38 G12 OEM Board & Sensor Reference Manual
of 10 (one meter of noise is reduced to 0.1 meter). The noise is further reduced
with each additional measurement.
If the lock to a satellite is lost, the noise goes back to one meter and smoothing
starts from the one-meter level. The loss of lock to a satellite is rare, and typically
happens only when the G12 antenna’s line of sight to the satellite is blocked by an
object, or when the satellite goes below the horizon.
Total position error (or error-in-position) is a function of the range errors (or errors-
in-range) multiplied by the PDOP (three-coordinate position dilution of precision).
PDOP is a function of satellite geometry; that is, the positions of the satellites in
relation to one another.
RTCM Messages
The G12 accepts differential correction messages in the RTCM format (refer to
RTCM Recommended Standards for Differential GNSS, version 2.2). The G12
can be set to output or receive RTCM messages using either of its two ports by
issuing the set command $PASHS,RTC,s1,c2 where s1 is either BAS (base
station mode) or REM (remote mode) and c2 is the port designator for the input or
output of differential corrections. The G12 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 G12.
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 G12 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.
G12RevD.book Page 38 Tuesday, April 2, 2002 4:33 PM
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