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adsb: fix BDS 2,1 registration character decoding

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Fix an incorrect bit shift when decoding aircraft registration
markings from BDS 2,1 Comm-B messages.

Coverity (CID 649088) reported that the expression
'(data[7] & 0x1) >> 3' always evaluates to zero, causing one bit of
the 6-bit character value to be discarded.

The BDS 2,1 aircraft registration marking format encodes seven
6-bit characters. Based on the bit layout defined in ICAO Doc 9871
(Technical Provisions for Mode S Specific Services) and verified
against independent implementations such as pyModeS/rs1090, the
remaining bit from data[7] is the most significant bit of the
character and must be shifted left, not right.

Update the extraction to preserve this bit when combining it with
the remaining bits from data[8].

Signed-off-by: Robin Getz <rgetz503@gmail.com>
This commit is contained in:
Robin Getz
2026-07-12 19:35:04 -04:00
parent 810b0524fb
commit b834b80f05
1 changed files with 1 additions and 1 deletions
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plugins/channelrx/demodadsb/adsbdemodgui.cpp
+1 -1
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@@ -3899,7 +3899,7 @@ void ADSBDemodGUI::decodeCommB(const QByteArray data, const QDateTime dateTime,
c[2] = ((data[5] & 0x7) << 3) | ((data[6] >> 5) & 0x7);
c[3] = ((data[6] & 0x1f) << 1) | ((data[7] >> 7) & 0x1);
c[4] = ((data[7] >> 1) & 0x1f);
c[5] = ((data[7] & 0x1) >> 3) | ((data[8] >> 3) & 0x1f);
c[5] = ((data[7] & 0x1) << 5) | ((data[8] >> 3) & 0x1f);
c[6] = ((data[8] & 0x7) << 3) | ((data[9] >> 5) & 0x7);
// Map to ASCII
for (int i = 0; i < 7; i++) {