AS 680/682
Supervisor Operation Description, Operation and Maintenance
ADS−B
5−22 Ed. 10.07
5.4.8 Page ’Telegram Check’
On this page, see Fig. 5−12, test criteria for telegram validation are defined. Any telegram that does
not pass these tests is discarded. This page is used to fine tune the probability of detection versus
the false alarm rate.
Fig. 5−12 Page ’Telegram Check’ (example)
5.4.8.1 Confidence Bits, AP Residuum and Signal Amplitude
Confidence bits are generated for all telegrams. The number of telegram bits, called confidence bits,
is counted and checked versus a given threshold. If the given confidence bits threshold is reached,
the telegram will be discarded. If the value of the parameter equals −1, the confidence check is disa-
bled.
The ’AP Field Residuum Check’ enables the check on a correct residuum of the parity check for squit-
ters. This check is only applicable of squitters containing an AP−field. If the residuum is equal to zero,
the information contained within the telegram data block is considered correct. The AP residuum
check is more reliable and overrules the confidence bit check.
The signal amplitude level determines the minimum amplitude of a signal to represent a valid tele-
gram. If the value of the parameter equals −1, the amplitude check is disabled.
5.4.8.2 Error Correction
Enabling error correction means that the SPB tries to correct damaged telegrams. Error correction
is only performed, if the AP−Field is not 0 and only one confidence bit is set, i.e. only one telegram
bit is bad.
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