The code in this repo lets you turn an ESP32 development board, a GNSS (Global Navigation Satellite System) module, and an IMU (Inertial Measurement Unit) module into a device that emulates the function of a RaceBox Mini streaming performance telemetry meter. The official RaceBox app and other RaceBox-compatible tools should be able to connect to it over BLE (Bluetooth Low Energy) and read live position, speed, and motion data at or near 25Hz.
This is a low-cost, hackable platform for experimenting with GNSS and IMU data logging, the RaceBox BLE protocol, and sensor fusion built from off-the-shelf parts.
I originally started this project as a streaming GNSS+IMU telemetry source for use with the AutoX Data Logger for iOS app.
Important
Unofficial project. This is an independent, educational, and non-commercial implementation. It is not affiliated with, endorsed by, or supported by RaceBox. "RaceBox" and related marks belong to their respective owner. Use this code for learning and personal purposes only, and at your own risk. Do not use this code to impersonate a genuine device for any commercial or fraudulent purpose.
- Reads a live GNSS fix (position, altitude, speed, heading, accuracy, fix status, satellite count) from a u-blox GNSS receiver at up to 25 Hz.
- Reads acceleration and rotation from a 6-axis IMU at 100Hz with data smoothing, and optional gyro-bias calibration at startup.
- Packs the GNSS and IMU data into a RaceBox Data Message (a u-blox UBX-framed binary packet) and streams it over BLE to any RaceBox-compatible client at or near 25Hz.
- Advertises a BLE Device Information Service (model, serial, firmware, hardware, manufacturer) so official apps recognize and pair with it.
- Prints a human-readable serial status line at 1Hz for debugging: packet rate, GNSS data rate, satellite count, fix type, horizontal accuracy, position, and IMU values.
flowchart LR
GNSS["u-blox GNSS module"] -- "UART · 115200 baud" --> ESP32["ESP32"]
IMU["accel + gyro"] -- "I²C" --> ESP32
ESP32 -- "BLE notify · RaceBox UBX packets" --> App["RaceBox-compatible app"]
| Part | Notes |
|---|---|
| ESP32 dev board | Developed on an AITRIP ESP32-WROOM-32 Development Board. |
| u-blox GNSS module | A u-blox M10-class GNSS receiver. Reference unit: HGLRC M100-5883. Other u-blox modules supported by the SparkFun library should work. |
| IMU module | I²C 6-axis accelerometer + gyroscope breakout. Reference unit: HiLetgo GY-521 based on the InvenSense MPU-6050. |
| Project Box | ABS Plastic Project Case, White, 80x50x26mm. You'll need to cut holes into this box to fit your specific board and component layout (see images below). |
| Nylon M2.5 hex standoffs | Nylon hex standoffs, washers, nuts, screws, to help with positioning the components within the project box. |
GNSS module → ESP32 (UART, Serial2)
| GNSS pin | ESP32 pin |
|---|---|
| TX | GPIO16 (RX2) |
| RX | GPIO17 (TX2) |
| VCC | 3V3 |
| GND | GND |
IMU module → ESP32 (I²C)
| IMU pin | ESP32 pin |
|---|---|
| SDA | GPIO21 (default I²C SDA) |
| SCL | GPIO22 (default I²C SCL) |
| VCC | VIN (5V pin) |
| GND | GND |
Status LED: the onboard LED (GPIO2) blinks while waiting for a BLE connection and stays solid when a client is connected.
Pin assignments for the GNSS UART and the LED are configurable in
src/Gnimu/config.h. The IMU uses the ESP32's default I²C pins.
Photos of the reference build, from loose components to the finished, enclosed unit. Several shots show an RF shield fitted over the electronics — a hardware counterpart to the firmware's reduced BLE power that further isolates the GNSS receiver from radio noise (see A note on BLE power and GNSS lock).
The completed emulator, decorated with the stickers that came with the GNSS module
and an indicator of which end points forward (for Gyro/Accelerometer).
 Components and enclosure prior to assembly. |
 GNSS module with mounting hole in the lid. |
 Components wired together, using header pins underneath the ESP32 board. Extra unused pins were clipped off. |
 Assembled, using hardening epoxy putty to firmly affix the components. |
 RF shield test fit, not yet grounded or affixed. |
 RF shield grounded; I used two-sided tape to mount the shield. |
 Enclosure closed up, ready to test (before stickers!). |
 Connected to power, LEDs lit up. |
- Arduino IDE (2.x recommended).
