uBlox 6M GPS NMEA message parsing using Python

nmea_ublox.py

# Script connects to uBlox 6M GPS receiver through serial interface, 
# reads received NMEA messages, parses and verifies
# them and finally prints parsed information to console (logfile).
#
# NOTE: change below configuration based on your device parameters

# configuration
port = "COM15"
baud = 9600

try:
	import serial
except:
	print "PySerial library is required to run this script, please install"
	exit(1)

import logging

# helpers
def num(x): 
	try: return int(x)
	except: return 0

# parse code
def parse_gptxt(p):
	logging.info({0:"ERROR",1:"WARNING",2:"NOTICE",7:"USER"}[num(p[3])]+" "+p[4])
	
def parse_gpgll(p):
	# $GPGLL,Latitude,N,Longitude,E,hhmmss.ss,Valid,Mode*cs<CR><LF>
	if p[6] != "" and (p[6] == "A" or p[6] == "D"):
		logging.info("GLL:")
		logging.info("  lat: "+p[1]+" "+p[2])
		logging.info("  lon: "+p[3]+" "+p[4])
		logging.info("  time: "+p[5])
	
def parse_rmc(p):
	# $GPRMC,hhmmss,status,latitude,N,longitude,E,spd,cog,ddmmyy,mv,mvE,mode*cs<CR><LF>
	if p[2] == "A": # status valid
		logging.info("RMC:")
		logging.info("  time: "+p[1])
		logging.info("  lat: "+p[3]+" "+p[4])
		logging.info("  lon: "+p[5]+" "+p[6])
		logging.info("  speed: "+p[7]+" knots")
		logging.info("  cog: "+p[8]+" deg")
		logging.info("  date: "+p[9])

def parse_gpvtg(p):
	# $GPVTG,cogt,T,cogm,M,sog,N,kph,K,mode*cs<CR><LF>
	if p[7] != "":
		logging.info("VTG:")
		logging.info("  cogt: "+p[1])
		logging.info("  sog: "+p[5]+" knots")
		logging.info("  sog: "+p[7]+" km/h")

def parse_gpgga(p):
	# $GPGGA,hhmmss.ss,Latitude,N,Longitude,E,FS,NoSV,HDOP,msl,m,Altref,m,DiffAge,DiffStation*cs<CR><LF>
	if p[7] != "" and num(p[7]) > 0 and p[6] != "" and num(p[6]) > 0: # fix is valid
		logging.info("GGA:")
		logging.info("  time: "+p[1])
		logging.info("  lat: "+p[2]+" "+p[3])
		logging.info("  lon: "+p[4]+" "+p[5])
		logging.info("  fix: "+p[6])
		logging.info("  sat: "+str(num(p[7])))
		
def parse_gsa(p):
	# $GPGSA,Smode,FS{,sv},PDOP,HDOP,VDOP*cs<CR><LF>
	if p[2] != "" and num(p[2]) > 1: # fix is valid
		logging.info("GSA:")
		logging.info("  sat: "+str(sum([1 for i in range(12) if p[3+i]!=""])))
		
def parse_gpgsv(p):
	# $GPGSV,NoMsg,MsgNo,NoSv,{,sv,elv,az,cno}*cs<CR><LF>
	if p[2] != "" and num(p[2]) == 1:
		logging.info("GSV:")
		logging.info("  sat: "+str(num(p[3])))

# known NMEA messages (parameter number, handler f-tion)
messages = {
	"$GPTXT":[5, parse_gptxt],
	"$GPRMC":[12, parse_rmc],
	"$GPVTG":[10, parse_gpvtg],
	"$GPGGA":[15, parse_gpgga],
	"$GPGSA":[18, parse_gsa],
	"$GPGSV":[14, parse_gpgsv],
	"$GPGLL":[8, parse_gpgll],
}

def chksum(inp): # message checksum verification
	if not inp.startswith("$"): return False
	if not inp[-3:].startswith("*"): return False
	payload = inp[1:-3]
	checksum = 0
	for i in xrange(len(payload)):
		checksum =  checksum ^ ord(payload[i])
	return ("%02X" % checksum) == inp[-2:]

def parse_nmea(parts):
	if len(parts) <= 0: return
	id = parts[0].upper()
	
	if id in messages:
		if len(parts) < messages[id][0]:
			parts.extend(["" for e in range(messages[id][0]-len(parts))])
		messages[id][1](parts)
	else:
		logging.info("Unknown GPS data: "+parts)

if __name__ == "__main__":
	logging.basicConfig(filename='nmea.log',level=logging.DEBUG,format="%(asctime)s [%(levelname)s] %(message)s")	
	ser = serial.Serial(port, baud)
	try:
		while True:
			line = ser.readline()
			line = line.strip()
			
			logging.info("Received: " + line)
			
			if not chksum(line):
				logging.error("INVALID checksum: "+line)
			else:
				parse_nmea(line[:-3].split(","))
	finally:
		ser.close()