Coordinate Accuracy

[FandRM]

I have an inexpensive but adequate GPS and try to save the battery life. Sometimes I am right on target and sometimes I'm 50 feet off. Does it make a difference in accuracy whether I put in the coordinates at home then turn it off or put in the coordinates while riding in the car and keeping the GPS on. Also can the satellites be tracked with any accuracy while holding the GPS out the window at 60 MPH?

[+admo1972]

My knowledge of the satellites and such is a bit limited, but I'll tell you what I think.

 

Entering the coordinates at home and turning it off for a while is fine. You do get better accuracy if it is left on, but only to a point. Your GPSr will be ready to navigate once it has 4 satellites I believe. But, the longer it is on, the better chance you have of acquiring that 5th or 6th satellite which will improve your accuracy. Of cource, you can always lose satellite if it drops below the horizon, block by a cliff or tree cover, etc. I generally put in coordinates at home, and turn off the GPS if I know I won't refer to it for a while. If the cache is 2 miles down a trail, I'll turn it off for 10 minutes or so, then turn it on and leave it on for the rest of the hunt.

 

The speed at which you are moving should not affect your accuracy. The car itself may inhibit good, accurate results, via blocking a good portion of the sky, or have satellite signals bouncing off your car or other cars. But this may be minimal.

[+StarBrand]

Most modern GPSr units are capable of calculating your position many times per second. (assuming the unit's "battery saver" option is off). The speed you are going should have very little effect on the accuracy of any of those individual readings.

 

Should be equally accurate whether or not it is turned off after the coordinates are in or whether left on during the trip.

 

Do you use rechargeable batteries - niMh is the way to go.

[+Renegade Knight]

The coordinates you input are exact by nature. They only reference one spot. How accurate they are when they were taken is another thing.

 

When I'm driving I can overshoot my cache and have to come back. The GPS's position is accurate within the GPSs tolerances the moment it was calculated. However since you keep moving it's quicly wrong. The more so the faster you go.

[+Prime Suspect]

Does it make a difference in accuracy whether I put in the coordinates at home then turn it off or put in the coordinates while riding in the car and keeping the GPS on.

 

It's completely irrelevant where you are, or what you are doing. Entering the coordinates for a waypoint is similar to adding a number to your cell phone's contact list. It's going to work, whether or not you've got a signal. You only need signals for the actual navigation.

 

Also can the satellites be tracked with any accuracy while holding the GPS out the window at 60 MPH?

You're GPS doesn't "track" satellites. It's a passive listener, like the FM radio in your car. Your GPS knows the position of the satellites, not because it tracks them, but because the satellites report their location in their signals. And BTW, the satellites are already orbiting the earth at about 6,250 miles per hour.

[+admo1972]

It's a passive listener, like the FM radio in your car. Your GPS knows the position of the satellites, not because it tracks them, but because the satellites report their location in their signals. And BTW, the satellites are already orbiting the earth at about 6,250 miles per hour.

 

That's actually not true. The GPS knows the rough position of the satellites because in your GPSr has an almanac of which sats are in the sky at any moment. This is why it takes the GPS so long to acquire sats on it's first use, or if you travel hundreds of miles between uses.

[+LeonW]

It has been proven in these forums that it would be best to turn off your Battery Save mode when you need good accuracy like near to a cache.

 

Would you buy your girlfriend the cheapest tickets to a game; if not, don't skimp on your new "girlfriend" - give her the best - Lithium; and she will spoil you no end.

 

[+Prime Suspect]

It's a passive listener, like the FM radio in your car. Your GPS knows the position of the satellites, not because it tracks them, but because the satellites report their location in their signals. And BTW, the satellites are already orbiting the earth at about 6,250 miles per hour.

 

That's actually not true. The GPS knows the rough position of the satellites because in your GPSr has an almanac of which sats are in the sky at any moment. This is why it takes the GPS so long to acquire sats on it's first use, or if you travel hundreds of miles between uses.

