MATT_2.py

# import required libraries
import serial
import crcmod
import os
import os.path
import time
 
GPS = serial.Serial('/dev/ttyAMA0', 9600) # 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

# 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)  # 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] != "$GPGGA": # if the sentence doesn't start with a valid sentence
        print "Invalid sentence"
    elif data[0] == "$GPGGA" and data[6] == "0": # if it does start with a valid sentence but with no fix
        print "No Lock"
    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
        send(datastring) # send the new telemetry sentence to the send function

# function to convert lats and lngs to decimal latitude and longitude
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(lats) - 1):
            decs2 = decs2 + position_data[i]
        position = float(decs) + float(str((float(decs2)/60))[:8])
        
        if orientation == "S" or "W": 
            position = 0 - position 
        
        return position
   
# function to search a directory and return the name of the largest file
def find_best_file():
    best_file = ""
    biggest_size = 0
    imgdir = '/home/pi/to_transmit/'
 
    for file in os.listdir(imgdir): # for all files in "to_transmit" folder
        size = os.path.getsize(imgdir + file) # get the size of the file
        if size > biggest_size: # if the size is bigger than the previous biggest size
            biggest_size = size # reset biggest size
            best_file = file # reset best file
    if biggest_size == 0:
        best_file == ""
    return best_file
    
    
while True: # forever

    best_file = find_best_file() # setting best_file variable by running previous function
 
    if best_file == "": # if there are no images yet in the directory
        read_gps() # only read the GPS, nothing else
        
    else: # if there are images and thus a largest image
        os.system('ssdv -e -c MATT-1 -i ' + str(counter) + ' /home/pi/to_transmit/' + best_file + ' packets') # create a packets file for the images
        #os.system('rm /home/pi/to_transmit/*') # remove all images from the directory
        packets_file = open("packets", "rb") # open the packets file
        packets = packets_file.read(256) # read the first packet (256 bytes)
      
        while packets != '': # while there are still packets to be sent
 
            read_gps() # read the GPS (which also sends the GPS data - see "read_gps" function)
            send(packets) # send latest packet to the send function
            print packets # for testing only
            packets = packets_file.read(256) # re-read the packets file for the next packet