Subject: WEFAX Image Format Modification From: Samuel Patterson Date: Wed, 25 Jun 1997 14:00:34 -0400 (EDT) The Information Processing Division plans to implement a change to the GOES WEFAX products (GOES-8 and GOES-9) on June 30, 1997, between 1315z and 1345z. The modification is to add a 16 step grayscale immediately after the Binary Coded Header. Testing has shown with the addition of the grayscale, the transmission window remains less than three minutes and forty seconds (maximum allotted time). The following will be the new format for GOES WEFAX imagery. GOES WEFAX Image Format: Start Tone The start tone is 5 seconds or 20 lines in duration and consists of the 2400 Hz sub-carrier being modulated by a 300 Hz square wave. Phase Signal The phasing signal is 5 seconds or 20 lines in duration. The modulation of the sub carrier during this portion of the transmissionis such that the first 12.5 milliseconds of each line is fully modulated. This is the minimum sub-carrier level (equivalent to black level). For the remainder of each line, 237.5 milliseconds, the sub-carrier is unmodulated and the 2400 Hz level is maximum or white. Binary Coded Header The WEFAX Binary-Coded Headers, BCH, sometimes referred to as computer readable headers, were designed around 1990 to provide a standard character encoded message to be used by receivers of WEFAX data for automatically accepting or rejecting incoming data. Each WEFAX line consists of a stream of data whose duration is 250 ms. Each line consists of two parts, the line synchronization and the image data. Each BCH consists of 50 characters evenly spaced across the image data part of a WEFAX product for 4 consecutive WEFAX lines immediately following the phasing signal. The line synchronization duration is %5 of the total line duration, ideally 12.5ms. However, due to equipment design and different image resolutions this varies from 12.35 ms for polar and AAA WEFAX products to 12.585ms for GVAR WEFAX products. This line synchronization is consistent throughout the U.S. WEFAX products. This differs from the METEOSAT WEFAX products which have a 5%, 12.5ms, synchronization during the phasing period but change to a 4.76%, 11.9ms, synchronization for the remainder of the image, including the binary coded header. Grayscale The grayscale is 5 seconds or 20 lines in duration. The grayscale has 16 segments going left to right from total black to total white. These segments are represented numerically by numbers going from 0 (black) to 255 (white) in a linear fashion. That is, the segments have values of 0, 17, 34, 51,...,255. To calculate the "percent black" of any segment, you would count the segment number starting from the right with the rightmost (white) segment taken as zero. Then, divide the segment number by the number 15 and multiply by 100 to get the percent of black. For example, the segment just to the right of the leftmost one would be segment number 14. To get the percent black, you would do: (14/15)*100 or approximately 93 percent. Data Signal: During the data portion of the transmission, approximately 200 seconds or approximately 800 lines in duration, approximately the first 12.5 milliseconds of each line is modulated by a rectangular signal that is 2/3 black and 1/3 white. The remainder of each line is modulated by the analog data signal. Stop Tone: The stop tone is 5 seconds or 20 lines in duration and consists of the 2400 Hz sub-carrier being modulated by a 450 Hz square wave. Samuel Patterson E/SP11 4700 Silver Hill Road Stop 9909 Washington DC, 20233-9909 Voice: 301-457-5205 Fax: 301-457-5199