ashtech DG14, Reference Manual

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Differential Operation 55
Differential Operation
supports six out of the 64 different types of RTCM messages. Message type 3
contains base station status information. Message type 16 contains a special
ASCII message of up to 90 characters. Type 16 messages are used to
communicate special information. For example, a base station operator may
wish to construct a message informing users that the base station will go
offline temporarily in order to perform routine maintenance or repairs.
Message types 1, 2, and 9 contain data used for position correction. The type
6 message is a null frame message which is used to establish and maintain
RTCM message frame synchronization for remote differential stations. RTCM
messages are processed automatically by the DG16. Although RTCM
messages are output in binary format, it is possible to convert them to ASCII
format through the
$PASHS,NME,MSG set command and the $PASHQ,MSG
query command.
All RTCM messages except type 1 are generated by the base station only if
they are enabled using the
$PASHS,RTC,TYP set command. In addition, if
type 1 or type 9 messages are enabled and a change of ephemeris occurs in
one or more satellites, the base station automatically generates a type 2
message with the delta IODE (Issue of Data, Ephemeris) information. The
type 2 message is generated regardless of whether the type 2 message has
been enabled for output. When a DG16 set in differential remote mode
receives a type 2 message, and the IODE information in type 1 or 9
messages does not match, the remote receiver uses the delta ephemeris
information in the received type 2 message until the IODE information in the
incoming type 1 or 9 messages matches the IODE information in the received
type 2 message.
On initial power-up, or after issuing the
$PASHS,RST command (restore
defaults), the DG16’s setting for differential mode is OFF, and the setting for
the maximum age of an RTCM differential correction is 60 seconds, meaning
that an incoming correction whose age is greater than 60 seconds is not used.
If automatic differential GPS mode is not enabled
($PASHS,DIF,AUT) , and if
the differential correction data is unavailable or is older than the maximum
age specified by the
$PASHS,RTC,MAX set command, a DG16 set as a
remote differential station will not output position data. If automatic differential
mode is enabled, a DG16 set as a remote differential station will output
uncorrected positions if differential correction data is unavailable or if the age
of correction exceeds the maximum age setting.
RTCM 104 Format, Version 3.0
When the RTCM base option is enabled and the DG16 is configured as a
reference station, it computes differential corrections for up to 12 satellites,
converts those corrections to RTCM format and outputs the converted
messages through its serial ports. The DG16 generates message types 1, 2,