nmea_navsat_driver /fix topic hangs indefinitely

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Hey everyone!

I'm running into an issue that I've previously resolved and have seen posted in a few different places, so apologies if it's a duplicate. This weekend, I started getting this problem again where I run the nmea navsat driver ROS package to read in GPS strings via USB, and can see the serial messages via "$ cat /dev/<device>", but when I try to echo the "/fix" topic it just hangs.

Goal: Have the robot read GPS data via USB from a GPS unit.

Setup:

• OS - Ubuntu 14.04
• More OS info - Linux CPR-J100-0057 3.16.0-77-generic #99~14.04.1-Ubuntu SMP Tue Jun 28 19:17:10 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
• ROS - Indigo 1.11.20
• GPS Package - nmea_navsat_driver 0.4.2
• GPS unit - Emlid Reach RS, ReachView app 2.11

Walkthrough:

• With the robot powered on, the ROS core is started with the following env vars using the robot's IP address:

  export ROS_MASTER_URI=http://192.168.131.11:11311
  

  export ROS_IP=192.168.131.11

• GPS device is plugged into the robot's USB port, I see the device shows up in /dev:

  ls /dev/ttyA*
  

  Listed devices: /dev/ttyACM0 /dev/ttyACM1

• I'm able to display the serial data coming into the robot:

  cat /dev/ttyACM1

  Example printout:

  $GPGSA,A,3,07,08,09,11,23,48,51,,,,,,3.1,1.7,2.6*3D $GLGSA,A,3,67,68,77,78,88,,,,,,,,15.2,4.7,14.4*2A $GPGSV,2,1,07,07,44,322,48,08,79,128,45,09,55,248,45,11,27,170,37,1*65 $GPGSV,2,2,07,23,39,190,42,48,26,246,37,51,45,220,43,,,,,1*58 $GLGSV,2,1,05,67,22,239,41,68,22,296,44,77,49,019,38,78,27,317,38,1*7B $GLGSV,2,2,05,88,36,166,37,,,,,,,,,,,,,1*4D $GNGST,173725.20,1.220,,,,4.514,4.529,10.741*5C $GNVTG,,T,,M,0.09,N,0.17,K,A*32 $GNRMC,173725.40,A,3128.5281612,N,08331.7091448,W,0.04,,300418,,,A*4A $GNGGA,173725.40,3128.5281612,N,08331.7091448,W,1,12,0.0,108.391,M,-28.500,M,0.0,*61

• Next, I try running the nmea navsat driver ROS package and match the GPS settings for port and baud rate. It seems like these settings being wrong is a classic way for this not to work, but given step 3 above, it seems like that isn't the issue here.

  rosrun nmea_navsat_driver nmea_serial_driver _port:=/dev/ttyACM1 _baud:=4800

  NOTE: I get this warning when I run the package, but I think it's only temporary at startup. I've had this same warning a week ago when everything was working. The warning:

  [WARN] [WallTime: 1525110076.762267] Received a sentence with an invalid checksum. Sentence was: '$GPGSV,2,1,08,07,45,323,43,08,80,117,46 ...