gistfile1.py

#!/usr/bin/python
 
import os
import serial
import crcmod
import time
import time as time_

trigger = False
setNav = bytearray.fromhex("B5 62 06 24 24 00 FF FF 06 03 00 00 00 00 10 27 00 00 05 00 FA 00 FA 00 64 00 2C 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 16 DC")
crc16f = crcmod.predefined.mkCrcFun('crc-ccitt-false') # function for CRC-CCITT checksum
counter = 0 # this counter will increment as our sentence_id
             
        
def send(data):
    NTX2 = serial.Serial('/dev/ttyAMA0', 300, serial.EIGHTBITS, serial.PARITY_NONE, serial.STOPBITS_TWO) # opening serial at 300 baud for radio transmission with 8 character bits, no parity and two stop bits
    NTX2.write(data) # write final datastring to the serial port
    print data
    NTX2.close()
    
#create function equivalent to arduino millis();
def millis():
    return int(round(time_.time() * 1000))
    
# function to convert latitude and longitude into a different format 
def convert(position_data, orientation):
        decs = "" 
        decs2 = "" 
        for i in range(0, position_data.index('.') - 2): 
            decs = decs + position_data[i]
        for i in range(position_data.index('.') - 2, len(position_data) - 1):
            decs2 = decs2 + position_data[i]
        position = float(decs) + float(str((float(decs2)/60))[:8])
        
        if orientation == ("S") or orientation == ("W"): 
            position = 0 - position 
 
        return position
    
def parse_nmea(NMEA_sentence):
    data = NMEA_sentence.split(",") # split sentence into individual fields
    
    if data[6] == "0": # if it does start with a valid sentence but with no fix
        print "No Lock"
        pass

    else: 
        # parsing required telemetry fields
        satellites = data[7]
        lats = data[2]
        northsouth = data[3]
        lngs = data[4]
        westeast = data[5]
        altitude = int(float(data[10]))
        callsign = "NORB_Test" 
        time = data[1]
        time = float(time) # ensuring that python knows time is a float
        string = "%06i" % time # creating a string out of time (this format ensures 0 is included at start if any)
        hours = string[0:2]
        minutes = string[2:4]
        seconds = string[4:6]
        time = str(str(hours) + ':' + str(minutes) + ':' + str(seconds)) # the final time string in form 'hh:mm:ss'
        
        latitude = convert(lats, northsouth)
        longitude = convert(lngs, westeast)
        
        if altitude >= 142:
            trigger = True
        
        string = str(callsign + ',' + time + ',' + str(counter) + ',' + str(latitude) + ',' + str(longitude) + ',' + str(trigger) + ',' + satellites + ',' + str(altitude)) # the data string
        csum = str(hex(crc16f(string))).upper()[2:] # running the CRC-CCITT checksum
        csum = csum.zfill(4) # creating the checksum data
        datastring = str("$$" + string + "*" + csum + "\n") # appending the datastring as per the UKHAS communication protocol
        counter += 1 # increment the sentence ID for next transmission
        return datastring # send the datastring to the send function to send to the NTX2
        
GPS = serial.Serial('/dev/ttyAMA0', 9600, timeout=1) # open serial port

# Disabling all NMEA sentences except GPGGA
GPS.write("$PUBX,40,GLL,0,0,0,0*5C\r\n")
GPS.write("$PUBX,40,GSA,0,0,0,0*4E\r\n")
GPS.write("$PUBX,40,RMC,0,0,0,0*47\r\n")
GPS.write("$PUBX,40,GSV,0,0,0,0*59\r\n")
GPS.write("$PUBX,40,VTG,0,0,0,0*5E\r\n")      

GPS.close()

def check_for_NMEA():
    GPS = serial.Serial('/dev/ttyAMA0', 9600, timeout=1)
    start_time = millis()
    while millis() - start_time <= 3000:
        a = GPS.read(1)
        if a == "$":
            NMEA_sentence = GPS.readline()
            if "GPGGA" in NMEA_sentence:
                GPS.close()
                send(parse_nmea(NMEA_sentence))

while True:
    check_for_NMEA()