NTRIP and CORS explained: how network RTK corrections work
Key takeaways
- NTRIP is an internet protocol for streaming GNSS corrections, typically in RTCM format, from reference stations to rovers.
- A CORS network is a set of permanent GNSS reference stations whose data feeds network RTK correction services.
- An NTRIP system has three parts: servers that send data, a caster that distributes it and clients (rovers) that receive it.
- Network RTK methods such as virtual reference stations model errors across several stations, allowing longer distances than single-base RTK.
- Before choosing a service, check coverage, datum, mountpoint types, cost and the mobile signal across your work areas.
What is NTRIP?
NTRIP stands for Networked Transport of RTCM via Internet Protocol. It is an open protocol, based on HTTP, that streams GNSS correction data across the internet so a rover can perform RTK positioning without a radio link to a base station.
RTCM is the standard data format for GNSS corrections, defined by the Radio Technical Commission for Maritime Services. NTRIP is the delivery method; RTCM is the content. Most services today stream RTCM version 3 messages, including multi-signal messages (MSM) that carry data for all constellations.
Because NTRIP uses ordinary internet connections, a rover with 4G or a phone hotspot can receive corrections from a station tens of kilometres away. That is why NTRIP RTK has become the default workflow wherever mobile coverage and a correction service exist.
How does NTRIP work? Servers, casters and clients
NTRIP works through three components that pass correction data from reference stations to rovers. Understanding them makes troubleshooting much easier.
- NTRIP server: software at the reference station that pushes its correction stream to the caster.
- NTRIP caster: an internet server that receives streams from many stations and distributes them to authorised users. It publishes a list of available streams, called the sourcetable.
- NTRIP client: software in the rover or controller that connects to the caster, authenticates and receives the stream.
Each stream on a caster has a mountpoint, a name the client selects. A mountpoint might be a single station, the nearest station to the rover, or a computed network solution. Two protocol versions exist, NTRIP 1.0 and 2.0; most modern equipment supports both.
To connect, the rover needs the caster address, port (often 2101), mountpoint, username and password. For network solutions, the rover also sends its approximate position to the caster as an NMEA GGA message, so the caster can compute corrections for that location.
What is a CORS network?
A CORS network is a set of continuously operating reference stations: permanent GNSS receivers with geodetic antennas on stable monuments, logging and streaming data around the clock. Their coordinates are known precisely in a national or regional datum.
CORS stations serve two purposes. They provide real-time corrections through NTRIP services for RTK users, and they archive raw data for post-processing, geodetic research and datum maintenance. Many national mapping agencies run CORS networks; others are operated by regional governments, universities or private correction providers.
Most operators publish a CORS map showing station locations and coverage. Checking that map is the first step before buying a network rover, because station density determines the accuracy and reliability you can expect.
Network RTK GPS: single-base vs network solutions
Network RTK GPS uses several surrounding CORS stations together rather than relying on the nearest one. By modelling how atmospheric and orbit errors vary across the region, the network can correct a rover accurately even when it is far from any single station.
| Mountpoint type | How it works | Typical use |
|---|---|---|
| Single base | Corrections from one specific station | Work close to a known station; simple and transparent |
| Nearest base | Caster picks the closest station from the rover position | Mobile work across a region |
| Virtual reference station (VRS) | Network creates a virtual base near the rover | General network RTK with stations spaced tens of km apart |
| Master-auxiliary (MAC) | Data from a master station plus correction differences to auxiliaries | Network RTK where the rover applies the model |
| Area correction parameters (FKP) | Broadcast error gradients across an area | Older or one-way network systems |
With single-base RTK, accuracy degrades by about 1 ppm, or 1 mm per kilometre, and fixing slows over long baselines. Network solutions reduce this effect, which is why CORS stations in many networks are typically spaced several tens of kilometres apart.
GNSS correction services: types and costs
GNSS correction services range from free public networks to commercial subscriptions. Which one you can use depends entirely on where you work.
- Public government networks: some are free with registration; others charge a fee to recover costs.
- Commercial RTK correction service providers: subscription plans, often with wider coverage and support.
- Community and shared networks: stations contributed by users; coverage and reliability vary.
- Your own NTRIP base: a base receiver streaming to your own caster for a project or region.
