Guide to WMO Table Driven Code Forms

the new table entries can be used operationally a full two years before the “Full Operational
Implementation” date. In this light, the significance of the “Full Operational Implementation” date
is to mark the publication of the changes in the WMO Manual on Codes.
1.3.4 Validation of Updates
Whether requiring corresponding modifications to processing software or not, all changes must be
validated by a procedure required by the CBS. Under this procedure, proposed changes should
be tested by the use of two independently developed encoders and two independently developed
decoders that incorporate the proposed change. However, where the data originated from a
necessarily unique source (e.g., the data stream from an experimental satellite), the successful
testing of a single encoder with at least two independent decoders is considered adequate.
For those recommendations that are considered by the full CBS for approval, CBS may either
approve or not approve but not alter them. All changes to the WMO table-driven code forms
BUFR, CREX, and GRIB are documented in the form of supplements to the WMO Manual on
Codes. However, these supplements are issued no more than once a year.
In the above discussion, a distinction was made between the validation process for those changes
that require corresponding changes to processing software and those changes that do not.
Existing software can be used to validate table additions that do not require software changes.
Consequently, they are relatively simple to validate. The first step is to verify the table entries are
consistent with the original request, a simple pencil and paper exercise. Then, sample data can
be prepared and tested with existing software as indicated above.
The effort needed to validate changes that do require corresponding modifications to processing
software, however, can vary widely. A fairly simple example is the addition of a new grid or
product definition template. Most computer software is sufficiently modular that such changes can
be isolated and tested as required by the CBS rather easily. On the other hand, the validating of
GRIB2 is perhaps one of the most complex examples of validation. Although sharing many
aspects with GRIB1, the structure of GRIB2 is sufficiently different that virtually new processing
software is required. This software had to be written and tested before the validations could be
done. The validations that were performed for the GRIB2 Code Form involved not two centres but
six. Furthermore, at least two independent encoders and two independent decoders tested each
of the 53 templates of GRIB2. Consequently, the GRIB2 validation process took many computer
runs by six centres over a period of over two years. Clearly, this testing effort was several orders
of magnitude greater that that which will be required for the addition of a single template.
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