Furuno FR-1500 - Manual (Page 106)

-8.6-
HDG - Heading, deviation and variation
IMO Resolution A.382 (X). Heading (magnetic sensor reading), which if corrected for deviation, will
produce magnetic heading, which if offset by variation will provide true heading.
$--HDG, x.x, x.x, a, x.x, a*hh
Checksum
Magnetic variation, degrees, E/W (see notes 2 and 3)
Magnetic deviation, degrees, E/W (see notes 1 and 3)
Magnetic sensor heading, degrees
* Not used
NOTES 1 To obtain magnetic heading:Add easterly deviation (E) to magnetic sensor reading
Subtract westerly deviation (W) from magnetic sensor reading
2 To obtain true heading: Add easterly variation (E) to magnetic heading
Subtract westerly variation (W) from magnetic heading
3 Variation and deviation fields will be null fields if unknown.
HDT - Heading True
$--HDT, x.x, T*hh
Checksum
Heading, degree true
* Not used
MTW - Water temperature
$--MTW, x.x, C*hh
Checksum
Temperature, degrees C
* Not used