Yeah, that's what I said. Where do you think the almanac data comes from?

Edited May 13, 2007 by Prime Suspect

[marnen]

It has been proven in these forums that it would be best to turn off your Battery Save mode when you need good accuracy like near to a cache.

I wonder. I've found all my caches in Battery Save mode.

Would you buy your girlfriend the cheapest tickets to a game; if not, don't skimp on your new "girlfriend" - give her the best - Lithium; and she will spoil you no end.

I use lithium batteries in my eTrex Legend, but not because I think the navigation's any better. The dadgum thing just eats battery power so fast that anything else doesn't last! Maybe I will try rechargeables at some point.

 

Edit: I did not type "dadgum". This language filter is a bit overzealous, methinks.

Edited May 13, 2007 by marnen

[marnen]

Yeah, that's what I said. Where do you think the almanac data comes from?

It has been my understanding that the almanac data is internal to the GPSr, based on its calculation of satellite orbits. That's why it can give approximate satellite positions even without a signal.

[+webscouter.]

It helps when you hold the GPS out the window if you make a wing out of it and pretend it is a really fast plane.

[+dcvchicago]

Also can the satellites be tracked with any accuracy while holding the GPS out the window at 60 MPH?

Don't know that I'd recommend doing that--too easy to lose the unit. If ypur unit supports an external antenna, you can get a Gilsson antenna for about $20 (check Amazon.com) that sticks magnetically to the roof of your car.

[+Prime Suspect]

Yeah, that's what I said. Where do you think the almanac data comes from?

It has been my understanding that the almanac data is internal to the GPSr, based on its calculation of satellite orbits. That's why it can give approximate satellite positions even without a signal.

No, the almanac data comes from the satellites. The GPS does retain it, which is why it doesn't have to wait for a complete almanac refresh (which can take up to 20 minutes) each time you turn it on.

[+trainlove]

If you have a Garmin and you have a software package called G7ToWin you can actually download to the PC the current almanac in your GPS and see that some satellites in it might have weeks old data while the satellites taht are up right now while you are geocaching are current. A full almanac recovery from the satellites requires a lot of time, but each particular satellite sends it's almanac information along with it's positioning info.

P.S. I have never seen satellite 15 and have had my GPS connected to a 12V Gel Cell Battery on my roof for periods of more than 24 hours.

[+Prime Suspect]

If you have a Garmin and you have a software package called G7ToWin you can actually download to the PC the current almanac in your GPS and see that some satellites in it might have weeks old data while the satellites taht are up right now while you are geocaching are current. A full almanac recovery from the satellites requires a lot of time, but each particular satellite sends it's almanac information along with it's positioning info.

P.S. I have never seen satellite 15 and have had my GPS connected to a 12V Gel Cell Battery on my roof for periods of more than 24 hours.

This seems to imply that each satellite sends out just its own almanac data. Each satellite sends out a complete almanac containing information for all satellites. The almanac data sent from the satellites may go months without changing.

 

The ephemeris data, however, is unique to each satellite, and is continually changing. That may be what you're looking at in the G7ToWin extraction file.

[+trainlove]

There are multiple series of data sent by the satellites. Each sends its position information "ephemeris" to GPS receivers and to the GPS uplink/downlink center for use in the WAAS system, one download takes maybe a minute. Each sends a full "almanac", a full download takes 12.5 minutes. Each sends its pseudo random signal with timing info "Coarse / Acquisition code" that we all use, one full length reception of this takes a millisecond. And each sends the "P signal" which the military and surveyors use, this signal takes a week to repeat.

 

Each time you turn on your GPS it thinks it knows where all the satellites are. It starts to receive signals from the satellites it thinks it can see and updates the position of those in addition to calculating your position, this is one reason your EPE goes down within the first few tens of seconds. While all this is done it tries to download the full almanac but hardly ever can do so unless you put your GPS on your roof and do not move for several minutes and several satellites have good signals.