Can a single gps reciever get a RTK accuracy?

I am starting to study the RTK concept for GPS recievers. Apparently the high accuracy (centimeters) is achieved by using a different carrier wave than normal GPS and comparing this with a reference reciever in real time.

I have a slow moving rover with 0.1m/sec max speed which works in a limited open outside space and returns with given cycles to a base position for charging.

Here are the givens:

slow speed of movement of rover regular return to the same position for charging / syncing no need for absolute positions, only relative position to the base point (charging point)

Would it be possible to achieve good accuracy without the need of a second GPS reciever and cross communication? If the rover keeps track against the carrier wave all the time and makes small deltas in position, could it not work in isolation? In a manner, "surfing the wave" as it moves about, never taking so big steps that it loses position to another wave? It will also have the chance to re sync every time it is back in docking.

Many thanks for input from the GPS experts in the Arduino community

Obviously not, as there would be no need for RTK system, would it? Why not ultrasound for final approach? Much more accurate at small ranges than GPS.

Thanks for your reply,

I guess I need to study RTK fundamentals a bit more.

Ultrasound could be an option although it will require a bit more hardware than I was hoping for with the proposed idea.

An interesting and instructive experiment is to leave a GPS receiver out-of-doors for a day or so, and continuously log the reported position.

I've done this a few times (as receiver modules have improved), and find that even under the best conditions, the reported position wanders around in a seemingly random path, forming over time a blob of points a few meters in diameter. Since there is no reason to choose any one point over the other, your only option seems to be to average them together.

even under the best conditions, the reported position wanders around in a seemingly random path

This is done by design. Its to prevent civilian uses of GPS getting a completely accurate position.

Errors are deliberately introduced by the transmitting satellites for the signals that commercial receivers can decode.

The military use e.g. by the USA, doesn't have these deliberate errors and hence has a much more stable position.

You can remove the errors using Differential GPS, i.e where you have one receiver in a fixed location, (known not to move), which has a GPS receiver, and it retransmits the position error e.g. on 433Mhz to the device which is moving. The device which moves, then subtracts the error to give a position without the deliberate random position error

Of course if you want absolute position, the only way to do this is to leave the base station running for many hours, averaging out the position errors. But if you are only interested in the position of the moving device relative to a fixed position DGPS would work straight away.

Thank you both for a very good reply. As I understand it, the errors are contiuous and I need a live stationary reference to cancel them out.

My propsed concept requires that the errors are static or at least very slow in variation in order to work which I take it is not the case.

Thanks Jonas

The errors would even out if you averaged the position over a long enough time if you knew that the GPS receiver was definitely not moving.

DGPS removes the deliberate errors, any other errors will be factors like the clock precision inside the GPS module.

Looking online for RTK it looks like you'd need a special receiver to be able to get access to the phase information of the signal.

You may be able to do that using some sort of software defined radio, I've been messing around with those cheap USB TV dongles which there is a hacked driver to enable SDR, and someone has received GPS using a modified dongle with a replacement crystal reference, but whether even that sort of thing would give you the signal phase to the desired accuracy is debatable, but still probably worth a look

rogerClark:

even under the best conditions, the reported position wanders around in a seemingly random path

This is done by design. Its to prevent civilian uses of GPS getting a completely accurate position.

It sounds like you're talking about selective availability which did indeed introduce position error for non-military users. According to wikipedia, that 'feature' has been disabled since 2000. The variation in position observed is due to (among other things) atmospheric & space interference.

Agreed on Selective Availability. For comparison, the Russian GLONASS satellite system gives no better accuracy than the satellites lofted by the U.S., but I understand that combining the two (possible now with some GPS receivers) can lead to slightly improved accuracy. If anyone could develop a cheap reference system that would give cm accuracy, it would be unbelievably popular.

As it is, GPS can be nearly useless for amateur robot navigation - especially in the vicinity of buildings or other large objects. Signal reflections and other sources of interference can introduce erratic positional errors greater than 20 meters.

The "NavSpark" line of GPS receivers includes a model (~$80) that will output "raw" data that they've demonstrated to be capable of ~20cm accuracy when used in pairs. The NavSpark site includes a number of examples of how differential GPS is set up.

You'll also want to investigate "RTKLib" and its associated tutorials.

For someone with the time and dedication (and about $300) I don't think it's unreasonable to expect that ~20cm accuracy.

I don't think it's unreasonable to expect that ~20cm accuracy.

That is a remarkably cautious statement!

The Indiegogo site https://www.indiegogo.com/projects/navspark-arduino-compatible-with-gps-gnss-receiver seems rather unclear on just what one can expect from the NavSpark boards.

But the question remains. An independent GPS unit capable of cm accuracy and costing less than (for example) $500 would command an enormous market, so why isn't it here already?

jremington: An independent GPS unit capable of cm accuracy and costing less than (for example) $500 would command an enormous market, so why isn't it here already?

The core of GPS system is not capable of that kind of accuracy. There is Gallileo, however. http://en.wikipedia.org/wiki/Galileo_(satellite_navigation)

RTK is based on relative positioning, using carrier phase (fraction of 19cm) to compute position, instead of C/A code (fraction of 300m). So it’s only possible to get cm-level RTK accuracy using one receiver if you have another base station as reference providing data over internet or other wireless link. Otherwise very expensive multi-frequency multi-GNSS receiver that can determine its location precisely by itself after put at a fixed position surveying for a long time is not using RTK technique.

L1 GPS only receiver may have 70% availability having ambiguity fixed solution with RTKLIB under open sky over 12 hour observation for example (varies with location and setup). This is due to needing enough satellites with strong signal. More satellite constellation used seems to improve availability according to this: http://www.navspark.com.tw/blog/single-frequency-receiver-rtk-result-comparison

jremington: But the question remains. An independent GPS unit capable of cm accuracy and costing less than (for example) $500 would command an enormous market, so why isn't it here already?

I don't think the market is that enormous. For people around here, sure, I'm sure you could sell quite a few. But for the guy that wants it for something like car navigation no; like RH314 said you need that data stream from a second, fixed receiver so that would require (most practically) a cellular connection, and that service would get expensive for everyday use. And you don't need cm-level accuracy to find the nearest gas station.

But there is a market, and I think if you built a turnkey system using Navsparks for under $500 you'd get a lot of buyers. For all those companies in construction, farming, etc. using the expensive RTK systems that we've seen thus far you won't be likely to grab that market -- those existing, high prices imply guarantees from the respective manufacturers (Caterpillar, Deere, etc.) that it'll work right and that the integration is seamless. It's similar to how a company like HP can sell a 1 TB disk drive for $300 for a Proliant server; the buyer is expecting guarantees and is thus willing to pay 3x the street price for the product. That's not to say the buyer is making a wise choice, mind you, but that's how the market works.

jremington: An interesting and instructive experiment is to leave a GPS receiver out-of-doors for a day or so, and continuously log the reported position.

I've done this a few times (as receiver modules have improved), and find that even under the best conditions, the reported position wanders around in a seemingly random path, forming over time a blob of points a few meters in diameter. Since there is no reason to choose any one point over the other, your only option seems to be to average them together.

I was on my sail boat in the Caribbean just north of the dominican republic and within 5 minutes my GPS had me close to 150 miles away "the track on my chart plotter looked great" which showed me how not to count only on the GPS system.

regards Poppy Ann.