What is GPS RTK?
Nate 
Where Do I Get RTCM Corrections?
There's the not-free but super easy solution, and there's the freeway.
Not-Free: Skylark
A company called SwiftNav offers a service called Skylark. As of writing, for $49 per month you will get corrections covering North America, Europe, and Asia Pacific. You'll be issued NTRIP credentials that can immediately be used with Lefebure, SW Maps, or any GIS app that supports NTRIP. One downside is that with a 'regional' provider such as Skylark the distance to the correction station may be larger than 10km. While we've always gotten an RTK fix, we often see accuracy of ~30mm instead of the 14mm when using our own GNSS reference station. Your mileage may vary.
Sort Of Free: Build You Own
You can build your own GNSS reference station. It takes some work, some hardware, and you'll need to stay1 within 10km, but there's no annual fee and you'll have control over your own system. Additionally, we get a very good reported horizontal accuracy of 14mm in RTK Fix mode.
1 As the distance to a reference station passes 10km the accuracy of the RTK fix increases by a few centimeters.
Free: UNAVCO
If you're lucky there's a station within 10km (6 miles) of you that is broadcasting RTCM 3.x data over the internet. We located one a little more than 10 km from SparkFun HQ that works really well.
Here’s a list of stations we’ve found. If there are more that you know of, please let us know:
- UNAVCO - Mostly USA
- NTRIP Listing - Global
- European Reference Stations
- RTK2go and SNIP - Runs a server where a few dozen stations can be accessed (see comments section)
Many of the stations that broadcast real time RTCM correction data require registration. It’s a wild hodge-podge of scientific and non-profit civil organizations across the globe. It feels very internet-circa-1995. If anyone has a more straightforward way of discovering and connecting to RTCM providers, please let us know in the comments section.
We recommend using RTKLIB to subscribe to the feed and output the stream over a serial port to a GPS RTK capable module. For the best accuracy, your GPS receiver will need to be within 10km (6 miles) of the broadcast station. If you are greater than 10km, the ZED-F9P can still create a location fixed but the accuracy is degraded and the receiver will output the following:
WARNING: DGNSS baseline big: 10km