Mode S Gillham altitude changes
Modify all the Mode=S Gillham altitude decoding to use a new function decodeGillhamField() Change the Mode-S signal strength so that it rounds to the nearest integer
This commit is contained in:
Malcolm Robb
parent
e161b7662c
commit
19c11509e7
139
dump1090.c
139
dump1090.c
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@ -56,7 +56,7 @@
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// MinorVer changes when additional features are added, but not for bug fixes (range 00-99)
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// DayDate & Year changes for all changes, including for bug fixes. It represent the release date of the update
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//
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#define MODES_DUMP1090_VERSION "1.04.2804.13"
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#define MODES_DUMP1090_VERSION "1.04.2904.13"
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#define MODES_DEFAULT_RATE 2000000
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#define MODES_DEFAULT_FREQ 1090000000
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@ -1316,41 +1316,58 @@ int bruteForceAP(unsigned char *msg, struct modesMessage *mm) {
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return (0);
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}
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//
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// In the squawk (identity) field bits are interleaved as follows in
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// (message bit 20 to bit 32):
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//
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// C1-A1-C2-A2-C4-A4-ZERO-B1-D1-B2-D2-B4-D4
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//
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// So every group of three bits A, B, C, D represent an integer from 0 to 7.
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//
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// The actual meaning is just 4 octal numbers, but we convert it into a hex
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// number tha happens to represent the four octal numbers.
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//
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// For more info: http://en.wikipedia.org/wiki/Gillham_code
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//
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int decodeGillhamField(int rawGillham) {
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int hexGillham = 0;
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if (rawGillham & 0x1000) {hexGillham |= 0x0010;} // Bit 12 = C1
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if (rawGillham & 0x0800) {hexGillham |= 0x1000;} // Bit 11 = A1
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if (rawGillham & 0x0400) {hexGillham |= 0x0020;} // Bit 10 = C2
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if (rawGillham & 0x0200) {hexGillham |= 0x2000;} // Bit 9 = A2
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if (rawGillham & 0x0100) {hexGillham |= 0x0040;} // Bit 8 = C4
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if (rawGillham & 0x0080) {hexGillham |= 0x4000;} // Bit 7 = A4
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//if (rawGillham & 0x0040) {hexGillham |= 0x0800;} // Bit 6 = X or Q
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if (rawGillham & 0x0020) {hexGillham |= 0x0100;} // Bit 5 = B1
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if (rawGillham & 0x0010) {hexGillham |= 0x0001;} // Bit 4 = D1
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if (rawGillham & 0x0008) {hexGillham |= 0x0200;} // Bit 3 = B2
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if (rawGillham & 0x0004) {hexGillham |= 0x0002;} // Bit 2 = D2
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if (rawGillham & 0x0002) {hexGillham |= 0x0400;} // Bit 1 = B4
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if (rawGillham & 0x0001) {hexGillham |= 0x0004;} // Bit 0 = D4
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return (hexGillham);
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}
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//
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// Decode the 13 bit AC altitude field (in DF 20 and others).
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// Returns the altitude, and set 'unit' to either MODES_UNIT_METERS or MDOES_UNIT_FEETS.
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//
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int decodeAC13Field(unsigned char *msg, int *unit) {
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int msg2 = msg[2];
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int msg3 = msg[3];
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int m_bit = msg3 & 0x40; // set = meters, clear = feet
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int q_bit = msg3 & 0x10; // set = 25 ft encoding, clear = Gillham Mode C encoding
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int decodeAC13Field(int AC13Field, int *unit) {
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int m_bit = AC13Field & 0x0040; // set = meters, clear = feet
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int q_bit = AC13Field & 0x0010; // set = 25 ft encoding, clear = Gillham Mode C encoding
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AC13Field &= 0x1FFF; // limit the field to 13 bits
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if (!m_bit) {
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*unit = MODES_UNIT_FEET;
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if (q_bit) {
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// N is the 11 bit integer resulting from the removal of bit Q and M
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int n = ((msg2 & 0x1F) << 6) |
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((msg3 & 0x80) >> 2) |
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((msg3 & 0x20) >> 1) |
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(msg3 & 0x0F);
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int n = ((AC13Field & 0x1F80) >> 2) |
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((AC13Field & 0x0020) >> 1) |
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(AC13Field & 0x000F);
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// The final altitude is resulting number multiplied by 25, minus 1000.
