JSON output for messages decoded by libacars

Hi Thierry,

I have added JSON output to libacars. All decoded fields are now available as JSON keys.This patch extends acarsdec JSON output with this data. Whenever libacars has decoded a message, a new "libacars" key is added to the JSON message produced by acarsdec and everything is stored there.

For example, the following media advisory message:

[#1 (F:131.725 L:-27 E:0) 25/01/2020 17:47:23.936 --------------------------------
Mode : G Label : SA Id : 9 Nak
Aircraft reg: A7-BAI Flight id: QR0001
No: S26A
0LS174722V
Media Advisory, version 0:
 Link Default SATCOM lost at 17:47:22 UTC
 Available links: VHF ACARS

produces the following JSON output:

{
   "ack" : false,
   "block_id" : "9",
   "channel" : 0,
   "error" : 0,
   "flight" : "QR0001",
   "freq" : 131.725,
   "label" : "SA",
   "level" : -27,
   "libacars" : {
      "media-adv" : {
         "current_link" : {
            "code" : "S",
            "descr" : "Default SATCOM",
            "established" : false,
            "time" : {
               "hour" : 17,
               "min" : 47,
               "sec" : 22
            }
         },
         "err" : false,
         "links_avail" : [
            {
               "code" : "V",
               "descr" : "VHF ACARS"
            }
         ],
         "version" : "0"
      }
   },
   "mode" : "G",
   "msgno" : "S26A",
   "station_id" : "rasp04",
   "tail" : "A7-BAI",
   "text" : "0LS174722V",
   "timestamp" : 1579974443.93625
}

Example ADS-C message:

[#2 (F:131.825 L:-26 E:0) 09/08/2019 14:17:14.327 --------------------------------
Mode : 2 Label : B6 Id : 1 Nak
Aircraft reg: A6-EFF Flight id: EK9771
No: F23A
/DXBEGEK.ADS.A6-EFF072300207602C9C410231F6312
ADS-C message:
 Basic report:
  Lat: 49.2194366
  Lon: 20.7440758
  Alt: 40000 ft
  Time: 1032.750 sec past hour (:17:12.750)
  Position accuracy: <0.05 nm
  NAV unit redundancy: OK
  TCAS: OK

gives this:

{
   "level" : -26,
   "ack" : false,
   "msgno" : "F23A",
   "tail" : "A6-EFF",
   "timestamp" : 1565360234.3276,
   "channel" : 1,
   "error" : 0,
   "freq" : 131.825,
   "block_id" : "1",
   "mode" : "2",
   "flight" : "EK9771",
   "text" : "/DXBEGEK.ADS.A6-EFF072300207602C9C410231F6312",
   "station_id" : "rasp04",
   "label" : "B6",
   "libacars" : {
      "arinc622" : {
         "air_addr" : ".A6-EFF",
         "adsc" : {
            "tags" : [
               {
                  "basic_report" : {
                     "alt" : 40000,
                     "nav_redundancy" : true,
                     "tcas_avail" : true,
                     "lon" : 20.744076,
                     "pos_accuracy_nm" : 0.05,
                     "ts_sec" : 1032.75,
                     "lat" : 49.219437
                  }
               }
            ],
            "err" : false
         },
         "msg_type" : "adsc_msg",
         "gs_addr" : "DXBEGEK",
         "crc_ok" : true
      }
   }
}

I had to enlarge the JSON buffer from 500 to 1400 bytes, because many messages simply didn't fit and the resulting JSON string got truncated. But it's still not 100% bulletproof solution - for example this beast encoded as JSON won't fit in 1400 bytes:

/DELCAYA.ADS.G-YMMG0722C4886DA0C9474E3F9D160186696928E02EE016010A69C128E04EE016042F643128E0D16016014A657128E0FA601600636A0133C906A0160033613138810CE016004361213881152016036F66793881850017241BE8346509C40CA50D230
ADS-C message:
 -- CRC check failed
 Basic report:
  Lat: 48.8920784
  Lon: 19.2705345
  Alt: 38004 ft
  Time: 911.875 sec past hour (:15:11.875)
  Position accuracy: <0.25 nm
  NAV unit redundancy: OK
  TCAS: OK
 Intermediate projection:
  Distance: 48.750 nm
  True track: 296.5 deg
  Alt: 38000 ft
  ETA: 375 sec
 Intermediate projection:
  Distance: 33.250 nm
  True track: 297.4 deg
  Alt: 38000 ft
  ETA: 631 sec
 Intermediate projection:
  Distance: 133.875 nm
  True track: 281.8 deg
  Alt: 38000 ft
  ETA: 1675 sec
 Intermediate projection:
  Distance: 41.250 nm
  True track: 285.3 deg
  Alt: 38000 ft
  ETA: 2003 sec
 Intermediate projection:
  Distance: 12.375 nm
  True track: 298.1 deg
  Alt: 39396 ft
  ETA: 2101 sec
 Intermediate projection:
  Distance: 6.375 nm
  True track: 273.3 deg
  Alt: 40000 ft
  ETA: 2151 sec
 Intermediate projection:
  Distance: 8.375 nm
  True track: 273.2 deg
  Alt: 40000 ft
  ETA: 2217 sec
 Intermediate projection:
  Distance: 109.875 nm
  True track: 288.2 deg
  Alt: 40000 ft
  ETA: 3112 sec
 Fixed projection:
  Lat: 50.7782936
  Lon: 9.2099762
  Alt: 40000 ft
  ETA: 3237 sec

A better option would be to drop the pkt buffer altogether and write the contents of jsonbuf to the socket directly. jsonbuf is 30000 bytes long, so this should be more than enough. What do you think?

TLeconte / acarsdec

JSON output for messages decoded by libacars #67