- ESP32 board support — install the
esp32package by Espressif via the Boards Manager. - Libraries (install via Library Manager):
- Adafruit MPU6050 (pulls in Adafruit Unified Sensor + Adafruit BusIO)
- SparkFun u-blox GNSS Arduino Library
- BLE support is built into the ESP32 Arduino core — no extra install needed.
- Install the ESP32 board package and the libraries listed above.
- Open
src/Gnimu/Gnimu.ino. - Edit
src/Gnimu/config.h(at minimum, set yourDEVICE_ID). - Select your board (e.g. ESP32 Dev Module) and the correct serial port.
- Click Upload.
- Open the Serial Monitor at 115200 baud to watch the startup and status output.
Important
If you are building on an Apple Silicon Mac, you can use the AS-native Arduino IDE but you must have Rosetta installed in order to correctly compile the ESP32 binary. Without Rosetta installed you will get a compilation error.
All user-tunable settings live in src/Gnimu/config.h, grouped into sections. Highlights:
| Setting | Purpose |
|---|---|
DEVICE_ID |
10-digit device serial as a quoted string (e.g. "3608675309"). Validated at compile time: exactly 10 digits, first digit 0–3. |
MODEL, FIRMWARE_VERSION, HARDWARE_VERSION, MANUFACTURER |
Values reported via the BLE Device Information Service. |
GNSS_RX_PIN, GNSS_TX_PIN, ONBOARD_LED_PIN |
Hardware pin assignments. |
GNSS_BAUD, FACTORY_GNSS_BAUD |
Serial baud rates. On first boot the firmware can detect a module at its factory baud, switch it to GNSS_BAUD, and save the config to flash. |
MAX_NAVIGATION_RATE |
GNSS update rate in Hz (1–25). |
ENABLE_GNSS_* |
Per-constellation toggles (GPS, Galileo, GLONASS, BeiDou, QZSS, SBAS). Enable only what your module/region supports. |
GYRO_CALIBRATION_ENABLED, GYRO_CALIBRATION_SAMPLES |
Startup gyro-bias calibration — keep the device still during the first second of boot. |
ACCEL_ALPHA, GYRO_ALPHA |
IMU smoothing (exponential moving average) strength. |
BLE_TX_POWER |
BLE transmit power. Lowering this reduces RF interference with the GNSS front end and can noticeably improve satellite lock — see notes below. |
Several values are checked with static_assert at compile time, so an invalid configuration fails the build with a clear message instead of misbehaving on the device.
GNSS reception is sensitive to nearby RF noise. On compact builds, the ESP32's BLE radio can desensitize the GNSS receiver. Dialing BLE_TX_POWER down to a low level (the default is ESP_PWR_LVL_N12, the minimum) keeps the radio quiet — the receiver is usually close by, so high power isn't needed — and can dramatically improve fix quality, including indoors.
The RF shield shown in the build gallery is the hardware counterpart to this: a grounded metal enclosure over the GNSS module that physically blocks radio noise from reaching the GNSS receiver. The two measures stack — lowering the BLE power quiets the source, while the shield blocks whatever remains. Either helps on its own; together they give the most reliable lock.
With the BLE power level set to -12db, I have seen simultaneuous lock on as many as 20 satellites with horizontal accuracy (HAcc) as low as 220mm and pDOP values under 2 (really good for a cheap consumer-grade GNSS module).
- Power the assembled device and give the GNSS module time to acquire a fix (faster outdoors / near a window). The onboard LED blinks while unconnected.
- In the RaceBox app (or another RaceBox-compatible client), scan for and connect to the device — it advertises using the
MODEL+DEVICE_IDname. - On connect, the LED goes solid and the device begins streaming data packets.
- Optional: keep a serial monitor open at 115200 baud to watch live diagnostics.
| Symptom | Things to check |
|---|---|
Failed to find IMU module |
I²C wiring (SDA/SCL), 3V3 power, board address. |
u-blox GNSS not detected |
UART wiring (note TX↔RX crossover), GNSS_BAUD / FACTORY_GNSS_BAUD, module power. The sketch will attempt to auto-configure the baud rate. |
| Few or no satellites | Move outdoors / near a window; lower BLE_TX_POWER; give it a cold-start minute. |
| App won't connect | Confirm DEVICE_ID is valid (10 digits, first digit 0–3); make sure no other client is already connected. |
Build fails with a static_assert message |
Read the message — it names the offending config.h value and the allowed range. |
This project is a major evolution of earlier work by Anchit Chandra Sekhar. Changes in this version include bug fixes, externalized configuration, a BLE transmit-power control, startup gyro calibration, and modularization of the codebase.
Protocol details follow the RaceBox BLE Protocol Description (rev 8), available from RaceBox.
Released under the GNU General Public License v3.0 — see LICENSE.