Navman 11 - Manual (Page 10)

10 MN002000A © 2004 Navman NZ Ltd. All rights reserved. Proprietary information and specifications subject to change without notice.
during the next output interval. Standard log
requests will be accepted if the log (L) bit is set
and if the required data parameters are present in
the data portion of the request message.
3.2.5 Message header word 5
Word 5 of the message header is the data
checksum, used to validate the header portion of
the message. It is computed by summing (modulo
216) all words (including the word containing
DEL and SOH) contained in the header and then
performing a two’s complement on the sum.
SUM = Mod 2 16 Σ 4 word(i)
The computation of the header checksum may be
expressed mathematically as:
if sum = –32768, header checksum = SUM; else
header checksum = –SUM
where:
a. Unary negation is computed as the two’s
complement of a 16-bit data word.
b. Mod 216 indicates the least 16 bits of an
arithmetic process. That is, carry bits from bit
position 16 are ignored.
c. The summation is the algebraic binary sum
of the words indicated by the subscript i.
d. The –32768 sum value must be treated as a
special case since it cannot be negated.
(NOTE: A CURRENT BUG CAUSES CHECKSUM
ERRORS FOR A VALUE OF ZERO or –32 768)
3.2.6 Log request messages
Figure 3-2 shows the format of the data portion
of standard log request messages. The ranges
for words 6, 7, and 8 of these messages are as
follows:
Trigger: 0 = on time, 1 = on update
Interval: 0 to 65535 seconds (an interval of zero
produces a query as if the query bit [Q] in word 4
of the message header has been set).
Offset relative to the next even minute, zero to
60 seconds. An offset of zero specifies an initial
output relative to the current time, an offset of 60
specifies an initial output seconds into the next
minute.
When the Trigger field is set to ‘on time’ (integer
value 0), the first output will occur at the next
‘offset’ seconds into the minute, and will repeat
every ‘interval’ seconds thereafter. When the
trigger field is set to ‘on update’, the specified
message will be output only when the data is
updated (e.g. when satellite almanac is collected).
3.3 Binary message data
The data portion of a binary message, if it exists,
can be variable in length, as specified by the
data word count found in the header. The data
checksum follows the data and is not included in
the data word count.
The data checksum is a 16-bit word used to
validate the data portion of the message. It is
transmitted as the last word of any message
containing data (see Figure 3-2).
When the word count field is zero, the data
checksum does not exist. It is computed by
summing (modulo 216) all words in the data
portion of the message and then complementing
that sum. The mathematical expression for the
data checksum is:
SUM = Mod 2 16 Σ 5+n word(i)
If sum = –32 768, data checksum = SUM; else data
checksum = –SUM
where:
a. Unary negation is computed as the two’s
complement of a 16-bit data word.
b Mod 216 indicates the least 16 bits of an
arithmetic process. That is, carry bits from bit
position 16 are ignored.
c. The summation is the algebraic binary sum
of the words indicated by the subscript (i).
d. The –32 768 sum value must be treated as a
special case since it cannot be negated.
(NOTE: A CURRENT BUG CAUSES CHECKSUM
ERRORS FOR A VALUE OF ZERO or –32 768)
Data elements identified as ‘reserved’ must be set
to 5+N zero for input messages and are undefined
for output messages. All data storage that is not
explicitly 1-6 defined should be handled as if
marked ‘reserved’. Unless otherwise stated, the
resolution of each numeric data item is one integer
unit, as specified by that item in the ‘units’ field.
3.4 NMEA messages, format, and
sentence structure
NMEA messages are output in response to
standard Query (Q) or proprietary Log Control
(ILOG) messages as described in Section 3.6. The
timing of output messages is synchronised with the
time mark output event.
3.4.1 NMEA output messages
The following supported NMEA output messages
comply with the NMEA-0183 version 2.01
standard:
GGA: GPS fix data
GSA: GPS DOP and active satellites
GSV: GPS satellites in view
i=1
i=1