4 SBF Reference
size of the sub-blocks is mentioned explicitly in the block description, SBF blocks that contain
sub-blocks also contain a
SBLength
field, which indicates the size of the sub-blocks in bytes.
4.1.5
Padding Bytes
Padding bytes are foreseen at the end of most SBF blocks and sub-blocks, so that their total
size is equal to
Length
or
SBLength
respectively. The padding bytes are just placeholders
and should not be looked at by the decoding software. Their value is not defined.
4.1.6
SBF Revision Number
Each SBF block has an associated revision number. The revision number is incremented each
time a backwards-compatible change is implemented.
As described in section 4.1.1, the block number is to be found in bits 0 to 12 of the
ID
field,
and the revision is in bits 13 to 15 of that field.
A backwards-compatible change consists of adding one or more fields in the padding bytes,
or in the fields marked as "reserved" in the block description. Such change should be unno-
ticed by properly written decoding software that ignore the contents of padding and reserved
fields (see also section 4.1.12). Each time such change happens, the revision number is in-
cremented. The revision at which a given field has been introduced is documented in the
block description in chapter 4.2, unless that revision is 0 (see the
block as
an example). It is guaranteed that if a given field exists in revision N, it will also exist in all
revisions after N: no fields are withdrawn from SBF.
4.1.7
Do-Not-Use Value
It might happen that one or more pieces of data in an SBF block are not known at block cre-
ation time. For example, when there are insufficient satellite measurements to compute a po-
sition solution, the position components found in the
X
,
Y
and
Z
fields of the
block will not be available. To indicate that a given data item is not available or is currently
not provided by the receiver, the corresponding field is set to a ’Do-Not-Use’ value that is
never reached in normal operation.
When applicable, the Do-Not-Use value is mentioned in the block description. The Do-Not-
Use value refers to the raw contents of the field, without applying the scale factor. A field set
to its Do-Not-Use value should always be discarded by the decoding software.
4.1.8
Output Rate
In general, the default output rate for each SBF block is the renewal rate of the information.
For instance, the
block is output each time a new ephemeris data set is received
from a given GPS satellite. The default output rates of GNSS measurement blocks, PVT blocks
and integrated INS/GNSS blocks depend on your permission set. These three rates can be
checked by the command
297
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