NTRIP Caster API Documentation

Overview

PrairieRTK operates an NTRIP (Networked Transport of RTCM via Internet Protocol) caster that distributes centimeter-level GNSS correction data across Alberta's base station network. The caster runs as a TCP server on port 2101 and supports up to 200 concurrent connections.

The system provides both single-base corrections (raw RTCM3 from physical stations) and Virtual Reference Station (VRS) corrections interpolated from the full network to your rover's exact position.

Protocol Support

PrairieRTK supports both NTRIP v1 and NTRIP v2 request formats.

Authentication

Rover clients authenticate via HTTP Basic Authentication. Encode your credentials as Base64(username:password) and include them in the Authorization header.

Request Format

GET /VRS_AB HTTP/1.1
Host: prairiertk.polsia.app
Authorization: Basic cm92ZXI6cnRrX3JvdmVy
Ntrip-Version: Ntrip/2.0
Ntrip-GGA: $GPGGA,142305.00,...*XX

Auth Modes

Error Responses

Source Table

Request the source table to discover available mountpoints. Send a GET / request (no mountpoint path) to the caster.

Request

# NTRIP v1
GET / HTTP/1.0
User-Agent: NTRIP MyClient/1.0

# NTRIP v2
GET / HTTP/1.1
Host: prairiertk.polsia.app
Ntrip-Version: Ntrip/2.0

Response

SOURCETABLE 200 OK
Content-Type: text/plain
Server: PrairieRTK-NTRIP/1.0
Content-Length: 432

STR;EDMO_RTCM3;Edmonton;RTCM 3.3;;2;GPS+GLO;PrairieRTK;CAN;53.5461;-113.4938;0;0;PrairieRTK;none;B;N;0;
STR;CALG_RTCM3;Calgary;RTCM 3.3;;2;GPS+GLO;PrairieRTK;CAN;51.0447;-114.0719;0;0;PrairieRTK;none;B;N;0;
STR;VRS_AB;VRS_AB;RTCM 3.3;;2;GPS+GLO;PrairieRTK;CAN;52.0000;-113.5000;0;0;PrairieRTK;none;B;N;0;
ENDSOURCETABLE

STR Record Fields

Mountpoints

PrairieRTK provides two types of mountpoints:

Physical Station Mountpoints

Direct RTCM3 streams from individual base stations. Each station broadcasts GPS and GLONASS observations. Best when your rover is within ~30 km of the station.

VRS Mountpoint

Rover Client Connection

To receive RTCM3 corrections as a rover client:

Step 1 — Connect via TCP

Open a TCP connection to the caster host on port 2101.

Step 2 — Send NTRIP Request

# v1 GET request
GET /VRS_AB HTTP/1.0
User-Agent: NTRIP MyRover/1.0
Authorization: Basic cm92ZXI6cnRrX3JvdmVy
Ntrip-GGA: $GPGGA,142305.00,5232.8086,N,11343.5180,W,4,12,0.8,857.0,M,0.0,M,,*5B

# v2 POST request (GGA in body)
POST /VRS_AB HTTP/1.1
Host: prairiertk.polsia.app
Ntrip-Version: Ntrip/2.0
Authorization: Basic cm92ZXI6cnRrX3JvdmVy

$GPGGA,142305.00,5232.8086,N,11343.5180,W,4,12,0.8,857.0,M,0.0,M,,*5B

Step 3 — Receive Corrections

On success, the caster responds with ICY 200 OK followed by a continuous binary RTCM3 stream. Each RTCM3 frame starts with the sync byte 0xD3, followed by a 10-bit length field, the message payload, and a 24-bit CRC.

Step 4 — Send Updated GGA (VRS)

When connected to a VRS mountpoint, periodically send updated GGA sentences as your rover moves. The caster re-computes corrections for the new position. Send the NMEA sentence as raw ASCII on the same TCP connection:

# Send updated position every 10-60 seconds
$GPGGA,142315.00,5232.8100,N,11343.5200,W,4,12,0.8,858.2,M,0.0,M,,*5A

The caster detects $GPGGA or $GNGGA prefixes and triggers new VRS corrections automatically.

