I'm trying to use GPS clocks to synchronize some measurements and I'm a little confused about the leap second correction between GPS time and UTC. I was wondering - does the GPGGA NMEA header have the 18s leap second offset between GPS time and UTC corrected already? Further, are there GPS NMEA headers that don't have that correction factored in? I'm using two different GPS systems to produce time stamps and they have an 18s offset between each other, indicating that one is correcting for it and the other is not.
GGA is supposed to be corrected for the difference between GPS and UTC. It may take some time before the almanac is downloaded and the device knows what the offset should be. Try letting it run for 15 minutes to see if it corrects itself.