Q: What is the correct from ... import ... statement for python for the /nmea_sentence/sentence topic as well as the rospy.Subscriber("/nmea_sentence/sentence", Sentence, callback) statement?

My launch file works fine

<launch>
  <node name="gps" pkg="nmea_navsat_driver" type="nmea_topic_serial_reader">
    <param name="port" value="/dev/gps" />
    <param name="baud" value="115200" />
  </node>

  <node name="nmea" pkg="nmea_navsat_driver" type="nmea_topic_driver">
    <param name="useRMC" value="True" />
  </node>
</launch>

And I can use rostopic echo /nmea_sentence/sentence

To get output on the screen of my nmea sentence/sentence topic.

…
"$GPRMC,171450.40,A,4020.7237762,N,08007.7303416,W,000.003,206.1,220320,0.0,W,D*3A"
--- "$GPGGA,171450.60,4020.7237758,N,08007.7303411,W,4,12,0.9,319.3637,M,-34.191,M,00,0004*6D"
--- "$GNGSA,M,3,04,16,09,26,27,08,,,,,,,1.8,0.9,1.6*24"
--- "$GNGSA,M,3,48,46,57,47,58,38,,,,,,,1.8,0.9,1.6*2E"
--- "$GPRMC,171450.60,A,4020.7237758,N,08007.7303411,W,000.004,177.6,220320,0.0,W,D*33"
--- "$GPGGA,171450.80,4020.7237760,N,08007.7303404,W,4,12,0.9,319.3683,M,-34.191,M,00,0004*63"
--- "$GNGSA,M,3,04,16,09,26,27,08,,,,,,,1.8,0.9,1.6*24"
--- "$GNGSA,M,3,48,46,57,47,58,38,,,,,,,1.8,0.9,1.6*2E"
--- "$GPRMC,171450.80,A,4020.7237760,N,08007.7303404,W,000.003,139.0,220320,0.0,W,D*39"

What I can’t seem to do is figure out the syntax of the python code to subscribe to the /nmea_sentence/sentence topic. Can I get some help?

You can see my commented out failed attempts.

#!/usr/bin/env python
import rospy
#from std_msgs.msg import Header
#from nmea_msgs.msg import Sentence
#from std_msgs.msg import String
# from nmea_msgs.msg import String
#from nmea_msgs.msg import Header
from nmea_msgs.msg import Sentence

rospy.init_node('read_nmea', anonymous = True)

def callback(message):
    #response = message.sentence
    response = message
    #rospy.loginfo("I heard %s",data.header.sentence)  # with from std_msgs.msg import Header and rospy.Subscriber("/nmea_sentence", Header, callback)
    #rospy.loginfo("I heard %s",header.sentence)        # with from std_msgs.msg import Header and rospy.Subscriber("/nmea_sentence", Header, callback)
    #rospy.loginfo("I heard %s",data.sentence)          # with from std_msgs.msg import Header and rospy.Subscriber("/nmea_sentence", Header, callback)
    #rospy.loginfo("I heard %s",sentence)               # with from std_msgs.msg import Header and rospy.Subscriber("/nmea_sentence", Header, callback)
    #rospy.loginfo("I heard %s",data.sentence)          # with from nmea_msgs.msg import Sentence and rospy.Subscriber("/nmea_sentence/sentence", Sentence, callback)
    #rospy.loginfo("I heard %s",data)                   # with from nmea_msgs.msg import Sentence and rospy.Subscriber("/nmea_sentence/sentence", Sentence, callback)
    #rospy.loginfo("I heard %s",sentence)               # with from nmea_msgs.msg import Sentence and rospy.Subscriber("/nmea_sentence/sentence", Sentence, callback)
    #rospy.loginfo("I heard %s",message.sentence)       # with from nmea_msgs.msg import Sentence and ('nmea_sentence/sentence', Sentence, callback)
    rospy.loginfo("I heard %s",response)        # with Sentence and Header; using message.Sentence; using "", '' and / on topic

def listener():
    rospy.loginfo("start listening")
    #rospy.init_node('read_nmea', anonymous = True)
    #rospy.Subscriber("/nmea_sentence", Header, callback)
    #rospy.Subscriber('nmea_sentence/sentence', Header, callback)
    #rospy.Subscriber("nmea_sentence/sentence", Header, callback)
    #rospy.Subscriber('nmea_sentence/sentence', Sentence, callback)
    #rospy.Subscriber("/nmea_sentence/sentence", Header, callback)  
    #rospy.Subscriber("/nmea_sentence/sentence", String, callback) 
    #rospy.Subscriber("/nmea_sentence/sentence", Header, callback) 
    rospy.Subscriber("/nmea_sentence/sentence", Sentence, callback)
    rospy.spin()
if __name__ == '__main__':
    rospy.loginfo("Initiating read_nmea.py.(h)..")
    listener()

Ultimately I want to parse the heading, speed, number of satellites and signal quality of the gps signal within ROS.

Ref: http://docs.ros.org/api/nmea_msgs/html/msg/Sentence.html

$ rostopic type /nmea_sentence | rosmsg show  

std_msgs/Header header
  uint32 seq
  time stamp
  string frame_id
string sentence

$ rostopic type /nmea_sentence/sentence | rosmsg show
uint32 seq
time stamp
string frame_id

#!/usr/bin/env python
import rospy
from nmea_msgs.msg import Sentence
rospy.init_node('read_nmea', anonymous = True)

def callback(message):
    response = message.sentence
    rospy.loginfo("I heard %s",response)

def listener():
    rospy.loginfo("start listening")
    rospy.Subscriber("/nmea_sentence", Sentence, callback)
    rospy.spin()
if __name__ == '__main__':
    rospy.loginfo("Initiating read_nmea.py...")
    listener()