239.254.2.4 Traffic

So I wanted to see what traffic was going to 239.254.2.4.  That is a UDP MultiCast IP address.  Looking back at the trace you can see that three packets are sent every second pretty continuously.  The three packets have three different lengths.  60, 150 and 134.  Here is a quick look at three consecutive packets payload content.  The 60 length packets look consistent each time.  The other two have minor changes.  My assumption is that these are GPS data.  So it should have some of the same data as these NEMA 2000 sentences:

Estimated Error Information (PGRME) 
$PGRME,<1>,M,<2>,M,<3>,M*hh<CR><LF> 
  1. <1> Estimated horizontal position error (HPE), 0.0 to 999.9 meters 
  2. <2> Estimated vertical position error (VPE), 0.0 to 999.9 meters 
  3. <3> Estimated position error (EPE), 0.0 to 999.9 meters

Map Datum (PGRMM) 
$PGRMM,<1> *hh<CR><LF> 
  1. <1> Current map datum (variable length field, for example, “WGS 84”)

Global Positioning System Fix Data (GGA)  
$GPGGA,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,M,<10>,M,<11>,<12>*hh<CR><LF> 
  1. <1> UTC time of position fix, hhmmss format 
  2. <2> Latitude, ddmm.mmmm format (leading zeros will be transmitted) 
  3. <3> Latitude hemisphere, N or S 
  4. <4> Longitude, dddmm.mmmm format (leading zeros will be transmitted) 
  5. <5> Longitude hemisphere, E or W 
  6. <6> GPS quality indication, 0 = fix not available, 1 = Non-differential GPS fix available, 2 = Differential GPS (DGPS) fix available, 6 = Estimated 
  7. <7> Number of satellites in use, 00 to 12 (leading zeros will be transmitted) 
  8. <8> Horizontal dilution of precision, 0.5 to 99.9 
  9. <9> Antenna height above/below mean sea level, -9999.9 to 99999.9 meters 
  10. <10> Geoidal height, -999.9 to 9999.9 meters 
  11. <11> Differential GPS (RTCM SC-104) data age, number of seconds since last valid RTCM transmission (null if not an RTCM DGPS fix) 
  12. <12> Differential Reference Station ID, 0000 to 1023 (leading zeros will be transmitted, null if not an RTCM DGPS fix)

So my Lattitude and Longitude when running these tests was N 42.359289, W 82.953903  I don't see the N and the W in the data!

So the 60 length packet could be either 1) and explanation of what is coming or 2) the map datum.  

All of the data below came from:  http://cloudshark.org/captures/824f2c3bc420  This is the Boat Three packet capture where I start up the Garmin with everything else already on and ready.

60 Length Packets

9b0000000400000003030100
9b0000000400000003030100
9b0000000400000003030100

150 Length Packets

99 00 00 00 64 00   .......t......d.
00 00 00 00 01 00 00 00 00 00 b9 06 00 00 dd 5a   ...............Z
6d 61 6a a8 e7 3f 47 58 a1 81 45 2a f7 bf 00 00   maj..?GX..E*....
00 00 a4 80 21 41 00 00 00 00 00 00 00 00 00 00   ....!A..........
00 00 00 00 00 00 00 00 00 00 bd 3f 29 43 00 00   ...........?)C..
00 00 00 00 00 00 51 59 04 69 51 59 04 69 51 59   ......QY.iQY.iQY
04 69 51 59 04 69 51 59 04 69 51 59 04 69 00 00   .iQY.iQY.iQY.i..
00 00 00 00 00 00                                 ......

99 00 00 00 64 00   .......t......d.
00 00 00 00 04 00 00 00 00 00 b9 06 00 00 99 24   ...............$
73 3f 6a a8 e7 3f e2 c9 a6 4a 45 2a f7 bf 4c 37   s?j..?...JE*..L7
89 81 60 81 21 41 00 00 00 00 00 00 00 00 00 00   ..`.!A..........
00 00 00 00 00 00 00 00 00 00 59 5a 07 43 8f c2   ..........YZ.C..
75 3d 80 45 54 bf ce c1 0e 41 e2 af b0 41 6c 8f   u=.ET....A...Al.
be 41 b8 1e a5 3f 29 5c ef 3f 85 eb 11 40 00 00   .A...?)\.?...@..
00 00 00 00 00 00                                 ......

99 00 00 00 64 00   .......t......d.
00 00 00 00 04 00 00 00 00 00 b9 06 00 00 99 24   ...............$
73 3f 6a a8 e7 3f e2 c9 a6 4a 45 2a f7 bf 00 00   s?j..?...JE*....
00 00 62 81 21 41 00 00 00 00 00 00 00 00 00 00   ..b.!A..........
00 00 00 00 00 00 00 00 00 00 59 5a 07 43 8f c2   ..........YZ.C..
75 3d 80 45 54 bf ce c1 0e 41 e2 af b0 41 6c 8f   u=.ET....A...Al.
be 41 b8 1e a5 3f 29 5c ef 3f 85 eb 11 40 00 00   .A...?)\.?...@..
00 00 00 00 00 00                                 ......

134 Length Packets

9d 00 00 00 54 00   .......d......T.
00 00 05 f0 0a 3b ce 00 05 02 fc 08 47 1f 00 05   .....;......G...
0d b8 0b 0f 2c 00 05 0c 60 09 24 ea 00 05 1d 98   ....,...`.$.....
08 17 36 01 05 04 80 0c 1c 4c 00 05 19 c4 09 22   ..6......L....."
18 01 05 0a fc 08 35 37 00 05 11 9c ff 01 83 00   ......57........
00 1a 9c ff 04 a1 00 00 ff ff 7f ff ff ff 00 ff   ................
ff 7f ff ff ff 00                                 ......

9d 00 00 00 54 00   .......d......T.
00 00 05 8c 0a 3b ce 00 05 02 fc 08 47 1f 00 05   .....;......G...
0d b8 0b 0f 2c 00 05 0c 60 09 24 ea 00 05 1d fc   ....,...`.$.....
08 17 36 01 05 04 80 0c 1c 4c 00 05 19 c4 09 22   ..6......L....."
18 01 05 0a fc 08 35 37 00 05 11 9c ff 01 83 00   ......57........
00 1a 9c ff 04 a1 00 00 ff ff 7f ff ff ff 00 ff   ................
ff 7f ff ff ff 00                                 ......

9d 00 00 00 54 00   .......d,m....T.
00 00 05 8c 0a 3b ce 00 05 02 fc 08 47 1f 00 05   .....;......G...
0d b8 0b 0f 2c 00 05 0c fc 08 24 ea 00 05 1d 98   ....,.....$.....
08 17 36 01 05 04 80 0c 1c 4c 00 05 19 c4 09 22   ..6......L....."
18 01 05 0a fc 08 35 37 00 05 11 9c ff 01 83 00   ......57........
00 1a 9c ff 04 a1 00 00 ff ff 7f ff ff ff 00 ff   ................
ff 7f ff ff ff 00                                 ......
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