Navman 11 - Manual (Page 69)

69 MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.
measurements before computing a position
solution. This technique effectively eliminates
much of the error due to SA as well as errors
due to unmodelled satellite clock errors, satellite
ephemeris errors, and atmospheric delays.
This ‘improved’ solution is present in all output
messages.
With a few minor exceptions outlined below,
DGPS is enabled by default, but may be disabled
by the OEM. Because SA changes with time,
the corrections deteriorate with time as well.
Therefore, DGPS operation will only occur when
enough current DGPS corrections are available.
The Jupiter receiver accepts RTCM SC-104
format DGPS correction messages directly on the
auxiliary serial port. The receiver also accepts
DGPS corrections data formatted as a binary data
input message (Message 1351) over its primary
serial port. (Detailed information on the format of
this message is provided in Section 3.5.2.19.)
4.7.4.1 The RTCM protocol
The Jupiter will accept 6-of-8 RTCM SC-104 data
directly from the auxiliary serial port. No external
formatting is required and the receiver handles
all parsing and verification of the data. The user
needs only to verify the integrity of the data sent to
the receiver to ensure that high bit errors are not
present in the detected RTCM raw data stream.
The user should be aware that RTCM SC-104
data will be used only if, for every 30-bit word, the
syndrome (6-bit parity) exactly matches the one
which should occur on the basis of the 24-bit data
seen in each word. No attempt will be made to
correct single bit errors; any syndrome mismatch
will cause rejection of that data word and rejection
of the message in which it exists.
The receiver will parse the incoming data bits
and decode all of the RTCM SC-104 messages.
Those messages required for DGPS operation will
be used to fill in the DGPS database within the
receiver. Those messages which are not used will
be discarded.
4.7.4.2 The RTCM message types
The receiver accepts DGPS correction data as a
subset of the 64 RTCM SC-104 messages found
in Table 4-2 of the RTCM SC-1 04 Version 2.1
standard. Though the receiver will accept and
decode all RTCM messages, not all messages are
necessary for DGPS operation.
The Data Sheet for each of the Jupiter GPS
receivers shows which of the messages defined
in the RTCM standard are used by the receiver
to form a DGPS position solution. Refer to the
standard for more detailed descriptions of these
and other RTCM SC-104 messages.
Type 1 message
Type 1 messages contain pseudo-range and
pseudo-range rate corrections for a complete
set of visible satellites. Currently, this is the
most common type of message transmitted by
commercial RTCM providers and base stations.
Type 2 message
Type 2 messages contain delta corrections and
are transmitted by reference stations to help
receivers during ephemeris cutovers. These Table
4-2. Parameters And Data Maintained In EEPROM
messages are used by the field receiver in
conjunction with Type 1 or Type 9 messages until
both the reference station and field receiver are
operating with the same set of ephemeris.
Type 9 message
Type 9 messages have the same format as Type
1 messages, but usually only contain corrections
for a subset of the visible constellation. These
messages are typically transmitted at a higher
rate than the Type 1 messages. Beacons, such
as those operated by the U.S. Coast Guard, are
currently the primary source for these corrections,
but they are also available from some commercial
service providers and base stations.
4.7.4.3 Compliance with RTCM SC-I04 requirements
The Radio Technical Commission for Maritime
Services (RTCM) has a special committee
numbered 104 (SC-I04). Its charter is to create
recommended standards for the transmission of
DGPS correction data.
4.7.4.4 DGPS initialisation and configuration.
At power-on, the receiver initialises its internal
DGPS database to indicate that no valid
DGPS data is available. If the user requests
the Differential GPS Status message (binary
Message 1005), the message will indicate that no
corrections have been processed. Some of the
position status messages (binary messages 1000
and 1001, and NMEA message GGA) will also
indicate that the receiver is not computing a DGPS
solution.
As sufficient valid RTCM data is passed to the
receiver, it will automatically produce DGPS
solutions. Other than supplying RTCM data and
ensuring that DGPS operation is not disabled, no
action is required on the part of the user to cause
DGPS operation.
The receiver will compute DGPS solutions
whenever all of the following conditions are
satisfied: