A simple snippet of Python to parse a few NMEA sentences read from a MTK3339 device on a Raspberry Pi v3.

activity1.JPG

layout.png

MTK3339.py

import serial #sudo apt-get install python-serial
import time
import RPi.GPIO as gpio

PMTK_ACK           = '$PMTK001'
PMTK_INVALID       = '0'
PMTK_UNSUPPORTED   = '1'
PMTK_VALID_FAILED  = '2'
PMTK_VALID_SUCCESS = '3'

PMTK_SET_NMEA_UPDATE_1HZ  = b'$PMTK220,1000*1F\r\n'
PMTK_SET_NMEA_UPDATE_5HZ  = b'$PMTK220,200*2C\r\n'
PMTK_SET_NMEA_UPDATE_10HZ = b'$PMTK220,100*2F\r\n'

PMTK_SET_NMEA_OUTPUT_RMC_ONLY = b'$PMTK314,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0*29\r\n'
PMTK_SET_NMEA_OUTPUT_RMC_GGA  = b'$PMTK314,0,1,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0*28\r\n'
PMTK_SET_NMEA_OUTPUT_ALL_ON   = b'$PMTK314,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,1,1*28\r\n'
PMTK_SET_NMEA_OUTPUT_ALL_OFF  = b'$PMTK314,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0*28\r\n'

PMTK_ERASE_FLASH = b'$PMTK184,1*22\r\n'
PMTK_STOP_LOGGER = b'$PMTK185,1*23\r\n'
PMTK_STRT_LOGGER = b'$PMTK186,1*20\r\n'
PMTK_DUMP_LOGGER = b'$PMTK622,0*28\r\n'

class MTK3339:
	def __init__(self, device, enable_pin):
		self.utc = 'unknown'
		self.lat_decimal = 0
		self.lng_decimal = 0
		self.en = enable_pin
              # Setup GPIO.
		gpio.setmode(gpio.BCM)
		gpio.setup(self.en, gpio.OUT)
		gpio.output(self.en, gpio.HIGH)
              # Setup the serial device.
		self.tx = serial.Serial(port = device, baudrate = 9600, timeout = 3.0)
		time.sleep(0.2)
		if self.IsOpen():
			print('GPS device successfully connected.')
		self.Enable()
		self.TurnOffAllOutput() # Turn of all output to receive a clear ACK.
		self.SetFastOutput()
		return

	def Remove(self):
		self.tx.close()
		if self.IsOpen():
			print('GPS device failed to close.')
		else:
			print('GPS device successfully closed.')
			gpio.cleanup()
		return

	def Enable(self, enable = True):
		if enable:
			gpio.output(self.en, gpio.HIGH)
		else:
			gpio.output(self.en, gpio.LOW)
		return

	def IsOpen(self):
		return self.tx.isOpen()

	def __AckFlag(self, command, timeout_seconds = 2.0):
		flg = PMTK_INVALID
		ack = False
		tout = time.time() + timeout_seconds
		while ack == False and time.time() < tout:
			line = self.GetNextSentence().split(',')
			ack = (line[0].startswith(PMTK_ACK) and line[1].startswith(command))
		if ack:
			flg = line[2].split('*')[0] # Fetch flag value since we have an ACK for the correct command.
		return flg

	def SetSlowOutput(self):
		self.tx.write(PMTK_SET_NMEA_UPDATE_1HZ)
		return self.__AckFlag('220')

	def SetFastOutput(self):
		self.tx.write(PMTK_SET_NMEA_UPDATE_5HZ)
		return self.__AckFlag('220')

	def SetVeryFastOutput(self):
		self.tx.write(PMTK_SET_NMEA_UPDATE_10HZ)
		return self.__AckFlag('220')

	def TurnOffAllOutput(self):
		self.tx.write(PMTK_SET_NMEA_OUTPUT_ALL_OFF)
		return self.__AckFlag('314')

	def TurnOnAllOutput(self):
		self.tx.write(PMTK_SET_NMEA_OUTPUT_ALL_ON)
		return self.__AckFlag('314')

	def TurnOnRmcGgaOutput(self):
		self.tx.write(PMTK_SET_NMEA_OUTPUT_RMC_GGA)
		return self.__AckFlag('314')

	def TurnOnRmcOutput(self):
		self.tx.write(PMTK_SET_NMEA_OUTPUT_RMC_ONLY)
		return self.__AckFlag('314')

	def GetNextSentence(self):
		return self.tx.readline()

	def GetLastLocation(self):
		return self.utc, self.lat_decimal, self.lng_decimal

	def GetLastLocationGoogleMapLink(self):
		s = 'http://maps.google.com/?ll=unknown,unknown'
		if self.utc != 'unknown':
			s = 'http://maps.google.com/?ll=' + str(self.lat_decimal)[:8] + ',' + str(self.lng_decimal)[:9]
		return s

	def GetLocation(self, timeout_seconds = 2.0):
		valid = False
		zulu = 'unknown'
		lat = '0'
		lng = '0'
		north = False
		east = False
		tout = time.time() + timeout_seconds
		while valid == False and time.time() < tout:
			line = self.GetNextSentence().split(',')
			if line[0] == '$GPRMC':
				valid = (line[2] == 'A')
				if valid:
					zulu = line[1].split('.')[0]
					lat = line[3]
					north = (line[4] == 'N')
					lng = line[5]
					east = (line[6] == 'E')
				elif line[0] == '$GPGGA':
					valid = (line[4] == '1' or line[4] == '2')
					if valid:
						zulu = line[1].split('.')[0]
						lat = line[2]
						north = (line[3] == 'N')
						lng = line[4]
						east = (line[5] == 'E')
		if valid == False:
			raise ValueError('Unable to detect fix.')
		utc = zulu[0:2] + ':' + zulu[2:4] + ':' + zulu[4:6] + 'Z'
		lat_degree = int(lat[0:2])
		lat_minute = float(lat[2:9]) / 60.0
		lat_decimal = lat_degree + lat_minute
		if north == False:
			lat_decimal = lat_decimal * -1.0
		lng_degree = int(lng[0:3])
		lng_minute = float(lng[3:10]) / 60.0
		lng_decimal = lng_degree + lng_minute
		if east == False:
			lng_decimal = lng_decimal * -1.0
		self.utc, self.lat_decimal, self.lng_decimal = utc, lat_decimal, lng_decimal
		return utc, lat_decimal, lng_decimal

	def EraseLog(self):
		self.tx.write(PMTK_ERASE_FLASH)
		return self.__AckFlag('184')

	def StopLogging(self):
		self.tx.write(PMTK_STOP_LOGGER)
		return self.__AckFlag('185')

	def StartLogging(self):
		self.tx.write(PMTK_STRT_LOGGER)
		return self.__AckFlag('186')

	def DumpLog(self):
		self.tx.write(PMTK_DUMP_LOGGER)
		return self.__AckFlag('622')

GPS_PIN_EN = 17

if __name__ == '__main__':
	my_gps = MTK3339('/dev/ttyS0', GPS_PIN_EN) # The default for Raspberry Pi 2 is: '/dev/ttyAMA0'
	my_gps.Enable(True)
	my_gps.EraseLog()
	my_gps.StartLogging()
	my_gps.TurnOnRmcGgaOutput() #or TurnOnRmcOutput()
	try:
		while True:
			try:
				print(my_gps.GetLocation())
				time.sleep(1.0)
			except Exception as e:
				print(e)
	except KeyboardInterrupt as kb:
		pass
	finally:
		print(my_gps.GetLastLocationGoogleMapLink())
		my_gps.StopLogging()
		my_gps.Remove()