I have 6 receivers running as rovers on various machines all running 2.22.4
I have noticed that the GGA sentence doesn’t appear to be as reliable as the RMC sentence. My application allows me to select which sentence I use so I’m able to switch to RMC and generally keep running. It is not uncommon for a second to pass and no GGA sentence is sent.
Using ROS and measuring the rate of the msg over time GGA comes out to 4.4 Hz and RMC comes out to 4.9.
I’m at a level where checksums aren’t being checked yet so that isn’t the problem.
So pretty basic question. What causes the receiver to send a GGA sentence and what causes the receiver to sent an RMC sentence? GSA, GSV, VTD, and ZDA all appear to be reliable. GGA is the only one that sometimes doesn’t come through.
I had created this and then later in the day deleted it because I thought the problem was on my end and I was dropping strings. As I looked at my logs and data more I realized that I don’t believe I am.
A little more context I’m using http://wiki.ros.org/nmea_navsat_driver to read my receivers. I’m using the serial topic driver. What I noticed last night that caused me to delete the topic is that I had occurrences where a GGA message was present but a NavSatFix wasn’t present in my bag file. Today I was looking closer and I found multiple instances where all the nmea strings are present except GGA. So the my initial inquiry is still relevant as to what controls when a GGA sentence is sent vs an RMC sentence.
May I ask you in what conditions you run your survey? Would you mind specifying if you noticed GNGGA messages missing when outputting at a lower frequency?
After looking at the nmea_navsat_driver code we noticed that the ROS queue size was set to 1. The burst of the GGA, RMC, GSV, GSA, etc could cause the GGA msg to be discarded. Our solution was to change the nmea_navsat_driver queue size to 10 and to ignore all msgs other than GGA. This fixed the problem for us. At some point we will likely leverage the GSV/GSA msgs but until that time what we are doing now is working.
