Concept · RTK Fundamentals
What is NTRIP and how does it work?
NTRIP is the internet protocol that delivers RTK correction data from reference stations to your rover over a mobile data connection. It replaced radio links as the dominant correction delivery method — and it is the reason a single receiver with a SIM card can achieve centimetre accuracy anywhere in a correction network's coverage area.
What NTRIP stands for
NTRIP stands for Networked Transport of RTCM via Internet Protocol. Unpacking the name tells you exactly what it does: it takes RTCM correction data — the standard format for RTK corrections — and delivers it over the internet using standard HTTP-based communication.
NTRIP was developed by the German Federal Agency for Cartography and Geodesy (BKG) in the early 2000s and became the global standard for internet-based RTK correction delivery. Today it is used by every major correction network, every modern RTK receiver and virtually every field survey software package.
2101
Standard TCP port
HTTP
Protocol base
RTCM3
Data format carried
<1 s
Correction latency
The three components
NTRIP has three distinct roles — caster, server and client — that together form the correction delivery chain. Understanding these helps you configure your equipment correctly and diagnose connection problems.
The NTRIP correction chain
Data flows left to right — from satellite to your receiver
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Reference station
NTRIP Server
📡
NTRIP Caster
Central hub
📱
Your device
NTRIP Client
NTRIP Server — the reference station side
The NTRIP Server is the software running at each physical reference station. It reads the raw GNSS observations from the receiver and pushes them continuously to the caster over the internet. A reference station broadcasts its data to the caster 24 hours a day, typically at 1-second intervals. You never interact with the server directly — it operates invisibly in the background.
NTRIP Caster — the central hub
The NTRIP Caster is the server infrastructure that receives data from all reference stations and distributes it to clients on request. It maintains a sourcetable — a list of all available correction streams (mountpoints) — and authenticates connecting clients. When you enter an NTRIP host address like ntrip.rtksub.com, you are connecting to a caster.
A caster can serve thousands of clients simultaneously from the same set of reference stations. This is why network correction services are economically viable — one physical infrastructure serves many subscribers.
NTRIP Client — your device
The NTRIP Client is the software on your side — built into Emlid Flow, SW Maps, Trimble Access, FieldGenius and every other modern field application. It connects to the caster, authenticates, selects a mountpoint and receives the correction stream. The client passes the incoming RTCM3 data to the receiver, which uses it to compute an RTK Fix.
How NTRIP works step by step
1
Reference stations observe satellites
Permanent GNSS receivers at fixed, precisely surveyed locations track all visible satellites continuously. They measure the difference between what the signals should look like at their known position and what they actually receive — this difference is the correction data.
2
Stations stream RTCM3 to the caster
Each reference station sends its correction data to the NTRIP caster in real time. The data is formatted as RTCM3 messages — the universal standard for RTK corrections. This stream flows continuously, every second of every day.
3
Your client connects and authenticates
Your field software (the NTRIP client) opens a TCP connection to the caster on port 2101. It sends an HTTP GET request including your username and password. The caster verifies your credentials and confirms the connection.
4
You select a mountpoint
The caster sends back a sourcetable listing all available correction streams. You select a mountpoint — the name of the specific correction stream you want. Your client sends the mountpoint name back to the caster, which begins streaming that correction data to you.
5
Corrections flow to your receiver
The caster streams RTCM3 correction messages continuously to your client. Your field software or NTRIP app passes these to your GNSS receiver — typically via Bluetooth, USB or directly over the network if the receiver has its own IP connection.
6
Your receiver computes RTK Fixed
The receiver combines the incoming corrections with its own satellite observations. It resolves carrier phase ambiguities — the mathematical step that unlocks centimetre accuracy — and outputs an RTK Fixed position. In good conditions this takes 10–60 seconds from the moment corrections begin flowing.
NTRIP is essentially streaming audio — but for position
A useful analogy: NTRIP works like an internet radio stream. The caster is the radio server. Your NTRIP client is the app on your phone. The mountpoint is the station you tune to. Corrections flow continuously as long as you are connected — and like a radio stream, a brief internet dropout interrupts it until the connection re-establishes.
NTRIP vs radio link
Before NTRIP became widespread, RTK corrections were delivered by UHF radio — a base station on site broadcast corrections to rovers within line-of-sight range. Radio is still used today, but NTRIP has become the dominant method for most professional applications.
UHF radio link
NTRIP
Range
2–10 km line of sight. Terrain and buildings block signal.
Unlimited within network coverage. Works across an entire country.
Infrastructure required
Base station, tripod, radio module, battery. 15–30 min setup per job.
One rover. Mobile data connection. No base station.
Works without internet
Yes — fully independent of internet or mobile coverage.
No — requires mobile data at the rover location.
Latency
<100 ms — very low latency, ideal for machine guidance.
<1 s over 4G — acceptable for all surveying applications.
Number of rovers served
Unlimited — radio broadcast reaches all rovers in range.
Each rover needs its own NTRIP connection and subscription.
Accuracy
Best within 10 km of base. Degrades at distance.
Consistent with VRS across the entire network coverage area.
Cost
High upfront hardware cost. No ongoing fees.
Low upfront (one receiver). Monthly subscription for corrections.
What you need to use NTRIP
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NTRIP-compatible receiver
Any modern multi-band RTK receiver. Emlid, Trimble, Leica, u-blox ZED-F9P and most others support NTRIP natively.
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NTRIP client software
Built into Emlid Flow, SW Maps, Trimble Access, FieldGenius, Lefebure and most field apps. You are unlikely to need a separate client.
🌐
Internet connection
Mobile data (4G or 3G) at the rover location. A few hundred kilobytes per hour — similar to a basic messaging app.
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NTRIP credentials
Host address, port (2101), mountpoint name, username and password from your correction service provider.
NTRIP data usage is very low
A typical NTRIP correction stream uses 50–200 KB per hour depending on the mountpoint and number of satellite constellations. Over a full 8-hour working day this is under 2 MB — negligible on any mobile data plan. NTRIP will not meaningfully affect your data allowance.
NTRIP v1 vs NTRIP v2
There are two versions of the NTRIP standard. Most users will never need to think about this distinction — your software handles it automatically — but it is useful to understand when troubleshooting.
- NTRIP v1 (2004) — the original standard. Uses basic HTTP 1.0. One-way communication: the caster sends corrections and receives nothing back from the client. Does not natively support GGA position transmission for VRS. Still widely supported by all servers and clients.
- NTRIP v2 (2007) — adds bidirectional communication over HTTP 1.1. Supports GGA transmission from client to caster within the protocol — this is how VRS mountpoints work. Supports chunked transfer encoding for more reliable streaming. Trimble Access automatically negotiates v2 if the server supports it.
Force v1 only if you have connection problems
Some older equipment or network configurations have problems with NTRIP v2 negotiation. If you cannot connect and everything else looks correct, try forcing your client to use NTRIP v1.0. In Trimble Access this is a checkbox in the survey style data link settings. In most other clients, look for an "NTRIP version" dropdown.
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