in the mountains, big cities or in higher latitudes the signal will either be obscured by terrain or too low in the horizon to be received consistently or with accuracy. Therefore, you might see a "Lost WAAS" or "Lost Satellite" report on your GPS. If accuracy is critical for your navigation needs, like if you want to hit the button on the runway instead of the taxi ramp, or if you need to stay in the deep channel of a harbour entrance in your boat, this would be important information to know. Thankfully, on a bike this is seldom a big issue, unless you overshoot the only gas station within 50 miles thanks to a lost signal.

In Europe there was so much concern for the reliability of the US-based GPS system (remember the US could turn it off or dither it at their whim) they developed their own system using European satellites.

Bottom line, GPS is not fool proof or as accurate as everyone complacently assumes. If accuracy is critical, you need to monitor the system, be aware of its faults and know its limitations.

[jerseygeorge]

I believe the Wide Area Augmentation Signal (WAAS) was developed to improve the accuracy of commercial navigation, particularly air navigation, at a time when the US was still applying "artificial dithering" to make GPS inaccurate for potential enemy use.

 

I think that you are pretty close. All aircraft used ground based signals for navigation and instrument approaches. Typically we can fly an approach to 200 feet above the ground, but with proper equipment and training can use ground based signals to a full auto landing. Then along comes satellite navigation. The FAA wanted some sort of ground system to check the performance, and that became WAAS. I thought incorrectly that it was ground based transmitters that augment the satellites, but it is a ground based system that corrects, or augments the satellite signals. With this system we get down to about 250 feet above the ground, only about 50 feet higher than the older, but reliable ground based systems. Actually quite impressive.

 

As far as speed goes, I have had my Garmin Zumo up to 700 mph, and down low at close to 300 (speed limits). It tries to figure out what road it is on, belive me, you have never seen the screen and map move so fast in your life. This units are capable of a lot more than we can ever do to them. If you unit is not calculating fast enough or accurately enough I think something else is at issue.

 

Also, I have turned WAAS of and on on my Zumo. I can not find any difference at all in start up acquisition time or road accuracy. I drive through NYC, past tall building, in and out of tunnels (of course no reception) but I can not see any difference at all.

 

My 2 cents!

[Joe Frickin' Friday]

It tries to figure out what road it is on, belive me, you have never seen the screen and map move so fast in your life.

 

Don't know about the Zumo, but on the 276c you can turn off the "lock on roads" feature in the menus. Very useful for going cross-country, whether flying, hiking, or riding an obstacle course in a parking lot.

[Sidmariner]

I'm stretching now, but I was under the impression that there are two WAAS communications satellites, one over mid-Atlantic and one mid-Pacific (could be wrong). The ability of a unit to receive WAAS depends on the sight-line to the satellites, hence my frequent loss of signal while riding west of the coastal mountain range and up Oregon/Washingon coast to BC. The WAAS satellite I needed to receive from was to the east, and at low altitude (elevation) relative to me.

In my early days of navigation I was lucky to receive one satellite fix every 11 hours using the "Transit" system. Over the last five years we have gone from 4, to 8, to 12 channel (satellite tracking) receivers. It's rare now that you will find yourself without at least 3-4 good strong signals to process; good enough to negate the need for WAAS.

You're right about the device though. I somehow have to reduce the drain on the processor, maybe by limiting graphics. I was hugely disappointed in the performance of the Quest 2 when I needed guidance to get through Nashville and St Louis last spring. After I blew past too many critical offramps I was ready to chuck the unit.

[David]

I believe the Wide Area Augmentation Signal (WAAS) was developed to improve the accuracy of commercial navigation, particularly air navigation, at a time when the US was still applying "artificial dithering" to make GPS inaccurate for potential enemy use.

 

I think that you are pretty close. All aircraft used ground based signals for navigation and instrument approaches. Typically we can fly an approach to 200 feet above the ground, but with proper equipment and training can use ground based signals to a full auto landing. Then along comes satellite navigation. The FAA wanted some sort of ground system to check the performance, and that became WAAS. I thought incorrectly that it was ground based transmitters that augment the satellites, but it is a ground based system that corrects, or augments the satellite signals. With this system we get down to about 250 feet above the ground, only about 50 feet higher than the older, but reliable ground based systems. Actually quite impressive.

 

As far as speed goes, I have had my Garmin Zumo up to 700 mph, and down low at close to 300 (speed limits). It tries to figure out what road it is on, belive me, you have never seen the screen and map move so fast in your life. This units are capable of a lot more than we can ever do to them. If you unit is not calculating fast enough or accurately enough I think something else is at issue.

 

Also, I have turned WAAS of and on on my Zumo. I can not find any difference at all in start up acquisition time or road accuracy. I drive through NYC, past tall building, in and out of tunnels (of course no reception) but I can not see any difference at all.

 

George, I think you're confusing WAAS with differential GPS.