NMEA Sentences and  EPIRB Interfacing

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The onboard ACR Rapidfix 406 EPIRB does not recognize digital GPS fix information from the Raymarine system.  This model has an infrared optical interface which transfers fix data from external GPS sources.  However, fix data from an external Garmin GPS (NMEA v2.3) was acknowledged.  ACR confirmed that only $GPGGA sentences can be interpreted and not the $IIGGA or $ECGGA sentences output by the Raymarine system.  

The correct $GPGGA format is shown below;


     GP              Talker ID
     GGA          Global Positioning System Fix Data
     162714       Fix taken at 16:27:14 UTC
     4144.0394,N   Latitudede 481deg 44.0394' N
     07117.7006, W  Longitude 7 deg 117.0006' W
     1            Fix quality: 0 = invalid, 1 = GPS fix (SPS), 2 = DGPS fix, 3 = PPS, 4 = Real Time Kinematic, 5 = Float RTK, 
                                    6 = estimated (dead reckoning) (NMEA 2.3 feature), 7 = Manual input mode, 8 = Simulation mode
     05           Number of satellites being tracked
     1.2          Horizontal dilution of position
     2.7,M      Altitude, Meters, above mean sea level
     -35.5,M       Height of geoid (mean sea level) above WGS8 ellipsoid
     (empty field) time in seconds since last DGPS update
     (empty field) DGPS station ID number
     *76          the checksum data, always begins with *

Of note, the output from the Raymarine Seatalk to NMEA interface is different in several ways.  First, the talker ID is changed from GP to II.  Then after the differential GPS gets a lock, the fix type changes from "0" to "2".  Lastly, there is not a comma before the checksum data after dGPS lock occurs. 

$IIGGA,143227,4143.978,N,07117.441,W,0,00,20,0,M,-34,M,,*53  <sentence before dGPS lock)
$IIGGA,143232,4143.979,N,07117.441,W,2,08,1,16,M,-34,M,0,0198*68 <sentence after dGPS lock)

A call back to Nico, from ACR technical support , brought further insight.  The older model EPIRB only recognizes a $GPGGA sentence with a fix quality of "1".  Apparently, differential GPS came after this EPIRB was designed.  While I was able to program the Brookhouse MUX to make the required conversions, Nico inquired how old the EPIRB was.  Given that the old style hazmat batteries needed to be replaced in 2012 anyway (~$300), I took the offer to trade in the old one for a new (albeit now discontinued) ACR Globalfix 406 unit.  The Globalfix 406 model has a built in GPS!  This now allows the Brookhouse MUX to be dedicated to multiplexing NMEA sentences coming from the Raymarine Seatalk <-> NMEA interface and a Digital Yachts ANT200 AIS receiver without any conversions.  An example data stream from the MUX is below.  Received AIS sentences are in yellow.  The combined MUX output is now fed to navigation software capable of displaying AIS data.  The EPIRB is now standalone.  A earlier discussion of the meaning of many of these NMEA sentences is here.

The combined MUX output looks like;

<$IIGGA sentence begin again, etc>

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