Satellite Accuracy

[saopaulo1@hotmail.com]

Placing my first cache in Brazil, I noticed the crappy accuracy I was getting. Taking account of the buildings around me, do you location on earth have anything to do with your sat. accuracy.

 

Even when I'm on the roof of my apartment building (with a clear view to the sky) I get a bad reception.

[+PDOP's]

Your geographic location in the world shouldn't matter as long as you have a clear view of the sky.

[+NightPilot]

The G in GPS stands for Global. Any place on earth is as good as another for receiving GPS signals, on average. The satellites are not geosynchronous, so you'll see different satellites in different places at different times, but if you're receiving poor signals it's likely a problem with your receiver, not the satellites. Make sure to hold the GPS away from your body, because your body will completely block the signals from the satellites in that directions. Your hand over the antenna will block everything.

[+Rockin Roddy]

The quality of the unit is another factor...I didn't realize how much better the reception could be until I bought the Magellan Explorist 500LE a while back. It NEVER has lost signal (yet) even in thick coverage or in our local Lowe's store (plenty of metal and concrete coverage there).

 

Our old Garmin Etrex loses signal if a cloud goes overhead! I use the Magellan, Tod (cache friend) uses the etrex...he's constantly fighting to obtain signal while I'm constantly yelling "mine says 40'....."!!

[saopaulo1@hotmail.com]

I dont think it's the unit. In the US I was getting an average of 15ft. Now it's about 40ft

[chuckr30]

I dont think it's the unit. In the US I was getting an average of 15ft. Now it's about 40ft

I noticed my geko 201 got great accuracy (15ft) in wide open areas, and even got a very quick sat lock on a large lake. But in any type of cover it got crappy reception, and tended to think I was still moving for the next 20 seconds, even though I had stopped. I think that is a software glitch, or bad software design.

 

Anyway, in open areas my Garmin 76csx works great. I wonder if smog might affect signal reception... I do think cloud cover affects reception. On a cloudy winter day 2 days ago, my 76csx took a long time to get a lock.

[+tozainamboku]

I dont think it's the unit. In the US I was getting an average of 15ft. Now it's about 40ft

 

WAAS

[saopaulo1@hotmail.com]

I dont think it's the unit. In the US I was getting an average of 15ft. Now it's about 40ft

 

WAAS

 

That explains it now.

[+Billk72]

The global positioning system was built by the D.O.D(Department Of Defense) for tracking of troops and giving troops accurate means of finding locations. The GPS system is still maintained by the DOD. Our civilian models are limited they don't use the system fully so minor deviations are expected. If your system was once more accurate then it is now I would suggest a hard reset. Just like on your computer there are times you need to delete your Internet cache to speed your Internet back up also a monthly power reboot of your modem will speed you up. Basically any computerized device that uses memory will need a reset eventually to clear the memory of clutter.

[+Prime Suspect]

The global positioning system was built by the D.O.D(Department Of Defense) for tracking of troops and giving troops accurate means of finding locations. The GPS system is still maintained by the DOD. Our civilian models are limited they don't use the system fully so minor deviations are expected. If your system was once more accurate then it is now I would suggest a hard reset. Just like on your computer there are times you need to delete your Internet cache to speed your Internet back up also a monthly power reboot of your modem will speed you up. Basically any computerized device that uses memory will need a reset eventually to clear the memory of clutter.

This is actually a bad idea, unless there is a real problem with your GPS. Over time, your unit builds up a table that is used for clock corrections over various temperature ranges. A hard reset can wipe out this table, causing the unit to take longer to get a 3D fix.

 

The idea that a GPS is like a PC is a flawed one, since a GPS is a closed system. All the software it runs is designed and tested by the manufacturer, so they can test for things like memory leaks (the leading reason you need to periodically reboot your PC).

Edited January 22, 2007 by Prime Suspect

[+Billk72]

I agree that by doing this the first time it will take longer to get its initial fix but will be fine after that. The built up table is gives the unit more to compute and cause the change in accuracy.

[saopaulo1@hotmail.com]

I'll wait on rebooting. I'm trying to get a hold of some local cachers to see how their accuracy is.

[+Prime Suspect]

I agree that by doing this the first time it will take longer to get its initial fix but will be fine after that.

No, it won't. It takes a long time to build up the table back up. If the current internal temperature of the unit hasn't been added to the table yet (with the appropriate adjustment factor), then it's going to take longer to get a 3D satellite fix.

 

The built up table is gives the unit more to compute and cause the change in accuracy.

