From 19c11509e79b106cb4dbb8fe54186ce84881a4fb Mon Sep 17 00:00:00 2001 From: Malcolm Robb Date: Mon, 29 Apr 2013 18:33:59 +0100 Subject: [PATCH] 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 --- dump1090.c | 139 +++++++++++++++++++++-------------------------------- 1 file changed, 55 insertions(+), 84 deletions(-) diff --git a/dump1090.c b/dump1090.c index c0c966d..92a9a68 100644 --- a/dump1090.c +++ b/dump1090.c @@ -56,7 +56,7 @@ // MinorVer changes when additional features are added, but not for bug fixes (range 00-99) // DayDate & Year changes for all changes, including for bug fixes. It represent the release date of the update // -#define MODES_DUMP1090_VERSION "1.04.2804.13" +#define MODES_DUMP1090_VERSION "1.04.2904.13" #define MODES_DEFAULT_RATE 2000000 #define MODES_DEFAULT_FREQ 1090000000 @@ -1316,41 +1316,58 @@ int bruteForceAP(unsigned char *msg, struct modesMessage *mm) { return (0); } // +// In the squawk (identity) field bits are interleaved as follows in +// (message bit 20 to bit 32): +// +// C1-A1-C2-A2-C4-A4-ZERO-B1-D1-B2-D2-B4-D4 +// +// So every group of three bits A, B, C, D represent an integer from 0 to 7. +// +// The actual meaning is just 4 octal numbers, but we convert it into a hex +// number tha happens to represent the four octal numbers. +// +// For more info: http://en.wikipedia.org/wiki/Gillham_code +// +int decodeGillhamField(int rawGillham) { + int hexGillham = 0; + + if (rawGillham & 0x1000) {hexGillham |= 0x0010;} // Bit 12 = C1 + if (rawGillham & 0x0800) {hexGillham |= 0x1000;} // Bit 11 = A1 + if (rawGillham & 0x0400) {hexGillham |= 0x0020;} // Bit 10 = C2 + if (rawGillham & 0x0200) {hexGillham |= 0x2000;} // Bit 9 = A2 + if (rawGillham & 0x0100) {hexGillham |= 0x0040;} // Bit 8 = C4 + if (rawGillham & 0x0080) {hexGillham |= 0x4000;} // Bit 7 = A4 + //if (rawGillham & 0x0040) {hexGillham |= 0x0800;} // Bit 6 = X or Q + if (rawGillham & 0x0020) {hexGillham |= 0x0100;} // Bit 5 = B1 + if (rawGillham & 0x0010) {hexGillham |= 0x0001;} // Bit 4 = D1 + if (rawGillham & 0x0008) {hexGillham |= 0x0200;} // Bit 3 = B2 + if (rawGillham & 0x0004) {hexGillham |= 0x0002;} // Bit 2 = D2 + if (rawGillham & 0x0002) {hexGillham |= 0x0400;} // Bit 1 = B4 + if (rawGillham & 0x0001) {hexGillham |= 0x0004;} // Bit 0 = D4 + + return (hexGillham); + } +// // Decode the 13 bit AC altitude field (in DF 20 and others). // Returns the altitude, and set 'unit' to either MODES_UNIT_METERS or MDOES_UNIT_FEETS. // -int decodeAC13Field(unsigned char *msg, int *unit) { - int msg2 = msg[2]; - int msg3 = msg[3]; - int m_bit = msg3 & 0x40; // set = meters, clear = feet - int q_bit = msg3 & 0x10; // set = 25 ft encoding, clear = Gillham Mode C encoding +int decodeAC13Field(int AC13Field, int *unit) { + int m_bit = AC13Field & 0x0040; // set = meters, clear = feet + int q_bit = AC13Field & 0x0010; // set = 25 ft encoding, clear = Gillham Mode C encoding + AC13Field &= 0x1FFF; // limit the field to 13 bits if (!m_bit) { *unit = MODES_UNIT_FEET; if (q_bit) { // N is the 11 bit integer resulting from the removal of bit Q and M - int n = ((msg2 & 0x1F) << 6) | - ((msg3 & 0x80) >> 2) | - ((msg3 & 0x20) >> 1) | - (msg3 & 0x0F); + int n = ((AC13Field & 0x1F80) >> 2) | + ((AC13Field & 0x0020) >> 1) | + (AC13Field & 0x000F); // The final altitude is resulting number multiplied by 25, minus 1000. return ((n * 25) - 1000); } else { // N is an 11 bit Gillham coded altitude - int n = 0; - if (msg2 & 0x10) {n |= 0x0010;} // Bit 20 = C1; - if (msg2 & 0x08) {n |= 0x1000;} // Bit 21 = A1; - if (msg2 & 0x04) {n |= 0x0020;} // Bit 22 = C2; - if (msg2 & 0x02) {n |= 0x2000;} // Bit 23 = A2; - if (msg2 & 0x01) {n |= 0x0040;} // Bit 24 = C4; - if (msg3 & 0x80) {n |= 