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ertnode

ertnode is an application that performs the actual tracking: it should be run in a device attached to the object that needs to be tracked, e.g. a weather balloon or a vehicle. It collects telemetry data and takes photographs (currently only with Raspberry Pi camera) and transmits them using LoRa radio modulation to ertgateway receivers.

Features

The current design consists of concurrently running threads performing data collection and data transmissions simultaneously and independently. This design allows independent configuration of data collection and transmission intervals/schedules, so that the application may store more fine-grained telemetry and image data locally than what it transmits to receivers.

There are two data collection threads in the application:

  • Telemetry data collector, which logs the collected telemetry to local disk

  • Image capture routine, which takes photographs and saves them to local disk

The respective data transmission threads are:

  • Telemetry sender, serializing the latest piece of telemetry data to MsgPack format and transmitting via radio

  • Image sender, transmitting a resized thumbnail version of the latest image captured

The transmission threads may transmit data simultaneously, so packets containing telemetry and image data will be interleaved in the radio transmissions.

In order to improve chances for successfully received telemetry data messages, there are two types of messages sent by the telemetry sender: messages with full data and description strings and messages with only the GPS data and a couple of sensor readings without string descriptions. The former amounts currently to about 1000 bytes of data, which results in 4 LoRa packets to be transmitted, and the latter, abbreviated telemetry data message is designed to fit in one 251-byte packet payload. The possibility of successfully receiving one of these one-packet messages is significantly higher than data spanning multiple packets, especially when the received radio signal is very weak.

In addition to the tracking-related functionality, ertnode runs also the same web server as ertgateway, providing HTTP and WebSocket APIs to monitor and inspect the data it transmits in real time. The ertgateway-ui-web web UI can be used with ertnode, although it is mainly useful for testing and debugging purposes.

Hardware requirements

The minimum hardware requirements for running ertnode are:

  • A Raspberry Pi model A+, B+, Zero, 2B, 3B, 3B+ or 4B (any model with the 40-pin GPIO connector)

    • Other single-board computers can be used by implementing support for accessing GPIO pins and registering GPIO interrupts.

  • A GPS receiver supported by gpsd — any receiver outputting NMEA format data through serial port should work.

  • A Semtech SX127x / HopeRF RFM9xW LoRa transceiver connected to Raspberry Pi SPI port.

  • Optional: I2C sensors supported by RTIMULib, such as the ones in Raspberry Pi Sense HAT

  • Optional: A Raspberry Pi camera connected via the CSI port

    • Support for other cameras is very easy to implement

Installation

The installation instructions assume use of Raspberry Pi OS (formerly Raspbian). The current codebase has been tested on Raspberry Pi OS version 10.8, which is based on Debian 10.8 released on February 6th, 2021.

Dependency installation

Install library and tool dependencies:

apt-get install -y build-essential git cmake ntp gpsd libgps23 libgps-dev libyaml-0-2 libyaml-dev wiringpi
apt-get install -y libraspberrypi-bin webp imagemagick jq

Raspberry Pi configuration

Enable peripheral interfaces in Raspberry Pi by adding the following to /boot/config.txt:

# Enable I2C
dtparam=i2c_arm=on

# Enable SPI
dtparam=spi=on

# Enable SPI1 1CS (for the SPI1 bus)
dtoverlay=spi1-1cs

# Enable serial port UART for GPS
enable_uart=1

# Enable use of PPS time signal through GPIO (if exposed by GPS receiver)
dtoverlay=pps-gpio,gpiopin=4

# Enable Raspberry Pi camera
start_x=1

GPSd and NTP configuration

Configure GPSd to use Raspberry Pi internal serial port (assuming GPS is connected to it). Replace the contents of file /etc/default/gpsd with the following configuration:

# Default settings for the gpsd init script and the hotplug wrapper.

# Start the gpsd daemon automatically at boot time
START_DAEMON="true"

# Use USB hotplugging to add new USB devices automatically to the daemon
USBAUTO="true"

# Devices gpsd should collect to at boot time.
# They need to be read/writeable, either by user gpsd or the group dialout.
DEVICES="/dev/ttyAMA0"

# Other options you want to pass to gpsd
GPSD_OPTIONS="-n"

The time data and signals from a GPS receiver can be used to feed the NTP daemon, so that Raspberry Pi can keep correct time as long as it has GPS lock.

Some GPS receivers expose PPS signal, which can be fed to Raspberry Pi GPIO for extra accuracy. There is more information about using PPS signal on these sites. To use the PPS signal, an additional utility needs to be installed:

git clone https://github.com/flok99/rpi_gpio_ntpd.git
cd rpi_gpio_ntpd
make
sudo make install

Run the utility at boot time by adding the following to /etc/rc.local:

# Use GPIO18 (pin 12) for GPS PPS signal
/usr/local/bin/rpi_gpio_ntp -N 1 -g 18

Add the following configuration to /etc/ntp.conf:

# GPS Serial data reference
server 127.127.28.0 minpoll 4 maxpoll 4
fudge 127.127.28.0 time1 0.0 refid GPS

# GPS PPS reference
server 127.127.28.1 minpoll 4 maxpoll 4 prefer
fudge 127.127.28.1 refid PPS

Enable GPSd and NTP daemon by executing:

systemctl enable gpsd
systemctl start gpsd

systemctl enable ntp
systemctl start ntp

Reboot Raspberry Pi to make all config changes take effect.

Building ertnode

Check out source code and build it:

git clone https://github.com/mikaelnousiainen/ert.git
cd ert
git submodule update --init --recursive
cd ..
mkdir -p build/ertnode
cd build/ertnode
cmake ../../ert/ertnode
make

Configuring ertnode

Configure the application by editing ertnode.yaml in the build/ertnode directory.

Running ertnode

Run ertnode: (uses sudo with root privileges, which are needed for GPIO access)

./ertnode-start-dev.sh # Run on foreground

./ertnode-start.sh # Run as a background daemon