- Satellite-delivered and PPP services: corrections broadcast by satellite or internet for areas without CORS; typically centimetre to decimetre accuracy after convergence.
Subscription prices vary widely by country, accuracy level and term, so compare plans locally. Also confirm which datum and epoch the network coordinates use, because RTK positions inherit the network's reference frame.
How to set up an NTRIP RTK rover
Setting up an NTRIP rover takes a few minutes once you have an account. The steps are similar on most receivers and controllers.
- Get credentials: register with a correction service and note the caster address, port, username and password.
- Connect to the internet: use the receiver's 4G modem, or the controller or phone data connection via Bluetooth or Wi-Fi.
- Configure the NTRIP client: enter the caster details and download the sourcetable.
- Choose a mountpoint: pick the network solution or nearest station that matches your receiver's signals and RTCM version.
- Send GGA if required: enable position reporting for network mountpoints.
- Wait for a fixed solution: check correction age, typically a second or two, and fixed status before measuring.
- Check a known point: measure a control mark to confirm datum, geoid model and coordinate settings.
Our guide to RTK base-rover vs network RTK helps you decide whether NTRIP or your own base suits a project. For background on the positioning method itself, see what RTK GPS is and how it works.
Common NTRIP problems and fixes
Most NTRIP problems are connection or configuration issues rather than GNSS issues. Work through them in order.
- Cannot connect: check the caster address, port, credentials and that mobile data is active.
- Connected but no fix: confirm the mountpoint supports your constellations and RTCM message types, and that GGA is being sent where required.
- Correction age climbing: mobile signal is weak or dropping; move to better coverage or use an external antenna.
- Fixed but coordinates offset: datum, epoch, geoid or projection settings do not match the project.
- Slow fixing far from stations: switch to a network mountpoint or a closer single base.
If mobile coverage is unreliable, a local base with a UHF radio avoids the dependency on the internet; see how to set up an RTK base station.
Choosing a network RTK rover
A good network RTK rover combines strong GNSS tracking with a dependable data connection. Built-in 4G simplifies the setup, while Bluetooth lets the rover use a controller's connection.
The Mars Geosystems Mini network RTK rover is a 409 g pocket CORS/NTRIP rover with a 1408-channel full-constellation engine, IMU tilt up to 60° and about 16 h working time. The 199 g P2 handheld GIS RTK rover works as a network RTK rover with 4G and Bluetooth for GIS and tracking. Both need a local CORS account. Where coverage is patchy, the E1 Lite GNSS receiver adds UHF radio with 3–15 km internal range depending on environment.
Equipment ships from one unit, with OEM options for distributors. Share your region and correction service to request a quote for network RTK rovers.
Frequently asked questions
What is an NTRIP caster?
An NTRIP caster is an internet server that collects GNSS correction streams from reference stations and distributes them to authorised rovers. It publishes a sourcetable listing available mountpoints. Rovers connect to the caster with an address, port, username and password, then select the mountpoint they want to receive corrections from.
What is the difference between NTRIP and CORS?
CORS refers to the permanent GNSS reference stations that measure satellite data continuously at known positions. NTRIP is the internet protocol used to deliver their correction data to rovers. In short, CORS stations generate the corrections and NTRIP carries them. A network RTK service usually combines both.
Is NTRIP free to use?
The NTRIP protocol itself is open and free. Access to correction data depends on the provider. Some government CORS networks offer free access after registration, others charge fees, and commercial services sell subscriptions. You also need a mobile data connection, which has its own cost.
How accurate is NTRIP RTK?
NTRIP RTK delivers the same accuracy as other RTK methods, typically about 1 to 2 cm horizontally and 2 to 3 cm vertically under good conditions. Accuracy depends on distance to stations, network quality, satellite visibility and correction latency. Network solutions reduce distance-dependent errors compared with a single distant base.
What is a mountpoint in NTRIP?
A mountpoint is the name of a specific correction stream on an NTRIP caster. It may represent one reference station, the nearest station to the rover, or a network solution such as a virtual reference station. Each mountpoint lists its format, message types and supported constellations in the caster's sourcetable.
Can I set up my own NTRIP service?
Yes. You can connect a GNSS base receiver to the internet and stream its corrections to an NTRIP caster, either your own server or a hosted one. Rovers then connect over mobile data. This gives a private correction service for a project area, though you must maintain the base and its known coordinates.
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