usage: ./udpgpstestpy [--noheading] --ip 127.0.0.1 --port 14401

udpgpstest.py

#!/usr/bin/python

import time
import socket
import argparse
from datetime import datetime
import operator
from functools import reduce
from math import floor

parser = argparse.ArgumentParser(description="Driver for the Water Linked Underwater GPS system.")
parser.add_argument('--ip', action="store", type=str, default="127.0.0.1", help="remote ip to query on.")
parser.add_argument('--port', action="store", type=str, default="14401", help="remote port to query on.")
parser.add_argument('--noheading', dest='noHeading', action='store_true', help="do not sent heading")
args = parser.parse_args()

# Nmea messages templates
# https://www.trimble.com/oem_receiverhelp/v4.44/en/NMEA-0183messages_GGA.html
gpgga = ("$GPGGA,"                                    # Message ID
       + "{hours:02d}{minutes:02d}{seconds:02.4f},"   # UTC Time
       + "{lat:02.0f}{latmin:02.6f},"                 # Latitude (degrees + minutes)
       + "{latdir},"                                  # Latitude direction (N/S)
       + "{lon:03.0f}{lonmin:02.6f},"                  # Longitude (degrees + minutes)
       + "{londir},"                                  # Longitude direction (W/E)
       + "1,"                                         # Fix? (0-5)
       + "06,"                                        # Number of Satellites
       + "1.2,"                                       # HDOP
       + "0,"                                         # MSL altitude
       + "M,"                                         # MSL altitude unit (Meters)
       + "0,"                                         # Geoid separation
       + "M,"                                         # Geoid separation unit (Meters)
       + "00,"                                        # Age of differential GPS data, N/A
       + "0000"                                       # Age of differential GPS data, N/A
       + "*67")                                       # Checksum

gpzda =  "$GPZDA,{hours:02d}{minutes:02d}{seconds:02.4f},04,07,2018,00,00*60"


# https://www.trimble.com/oem_receiverhelp/v4.44/en/NMEA-0183messages_RMC.html
gprmc = ("$GPRMC,"                                  # Message ID
       + "{hours:02d}{minutes:02d}{seconds:02.4f}," # UTC Time
       + "A,"                                       # Status A=active or V=void
       + "{lat:02.0f}{latmin:02.6f},"               # Latitude (degrees + minutes)
       + "{latdir},"                                # Latitude direction (N/S)
       + "{lon:03.0f}{lonmin:02.6f},"                # Longitude (degrees + minutes)
       + "{londir},"                                # Longitude direction (W/E)
       + "0.0,"                                    # Speed over the ground in knots
       + "{orientation:03.2f},"                     # Track angle in degrees
       + "{date},"                                  # Date
       + ","                                        # Magnetic variation in degrees
       + ","                                        # Magnetic variation direction
       + "A"                                        # A=autonomous, D=differential,
                                                    # E=Estimated, N=not valid, S=Simulator.
       + "*76")                                     # Checksum

def fixChecksum(string):
    """
    Fixes checksum of an Nmea string
    """
    data, checksum = string.split("*")
    original_checksum = int(checksum.strip().lower(), 16)
    calculated_checksum = reduce(operator.xor, bytearray(data[1:], encoding="utf-8"), 0)
    return (data + "*%02x\r\n" % calculated_checksum).upper()

def format(message, now=0, lat=0, lon=0, orientation=0):
    """
    Formats data into nmea message
    """
    now = datetime.now()
    latdir = "N" if lat > 0 else "S"
    londir = "E" if lon > 0 else "W"
    lat = abs(lat)
    lon = abs(lon)
    print (lat, (lat % 1.0) * 60.0)
    msg = message.format(date=now.strftime("%d%m%y"),
                       hours=now.hour,
                       minutes=now.minute,
                       seconds=(now.second + now.microsecond/1000000.0),
                       lat=floor(lat),
                       latmin=(lat % 1.0) * 60.0,
                       latdir=latdir,
                       lon=floor(lon),
                       lonmin=(lon % 1.0) * 60.0,
                       londir=londir,
                       orientation=orientation)
    
    return fixChecksum(msg)

qgcNmeaSocket = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
qgcNmeaSocket.setblocking(0)

orientation = 0.0
while True:
    time.sleep(0.1)
    orientation = (orientation + 3) % 360.0
    messages = [gpgga, gpzda] if args.noHeading else [gpgga, gprmc]
    for message in messages:
        msg = format(message, 
                     datetime.now(), 
                     -27.589743, 
                     -48.52129, 
                     orientation=orientation)
        print(msg)
        qgcNmeaSocket.sendto(bytearray(msg, encoding="utf-8"), (args.ip, int(args.port)))