987654321098765432109876 543210
  +------------------------+------+
  |dddddddddddddddddddddddd|pppppp|   RTCM word
  +------------------------+------+
    d data
    p parity    same algorithm as in GPS data

Die untersten beiden CRC-Bits des vorhergehenden Wortes gehen in die aktuelle CRC-Berechnung ein, ich verwende daher ein erweitertes 32-Bit RTCM-Wort mit einer Kopie der alten CRC-Bits in oberen beiden Bits. Wenn das unterste CRC-Bit des vorhergehenden Wortes gesetzt ist, werden die 24 Datenbits vor dem Senden invertiert.

Das 30-Bit RTCM-Wort wird für die serielle Übertragung in fünf 6-Bit Zeichen unterteilt, bei denen die Bitreihenfolge umgedreht ist. Zu diesem Wert wird 0x40 addiert und das Zeichen über die serielle Schnittstelle zum GPS-Empfänger gesendet. Als erstes werden die obersten 6 Bits des RTCM-Wortes gesendet.

Jede RTCM-Nachricht besteht aus zwei Header-Worten und Null oder mehreren Datenworten.

RTCM Header
   10 98765432 109876 5432109876 543210
  +--+--------+------+----------+------+
 0|--|01100110|mmmmmm|iiiiiiiiii|......|
  +--+--------+------+----------+------+
      preamble 0x66
      m message type 0..63
      i station id   0..1023

   10 9876543210987 654 32109 876 543210
  +--+-------------+---+-----+---+------+
 1|--|zzzzzzzzzzzzz|sss|lllll|hhh|......|
  +--+-------------+---+-----+---+------+
      z z-count
      s sequence                incremented modulo 8
      l message length 0..31    number of following data words
      h station health
          111 unhealthy broadcast
          110 unmonitored broadcast
          101 udre scale factor = 0.10
          100 udre scale factor = 0.20
          011 udre scale factor = 0.30
          010 udre scale factor = 0.50
          001 udre scale factor = 0.75
          000 udre scale factor = 1.00

RTCM Msg 1    Differential GPS Corrections               (all viewable sats)
RTCM Msg 2    Delta Differential GPS Corrections                 (??)
RTCM Msg 9    Partial Satellite Set Differential GPS Corrections (some sats)
---- 0 ----
   10 9 87 65432 1098765432109876 543210
  +--+-+--+-----+----------------+------+
 2|--|s|uu|ppppp|rrrrrrrrrrrrrrrr|......|
  +--+-+--+-----+----------------+------+
      s scale
      u UDRE user differential range error (estimate of PRC error)
          0 1-sigma error <= 1m
          1     "         <= 4m
          2   	"         <= 8m
          3   	"         >  8m
      p PRN
      r PRC (signed)      error if 0x8000
  
   10 98765432 10987654 3 21 09876 543210
  +--+--------+--------+-+--+-----+------+
 3|--|tttttttt|iiiiiiii| |  |     |......|
  +--+--------+--------+-+--+-----+------+
      t RRC (signed)        error if 0x80
      i iod

---- 1 ----
   10 98765432 10987654 3 21 09876 543210
  +--+--------+--------+-+--+-----+------+
 3|--|********|********|s|uu|ppppp|......|
  +--+--------+--------+-+--+-----+------+
      * previous satellite data
      s scale
      u udre
      p prn
  
   10 9876543210987654 32109876 543210
  +--+----------------+--------+------+
 4|--|rrrrrrrrrrrrrrrr|tttttttt|......|
  +--+----------------+--------+------+
      r range (signed)
      t range rate (signed)
  
   10 98765432 1 09 87654 32109876 543210
  +--+--------+-+--+-----+--------+------+
 5|--|iiiiiiii| |  |     |        |......|
  +--+--------+-+--+-----+--------+------+
      i iod
      
---- 2 ----
   10 98765432 1 09 87654 32109876 543210
  +--+--------+-+--+-----+--------+------+
 5|--|********|s|uu|ppppp|rrrrrrrr|......|
  +--+--------+-+--+-----+--------+------+
      * previous data
      s scale
      u udre
      p prn
      r range (high byte)
      
   10 98765432 10987654 32109876 543210
  +--+--------+--------+--------+------+
 6|--|rrrrrrrr|tttttttt|iiiiiiii|......|
  +--+--------+--------+--------+------+
      r range (low byte)
      t range rate (signed)
      i iod

RTCM Msg 3    Reference Station Data
   10 987654321098765432109876 543210
  +--+------------------------+------+
 2|--|xxxxxxxxxxxxxxxxxxxxxxxx|......|
  +--+------------------------+------+
  
   10 98765432 1098765432109876 543210
  +--+--------+----------------+------+
 3|--|xxxxxxxx|yyyyyyyyyyyyyyyy|......|
  +--+--------+----------------+------+

   10 9876543210987654 32109876 543210
  +--+----------------+--------+------+
 4|--|yyyyyyyyyyyyyyyy|zzzzzzzz|......|
  +--+----------------+--------+------+

   10 987654321098765432109876 543210
  +--+------------------------+------+
 5|--|zzzzzzzzzzzzzzzzzzzzzzzz|......|
  +--+------------------------+------+
      x x-coordinate
      y y-coordinate
      z z-coordinate

RTCM Msg 4    Datum            ????

RTCM Msg 5    Constellation Health
  ein Wort pro Satellit

   10 9 87654 3 210 98765 4 3 2 1098 76 543210
  +--+-+-----+-+---+-----+-+-+-+----+--+------+
 2|--|a|ppppp|i|hhh|ccccc|e|n|l|tttt|bb|......|
  +--+-+-----+-+---+-----+-+-+-+----+--+------+
   a ???
   p PRN
   i iodl
   h health
   c carrier/noise ratio (+24dB if >0)
   e health enable
   n new nav data soon
   l loss of warning
   t time to unhealthy
   b ???

RTCM Msg 6    Null Message

RTCM Msg 7    Radio Beacon Almanac

RTCM Msg 16   Special Message (ASCII Text)

   10 98765432 10987654 32109876 543210
  +--+--------+--------+--------+------+
 2|--|aaaaaaaa|bbbbbbbb|cccccccc|......|      ????
  +--+--------+--------+--------+------+

References:
  rtcm.c        John Sager [email protected],
                found @ W.Rupprecht http://www.wsrcc.com/wolfgang/gps
  dgpsbst.pdf   US DoT Broadcast Standard for the USCG DGPS navigation service

Bixby
  Bauanleitung Langwellen-Rx, 2x16 LCD, MC3362, MC145162 (dual PLL), PIC16F84

Heineck
  Bauanleitung mit billigem AM-Radio, PIC16Cxx

Cochran
  rcdgps2.zip    Dekodierungs-Software  LW -> RTCM
  dgps-co.cpp