GPS accuracy

[Cougar]

I understand that the signals from the GPS satellite system are intentionally degraded by the Department of Defense for non-military or governmental uses. No complaint there, since they put up all the satellites and let everyone use the system for free.

So the non-military system is reported to have the following accuracy:

* 100 meter horizontal accuracy
* 156 meter vertical accuracy
* 340 nanoseconds time accuracy

I have a hand-held GPS device (which I haven't really used that much), and as I recall, it seemed to give me a fairly accurate velocity reading as I was walking or jogging, certainly accurate to a couple mph.

Therefore, the above-cited "100 meter horizontal accuracy" figure doesn't seem to square with the pretty darn accurate velocity reading I was getting from my little hand-held GPS.

My question is, how accurate are the velocity readings on a typical high-end but non-military GPS system? And perhaps more importantly in this case, what proof or authority can I refer to as to that level of accuracy?

[cjl]

I've typically seen much more accurate than 100 meter accuracy on the handheld GPS, as well as pretty good vertical accuracy too. When comparing with the actual topo data, the vertical is typically within 50 feet on mine, and horizontal positional is within 20-25 feet.

[NEOWatcher]

I understand that the signals from the GPS satellite system are intentionally degraded...
Therefore, the above-cited "100 meter horizontal accuracy" figure doesn't seem to square with the pretty darn accurate velocity reading I was getting from my little hand-held GPS.
It was my understanding that the intentional degradation was lifted a few years ago, or modified with a different type of signal, and the difference now between military and civilian is some sort of security to prevent false signals.

[01101001]

It was my understanding that the intentional degradation was lifted a few years ago, or modified with a different type of signal, and the difference now between military and civilian is some sort of security to prevent false signals.

Wikipedia: Global Positioning System :: Selective availability (http://en.wikipedia.org/wiki/Global_Positioning_System#Selective_availability)

GPS includes a (currently disabled) feature called Selective Availability (SA) that can introduce intentional, slowly changing random errors of up to a hundred meters (328 ft) into the publicly available navigation signals to confound, for example, guiding long range missiles to precise targets. [...] Prior to being turned off, SA typically added signal errors of up to about 10 meters (32 ft) horizontally and 30 meters (98 ft) vertically.

:: Accuracy and error sources (http://en.wikipedia.org/wiki/Global_Positioning_System#Accuracy_and_error_sourc es)

When [error sources] taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft).

[Torsten]

Yes, the error that was added to the signals under "selective availability" was set to zero in 2000.

Your GPS unit probably also benefits from having Wide Area Augmentation System (WAAS) capability, explained here (http://www8.garmin.com/aboutGPS/waas.html). It's quite a remarkable feature, and the improvement in positional accuracy that it allows cheap GPS units to achieve is impressive.

ETA: Oops, a little late, as usual. Bitman beat me to it.

[Larry Jacks]

Others have already correctly addressed the issue of GPS accuracy. Ordinary GPS units without WAAS are probably accurate to 10 meters or less in horizontal positioning. Their vertical accuracy isn't so great. However, WAAS greatly increases vertical accuracy, so much so that properly equipped and certified aircraft use GPS/WAAS receivers to shoot precision instrument approaches to a 200 foot (~60 meters) decision height. You couldn't do that with selective availability engaged.

My question is, how accurate are the velocity readings on a typical high-end but non-military GPS system? And perhaps more importantly in this case, what proof or authority can I refer to as to that level of accuracy?

From what I've seen with even low end GPS receivers, velocity readings are very accurate. I don't have an authority to cite for you but I use handheld GPS units while flying and I'm very happy with the results. I'd love to get a certified panel mounted GPS receiver with WAAS for my plane but the cheapest price (http://www.advancedaerotech.com/servlet/StoreFront) I've seen is $9500 installed. That's almost 1/3rd the price I paid for my plane and kind of hard to justify, despite being a terrific unit.

[Torsten]

Oh, I totally forgot to address this in my post above:

My question is, how accurate are the velocity readings on a typical high-end but non-military GPS system? And perhaps more importantly in this case, what proof or authority can I refer to as to that level of accuracy?

I have no authority to quote on this question, but a few months ago I did a little test with a Garmin e-trex (one of the cheapest units around) in which I had it connected to my laptop while I drove along a relatively level stretch of road with the cruise control on. I transferred the instantaneous speeds it recorded at one second intervals (to 0.1 km/h) to a spreadsheet and plotted them. They clearly showed the small changes in speed caused by the cruise control working. That is, they showed the variations in this homeostatic system. I thought it was pretty cool.

[orionjim]

...

I have a hand-held GPS device (which I haven't really used that much), and as I recall, it seemed to give me a fairly accurate velocity reading as I was walking or jogging, certainly accurate to a couple mph.

Therefore, the above-cited "100 meter horizontal accuracy" figure doesn't seem to square with the pretty darn accurate velocity reading I was getting from my little hand-held GPS.

