Precise GPS

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Dear All
I want to know if you know some way or module for achieving 1cm accuracy for GPS positioning with a reasonable cost,
Any Ideas are welcome

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How accurate do you get with differential GPS?

When GPS was first being trialled for automated landing systems I believe they used differential GPS. It involves setting up a second unit at a fixed, known location in the airport. It knows that the exact GPS coordinate should be and then it compares this to what is calculated from the received satellite signals. This may tell it the it's North by 5.2m and East by -0.7m so it transmits data to the device in the aircraft to say "readings in this vicinity are inaccurate by 5.2m N, -0.7m E" and the aircraft makes this adjustment to the signal it receives. I'm guessing Wikipedia knows more about this.

EDIT: it does...

http://en.wikipedia.org/wiki/Dif...

That says it improves accuracy from 15m to 10cm.

(I do wonder about how accurate you need things. I suppose if the aircraft wheels are 5.2m off to the left of runway centre line then maybe that's an issue (but how wide is a runway anyway)? But say you are trying to strike a target with a ballistic missile, if it lands 5.2 metres to the right does it actually matter?)

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If you wanted to map some city terrain for construction,then there is no missile! and 5.2M matters, you are in your neighbor house bedroom then!

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hehe As we say, "close only counts in horseshoes and nuclear bombs."

Long ago, I read that the designers figured a few hundred feet would be plenty accurate for civilian navigation, close enough to see your destination, and encrypted the more precise bit of time info so just our military could use it more precicely. (It's not encrypted now.) This looked rather foolish when geologists realized you don't need the encrypted data, but that by comparing the phase of the carrier waves, you could figure to within a few mm. I have no idea if there are any off-the-shelf gadgets that do this, but geologists use(d) this idea to monitor fault lines and the like.

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Land surveyors do use GPS. And, they do get better than 5m. How? Not certain but I have heard rumors that there is averaging over a period of time (some minutes, maybe longer). One of the companies that makes such equipment is Trimble. Maybe this page will lead you to some ideas:

http://www.trimble.com/survey/

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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P-Y codes (and M codes) are still encrypted. The C/A codes are not. However, sometime back in the late nineties, they turned off the Selective Availability -- a feature that allowed them to intentionally distort hte C/A signals, resulting in less positional precision.

Here's a neat picture of the event (turning it off): http://www.gps.gov/systems/gps/modernization/sa/data/timeline.gif

I guess it was in 2000...

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Quote:

If you wanted to map some city terrain for construction

Sounds like "differential" would work nicely for that but I guess you don't even need that. Just use relative (x,y) from some fixed datum. If you make all measurements from the south-west corner of the building plot (say) then the fact that both (x1,y1) and (x2,y2) are both off by (5.2, -0.7) shouldn't matter as long as the GPS error is consistent over a local vicinity (and I think it is).

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Surveyors use "real time kinetic" technology to get close to cm level accuracy with GPS. This approach compares the GPS carrier phases received at one location with the phase received at a reference location.

I'm not a surveyor, so I don't know if RTK can be achieved at "reasonable cost".

- S

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I just spoke with a fellow researcher in the lawn in front of our building while she was checking out a precision GPS. This is a "topcon" (brand) GPS and is the kind that surveyors commonly use.

This is a differential GPS. You have a master and up to several slaves. One mode is when the slaves measure their position relative to the position of the master. It takes about 15 minutes to reach cm level precision.

Another mode is cellular based. It uses a network of differential masters run by the state highway department. The network provides offset information of the GPS positions with respect to the "true grid". The units use this correction to estimate the true position of the device. Again, it takes 15 minutes or more. Longer measurement (in both cases) gives higher precision, up to a point.

It seems likely that, in both cases, simple averaging is used. That gets the noise level down and gives x,y,z numbers that can then be corrected based on the differential data.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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RTK ( real time kinematic) will get better than 2cm in the right conditions. You'll pay for the accuracy asceach receiver is over $10000usd each along with an antenna at over $1000 . For stand alone operation you need two receivers. These are what the surveyors use. In many places you can use one revceiver and subscribe to satellite corrections or a land based network via the internet.

The interesting business model most of the big GNSS companies use is for a particular model of receiver, the performance is based on the options you purchase. This is purely a magic value that unlocks the features. The base option level of the receiver might only be 1Hz and 2m resolution and cost only $1000 but add features and that same receiver can cost over $10000. The options are usually speed and accuracy.
These days GPS is not the only service, there is the Russian Glonass system and the European and Chinese systems are coming online. The more satellites you can see, the better the accuracy.

Note that commercial receivers are limited to maximum height and speed so you can't use them in rockets, missiles or supersonic aircraft.

When the low end receivers start to use the other satellite services, the accuracy will be better than 2m and better availability.

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The only high end manufacturer that i have found that has online pricing is javad.
www.javad.com

Read and weep!

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Thanks for your answers, I have found this page, But I do not understand what they have done,http://hacknmod.com/hack/diy-rea...

Do you have any Idea?

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That is just a news article. Did you notice that they link to the project site?

http://gpspp.sakura.ne.jp/rtklib...

As of January 15, 2018, Site fix-up work has begun! Now do your part and report any bugs or deficiencies here

No guarantees, but if we don't report problems they won't get much of  a chance to be fixed! Details/discussions at link given just above.

 

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It is all explained in the site linked. The rtklib is an open source
Ibrary to do the necessary computation. The commercial receivers do much the same with their on board processors. The next piece of the puzzle is the receiver. Most of the receivers mentioned are high end receivers and there's mention of a software defined radio. So its basically,doing all the stuff the commercial receivers do but open source.

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Kartman wrote:

Note that commercial receivers are limited to maximum height and speed so you can't use them in rockets, missiles or supersonic aircraft.

Only if they're produced in and exported from the US: the 515m/s and 18,000m limits are required to prevent them being classified as armaments.

The Ublox6 is capable of altitudes up to 50,000 metres. It's what we're using for our balloon rocket launch system (testing with a paper plane this weekend - those Americans are getting too close to our record!).

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Quote:
I do not understand what they have done,http://hacknmod.com/hack/diy-rea...

The details are in:

http://gpspp.sakura.ne.jp/paper2...

They use the ublox GPS chipset, in an undocumented mode, to get the carrier phase measurements. These measurements are processed by rtklib to do the real-time kinematic calculations.

- S

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might be worth mentioning http://www.gnss-sdr.org/

you can even use a DVB-T receiver based on the Realtek RTL2832U ($20 on dx.com) as an SDR receiver.

http://sdr.osmocom.org/trac/wiki...

it's cheap, not sure about DGPS support in that library though..

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