GPS Beacon

About

This project turns the nRF52840 DK board into a Bluetooth Low Energy (BLE) beacon that broadcasts GPS coordinates supplied to it over a UART connection. The project was completed as an embedded C and Bluetooth coding exercise.

Usage

In order to use the GPS beacon, the nRF52840 development kit must be powered on and flashed with the firmware from this project (see Building for instructions on how to build the image and flash the board).

Serial interface

The UART interface on the board was chosen as the means for supplying GPS coordinates to the device. This is a convenient interface for both other MCUs and PCs, as a connection can be made via the exposed UART pins or via the onboard USB-to-UART converter.

The UART interface is configured to use the following settings:

• Baud rate: 115200
• Data bits: 8
• Parity: None
• Stop bits: 1
• Flow control: None

Once powered on, the GPS beacon will set up the UART and Bluetooth interfaces and transmit the following message over UART:

Boot complete

It will not start broadcasting the advertising data immediately, as the GPS coordinates need to be specified beforehand. In order to set and update the coordinates being broadcast, send the pos-set <latitude> <longitude> <altitude> followed by a newline character (\n) over the UART interface.

The parameters are as follows:

• latitude is the latitude in decimal degrees, represented as a floating-point number in the range [-90, 90]. Positive values are on the northern hemisphere, negative values on the southern hemisphere. For example, 38° 53′ 23″ N is expressed as 38.8897.
• longitude is the longitude in decimal degrees, represented as a floating-point number in the range [-180, 180]. Positive values are on the eastern hemisphere, negative values on the western hemisphere.
• altitude is the altitude in meters, represented as an integer in the range [-32768, 32767]. Positive values are above sea level, negative values below sea level.

Example command with response:

> pos-set 59.4367073 24.7428397 21
OK: Set position to 594367073, 247428397, 21

Once the coordinates have been set, you can send ad-start to start the flow of Bluetooth advertising data:

> ad-start
OK: Advertising started

Note that advertising cannot be started until the coordinates have been set. If you attempt to start advertising before setting the coordinates, the following error message will be returned:

> ad-start
ERROR: Failed to start advertising (code -1)

You may update the coordinates being transmitted at any time using pos-set. If you wish to deactivate the beacon, use the command ad-stop. You can also use help to list the commands supported by the device:

> help
Available commands:
  ad-start - Start advertising
  ad-stop - Stop advertising
  help - Print this help message
  pos-set lat lon alt - Set latitude, longitude and altitude

Bluetooth behaviour

Once the coordinates have been set and advertising has been started, the device will start broadcasting custom advertising data packets. nRF Connect is a useful mobile app that can be used to monitor the advertising data packets and verify the correct operation of the beacon.

Given example coordinates from the Serial interface section above, the beacon will appear as follows in nRF Connect:

The device is a BLE Broadcaster that does not accept connections. It transmits the supplied GPS coordinates in the manufacturer-specific data section of the advertising data packet. In this example, the data payload is:

23 6D 52 61	0E BF 75 2D	00 15
Latitude	Longitude	Altitude

Decoding the data:

• Latitude: transmitted as a 32-bit signed integer in units of 1/10,000,000 degrees. This yields a worst-case resolution of 11.1 mm at the equator (calculation).
  0x236D5261 → 594367073 → 59.4367073 N
• Longitude: transmitted as a 32-bit signed integer in units of 1/10,000,000 degrees like the latitude.
  0x0EBF752D → 247428397 → 24.7428397 E
• Altitude: transmitted as a 16-bit signed integer in units of 1 meter.
  0x0015 → 21 → 21 m

Development

This section contains information relevant to developers that wish to modify the project.

Prerequisites

The project has been developed using the nRF Connect SDK which is the SDK that Nordic Semiconductor recommend for new projects using their hardware going forward.

In order to set up a development environment, the below software is required. The Get started page by Nordic Semiconductor has the relevant download links.

• nRF Connect for Desktop
• nRF Command Line Tools
• Toolchain Manager (installed via nRF Connect for Desktop)
• nRF Connect SDK v2.2.0 (installed via Toolchain Manager)
• Optional: Visual Studio Code + nRF Connect extensions (for ease of development; installed via Toolchain Manager)

Building

It is easiest to develop and build the project using VS Code. See: Instructions.

If you choose to use a different IDE, you may follow the Building on the command line instructions in the nRF Connect SDK documentation to use the west tool for building and flashing the board.

Build command:

west build --build-dir build --board nrf52840dk_nrf52840

Flash to the board connected via USB:

west flash --erase