If you understand how a Differential GPS works then you will understand why the velocity readings were so good. The accuracy of a GPS unit depends upon many things, but most of the variation comes from the satellite system itself. While it uses atomic clocks and sophisticated hardware to make corrections all of this reduces the errors we see to about 10 meters. But if you have two units close to each other the difference between the readings of the two will be very small. In other words most of the variation comes from the complex satellite GPS system itself.

A Differential GPS system has one fixed receiver at a known location and sends the signal it receives out with its location. The differential GPS picks up this signal and location and subtracts it’s location from the fixed unit and “walla” you can get an accuracy of a couple of centimeters.

When you are walking or driving and measuring velocity you are in effect using your GPS unit as a differential GPS system, since you are comparing the new reading with your old readings you are not seeing the total variation in the whole satellite system, only its bias, and the small amount of change in your unit and of course the distance you have moved.

Quote:
What proof or authority can I refer to as to that level of accuracy?

I can't answer this question, but if you read up on a differential GPS and think about what is going on I think you will be able to convince yourself the accuracy should be pretty good.

Jim

[nauthiz]

I used to work with non-military GPS equipment that could get sub-centimeter accuracy in good conditions (out in the open, no trees or buildings nearby) and sub-meter in more realistic conditions. On the system in question we used different (much more precise) equipment to get velocity readings, so I'm not sure what kind of data the GPS system could provide. I imagine with that kind of precision you could get pretty good instantaneous velocity readings though, somewhere in the neighborhood of +/- 0.1m/s.

Granted, that was survey-grade equipment, so it might be a bit more high-end than you were thinking.

[Larry Jacks]

In some places, GPS receivers are being used to monitor seismic activity. For example, there's the Southern California Integrated GPS Network (http://www.scign.org/). That requires not only very good horizontal resolution but also the ability to measure velocity with high accuracy.

[hhEb09'1]

In some places, GPS receivers are being used to monitor seismic activity. For example, there's the Southern California Integrated GPS Network (http://www.scign.org/). That requires not only very good horizontal resolution but also the ability to measure velocity with high accuracy.Even fifteen years ago, before selective availability was turned off, geophysicists would use integrated gps, where they could average out the SA and other systematic errors over time and measure 1000 km baselines down to the cm

[George]

Survey equipment uses GPS, also.

This site (http://www.stephensenv.com/pages/gps.html) gives a range of accuracies, depending on which system is used.

Different technologies with different accuracy are available for different applications. Hand held units costing from a few hundred to a few thousand dollars are available to the public, and with accuracies ranging from +/- 100 meters to +/- 5 meters, are suitable only for navigation and orienteering. The position of the unit is displayed "on-the-fly". Training and expertise required is minimal.

...

Real Time Kinematic (RTK) GPS, the positional data are displayed and recorded immediately. Subcentimeter to millimeter level accuracies (both horizontal and vertical) are obtainable with both single and dual frequency technologies.
To my knowledge, there is no consistent accuracy 3mm or better available. This is the accuracy range for conventional stringline control systems (eg curb machines and asphalt pavers w/o skis). Stringline set-up is costly and gps should be a good alternative once accuracy is achieved.

[Jeff Root]

Ten or fifteen years ago I heard that GPS was being used to
steer automated tractors in large farm fields with 8-inch accuracy.

-- Jeff, in Minneapolis

[Halcyon Dayz]

There is a way around the build-in inaccuracy.
But it doesn't work in real-time. It is used by the Netherlands Archaeology Service.
They record their measurements and later compare it with a recording made at a precisely known place at precisely the same time so all the deviations can be compensated for.

[sarongsong]

...if you read up on a differential GPS and think about what is going on I think you will be able to convince yourself the accuracy should be pretty good.Yup! http://www.bautforum.com/images/icons/icon10.gif...DGPS accuracy and integrity are better than GPS...Positions of 10 meters or better are achievable using DGPS (USCG signals) vs. 100 meters or better for GPS...
U.S. Coast Guard (http://www.navcen.uscg.gov/dgps/dgpsfaq.htm)

[KaiYeves]

So how do all of those geocachers find the hidden items?

[Cougar]

Thanks to everyone for all the excellent responses. I was fumbling around in the forest of GPS info on the web and should have known just to focus in on the wiki-tree :) , which had a lot of great info.

A Differential GPS system has one fixed receiver at a known location and sends the signal it receives out with its location. The differential GPS picks up this signal and location and subtracts it’s location from the fixed unit and “walla” you can get an accuracy of a couple of centimeters.... When you are walking or driving and measuring velocity you are in effect using your GPS unit as a differential GPS system, since you are comparing the new reading with your old readings....

This is pretty much exactly what I was looking for. Thanks, Orionjim. The system I am researching (not to acquire, but to... understand the accuracy) is a non-differential Automatic Vehicle Location system. From what you say, the velocity readings will still be extremely accurate.

[mugaliens]

I understand that the signals from the GPS satellite system are intentionally degraded by the Department of Defense for non-military or governmental uses.

