Training on Meteorological Telecommunications Alanya, Turkey, September 2010 General Philosophy of Table Driven Code Forms Simon Elliott, EUMETSAT Eva Červená, CHMI

Training on Meteorological Telecommunications Alanya, Turkey, September 2010  Code Structures  BUFR and CREX Tables  Features Common to BUFR and CREX  Differences Between BUFR and CREX

Training on Meteorological Telecommunications Alanya, Turkey, September 2010 Table driven codes GRIB - Processed data in the form of grid- point values expressed in binary form. –Forecast, analysis, climatology, satellite images and products BUFR - Binary universal form for the representation of meteorological data. –Observations, satellite products CREX - Character form for the representation and exchange of data. –Like simple BUFR but human readable ASCII

Training on Meteorological Telecommunications Alanya, Turkey, September 2010 General structure Indicator section: GRIB/BUFR/CREX Identification section: Date, time, originator, table versions... Optional section: Metadata (potentially XML), private data … Data description: What sort of data follows Data section: actual data here Closure: “7777” Table driven codes generally have this structure 

Training on Meteorological Telecommunications Alanya, Turkey, September 2010 Some features Data are self defining: type, content, edition, section lengths, etc. Data identification is at fixed offsets for quick access without decoding Binary data (GRIB and BUFR) are not really human readable Optional section can contain anything

Training on Meteorological Telecommunications Alanya, Turkey, September 2010 Consider BUFR Table B - Element descriptors –Start with ‘0’, temperature, pressure, etc. Table C - Operator descriptors –Start with ‘2’, change of scale, qc follows, etc. Table D - Sequence descriptors –Start with ‘3’, wind sequence, high accuracy location sequence, etc. Code and flag tables - –Type of cloud, vertical sounding significance, etc.

Training on Meteorological Telecommunications Alanya, Turkey, September 2010 Table B Example: Cloud cover (total) % Data width (bits) Element name Element descriptor Unit Class 20, observed phenomena Reference value Scale Entry in class

Training on Meteorological Telecommunications Alanya, Turkey, September 2010 Scale, reference value and data width Encoding: value = (obs x [10^”scale”]) - “reference” e.g.: Let scale=1, reference=-50, width=7 then, if obs=-1.5, encoded value is 35 allows obs from -5.0 to +7.6, steps of 0.1 Decoding: obs = (value + “reference”) ÷ [10^”scale”] e.g.: Let scale=-2, reference=5, width=4 then, if value=14, decoded value is 1900 allows obs from 500 to 1900, steps of 100

Training on Meteorological Telecommunications Alanya, Turkey, September 2010 Table C Operator descriptors for more complex tasks, e.g. –Change width –Change scale –Change reference –Quality control information –First order statistics follow –Define “data present” bit map –Re-use “data present” bit map –Cancel “data present” bit map

Training on Meteorological Telecommunications Alanya, Turkey, September 2010 Table D Not strictly required Saves space in Section 3 (list of descriptors) One table D entry represents an agreed list of table B and/or C and/or D descriptors = Year Month Day

Training on Meteorological Telecommunications Alanya, Turkey, September 2010 Differences between BUFR and CREX BUFR is binary, better for computers BUFR has a compression scheme Encoded values are non- negative in BUFR Wide range of operators is available in BUFR BUFR mostly uses SI units (for OPMET data, kt, ft, ˚C are allowed) CREX is ASCII, easier for humans to read CREX has no explicit compression scheme Encoded values may be negative in CREX Only a subset of BUFR operators is used in CREX CREX allows other units (temperature can be Celsius or Kelvin) In general, BUFR and CREX are quite similar. CREX may be used as an interim solution if computers are not available or telecoms are inadequate...