NTRIP Protocol

NTRIP (Networked Transport of RTCM via Internet Protocol) streams RTK correction data from reference stations to rovers over the internet. It is the standard mechanism for delivering RTCM3 corrections to the LC29H(BA) and LC29H(DA).

Protocol Overview

NTRIP is built on HTTP/1.0 over TCP with three roles:

Caster — Receives RTCM3 streams from base stations and redistributes them to clients. Typically listens on port 2101.
Server — A base station that pushes RTCM3 data to the caster.
Client — A rover that pulls RTCM3 data from a specific mount point and forwards it to the GNSS module over serial.

The client opens a TCP socket, sends an HTTP/1.0 GET request, and reads a continuous binary RTCM3 stream from the response body. No chunked encoding, no keep-alive, no TLS in the original spec (some modern casters support HTTPS).

NTRIP 2.0 adds the Ntrip-Version: Ntrip/2.0 header and uses standard HTTP response codes. NTRIP 1.0 uses ICY 200 OK as the success response.

Source Table Request

To discover available mount points:

GET / HTTP/1.0
Host: caster.example.com
User-Agent: NTRIP lc29h
Ntrip-Version: Ntrip/2.0

The caster responds with a source table. Stream entries use the STR prefix:

STR;mountpoint;identifier;format;format_details;carrier;nav_system;network;country;lat;lon;nmea;solution;generator;compression;auth;fee;bitrate;misc

Mount Point Connection

To connect to a specific mount point with Basic authentication:

GET /MOUNT_POINT HTTP/1.0
Host: caster.example.com
User-Agent: NTRIP lc29h
Authorization: Basic base64(username:password)
Ntrip-Version: Ntrip/2.0

On success, the caster responds with ICY 200 OK (NTRIP 1.0) or HTTP/1.1 200 OK (NTRIP 2.0), followed by continuous binary RTCM3 frames.

GGA Position for VRS Networks

Virtual Reference Station (VRS) networks require the rover’s approximate position to generate corrections. The client periodically sends GGA sentences to the caster:

$GPGGA,hhmmss.ss,ddmm.mmmmm,N,dddmm.mmmmm,E,q,ss,h.h,a.a,M,g.g,M,t.t,rrrr*hh

RTCM3 Binary Framing

RTCM3 messages arriving from the caster use binary framing:

D3 [reserved:6][length:10] [message_data] [CRC24Q:3]

Field	Bytes	Description
Preamble	1	`0xD3`
Reserved + Length	2	6 reserved bits + 10-bit data length (0–1023)
Message Data	variable	RTCM3 message payload
CRC	3	CRC-24Q integrity check

Common RTCM3 Messages

Message	Constellation	Description
1005	—	Base station ARP (ECEF position)
1074	GPS	MSM4 compact observables
1077	GPS	MSM7 full observables
1084	GLONASS	MSM4
1087	GLONASS	MSM7
1094	Galileo	MSM4
1097	Galileo	MSM7
1124	BeiDou	MSM4
1127	BeiDou	MSM7
1230	GLONASS	Code-phase bias

Data Flow

Base Station (BS variant)
    │ RTCM3 over serial
    ▼
NTRIP Server (STRSVR / rtkbase)
    │ TCP to port 2101
    ▼
NTRIP Caster (rtk2go.com)
    │ HTTP/1.0 response
    ▼
NTRIP Client (lc29h ntrip / QGNSS)
    │ RTCM3 over serial
    ▼
RTK Rover (DA/BA variant)

Connection Lifecycle

Resolve caster hostname to IP address
Open TCP connection to caster port (default 2101)
Send HTTP GET with mount point path and Basic auth
Read response status (ICY 200 OK or HTTP/1.1 200 OK)
Begin reading continuous RTCM3 binary stream
Forward each complete RTCM3 frame to the GNSS module via serial
Periodically send GGA position to caster (for VRS networks)
Monitor connection health — reconnect on timeout or TCP error
Close TCP socket when done

Timeout Recommendations

Parameter	Value	Notes
Connection timeout	10 seconds	TCP connect to caster
Response timeout	15 seconds	Waiting for `ICY 200 OK`
Data timeout	30 seconds	No RTCM3 data received
GGA send interval	10–60 seconds	Required for VRS
Reconnect backoff	5 seconds initial, exponential	Avoid overwhelming caster

Public Casters

Caster	Host	Port	Auth	Notes
rtk2go	rtk2go.com	2101	email (free)	Largest public caster
UNAVCO	caster.unavco.org	2101	varies	US academic network
HKGS	landsd-gncaster.realtime.data.gov.hk	2101	psi_user/psi	Hong Kong