=============================================================================== Robb Kambic Unidata Program Center Software Engineer III Univ. Corp for Atmospheric Research address@hidden WWW: http://www.unidata.ucar.edu/ =============================================================================== ---------- Forwarded message ---------- Date: Mon, 16 Aug 1999 17:29:58 -0400 From: Roland Blanchard <address@hidden> To: address@hidden Subject: LDM/Noaaport GRIB I have analyzed the reprocessed FOS GRIB data (Alden HRS feed) and the NOAAPORT/LDM GRIB data (LDM pqcat feed) and have made the following observations. After viewing the binary output of both GRIB bulletin feeds, I determined that both ALDEN HRS and LDM do not start the same at the beginning of the GRIB bulletin. Alden HRS feed starts <SOH> followed by <NL> followed by the three byte sequence number followed by <NL> followed by TTiiaa NNNN ddhhmm. LDM HRS feed starts <SOH> followed by <NL> followed by the three byte sequence number followed by a space character followed by <NL> followed by TTiiaa NNNN ddhhmm. The addition of a space character (hex 20) may be causing our decoder to not properly recognize the WMO header (TTiiaa NNNN ddhhmm). At the end of the GRIB bulletin, I noticed another difference between each feed. Alden HRS feed ends with 7777 <NL> <ETX> two-byte checksum. LDM HDS feed ends with 7777 <ETX> <NL> <ETX> two-byte checksum. Each 7 in the 7777 has a hex value of 37. The NL (newline) has a hex value of 0a. The ETX code has a hex value of 03. It is my understanding that the GRIB data is enveloped in a WMO header structure. If this is true then I do not understand why there is an extra <ETX> code. Is this possibly part of the NOAAPORT data transmission format? I have attached a sample listing of both the Alden HRS feed in a hexdump format and the LDM pqcat HDS feed in a hexdump listing. I deleted most of the data of the GRIB bulletin to show the format of the beginning and the end of each bulletin. I hope this can be of help in determining the cause of the differences between Alden HRS and LDM HDS. If you need any additional information, please let me know. Thanks, Roland
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Xdumpfos.txt
Description: Xdumpfos.txt (Text Document)
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Xdumpnp.txt
Description: Xdumpnp.txt (Text Document)