Since version 2.0.0:

All OEM7 NMEA sentences can be published as nmea_msgs/Sentence. Consult the OEM7 manual of the list of supported NMEA sentences.

To publish NMEA sentences using nmea_msgs/NMEASentence, configure the Oem7 receiver to output the desired sentence by modifying the receiver initialization commands.

The recommended way to achieve this is to specify the "ext_init_commands" parameter, using the same syntax as the "receiver_init_commands" parameter. For example, create a custom nmea.yaml parameter file with the following contents:

receiver_ext_init_commands: 
- "LOG GPGGA ONTIME 1"
- "LOG GPGSA ONTIME 1"
- "LOG GPGSV ONTIME 1"
- "LOG GPRMC ONTIME 1"

NMEA_Sentence: {topic: /gps/nmea_sentence,    frame_id: gps}

Load these parameters into /novatel/oem7/receivers/main namespace. For example, in your custom .launch file:

<rosparam file="nmea.yaml" ns="/novatel/oem7/receivers/main"/> 

OEM7 NMEA Standard logs: https://docs.novatel.com/OEM7/Content/Logs/NMEA_Standard_Logs_.htm