Base Station Connection

To push RTCM3 observation data from a base station (NTRIP server/source), use the SOURCE command:

SOURCE <password> /<mountpoint>
Source-Agent: NTRIP MyBase/1.0

Example

SOURCE prairie2024 /EDMO_RTCM3
Source-Agent: NTRIP Trimble/5.0

On success, the caster responds with HTTP/1.1 200 OK. After that, send raw RTCM3 binary frames on the connection. The caster relays data to all subscribed rover clients on that mountpoint.

GGA Sentence Integration

NMEA GGA (Global Positioning System Fix Data) sentences tell the VRS engine where your rover is located. This is essential for VRS mountpoints — the caster interpolates corrections specific to your position.

Supported GGA Talker IDs

• $GPGGA — GPS only
• $GNGGA — Multi-GNSS (GPS + GLONASS + …)
• $GLGGA — GLONASS

GGA Field Format

$GPGGA,HHMMSS.ss,DDMM.MMMMM,N|S,DDDMM.MMMMM,E|W,Q,NN,H.H,AAA.A,M,GG.G,M,A.A,RRRR*CC

Field breakdown:
 1  HHMMSS.ss       UTC time
 2  DDMM.MMMMM     Latitude (degrees + minutes)
 3  N|S             North or South
 4  DDDMM.MMMMM    Longitude (degrees + minutes)
 5  E|W             East or West
 6  Q               Fix quality (1=GPS, 2=DGPS, 4=RTK fixed, 5=RTK float)
 7  NN              Number of satellites
 8  H.H             HDOP
 9  AAA.A           Altitude above MSL (meters)
10  M               Altitude units
11  GG.G            Geoid separation (meters)
12  M               Geoid units
13  A.A             Age of DGPS data
14  RRRR            Reference station ID
 *  CC              Checksum (XOR of chars between $ and *)

How to Include GGA

You can send your GGA position in two ways:

1. Ntrip-GGA header — Include Ntrip-GGA: $GPGGA,... as an HTTP header in your initial connection request.
2. In-band NMEA — After connection, send raw GGA sentences on the TCP stream. The caster detects lines starting with $GPGGA or $GNGGA and re-triggers VRS computation.

RTCM3 Message Types

PrairieRTK encodes and decodes the following RTCM3 message types:

RTCM3 Frame Structure

┌──────────┬────────────┬──────────────┬──────────┐
│ Preamble │   Length   │   Payload    │  CRC-24Q │
│  0xD3    │  10 bits   │  N bytes     │  3 bytes │
│  1 byte  │  2 bytes   │  (variable)  │          │
└──────────┴────────────┴──────────────┴──────────┘

Total frame size = 3 (header) + N (payload) + 3 (CRC) = N + 6 bytes

VRS Corrections

The Virtual Reference Station engine creates a synthetic base station at your rover's location by interpolating observations from the surrounding physical network.

How It Works

1. Rover sends its position via GGA sentence
2. VRS engine identifies the 3+ nearest base stations (configurable via minStations)
3. Ionospheric delays are interpolated using Inverse Distance Weighting (IDW)
4. Tropospheric delays are computed via the Saastamoinen model
5. Geometric corrections account for satellite angle differences between base and VRS
6. Synthetic observations are encoded as RTCM3 messages (1005 + 1004 + 1012) and sent to the rover

VRS Output Messages

When you connect to VRS_AB, you receive:

Requirements

• Minimum 3 active base stations within the network (default, configurable)
• Satellites must be above 10° elevation to be included
• Rover must provide a valid GGA position

Configuration

The caster reads from config/config.json on startup:

