Coding the Radio Call Sign User Protocol with the Radio Call Sign

Radio call signs up to seven characters can be encoded in the Radio Call Sign User Protocol if the call sign complies with the ITU Radio Regulations on the formation of call signs (Article S19 of the Radio Regulations) whereby alphabetic characters are allowed only in the first four characters of the sequence. The first four characters are coded with the modified-Baudot code in bits 40 to 63. The final three digits are coded in binary-coded decimal form in bits 64 to 75.

If the Radio Call Sign User Protocol is used for a call sign of less than seven characters, the call sign shall be left justified (i.e. data on the left) and padded to the right of the last character with "space characters" (represented by "1010") - see example. Note this is different from the procedure used for padding spaces of the Maritime User Protocol when encoded with the vessel's radio call sign.

Characters coded with the modified-Baudot code can be either alphabetic or numeric; characters coded in binary-coded decimal (BCD) must be numeric.

Note: Binary Coded Decimal (BCD) is not the same as the decimal value of the bits. Rather the BCD value of bits 64 to 75 is determined by grouping the bits into blocks of 4 binary digits each. The BCD value is the concatenation of the decimal value of each group. For example the radio call sign "ABC123" would be encoded as follows:

A B C 1 2 3

111000 110011 101110 011101 0010 0011 1010