I had a play with a few 1cm accurate GPS today. Almost every tractor and Header/Cotton picker on my Cousins farm has a survey grade, differential RTK GPS built in with auto steer (and much more!) No wonder the latest Cotton Picker cost a Million Dollars!!!!
There is no getting away from a decent sized antenna just yet:
It uses both GPS and Glosnas systems. here is the GPS sat display:
The Cotton Picker on auto steer following a plot within 1cm. They till, plant and harvest on the exact same track!
It is interesting that RTK GPS has been demonstrated now with small patch antennae like we use to give 2cm accuracy with a base station. Hold on to your hats people. the technology is advancing at a rapid rate! 
Select to expand quote
There is no getting away from a decent sized antenna just yet:
Should still fit on top of a helmet!
Does each farm have its own base station for differential gps or are they using a nearby public one? Do they have accelerometers as well? I think these things genrrally do a fair bit of time averaging and hook intp wheel speed and steering angle so they combine a bit of traditional dead reckoning nav and time averaged gps to get better accuracy than you could with a single reading. It would probably be way worse than 1cm if you took it off the tractor and strapped it on your back and did a speed run. Pretty cool tech and it gets better every year. Dont worry about antennas, back in 1990 when the first gpss hit the market there was all hype about the physics will never let the antennas get smaller than a softball. Then some clever guy invented fractal antennas and now they fit in a watch. It will be in toys soon....
There are base stations set up by the machinery companies dotted over the Darling Downs. The one in use here as a couple of KM away and has a radius of 10 KM. Since everyone uses them, there is almost complete coverage as I understand it. There are also a couple of different brands of GPS used. I don't know if they have separate base stations or use a common one.
As far as I could tell, these ones had that cm accuracy without any 'assistance' and there was no mention of accelerometers. The track lines are saved for the paddocks and the tractors follow them exactly. There was one screen that showed real time accuracy and it was varying between 0 and 1-2 cm. They can even offset a second planting 5 or 10cm next to a failed crop. I saw where Corn was planted over a hail damaged Cotton Crop that way. The badly damaged Cotton was killed off with herbicide and the Maize was planted a few CM offset over it so there was no need to mulch and cultivate again before planting. And at the other end of the 2km row it was still the same offset!
Apparently, the increased power of modern processors has helped the use of much smaller and 'noisier' antennae. The power allows solutions of ambiguities to be resolved much faster to a higher level of accuracy. This has allowed better than 2cm resolution from small patch antennae to be demonstrated in experimental conditions. They were confident that that could be applied to consumer devices in the near future and were aiming for RTK accuracy from tiny cellphone type antennae.
Trimble have built a RTK network in Perth, you can get a subscription to use it for $4000 /year.
Access to the commercial base stations can be costly. If the devices are designed properly, you can set up your own though with little more than another GPS device and a radio link. There are also a few free ones in various places, but they may be too far away for best accuracy.
I see Skytraq has recently released a 20 Hz chip which can be used in RTK mode at 10Hz using another S2525F8-RTK chip as the base station for centimetre accuracy.
On their Skytraq site they say you can get an evaluation board for $50 but I see Navspark is selling them for 3-6 times the price (depending on update rate)
Interesting!
I already received an offer for a different 40hz brand, but some Skytracks seems to support binary output also.
www.sparkfun.com/datasheets/GPS/Modules/AN0003_v1.4.14_FlashOnly.pdf
Binary + high hz
www.skytraq.com.tw/products/Venus816_PB_v1.pdf
Binary + high hz
www.skytraq.com.tw/products/Venus838FLPx_PB_v1.pdf
@1hz they are not that expensive, but @10hz they price is quite high (times 2!).
What I can't find on the site is if the relation is 1-on-1 or 1-to-many, second one could be funny with a solar powered design ;)
As far as I can see, the Skytack Binary output data does not include any sentence with Velocity accuracy estimate.
But it seems these GPS may be able to do RTK which could trump Doppler accuracy anyhow.
