disaster.py

# import required libraries
import serial
import crcmod
import os
import os.path
import time
 
def disable_sentences():
    GPS = serial.Serial('/dev/ttyAMA0', 9600, timeout=1) # open serial to write to GPS
 
    # 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() # close serial ready for re-opening in loop
 

crc16f = crcmod.predefined.mkCrcFun('crc-ccitt-false') # function for CRC-CCITT checksum
counter = 0 # sentence ID start value
disable_sentences()
 
 
# function to send both telemetry and packets
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
    NTX2.close()
 
 
# function to read the gps and process the data it returns for transmission
def read_gps():
    global counter # making the counter variable global to this function
    gps = serial.Serial('/dev/ttyAMA0', 9600, timeout=1)  # open serial at 9600 baud
    NMEA_sentence = gps.readline() # read the $GPGGA sentence from GPS
    gps.close() # close the serial port
    data = NMEA_sentence.split(",") # split sentence into individual fields
 
    if data[0] != "$GPGGA": # if the sentence doesn't start with a valid sentence
        print "Invalid sentence"
        return "SENTENCE INVALID"
        
    elif data[0] == "$GPGGA" and data[6] == "0": # if it does start with a valid sentence but with no fix
        print "No Lock"
        return "WAITING FOR LOCK :(\n"
    elif data[0] == "$GPGGA" and data[6] != "0": # if it does start with a valid sentence and has a fix
   
        lats = data[2]
        northsouth = data[3]
        lngs = data[4]
        westeast = data[5]
        altitude = data[9]
        callsign = "MATT_test" # this will be the callsign used primarily for checksum
        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)
        
        string = str(callsign + ',' + time + ',' + str(counter) + ',' + str(latitude) + ',' + str(longitude) + ',' + 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
        print datastring # for testing only
        return datastring
 
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








while True:
    datastring = read_gps()
    print datastring
    send(datastring)