What is NTRIP?

NTRIP — Networked Transport of RTCM via Internet Protocol — is how GNSS correction data gets from a base station to your rover over the internet. It is what makes centimetre-level RTK possible with nothing more than a mobile data connection.

Last reviewed 2026-10-07.

Why corrections are needed

A GNSS receiver on its own is accurate to a few metres, mostly because of errors in satellite orbits and clocks and delays in the atmosphere. A base station at a precisely known position sees almost the same errors. If it sends its measurements to a nearby rover, the rover can cancel those errors out — that is RTK (Real-Time Kinematic) positioning. The data is sent in the RTCM format; NTRIP is the way it is carried over the internet.

The three parts of NTRIP

• NTRIP server — runs at the base station and pushes its RTCM stream to a caster.
• NTRIP caster — an internet server that receives many streams and hands them out. Each stream has a name called a mountpoint.
• NTRIP client — runs on or next to your rover, logs in to the caster, asks for one mountpoint and feeds the data to the receiver.

NTRIP was developed by Germany's Federal Agency for Cartography and Geodesy (BKG) and standardised by RTCM: version 1.0 in 2004 and version 2.0 in 2009. Version 1 looks like a simple HTTP request; version 2 is proper HTTP/1.1. Casters normally listen on port 2101.

The source table

Ask a caster for its root address and it answers with a source table: one line per stream. Stream lines start with STR and list, separated by semicolons, the mountpoint, an identifier (often the town), the data format (e.g. RTCM 3.2), the message types, the carrier phase, the satellite systems, the network, the country, latitude and longitude, whether the client must send its position (NMEA), whether it is a single-base or network solution, and whether access needs a login or a fee.

SurvSupport re-reads these source tables from public casters every 30 to 120 minutes and puts every stream with a position on the map. That is the whole trick: we don't relay any corrections, we just show you what is out there.

What's inside the stream

Modern streams use RTCM 3. The messages a rover cares about most:

• 1005 / 1006 — the base station's antenna position. Without it the rover can't use the rest.
• MSM4 or MSM7 observations — 1074/1077 for GPS, 1084/1087 for GLONASS, 1094/1097 for Galileo, 1124/1127 for BeiDou. MSM7 carries more detail, MSM4 is lighter.
• 1230 — GLONASS code-phase biases, needed to mix receivers from different manufacturers on GLONASS.
• 1033 — the receiver and antenna description of the base.

Single base vs network RTK

With a single base, errors grow with distance — roughly 1 mm extra per km — and ambiguity resolution gets slower, so most users stay within 20–30 km of the base. Network RTK services (VRS, MAC, FKP) combine many bases to model the errors across a region; the rover sends its approximate position (an NMEA GGA sentence) and gets corrections tailored to it. State networks like SAPOS work this way, which is why they publish few mountpoints but cover a whole state.

Where to get corrections

Paid networks from receiver manufacturers and national agencies cover most developed countries, but a lot is free: see free public NTRIP casters, then connect your rover.