Update
Now I can see the gps data e.g., time, latitude, and longitude, from xgps. But there is still no data shown in cgps.
As instructed in gpsd_client/Tutorials/Getting Started with gpsd_client - ROS Wiki
I have run roscore, “gpsd /dev/ttyUSB0” and “rosrun gpsd_client gpsd_client _host:=localhost _port:=2947”, but when I do “rostopic echo /fix” there is no data coming out. Is it because the data format issue? I am using Garmin USB GPS 18x.