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MiLLennium Command Descriptions Manual 9
to generate, process and decode. The RTCA messages are therefore smaller, they have a 24 bit CRC that is much
more robust than
RTCM messages, and they permit the use of a four-alpha-character station ID.
RTCA Standard logs can be received through the COM1 or COM2 port of the rover GPSCard. The remote
GPSCard must issue the “ACCEPT port RTCA” command to dedicate a port before it will accept the
RTCA data
input to that port. The
RTCA logs cannot be intermixed with other logs.
The RTCAA log can be intermixed with other NovAtel ASCII data over a common communications port. It will
be directly interpreted by a rover GPSCard as a Special Data Input Command ($RTCA). “ACCEPT port
commands” must be used with this input command. A non-NovAtel rover station will need to strip off the header
($RTCA) and terminator (*xx), then convert the hexadecimal data to binary before the RTCA Standard can be
retrieved.
The RTCAB log can be intermixed with other NovAtel binary data. The COM1 or COM2 port of the remote
GPSCard must be dedicated to receiving
RTCA data only, and so the “ACCEPT port RTCA” command must be
issued. The remote GPSCard identifies the
RTCAB log by the message block identifier contained in the message,
and will interpret only the
RTCA data portion of the log.
NOTE: The CDSA/B logs may be used to monitor the COM port activity and differential data decode success.
Initialization - Rover Station
It is necessary to initialize the rover receiver to accept observation data from the reference station. If the receiver
is not correctly initialized, it will proceed to compute solutions in single point positioning mode.
Before initializing, ensure that the data link with the reference station has been properly set up. As well, ensure that
the COM port which is to receive the differential data is set up to match the bit rate and protocol settings of the
reference station broadcast data.
Establishing differential mode of operation at the rover receiver is primarily a one-step process whereby the accept
command is used to enable reception of observation data from the reference station.
ACCEPT COMMAND
The accept command is primarily used to set the GPSCard’s COM port command interpreter for acceptance of
various data formats. (see the
ACCEPT command in Chapter 2 and Appendix C)
Syntax
Example:
accept com2 rtcm
Once intitialized, the rover GPSCard receiver will operate in single point mode until the differential messages are
received. If the data messages are lost, the GPSCard will revert to single point positioning until the pseudorange
correction messages are restored.
NOTES: Ensure that the GPSCard RTCMRULE settings agree with the bit rule being transmitted by the RTCM
reference station. Unless otherwise set, all GPSCards default to 6CR.
LOG POSITION DATA AND OTHER USEFUL DATA
The GPSCard remote receiver has many options for information data logging. To monitor position status, the user
may find the
PRTKA/B logs to be the most informative. Other options exist, such as POSA/B and GPGGA. As well,
velocity data can be found in the
VLHA/B, SPHA/B and GPVTG logs. It is really up to the user’s specific applications
as to the full range of logs required by the user.
ACCEPT port mode