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return ((n * 25) - 1000);
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} else {
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// N is an 11 bit Gillham coded altitude
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int n = 0;
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if (msg2 & 0x10) {n |= 0x0010;} // Bit 20 = C1;
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if (msg2 & 0x08) {n |= 0x1000;} // Bit 21 = A1;
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if (msg2 & 0x04) {n |= 0x0020;} // Bit 22 = C2;
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if (msg2 & 0x02) {n |= 0x2000;} // Bit 23 = A2;
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if (msg2 & 0x01) {n |= 0x0040;} // Bit 24 = C4;
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if (msg3 & 0x80) {n |= 0x4000;} // Bit 25 = A4;
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if (msg3 & 0x20) {n |= 0x0100;} // Bit 27 = B1;
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if (msg3 & 0x08) {n |= 0x0200;} // Bit 29 = B2;
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if (msg3 & 0x04) {n |= 0x0002;} // Bit 30 = D2;
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if (msg3 & 0x02) {n |= 0x0400;} // Bit 31 = B4;
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if (msg3 & 0x01) {n |= 0x0004;} // Bit 32 = D4;
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n = ModeAToModeC(n);
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int n = ModeAToModeC(decodeGillhamField(AC13Field));
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if (n < -12) {n = 0;}
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return (100 * n);
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@ -1364,33 +1381,20 @@ int decodeAC13Field(unsigned char *msg, int *unit) {
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//
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// Decode the 12 bit AC altitude field (in DF 17 and others).
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//
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int decodeAC12Field(unsigned char *msg, int *unit) {
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int msg5 = msg[5];
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int msg6 = msg[6];
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int q_bit = msg5 & 1; // Bit 48 = Q
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int decodeAC12Field(int AC12Field, int *unit) {
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int q_bit = AC12Field & 0x10; // Bit 48 = Q
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AC12Field &= 0x0FFF; // limit the field to 12 bits
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*unit = MODES_UNIT_FEET;
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if (q_bit) {
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/// N is the 11 bit integer resulting from the removal of bit Q
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int n = ((msg5 & 0xFE) << 3) | ((msg6 & 0xF0) >> 4);
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int n = ((AC12Field & 0x0FE0) >> 1) |
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(AC12Field & 0x000F);
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// The final altitude is the resulting number multiplied by 25, minus 1000.
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return ((n * 25) - 1000);
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} else {
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// N is an 11 bit Gillham coded altitude
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int n = 0;
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if (msg5 & 0x80) {n |= 0x0010;} // Bit 41 = C1;
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if (msg5 & 0x40) {n |= 0x1000;} // Bit 42 = A1;
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if (msg5 & 0x20) {n |= 0x0020;} // Bit 43 = C2;
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if (msg5 & 0x10) {n |= 0x2000;} // Bit 44 = A2;
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if (msg5 & 0x08) {n |= 0x0040;} // Bit 45 = C4;
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if (msg5 & 0x04) {n |= 0x4000;} // Bit 46 = A4;
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if (msg5 & 0x02) {n |= 0x0100;} // Bit 47 = B1;
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if (msg6 & 0x80) {n |= 0x0200;} // Bit 49 = B2;
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if (msg6 & 0x40) {n |= 0x0002;} // Bit 50 = D2;
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if (msg6 & 0x20) {n |= 0x0400;} // Bit 51 = B4;
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if (msg6 & 0x10) {n |= 0x0004;} // Bit 52 = D4;
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n = ModeAToModeC(n);
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int n = n = ModeAToModeC(decodeGillhamField(AC12Field));
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if (n < -12) {n = 0;}
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return (100 * n);
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@ -1515,41 +1519,7 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
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mm->um = ((msg[1] & 7)<<3)| /* Request extraction of downlink request. */
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msg[2]>>5;
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/* In the squawk (identity) field bits are interleaved like that
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* (message bit 20 to bit 32):
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*
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* C1-A1-C2-A2-C4-A4-ZERO-B1-D1-B2-D2-B4-D4
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*
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* So every group of three bits A, B, C, D represent an integer
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* from 0 to 7.
