Why is output of GPS using python zero or Nan? -

i'm having troubles gps output. when type:

$ cat /dev/ttyusb0 

i have nmea sentences coming out (it's want)

$gpgga,134131.000,4548.0018,n,01557.1026,e,1,06,1.5,123.8,m,42.4,m,,0000*56  $gpgsa,a,3,12,15,24,17,22,18,,,,,,,2.3,1.5,1.8*30  $gpgsv,3,1,12,24,76,320,29,12,49,254,44,15,42,195,17,17,38,057,15*7a  $gpgsv,3,2,12,25,15,249,,09,12,112,,26,11,162,,18,09,267,19*75  $gpgsv,3,3,12,22,08,297,23,14,08,323,,04,06,114,,28,03,061,*78  $gprmc,134131.000,a,4548.0018,n,01557.1026,e,1.71,291.64,210513,,,a*67 

but, when try see output python (this basic code):

import gps session=gps.gps('localhost','2947') session.stream() print session 

i got this:

time:      (nan) lat/lon:  0.000000 0.000000 altitude: ? speed:    ? track:    ? status:   status_no_fix mode:     mode_no_fix quality:  0 p=0.00 h=0.00 v=0.00 t=0.00 g=0.00 y: 0 satellites in view: 

i tried else's code nan or 0:

    #! /usr/bin/python  # written dan mandle http://dan.mandle.me september 2012  # license: gpl 2.0  import os  gps import *  time import *  import time  import threading  gpsd = none #seting global variable  os.system('clear') #clear terminal (optional)  class gpspoller(threading.thread):   def __init__(self):      threading.thread.__init__(self)      global gpsd #bring in scope      gpsd = gps(mode=watch_enable) #starting stream of info       self.current_value = none      self.running = true #setting thread running true   def run(self):      global gpsd      while gpsp.running:       gpsd.next() #this continue loop , grab each set of gpsd info to$  if __name__ == '__main__':   gpsp = gpspoller() # create thread   try:      gpsp.start() # start      while true:       #it may take second or 2 data       #print gpsd.fix.latitude,', ',gpsd.fix.longitude,'  time: ',gpsd.utc       os.system('clear')       print       print ' gps reading'      print '----------------------------------------'       print 'latitude    ' , gpsd.fix.latitude     print 'longitude   ' , gpsd.fix.longitude       print 'time utc    ' , gpsd.utc,' + ', gpsd.fix.time       print 'altitude (m)' , gpsd.fix.altitude       print 'eps         ' , gpsd.fix.eps       print 'epx         ' , gpsd.fix.epx       print 'epv         ' , gpsd.fix.epv       print 'ept         ' , gpsd.fix.ept       print 'speed (m/s) ' , gpsd.fix.speed      print 'climb       ' , gpsd.fix.climb       print 'track       ' , gpsd.fix.track       print 'mode        ' , gpsd.fix.mode       print       print 'sats        ' , gpsd.satellites       time.sleep(5) #set whatever 

output:

 gps reading ---------------------------------------- latitude     0.0 longitude    0.0 time utc       +  nan altitude (m) nan eps          nan epx          nan epv          nan ept          nan speed (m/s)  nan climb        nan track        nan mode         1  sats         [] 

so know why zeros , unknown?

please help, i'm trying fix week now.

thanks