What NTRIP actually delivers
NTRIP — Networked Transport of RTCM via Internet Protocol — is the cellular transport layer that carries GNSS corrections from a fixed reference (a base station, or a modeled network of bases) to the rover in your hand. The corrections themselves are RTCM data; NTRIP is just the postal service.
Strip away the acronyms and there are three pieces of equipment:
- Base / CORS. A GNSS receiver on a published coordinate, logging raw observations. State and federal CORS sites do this 24/7.
- Caster. The internet server that brokers between bases and rovers. The caster publishes a list of mountpoints — each one a stream of corrections tagged to a station or to a virtual reference station modeled at the rover's location.
- Rover. Your handheld receiver, authenticated as an NTRIP client. It POSTs its NMEA position up to the caster, pulls the chosen mountpoint's RTCM down, and folds the corrections into its on-board RTK engine.
From satellites to centimetres
The full path from a satellite signal to the fix on your controller is short on paper and full of latency budgets in practice.
Total round-trip from the satellite's broadcast to your rover's fix sits in the 300–1500 ms window in normal conditions. RTK's ambiguity solve degrades past about 2 seconds of correction age — longer than that and the receiver falls back to a float or DGPS solution, and your reported coordinates start to drift.
Integer ambiguities solved. Centimetre-grade work.
Free, paid, and GEODNET
There's no single national network. What's available where you work depends on who runs it — a state DOT, a university consortium, a private vendor, or the decentralized GEODNET token-incentivized mesh. Each type comes with different access, cost, and reliability tradeoffs.
| Network | Access | Coverage | Latency | When to use |
|---|---|---|---|---|
| State DOT / CORS | Free, account-gated | Patchy — depends on state staffing | Low (state-fiber backed) | Best when you live in a state that maintains its own network. Check the directory below. |
| Commercial (eGPS, KeyNet, etc.) | $60–$200 / mo per rover | Regional, dense base spacing | Low, with SLA | When uptime matters — construction layout, ALTA boundary — pay for the SLA. |
| GEODNET | Token-paid, ~$3/day pricing | Continental, growing fast | Variable by node density | Fills gaps in states without a free network. Plus you can host a base and earn back token. |
Find your state's NTRIP access
Tap any state to reach its RTK / CORS network signup or login portal.
States without a unified public network link to GEODNET, a nationwide RTK service. Links independently verified — if you find a broken one, let us know.
VRS, MAX, i-MAX, and nearest base
Once you're connected to a caster, the mountpoint name decides which math the network runs to build your corrections. The differences look academic on a screen but matter for initialization time and accuracy at distance from the nearest physical base.
| Mountpoint | What it sends | Strengths | Tradeoffs |
|---|---|---|---|
| Nearest single base | Streams RTCM from the physical CORS closest to your location | Within ~10 km of the base — fastest initialization | Accuracy and reliability fall off past ~30 km baseline |
| VRS (Virtual Reference Station) | Network synthesizes a base "at" your location from multiple real bases | Consistent accuracy anywhere inside the network polygon | Requires the rover to send NMEA GGA up; ~10 s longer to initialize |
| MAX / i-MAX | Sends master + auxiliary base data, rover does the network solve | Standardized, vendor-agnostic, robust to base outage | Slightly more bandwidth, slightly more rover CPU |
| FKP (Area Correction) | Single base plus area-correction parameters | Lower bandwidth than MAX | Less common in U.S. networks; older format |
When the fix won't hold
- No fix at all. Check the mountpoint name, the NTRIP username / password, and that the rover is sending an NMEA GGA up to the caster. Most VRS networks will silently refuse to stream until they see a position.
- Floats only. Sky obstruction or correction age. Walk to clear sky and watch the age-of-corrections value — if it's climbing past 5 seconds, the cellular path is failing, not the sky.
- Fix that drifts after minutes. Multipath. Move 2 metres laterally; the new geometry should re-solve. If it does, the original position was being reflected off a building or a vehicle.
- One satellite constellation only. Some older networks broadcast GPS-only corrections via MSG 1004; switch the rover and the mountpoint to a multi-constellation stream (MSG 1077 / 1087 / 1097 / 1127) to pull in GLONASS, Galileo, and BeiDou.
How well did it stick?
A quick 5-question check on NTRIP & RTK Corrections. See where you stand and what to review.