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*
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* The actual meaning is just 4 octal numbers, but we convert it
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* into a hex number tha happens to represent the four
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* octal numbers.
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*
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* For more info: http://en.wikipedia.org/wiki/Gillham_code */
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{
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int hexSquawk = 0;
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unsigned char rawSquawk;
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rawSquawk = msg[2];
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if (rawSquawk & 0x01) {hexSquawk |= 0x0040;} // C4
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if (rawSquawk & 0x02) {hexSquawk |= 0x2000;} // A2
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if (rawSquawk & 0x04) {hexSquawk |= 0x0020;} // C2
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if (rawSquawk & 0x08) {hexSquawk |= 0x1000;} // A1
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if (rawSquawk & 0x10) {hexSquawk |= 0x0010;} // C1
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rawSquawk = msg[3];
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if (rawSquawk & 0x01) {hexSquawk |= 0x0004;} // D4
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if (rawSquawk & 0x02) {hexSquawk |= 0x0400;} // B4
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if (rawSquawk & 0x04) {hexSquawk |= 0x0002;} // D2
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if (rawSquawk & 0x08) {hexSquawk |= 0x0200;} // B2
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if (rawSquawk & 0x10) {hexSquawk |= 0x0001;} // D1
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if (rawSquawk & 0x20) {hexSquawk |= 0x0100;} // B1
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if (rawSquawk & 0x80) {hexSquawk |= 0x4000;} // A4
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mm->modeA = hexSquawk;
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}
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mm->modeA = decodeGillhamField((msg[2] << 8) | msg[3]);
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/* DF 11 & 17: try to populate our ICAO addresses whitelist.
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* DFs with an AP field (xored addr and crc), try to decode it. */
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@ -1572,10 +1542,10 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
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}
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}
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/* Decode 13 bit altitude for DF0, DF4, DF16, DF20 */
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if (mm->msgtype == 0 || mm->msgtype == 4 ||
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// Fields for DF0, DF4, DF16, DF20 13 bit altitude
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if (mm->msgtype == 0 || mm->msgtype == 4 ||
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mm->msgtype == 16 || mm->msgtype == 20) {
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mm->altitude = decodeAC13Field(msg, &mm->unit);
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mm->altitude = decodeAC13Field(((msg[2] << 8) | msg[3]), &mm->unit);
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}
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/* Decode extended squitter specific stuff. */
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@ -1598,7 +1568,7 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
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/* Airborne position Message */
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mm->fflag = msg[6] & (1<<2);
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mm->tflag = msg[6] & (1<<3);
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mm->altitude = decodeAC12Field(msg,&mm->unit);
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mm->altitude = decodeAC12Field(((msg[5] << 4) | (msg[6] >> 4)), &mm->unit);
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mm->raw_latitude = ((msg[6] & 3) << 15) |
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(msg[7] << 7) |
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(msg[8] >> 1);
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@ -2083,8 +2053,9 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
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}
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}
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// Don't forget to add 4 for the preamble samples. This also removes any risk of dividing by zero.
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sigStrength /= 60;
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// We measured signal strength over the first 56 bits. Don't forget to add 4
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// for the preamble samples, so round up and divide by 60.
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sigStrength = (sigStrength + 29) / 60;
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/* If we reached this point, and error is zero, we are very likely
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* with a Mode S message in our hands, but it may still be broken
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