Furuno FA Manual page 241

7. Press the [F1] key to change the display. The buzzer stops and the screen
shows the sentences being input to the radar, under the categories of GPS1
and GPS2 (position), speed, heading and nav data. Below is an example.
(The sentences shown depend on your system settings.)
Sentence input display
8. To terminate the test, press the [F1] key.
6. MAINTENANCE, TROUBLESHOOTING
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