{
  "caster": {
    "port": 2101,              // TCP listen port
    "maxConnections": 200      // Max simultaneous clients
  },
  "vrs": {
    "updateInterval": 1000,    // VRS refresh interval (ms)
    "minStations": 3,          // Minimum bases for VRS computation
    "idwPower": 2              // IDW interpolation exponent
  },
  "logging": {
    "level": "info",           // error | warn | info | debug
    "directory": "logs"
  },
  "auth": {
    "usersFile": "config/users.json"
  }
}

User Configuration

Define users in config/users.json:

{
  "users": [
    { "username": "rover1", "password": "secure_pass" },
    { "username": "survey", "password": "another_pass" }
  ]
}

Station Registry

Base stations are registered in config/stations.json:

{
  "stations": [
    {
      "id": "EDMO",
      "name": "Edmonton",
      "lat": 53.5461,
      "lon": -113.4938,
      "height": 668.0,
      "mountpoint": "EDMO_RTCM3",
      "expected_messages": [1004, 1012, 1077, 1087, 1005],
      "active": true
    }
  ]
}

Client Configuration Examples

RTKLIB (str2str)

# Connect to VRS mountpoint
str2str -in ntrip://rover:rtk_rover@prairiertk.polsia.app:2101/VRS_AB \
  -out serial://ttyUSB0:115200

# Connect to single-base mountpoint
str2str -in ntrip://rover:rtk_rover@prairiertk.polsia.app:2101/EDMO_RTCM3 \
  -out serial://ttyUSB0:115200

RTKLIB (RTKNAVI)

Trimble / Spectra / TopCon Receivers

u-blox F9P (via NTRIP client)

# Using ntripclient (Linux)
ntripclient \
  -s prairiertk.polsia.app \
  -r 2101 \
  -m VRS_AB \
  -u rover \
  -p rtk_rover \
  -D /dev/ttyACM0 \
  -n "$GPGGA,120000.00,5232.8086,N,11343.5180,W,4,12,0.8,857.0,M,0.0,M,,*5B"

Node.js (programmatic)

const net = require('net');

const socket = net.createConnection({
  host: 'prairiertk.polsia.app',
  port: 2101
}, () => {
  const auth = Buffer.from('rover:rtk_rover').toString('base64');
  const gga = '$GPGGA,142305.00,5232.8086,N,11343.5180,W,4,12,0.8,857.0,M,0.0,M,,*5B';

  socket.write(
    `GET /VRS_AB HTTP/1.1\r\n` +
    `Host: prairiertk.polsia.app\r\n` +
    `Authorization: Basic ${auth}\r\n` +
    `Ntrip-Version: Ntrip/2.0\r\n` +
    `Ntrip-GGA: ${gga}\r\n\r\n`
  );
});

socket.on('data', (data) => {
  // First response: ICY 200 OK\r\n\r\n
  // Then: continuous RTCM3 binary frames
  console.log(`Received ${data.length} bytes`);
});

socket.on('error', (err) => console.error(err.message));

Python

import socket
import base64

host = 'prairiertk.polsia.app'
port = 2101
mount = 'VRS_AB'
auth = base64.b64encode(b'rover:rtk_rover').decode()
gga = '$GPGGA,142305.00,5232.8086,N,11343.5180,W,4,12,0.8,857.0,M,0.0,M,,*5B'

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((host, port))

request = (
    f'GET /{mount} HTTP/1.1\r\n'
    f'Host: {host}\r\n'
    f'Authorization: Basic {auth}\r\n'
    f'Ntrip-Version: Ntrip/2.0\r\n'
    f'Ntrip-GGA: {gga}\r\n\r\n'
)
s.send(request.encode())

# Read response header
header = s.recv(256)
print(header.decode())

# Read RTCM3 binary frames
while True:
    data = s.recv(4096)
    if not data:
        break
    print(f'Received {len(data)} bytes of RTCM3 data')