In every GPS forum on the internet, the question is often asked, "How accurate is the model XYZ-999 GPS?" There's no simple answer. Or, actually, there are multiple answers. .

What aspect of the GPS do you want to measure?

It turns out that it's not terribly accurate. Most manufacturers will claim between 10 meters and 5 meters. I have occasionally seen as good as 2.5 meters with WAAS enabled.

So, what does that mean, really? Well, if the GPS reports that it currently has an accuracy of 7 meters, if you draw a circle with a diameter of 7 meters centered on the coordinates being reported, odds are that you are somewhere in that 7 meter circle. That's plenty good enough to help you find Ikea in a strange city, find the nearest donut shop or to get you home or back to the campsite. But it's not likely to get a survey engineer very excited.

This is as accurate as speed. Has to be, if you think about it. Speed is calculated by taking two readings, marking the location and time at each reading, computing the distance between the two locations and the time it took to travel that distance, then dividing the distance by the time to get the speed. So to have very high accuracy for speed, you must have equally high accuracy for the distance travelled. And for the time.

Which brings us to the final measurement of GPS accuracy:

As with the relative positional accuracy, this factor has to be very accurate to get the kind of speed accuracy claimed (and delivered). This is largely a function of the GPS satellites themselves, plus a little computation in the GPS receiver. Each GPS satellite has a cesium clock in it that is hugely accurate. Just one step short of the nuclear clocks like at Greenwich or Denver. It doesn't get much better than this. Definitely better than your average Timex (or Rolex, for that matter).

Y'er welcome.

...ken...

Good information. Thanks.

On a systemwide level, there are more active GPS satellites than required to operate the system. In addition to this active redundancy, there are also inactive spares that can be brought up if necessary.

This question will soon (within the next 10 years) become moot because we will have 4 independent satellite navigation constellations in place -- GPS (US), GLONASS (Russia), Galileo (EU), and Compass (China) -- and each of these systems will also have internal redundancy. Of course, it may take longer for consumer multisystem receivers to become widespread.

That scenario is also covered, multiple ways. Each satellite contains 3 cesium clocks. The timing signal that is transmitted is the result of a "majority rules" system. Simply, that means that if there is ever a discrepancy between the three clocks, the signal that at least two of the clocks agree on is the one that is transmitted. That triple redundancy in each satellite makes the system pretty reliable.

But if an entire satellite does go nuts and somehow starts to transmit junk, your GPS receiver will see a huge variance from that one signal compared to the others it's receiving and ignore it. No harm, no foul.

Tao also makes a good point about the number of satellites you are receiving from. There are situations where your accuracy will be much better with a good constellation of only three or four satellites than with a bad lineup of eight or nine. Your GPS receiver is trying to determine your location by triangulation. So the accuracy will be better if you have signals from a few satellites that are spread widely apart than from a bunch that are all in a neat line with each other.

...ken...

Why? For GPS the parameters that go into a fix a constantly changing:

1) Satellites are moving from fix to fix affecting the geometry of crossing lines of position. Additionally, the multipath environment is changing as the signals arrive from time varying directions. Reflected signals confuse the range to satellite calculation.

2) There is a basic satellite error caused by the the very small, but extremely significant, drift of its atomic frequency standard. That error combines with errors in the orbital parameters the receiver requires to calculate the satellite position and subsequent range. Each satellite has its own basic error that must be monitored by a network of ground stations and then corrected by the system operators on a daily basis.

3) Signal time transit through the ionosphere depends on 'space weather'.

4) Satellites are routinely are removed from service for maintenance or repositioning. Accuracy statistics can depend more upon which satellite is not available than the actual number available.

5) There is also a receiver implementation error. Some high quality sets are significantly better at rejecting multipath signals.

The attached scatter plot shows about twelve hours of fixes collected at my coffee table using a BU-353 without WAAS. Twenty nine satellites were set usable during the test. The constellation closely repeats itself over the earth every twelve hours.

You might see an error of over a hundred feet if you were unlucky enough to turn on the receiver during one of those excursions away from center zero but, in general for this particular sampling, one half of your fixes will be within ten feet of center zero.

--- CHAS

Quote:

Some high quality sets are significantly better at rejecting multipath signals. |

The main advantage of WAAS is that if your receiver can see one of the WAAS satellites it can download adjustment factors for those problems you mentioned. (I know you know that. I'm just adding it for those who read these threads to learn stuff.)

...ken...

Man, a ski hill out your back door and what looks like a pretty nice golf course about five minutes away. Life is tough!

...ken...

Quote:

Originally Posted by Ken in Regina Man, a ski hill out your back door and what looks like a pretty nice golf course about five minutes away. Life is tough! ...ken... |

The strategy is:

a) Use the first fix as a reference

b) Determine the scale of feet vs degree (WGS84 ellipsoid) at the reference fix

c) Plot successive fixes and calculate the mean center of the scatter plot; indicated by the + mark. As fixes are averaged that + approaches the true position.

d) Normalize the entire plot centering the + in the range circles; adjust the reference Lat and Lon by the amount of the plot centering shift to remove the bias.

I'm not sure how my methodology stands up to statistical rigor, but observations with Google Earth have consistently shown excellent congruence.

The histogram depicts range error counts at five foot intervals. You can intuitively judge how the process is progressing by its appearance. By the way, that's a Rayleigh Distribution.

The software reads NMEA GPRMC at 4800 baud. It's attached if anyone who would like to try it with no guarantees of performance.

--- CHAS

...ken...

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