21-Bit First BCH Error Correcting Field (Bits 86 - 106)

The 61 bits forming the first protected data field (PDF-1, bits 25 to 85) are followed by a 21-bit Bose-Chaudhuri-Hocquenghem (BCH) error-correcting code, in a field defined as the first BCH error correcting field (BCH-1, bits 86 to 106). This code can detect and correct up to three bit errors in the 82 bits of the first protected field (FPF=PDF-1 + BCH-1). The error-correcting code is generated from the content of PDF-1 as described in Annex B of document C/S T.001 "Cospas-Sarsat Beacon Specification". Therefore, it must be computed and encoded together with the PDF-1.

In the case of location protocol beacons, when updates to the PDF-1 are required for encoding position data, the error-correcting code (BCH-1) must be regenerated after each update.

The terminology used by Cospas-Sarsat to define the first protected field (FPF), the first protected data field (PDF-1) and first BCH error correcting field (BCH-1) is illustrated below.

PFD