I do caving. Sometimes, I do cave surveying. Underground we measure everything to the centimetre, but the whole survey hangs off one point on the surface, the cave entrance, and that point usually comes from a phone GPS that’s off by 5–10 metres.

So I built a cheap RTK rover: a Quectel LC29H GNSS module plus an ESP32 that talks to the phone over BLE, Bluetooth SPP or Wi‑Fi. You set it up in a small web UI, and it works with apps like SW Maps and QField. With an RTK fix it gets ~1 cm horizontally, and the parts cost about $80–140.

Then the next problem showed up: cave entrances like deep valleys with no mobile signal. So two more ESP32s became a LoRa bridge. One sits on the ridge with LTE and sends the corrections down by radio, the other serves them over Wi‑Fi at the entrance.

OSS-RTK -> https://github.com/infrat/oss-rtk