Turn an ESP32 with a GPS module into a GNSS RTK base station and NTRIP caster: it surveys in its own antenna position and serves RTCM 3.x corrections to rovers over the network. A second board runs the rover role — an NTRIP client that subscribes to the base and decodes its position stream.

This one sketch builds either role. Flip NTRIP_ROLE_BASE at the top:

#define NTRIP_ROLE_BASE 1 → base: survey-in + caster on port 2101, mountpoint BASE1.
#define NTRIP_ROLE_BASE 0 → rover: NTRIP client (set BASE_IP to the base board's IP).

Enable the feature for the whole build with -DPC_ENABLE_NTRIP_CASTER=1 (already in build_opt.h here for the Arduino IDE). It implies PC_ENABLE_NMEA0183 (the base surveys from GGA fixes). With PlatformIO:

pio ci examples/L7-Application/NtripCaster --board esp32dev --lib "." \

--project-option="build_flags=-DPC_ENABLE_NTRIP_CASTER=1"

Hardware

Two ESP32-S3 boards, each with a GT-U7 (u-blox 6/7-class) GPS:

GPS pin	ESP32-S3 pin	Notes
TX	GPIO18	GPS → board RX (Serial1 RX); NMEA at 9600 8N1
RX	GPIO17	GPS ← board TX (Serial1 TX)
PPS	GPIO4	1 pulse/second; counted via interrupt
VCC/GND	3V3 / GND

Run it

Flash board A with NTRIP_ROLE_BASE 1. Open Serial @ 115200. It connects to Wi-Fi, prints BASE IP: x.x.x.x, then surveys in — averaging GT-U7 fixes until it has SURVEY_MIN_OBS observations with a 3-D spread under SURVEY_ACC_LIMIT_M. It prints progress, then survey COMPLETE and begins broadcasting a 1005 station-reference message once per second.
Flash board B with NTRIP_ROLE_BASE 0 and BASE_IP set to board A's IP. It connects to the caster, subscribes to /BASE1, and prints each decoded frame:

stream started; decoding RTCM...

RTCM 1005 sta=2003 ECEF -2706196.881, -4261094.185, 3885757.343 m -> 37.7749000, -122.4194000 h=52.00 m [pps=42]

Inspect the caster with standard tools

The caster speaks NTRIP 1.0 and 2.0. From any machine on the network:

# Source table (list of mountpoints):
curl -s http://<BASE_IP>:2101/
 
# Subscribe to the RTCM stream (raw bytes; Ctrl-C to stop):
curl -s http://<BASE_IP>:2101/BASE1 | xxd | head

str2str (RTKLIB) and any NTRIP client (e.g. ntripclient, u-center) can also connect to <BASE_IP>:2101/BASE1.

Why both boards still won't get a cm-level RTK fix

RTK needs the base to broadcast observation messages (RTCM MSM: 1074/1077/1084/…), which are built from the receiver's raw carrier-phase / pseudorange measurements. The GT-U7 (like the NEO-6/7) does not output raw measurements (no UBX-RXM-RAWX), so it can only serve the surveyed reference point (1005/ 1006) — not the observations a rover needs to fix ambiguities. A true RTK base needs an F9P/M8T-class receiver; with one, the base would add MSM generation and the rover would compute a real fix.

What this example does prove, end to end on real hardware: survey-in position averaging → geodetic→ECEF → RTCM3 framing/CRC → NTRIP caster → NTRIP client → RTCM3 decode. The correction-transport pipeline is complete; only the raw-measurement source is missing.

Notes

PPS (GPIO4) is counted via a rising-edge interrupt and printed for observability. It marks the GPS second boundary and can gate precise survey sampling or a stratum-1 NTP tie-in (see NtpServer).
Security: the caster supports optional HTTP Basic auth per mountpoint — pass the base64 of user:pass as the third argument to pc_ntrip_caster_add_mount() (null = open access).
Tune SURVEY_MIN_OBS / SURVEY_ACC_LIMIT_M for how long/tight the survey-in must be before serving.