# Very simple GPS simulator
# Arlotto 2019
#first install python3-serial : sudo apt install python3-serial

gpsPort = '/dev/ttyUSB0'
gpsSpeed = 9600

timeBetweenSentences = 1 # value in seconds
gpsEndChars = '\r\n' # NMEA0183 use CR+LF https://en.wikipedia.org/wiki/NMEA_0183
gpsLat='4300.000,N'
gpsLong='006.000,E'
gpsType = 'GPGGA'
gpsSentenceEnd=gpsLat+gpsLong+\
                ',1,08,0.9,100.1,M,46.9,M,,*'

"""
GGA - essential fix data which provide 3D location and accuracy data.

 $GPGGA,123519,4807.038,N,01131.000,E,1,08,0.9,545.4,M,46.9,M,,*47

Where:
     GGA          Global Positioning System Fix Data
     123519       Fix taken at 12:35:19 UTC
     4807.038,N   Latitude 48 deg 07.038' N
     01131.000,E  Longitude 11 deg 31.000' E
     1            Fix quality: 0 = invalid
                               1 = GPS fix (SPS)
                               2 = DGPS fix
                               3 = PPS fix
			       4 = Real Time Kinematic
			       5 = Float RTK
                               6 = estimated (dead reckoning) (2.3 feature)
			       7 = Manual input mode
			       8 = Simulation mode
     08           Number of satellites being tracked
     0.9          Horizontal dilution of position
     545.4,M      Altitude, Meters, above mean sea level
     46.9,M       Height of geoid (mean sea level) above WGS84
                      ellipsoid
     (empty field) time in seconds since last DGPS update
     (empty field) DGPS station ID number
     *47          the checksum data, always begins with *

If the height of geoid is missing then the altitude should be suspect.
Some non-standard implementations report altitude with respect to the
ellipsoid rather than geoid altitude. Some units do not report negative
altitudes at all. This is the only sentence that reports altitude. 
"""


import sys
import time


def computeCheckSum(sentence) :
    chk = 0
    for c in sentence :
       chk ^=ord(c)
    
    chk = hex(chk)[2:]
    if len(chk)<2 :
         chk='0'+chk
    return chk

try :
    import serial
except ModuleNotFoundError:
    print('first install python3-serial : $sudo apt install python3-serial')
    sys.exit(-1)

try :
    gps = serial.Serial(port=gpsPort,baudrate=gpsSpeed,\
      bytesize=8, parity='N', stopbits=1, timeout=None,\
      xonxoff=0, rtscts=0)
except :
    print('unable to open gps port',gpsPort);
    sys.exit(-1)
    
if not gps.isOpen() :
    print('unable to open gps port',gpsPort);
    sys.exit(-1)

while 1 :
    # read gmt time
    gmt = time.gmtime()
    gpsTime = f'{gmt.tm_hour:02d}{gmt.tm_min:02d}{gmt.tm_sec:02d}'
    gpsSentence = '$'+gpsType+','+gpsTime+','+gpsSentenceEnd
     
    chk=computeCheckSum(gpsSentence[1:-1])
    gpsSentence += chk+gpsEndChars
    print(gpsSentence)
    #print(chk)
    
    gps.write(gpsSentence.encode('ASCII'))
    time.sleep(timeBetweenSentences)  # parameter in second !!