gps nmea sentence subscriber format for python

asked 2020-03-22 13:38:18 -0500

Al53 gravatar image

updated 2020-03-22 17:16:43 -0500

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 ...
(more)
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Comments

You don't actually tell us about any error/problem.

gvdhoorn gravatar image gvdhoorn  ( 2020-03-22 13:51:52 -0500 )edit

Sorry for that. I have a window open echoing /nmea_sentence/sentence so I know sentences are being produced. I start the subscriber and the loginfo statements are printed, but no further output is provided. $ rosrun beginner_tutorials read_nmea.py --debug

Al53 gravatar image Al53  ( 2020-03-22 13:59:47 -0500 )edit

Figured it out. Here is the solution. 

#!/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()
Al53 gravatar image Al53  ( 2020-03-22 18:37:28 -0500 )edit

What is/was the solution?

Please try to clearly describe both the problem and whatever you did to solve it.

It's quite unclear what you did that fixed the problem you apparently had.

gvdhoorn gravatar image gvdhoorn  ( 2020-03-23 04:20:24 -0500 )edit

The problem was no loginfo output in the callback function even though I knew sentences were being produced. The solution was to use the correct syntax for the key statements:

from nmea_msgs.msg import Sentence
response = message.sentence
rospy.Subscriber("/nmea_sentence", Sentence, callback)

Al53 gravatar image Al53  ( 2020-03-23 15:22:55 -0500 )edit
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