[Android General] ronbo76:
Inherent GPS errors

I am not sure what the difference is between your three GPS receivers but will attempt to give you a small primer at no charge (normally I teach this course at $80USD per hour and the prep puts me at just about one hour).

A GPS at ground speed zero is at its most accurate receiving position based upon signal correlation. A minimum of three satellites are needed for 2D navigation. Four gives 3D SATNAV.

Layered with the US GPS system is an an inherent error that a fudge factor used by civil authority to prevent using our system to bomb accuracately our nation. At one time that error was over 250 feet but supposedly is now down to three feet or less. SUPPOSEDLY.

If you look at the first linked article, it mentions that the the best four satellite model positions are used to determine the stand alone GPS receiver position. If you think about those four models as four lines with all four lines directly intersecting in one perfect center position (picture this as a circle intersected by four lines forming a perfect eight pie segment with the interior angles all equal to one another) and you would be living in Fantasyland.

To go back to determing your position by shooting a sextant with four celestial bodys, one would mostly likely get a "H - cut". Three celestial bodies used to be considered optimum as if you got a somewhat perfect triangle, your position would most likely be in the center of the triangle.

To illustrate an H - cut, I take my students out to an empty quad and throw four marbles into the air. Where they fall might indicate our SATNAV position.

Now, I am being somewhat general as this would be at least the third hour of this Intro into SATNAV GPS, but it gives you a basic idea of what your GPS is up against in determing position at ground speed zero. Now, throw in errors caused by the receiver being in motion, time arrival error, atmospheric errors and datum based errors and you are adding positional error to the "basic 18 feet inherent error". You can read about these at GPS errors.

Glad you mentioned being in the city. There is a special error for http://en.wikipedia.org/wiki/Gotham_City or large cities like San Francisco. It's called multipath errors caused by the signals bouncing off buildings, blocked by terrain or absorbed by material. I defy anyone to get an accurate signal in dense cities like New York, London or Paris.

Errors can also be introduced by what software program you are using to determine your postion. The worst case IMHO would be to use cell towers combined with an online database. Next, the worst would be to used an out-dated program combined with out-dated maps. Good mapping software combined with a good stand alone GPS receiver are best IMHO.

To get back to my own direct experience, I have a great feel for how accurate my Haipai is. Attached to this is a screen shot of my current GPS position.

LEGEND:

The small blue dot shows exactly where I am. The blue triangle is where the GPS places me on the street.

The numbers in the corner are where the GPS sofware thinks I am (Red square).

The green square shows where I actually am physically.

My GPS program when used for driving thinks I am at the blue triangle which about 150 feet in error from where I actually am. Through a special code I inserted into the program, I can see the actual blue dot position where the GPS thinks I am. That is about 40 feet in error. Now, if you look at the address numbers that the program interprets me to be - that would be an error of approximately 240 feet from the street model positon (90 feet from where I actually am).

On the road I can watch the blue dot and see it "lag" due to signal error. But, it is still the most accurate position about 50 feet plus or minus. To sum this all up, based upon where I used to live and where I now live, I know what the inherent GPS errors are in my area and know that they are multiplied by whatever error the maps have in relation to the actual city or highway place I am at.