No, the table has no affect on the accuracy of the unit. Once the unit gets a 4 satellite fix, it can correct the clock from the signals, and doesn't use the table at all (except to add data to the table, for future corrections).

 

Again, it's not necessary or desirable to perform routine hard-resets. It should only be done when there is a real problem with the unit.

Edited January 22, 2007 by Prime Suspect

[+JTA]

GPS accuracy can get pretty scientific. In college I did many research papers and presentations on GPS use in agriculture and precision farming. One thing we found was on tractors, if the GPS was mounted on a flat surface, the signals could reflect causing small positional changes. The atmosphere also makes a difference. If you are getting signals from a SAT low in the horizon, the atmosphere can delay the signal from the SAT to the GPSr very slightly, but "slightly" means a lot when a signal is traveling at the speed of light. If you log the NMEA strings for 24 hours from your GPSr and plot the points in excel, you will see a great deal of variation from hour to hour. So if you think about it, it's amazing we can get the accuracy we have now.

 

JTA

[saopaulo1@hotmail.com]

Today I was talking to a fellow business man who has a Extrex Venture in Brazil..anyway he said he gets about 12 meters (about 30+ft) This is in the country side.

[+NightPilot]

It's not just atmospherics. The effects of relativity, caused by the speed of the satellites and the lack of gravity, have to be corrected for. When the first satellites were ready to be launched, there was some debate among the scientists and engineers about whether relativity would be a factor. It was decided to install the ability to make the corrections, but leave them turned off. It turned out that the corrections were necessary, and they had to turn them on. It doesn't take light (or radio signals, which are light, but at wavelengths we can't see) very long to travel 10 meters. It's an amazing feat of engineering that we can get the accuracy we do.

[+Thomas D]

OK.. not that it helps the OP but,

 

some time, somewhere, in the past year, I read that one nanosecond of variation in signal timing equals 200 feet of error....

 

If true that kind of puts it in perspective but, is that anywhere near an accurate statement or not?

 

Was just wondering, someone else can do the math LOL! there seem to be some quite knowledgeable folks responding here.

[+Prime Suspect]

Old-tyme coders like me, all know the story of Admiral Grace Hopper and her "nanoseconds". Hopper was one of the developers of COBOL, the first real computer language. She also may have been the person who coined the phrase It's easier to ask forgiveness than it is to get permission. To give non-technical people an idea of the limitations they were working under, she needed a visual aid to represent the idea of a nanosecond. She had some pieces of wire cut to just under a foot's length. That's how far electricity travels in 1 nanosecond.

[+Thomas D]

Old-tyme coders like me, all know the story of Admiral Grace Hopper and her "nanoseconds". Hopper was one of the developers of COBOL, the first real computer language. She also may have been the person who coined the phrase It's easier to ask forgiveness than it is to get permission. To give non-technical people an idea of the limitations they were working under, she needed a visual aid to represent the idea of a nanosecond. She had some pieces of wire cut to just under a foot's length. That's how far electricity travels in 1 nanosecond.

 

OH WELL THEN that altogether different!!! That statment was WAY off base, thanks, wish I could remember where I heard it. So it will take an inordinately LONG 200 nanoseconds to introduce 200 feet of error!!! LOL! + or - the effect of... what the heck did they call that? The difference between the speed of light and the actual speed of radio signals.

 

It is truly amazing...

[+Lovey Pigs]

OK, all these replies and no mention of: GLOBAL Warming! can't we blame everything on global warming, esp. with GPS units??? Hmmm...

[+tomfuller & Quill]

As I wrote in the OTHER discussion (More accuracy problems). The best place I have found to answer all your questions about GPS accuracy, WAAS, CORS, DGPS, ephemeris from satellites etc. can be found at:

 

http://edu-observatory.org/gps/dgps.html The link by Dennis Milbert is very good.

 

tomfuller & Quill

[chuckr30]

The atmosphere also makes a difference. If you are getting signals from a SAT low in the horizon, the atmosphere can delay the signal from the SAT to the GPSr very slightly, but "slightly" means a lot when a signal is traveling at the speed of light. If you log the NMEA strings for 24 hours from your GPSr and plot the points in excel, you will see a great deal of variation from hour to hour. So if you think about it, it's amazing we can get the accuracy we have now.

 

AHA! So my theory was correct, the weather conditions (and atmosphere) DO make a difference in GPS accuracy.

[+NightPilot]

Yes, and that's what WAAS is designed to do - compensate for atmospheric conditions, among other things. If you have a good WAAS signal and are receiving corrections, then the atmospheric conditions have already been corrected for, and the effect is mostly nullified.