Free RTK Corrections: State CORS Networks, RTK2go, and Public NTRIP Casters

An RTK receiver without corrections is just an expensive GPS. The corrections are what cancel atmospheric and satellite errors and take you from meters to centimeters, and while commercial correction subscriptions can cost more per year than a budget rover costs outright, a large and growing share of the United States is covered by correction streams that cost nothing. Here is where to find them and how to judge them.

NTRIP in one minute: casters, mountpoints, sourcetables

Corrections travel over the internet using NTRIP. Three terms cover almost everything you will encounter:

• Caster. A server that relays correction streams. Your field app connects to it with a hostname, port, and usually a username and password.
• Mountpoint. One named stream on that caster, typically corresponding to a single base station or a network solution. You pick the mountpoint appropriate for your location and receiver.
• Sourcetable. The caster's directory of its mountpoints, listing each stream's location, format, and constellations, which is how apps show you a pick list.

If the base/rover mechanics behind all this are fuzzy, our RTK GNSS explainer covers them from first principles.

State CORS networks: the best free option in 22 states

Many US state departments of transportation operate networks of Continuously Operating Reference Stations (CORS) to support their own construction and machine control, and open them to the public. As of mid-2026, 22 US states run free public RTK/CORS networks. Registration is usually just a form; you receive caster credentials and access to dozens or hundreds of professionally maintained stations, often with network (VRS-style) solutions that keep your effective baseline short anywhere in the state.

If you work in one of those states, this should be your default. These are permanent monuments on published coordinates, maintained by agencies with an institutional interest in uptime.

Other free casters

RTK2go

RTK2go is a community caster: anyone running a base station can push their stream to it, and anyone can connect. Coverage is a patchwork that reflects where hobbyists and small operators happen to live, and quality varies with the operator's antenna, mount stability, and how carefully they surveyed their base coordinate. It can be excellent; verify before you trust it.

Emlid Caster

Emlid runs a free caster intended primarily for users connecting their own base to their own rover over the internet, without running server infrastructure. It is a clean solution for the own-base workflow, and it is not limited to Emlid hardware.

GEODNET

GEODNET is a decentralized network of stations whose coverage has been expanding rapidly. It is another option worth checking for your area, particularly where state coverage is thin.

How to evaluate a free caster

Free does not mean interchangeable. Three questions determine whether a stream is usable for real work.

The datum question deserves emphasis because it produces the classic "everything is shifted a meter and a half" support ticket. ITRF2020 at the current epoch and NAD83(2011) epoch 2010.00, the frame most US plans use, differ by roughly 1 to 2 meters. If your corrections are in one frame and your project expects the other, every point carries that offset while looking centimeter-precise. This is also a moving target: the NATRF2022/NSRS modernization is landing in 2026. Know your stream's datum, and use software that stamps datum and epoch on every export rather than leaving it implicit.

Practical setup checklist

1. Find your source. Check whether your state runs a free CORS network; otherwise scan RTK2go and GEODNET coverage for a station near your work area, or stand up your own base via a free caster.
2. Register and save credentials. Hostname, port, mountpoint, username, password.
3. Confirm the datum of the mountpoint and how it maps to your project's coordinate system.
4. Connect and watch correction age. A healthy stream delivers corrections every second; if age climbs past a few seconds, your fix quality is quietly degrading.
5. Verify before you survey. Occupy a known point, or shoot the same point twice through separate fix acquisitions, and confirm the numbers agree.

The bottom line

Between 22 state networks, RTK2go, Emlid Caster, and GEODNET's growing footprint, many US users can run centimeter RTK with zero correction subscription, which pairs naturally with the sub-$1,000 hardware we cover in Budget RTK Receivers in 2026 and a phone as the controller, as described in our iPhone GPS accuracy guide. Open Survey is being built for exactly this workflow: an NTRIP client (v1 and v2) with auto-reconnect and a directory of free casters built in, so finding a stream is part of the app, not a research project.