What is the maximum Theoretical accuracy of GPS?
I was talking with a potential client, and they requested that we plot some points with GPS, with a maximum (or should that be minimum?) accuracy of 2 m.
This is an area with no WAAS, and I was under the impression that even in the best of conditions, a single gps point can be accurate up-to only 15 meters(Horizontal field). Is this correct?
What is the maximum theoretical accuracy of GPS without using WAAS or differential GPS?
It is receiver dependent of course, see what a low end gps provides in this thread http://gis.stackexchange.com/questions/12011/gps-garmin-in-field-data-collection/12092#12092
I'm not interested in any specific instrument. I'm concerned with the Theoretical Accuracy in Typical Environmental conditions.
@DanPatterson I think that particular instrument and readings are using WAAS, and are not pure Satellite based GPS positioning.
Not WAAS enabled @Devdatta, 7 year old Garmin Etrexs. Sometimes stationarity at a site yields better results for any GPS. The link was just food for thought.
How long are you willing to wait in the same location to collect a reading?
@Kirk It depends on the application. The plate tectonics guys measure to the millimeter by keeping a unit at a fixed location year round. Roughly speaking, there's an inverse square root law in play: to double the accuracy, you have to stand around four times as long. (This law really applies only for fairly long durations, because the errors have high temporal correlations.)
Accuracy describes how closely the calculated value compares to the known value. Precision is the measure of repeatability. http://www.spatial-ed.com/gps/gps-basics/133-effect-gps-accuracy.html
The United States government currently claims 4 meter RMS (7.8 meter 95% Confidence Interval) horizontal accuracy for civilian (SPS) GPS. Vertical accuracy is worse. Mind you, that's the minimum. Some devices/locations reliably (95% of the time or better) can get 3 meter accuracy. For a technical document on that specification you can go here.
For more general GPS accuracy information, head to GPS.gov's website. That website also includes data and information on WAAS-enabled systems and accuracy levels depending on location. It's a great resource.
Basically, you can't get 2 meter accuracy reliably without some form of correction.
Edit: Something else to contemplate is using a device that can communicate with both GPS and GLONASS satellites. I'm not aware of any accuracy articles or studies that combine both systems to improve accuracy, but at the very least, it increases the potential satellites that may be available at one particular location/time, especially near the poles.
I had looked at the 2008, SPS standard before, but it is very dense and a difficult read. I can see that they have mentioned 4m as the 95 percentile for the error; But I was rather hoping for some kind of formula or equation that proved it.
why do you insist on "theoretical" best performance, or want to see the math behind the answer above (esp after you say that the standard that lays it out is "dense and a difficult read")? Your potential client has a budget in mind which will determine what grade of GPS units you can take to the field. Your readings will not be any better than the vendor claims for the unit, and probably will be worse. As several answers/comments have noted, a consumer grade GPS will get you 10-15M. Without WAAS or differential GPS, you won't achieve your goal, regardless of the equations.
For clarity, the advertised 7.8m (95% ci) is of the *pseudorange* (satellite-to-receiver range), not necessarily user location accuracy. Though they are strongly related, they are not the same thing. The horizontal accuracy of a receiver is dependent on where the receiver is to all of the satellites, the angles to each satellite, etc.
@prosti - on handheld device, the major limitations are a) antenna length/quality and b) battery consumption tradeoffs in the circuitry.
This answer is pretty old (7 years!). So, can we say that we've improved in the accuracy for the civilian GPS? If yes, then what's the maximum accuracy we can reach?
GPS.gov's SPS performance doc hasn't changed since 2008. They make no greater claim today than they did 7 years ago. However, if you go read their website today, it does say that global average URE was 0.716 meters at 95% on May 11, 2016. But that was just one day. They also mention that they are working on an improvement to accuracy of the GPS system, but that it isn't implemented yet. Additionally, they mention that most smartphones can get around 4.9 meter accuracy. Either way, the answer to this question is still valid: You can't reliably get <2 meters without some form of correction.