Interfacing Ublox GPS M8N with Raspberry Pi

Ublox GPS M8N module support UART communication and its fairly integrate with your Raspberry Pi. Just connecting to power and Rx wires will allow you to read the GPS NMEA messages. However the module is much more capable than just sending messages in raw text format. The module supports lot of configurations and binary mode which is much easier to handle.

Wiring:

In my case, I was using Tx/Rx wires for another module so I ran out of UART ports in my Raspberry Pi Zero. As the Zero has only one UART pair and also does not have an inbuilt USB hub, I decided to use the micro USB port and hard wired a USB UART module directly to the Pi Zero board.

Code:

The GPS Class in my project imports the ublox python module which is available here. This module allows us to configure to work in binary mode and give use the function to configure frequency of various messages. You can configure dynamic model which is very essential for my project as altitude calculation is more important for my project.

#!/usr/bin/env python3

import logging, math
from ublox import *
from threading import Thread

class GPSModule(Thread):
    gps = None
    latitude = 0.0
    longitude = 0.0
    altitude = 0.0
    fix_status = 0
    satellites = 0
    healthy = True
    onHighAltitude = False

    def __init__(self, portname="/dev/ttyUSB0", timeout=2, baudrate=9600):
        logging.getLogger("HABControl")
        logging.info('Initialising GPS Module')
        try:
            self.gps = UBlox(port=portname, timeout=timeout, baudrate=baudrate)
            self.gps.set_binary()
            self.gps.configure_poll_port()
            self.gps.configure_solution_rate(rate_ms=1000)
            self.gps.set_preferred_dynamic_model(DYNAMIC_MODEL_PEDESTRIAN)
            self.gps.configure_message_rate(CLASS_NAV, MSG_NAV_POSLLH, 1)
            self.gps.configure_message_rate(CLASS_NAV, MSG_NAV_SOL, 1)

            Thread.__init__(self)
            self.healthy = True
            self.start()
        except Exception as e:
            logging.error('Unable to initialise GPS: %s' % str(e), exc_info=True)
            self.gps = None
            self.healthy = False

    def run(self):
        while self.healthy:
            self.readData()
            time.sleep(1.0)

    def checkPressure(self, pressure):
        alt = 0.0
        if pressure is not 0:
            alt = 44330.0 * (1.0 - math.pow(pressure / 1013.25, 0.1903))
        self.checkAltitude(alt)

    def checkAltitude(self, altitude):
        if altitude is not 0:
            if altitude > 9000 and not self.onHighAltitude:
                self.onHighAltitude = True
                self.gps.set_preferred_dynamic_model(DYNAMIC_MODEL_AIRBORNE1G)
            if self.onHighAltitude and altitude < 8000:
                self.onHighAltitude = False
                self.gps.set_preferred_dynamic_model(DYNAMIC_MODEL_PEDESTRIAN)

    def readData(self):
        try:
            msg = self.gps.receive_message()
    
            if msg is not None:
                logging.debug(msg)
                if msg.name() == "NAV_SOL":
                    msg.unpack()
                    self.satellites = msg.numSV
                    self.fix_status = msg.gpsFix
                elif msg.name() == "NAV_POSLLH":
                    msg.unpack()
                    self.latitude = msg.Latitude * 1e-7
                    self.longitude = msg.Longitude * 1e-7
                    self.altitude = msg.hMSL / 1000.0

                    if self.altitude < 0.0:
                        self.altitude = 0.0
        except Exception as e:
            logging.error("Unable to read from GPS Chip - %s" % str(e), exc_info=True)
            self.healthy = False

    def close(self):
        logging.info("Closing GPS Module object")
        self.healthy = False
        self.gps.close()
        self.gps = None

The python class has been tested for various scenarios for many days continuously. Even the model switching works fine realtime and does not need reset. Please feel free to modify or improve the class for your use case. The entire project source is available at my github project.