12-Bit Second BCH Error Correcting Field (Bits 133 - 144)

The 26 bits forming the second protected data field (PDF-2, bits 107 to 132) are followed by a 12-bit Bose-Chaudhuri-Hocquenghem (BCH) error-correcting code, in a field defined as the second BCH error correcting field (BCH-2, bits 133 to 144). This code can detect and correct up to two bit errors in the 38 bits of the second protected field (SPF=PDF-2 + BCH-2). The error-correcting code is generated from the content of PDF-2 as described in Annex B of Cospas-Sarsat Beacon Specification C/S T.001 (click here to download a copy of document C/S T.001 in PDF format). Therefore, it must be computed and encoded together with the PDF-2.

In the case of location protocol beacons, when updates to the PDF-2 are required for encoding position data, the error-correcting code (BCH-2) must be regenerated after each update.

The 12-bit BCH error-correcting code must be computed and encoded in the second BCH error correcting field (BCH-2) of all protocols using the long message format (with the exception of orbitography protocol which are intended for use only by the Cospas-Sarsat System operators).

The terminology used in Cospas-Sarsat to define the second protected field (SPF), the second protected data field (PDF-2) and second BCH error correcting field (BCH-2) is illustrated below.