ashtech DG16 Reference Manual page 274

The MSG message generated for RTCM message type 6 contains the common
data fields only. This message is typically used as "transmission fill" to assist in
establishing and maintaining message frame synchronization. This message can
be set for output in one second increments.
The MSG message generated for RTCM message type 9 is an abbreviated version
of the MSG message for RTCM message type 1. It contains the common data fields,
but contains PRC, RRC, UDRE, and IODE data for a maximum of three satellites
per message. This message can be set for output in one second increments.
The MSG message generated for the RTCM type 16 message contains the
common data fields and a text message of up to 90 characters, spaces included.
The text in the type 16 message is entered through the command
$PASHS,RTC,MSG. See the next section, RTCM Commands, for more information
on set commands for differential operation.
MSG Formats
The MSG format for RTCM message type 1 is in the format:
$GPMSG,d1,d2,f3,d4,d5,d6,m7,n*(d8,d9,f10,f11,d12)*hh
The MSG format for RTCM message type 2 is in the format:
$GPMSG,d1,d2,f3,d4,d5,d6,m7,n*(d8,d9,f10,f11,d12)*hh
The MSG format for RTCM message type 3 is in the format:
$GPMSG,d1,d2,f3,d4,d5,d6,m7,f8,f9,f10*hh
The MSG format for RTCM message type 6 is in the format:
$GPMSG,d1,d2,f3,d4,d5,d6,m7*hh
The MSG format for RTCM message type 9 is in the format:
$GPMSG,d1,d2,f3,d4,d5,d6,m7,(3*(d8,d9,f10,f11,d12))*hh
The MSG format for RTCM message type 16 is in the format:
$GPMSG,d1,d2,f3,d4,d5,d6,m7,s8*hh
The MSG format for RTCM message type 18 is in the format:
$GPMSG,d1,d2,f3,d4,d5,d6,m7,d13,d14,f15,n(d16,d17,d18,d19,f20)*
hh
The MSG format for RTCM message type 19 is in the format:
$GPMSG,d1,d2,f3,d4,d5,d6,m7,d13,d14,d21,f15,n(d16,d22,d18,d23,f
24)*hh
In MSG messages generated for RTCM message types 1 and 2, fields d8 through d12 are repeated
"n" times; n = the number of locked satellites. For RTCM message type 9, fields d8 through d12 are
repeated 3 times per message.
254 DG14 and DG16 Board & Sensor Reference Manual