The capability to degrade the non-encoded portion of the signal remains, although the system has not been degraded in more than a decade, primarily because it's in such widespread use by the aviation communitity. That was by order of President Clinton during his first term, and has stuck ever since.

So the non-military system is reported to have the following accuracy:

* 100 meter horizontal accuracy
* 156 meter vertical accuracy
* 340 nanoseconds time accuracy

That's with WAAS (Wide Area Augmentation System (http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System)). With WAAS, the accuracy is far greater. Most handhelds and car nav systems these days have WAAS.

My question is, how accurate are the velocity readings on a typical high-end but non-military GPS system? And perhaps more importantly in this case, what proof or authority can I refer to as to that level of accuracy?

Visit the article on WAAS (http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System) - it contains some detailed information on various accuracies, as well as a very good comparison of accuracies as compared to regular GPS and other navigation systems.

[Tog_]

Mine is the cheapest version of the e-Trex that was available in 1999 or so. I tested mine by marking a waypoint where I parked at home, then doing it again when I parked at work. I repeated this when I left for the day. After two weeks I had 20 way points for where I parked at my house, and 20 where I parked at work. I plotted these on a mapping program I had and found that I was within 10 meters horizontally, with an average of about 7. Oddly, nearly all of them had me to the north west of where the map said I should have been. The plots at work matched the ones at the house pretty closely, so I don't think there was a great deal of change over the 10 minute drive.

As for the question on geo-caching, the coords will get you close, then you look for something odd, like a pile of rocks, or sticks lined up to point at a part of the ground. Best way to do it is once you find where your unit thinks the right spot is, mark that spot, then walk in a spiral out looking for the likely place.

[mugaliens]

Geocaching with WAAS will almost always get you to within 1 meter.

Sort of cheating, in my book!

[01101001]

Geocaching with WAAS will almost always get you to within 1 meter.

Within 1 meter of the reading (or purported reading) the cacher's GPS gave?

[Cougar]

What about the time-step in typical GPS systems, or AVL systems in particular? How often are locational coordinates determined in order to calculate the change in distance so as to divide by the time-step and arrive at an instantaneous velocity? Is this modifiable by the outfit that acquires and uses the system?

[nauthiz]

There is a way around the build-in inaccuracy.
But it doesn't work in real-time. It is used by the Netherlands Archaeology Service.
They record their measurements and later compare it with a recording made at a precisely known place at precisely the same time so all the deviations can be compensated for.

What you're describing sounds like a differential GPS system. There are a number of services (such as OmniSTAR) that broadcast DGPS data over the airwaves so that the system can be used in real-time. Many countries also maintain their own DGPS systems.

[Halcyon Dayz]

There is a way around the build-in inaccuracy.
But it doesn't work in real-time. It is used by the Netherlands Archaeology Service.
They record their measurements and later compare it with a recording made at a precisely known place at precisely the same time so all the deviations can be compensated for.

What you're describing sounds like a differential GPS system. There are a number of services (such as OmniSTAR) that broadcast DGPS data over the airwaves so that the system can be used in real-time. Many countries also maintain their own DGPS systems.

I'm sure they've been using that since it became available.
For surveys you'd want to use what ever technology is the most accurate.

GPS was a revolutionary breakthrough, it made lots of things a lot easier to do.

[Torsten]

What about the time-step in typical GPS systems, or AVL systems in particular? How often are locational coordinates determined in order to calculate the change in distance so as to divide by the time-step and arrive at an instantaneous velocity? Is this modifiable by the outfit that acquires and uses the system?

Again, using the e-Trex as an example, the tracklog can be programmed to record position at one second intervals. For the little experiment I described above, I used a program called GPS Trackmaker (http://www.gpstm.com/index.php) to record the positions on the laptop as the e-Trex determined them, rather than as a subsequent download from its internal tracklog.

Regarding geocaching, I have used these cheap gps units to record the location of "pigtails" - steel pins stuck in the ground with a numbered plastic tag attached to them - that marked the centre of research plots established in clearcuts. I was able to find them three years later, but the subsequent navigation was usually off by a few metres. Nevertheless, in most cases it was close enough to see the pin immediately. Now and then I had to use the spiral search pattern described by Tog_, especially if a lot of brush had developed in the intervening years. And on one occasion, the spot the device led me to was more than 20 m from the actual plot. Fortunately, even after doing more than 500 of these installations, I remembered the area and realized where I ought to be looking.

[Torsten]

I'm sure they've been using that since it became available.


In the forest industry in BC, differential GPS has been used for many years for making maps of areas prior to being logged. Typically, boundaries and roads will be field-located and marked by one individual, and some time later when that phase is complete, another will follow along with a GPS device. These are high-end units, with an external antenna and large memories. The data are downloaded into a program that corrects the recorded locations against those that were recorded for a stationary receiver at the same time. We used to have to pay for the stationary receiver data by subscribing to a service, but it may be public domain now. So, this method does not work in real time (as you said), but it gives very accurate results. We also have a real time DGPS service available here that purports to have 1-2 metre accuracy.