Parity Information (PI)
Parity Information (PI) is used both on uplink and downlink to check the quality of the decoded data. The parity field are 24 bits calculated on the sending end, used to detect errors in data transmission between interrogator and transponder. It uses a modified cyclic redundancy code based on the polynomial:
| G(x) = x24 + x23 + x22 + x21 + x20 + x19 + x18 + x17 +
+ x16 + x15 + x14 + x13 + x10 + x3 + 1
The parity information is combined with the 24-bit aircraft address and added to the message. The parity information will be recalculated at the receiving end and then compared with the combined parity and aircraft address. This allows to confirm, that the message has been transmitted without error.
The used polynomial G(x) supports both error detection and error correction. To keep the transponder design as simple as possible, only error detection is used on the uplink.