0x4000;} // Bit 25 = A4; - if (msg3 & 0x20) {n |= 0x0100;} // Bit 27 = B1; - if (msg3 & 0x08) {n |= 0x0200;} // Bit 29 = B2; - if (msg3 & 0x04) {n |= 0x0002;} // Bit 30 = D2; - if (msg3 & 0x02) {n |= 0x0400;} // Bit 31 = B4; - if (msg3 & 0x01) {n |= 0x0004;} // Bit 32 = D4; - - n = ModeAToModeC(n); + int n = ModeAToModeC(decodeGillhamField(AC13Field)); if (n < -12) {n = 0;} return (100 * n); @@ -1364,33 +1381,20 @@ int decodeAC13Field(unsigned char *msg, int *unit) { // // Decode the 12 bit AC altitude field (in DF 17 and others). // -int decodeAC12Field(unsigned char *msg, int *unit) { - int msg5 = msg[5]; - int msg6 = msg[6]; - int q_bit = msg5 & 1; // Bit 48 = Q +int decodeAC12Field(int AC12Field, int *unit) { + int q_bit = AC12Field & 0x10; // Bit 48 = Q + AC12Field &= 0x0FFF; // limit the field to 12 bits *unit = MODES_UNIT_FEET; if (q_bit) { /// N is the 11 bit integer resulting from the removal of bit Q - int n = ((msg5 & 0xFE) << 3) | ((msg6 & 0xF0) >> 4); + int n = ((AC12Field & 0x0FE0) >> 1) | + (AC12Field & 0x000F); // The final altitude is the resulting number multiplied by 25, minus 1000. return ((n * 25) - 1000); } else { // N is an 11 bit Gillham coded altitude - int n = 0; - if (msg5 & 0x80) {n |= 0x0010;} // Bit 41 = C1; - if (msg5 & 0x40) {n |= 0x1000;} // Bit 42 = A1; - if (msg5 & 0x20) {n |= 0x0020;} // Bit 43 = C2; - if (msg5 & 0x10) {n |= 0x2000;} // Bit 44 = A2; - if (msg5 & 0x08) {n |= 0x0040;} // Bit 45 = C4; - if (msg5 & 0x04) {n |= 0x4000;} // Bit 46 = A4; - if (msg5 & 0x02) {n |= 0x0100;} // Bit 47 = B1; - if (msg6 & 0x80) {n |= 0x0200;} // Bit 49 = B2; - if (msg6 & 0x40) {n |= 0x0002;} // Bit 50 = D2; - if (msg6 & 0x20) {n |= 0x0400;} // Bit 51 = B4; - if (msg6 & 0x10) {n |= 0x0004;} // Bit 52 = D4; - - n = ModeAToModeC(n); + int n = n = ModeAToModeC(decodeGillhamField(AC12Field)); if (n < -12) {n = 0;} return (100 * n); @@ -1515,41 +1519,7 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) { mm->um = ((msg[1] & 7)<<3)| /* Request extraction of downlink request. */ msg[2]>>5; - /* In the squawk (identity) field bits are interleaved like that - * (message bit 20 to bit 32): - * - * C1-A1-C2-A2-C4-A4-ZERO-B1-D1-B2-D2-B4-D4 - * - * So every group of three bits A, B, C, D represent an integer - * from 0 to 7. - * - * The actual meaning is just 4 octal numbers, but we convert it - * into a hex number tha happens to represent the four - * octal numbers. - * - * For more info: http://en.wikipedia.org/wiki/Gillham_code */ - { - int hexSquawk = 0; - unsigned char rawSquawk; - - rawSquawk = msg[2]; - if (rawSquawk & 0x01) {hexSquawk |= 0x0040;} // C4 - if (rawSquawk & 0x02) {hexSquawk |= 0x2000;} // A2 - if (rawSquawk & 0x04) {hexSquawk |= 0x0020;} // C2 - if (rawSquawk & 0x08) {hexSquawk |= 0x1000;} // A1 - if (rawSquawk & 0x10) {hexSquawk |= 0x0010;} // C1 - - rawSquawk = msg[3]; - if (rawSquawk & 0x01) {hexSquawk |= 0x0004;} // D4 - if (rawSquawk & 0x02) {hexSquawk |= 0x0400;} // B4 - if (rawSquawk & 0x04) {hexSquawk |= 0x0002;} // D2 - if (rawSquawk & 0x08) {hexSquawk |= 0x0200;} // B2 - if (rawSquawk & 0x10) {hexSquawk |= 0x0001;} // D1 - if (rawSquawk & 0x20) {hexSquawk |= 0x0100;} // B1 - if (rawSquawk & 0x80) {hexSquawk |= 0x4000;} // A4 - - mm->modeA = hexSquawk; - } + mm->modeA = decodeGillhamField((msg[2] << 8) | msg[3]); /* DF 11 & 17: try to populate our ICAO addresses whitelist. * DFs with an AP field (xored addr and crc), try to decode it. */ @@ -1572,10 +1542,10 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) { } } - /* Decode 13 bit altitude for DF0, DF4, DF16, DF20 */ - if (mm->msgtype == 0 || mm->msgtype == 4 || + // Fields for DF0, DF4, DF16, DF20 13 bit altitude + if (mm->msgtype == 0 || mm->msgtype == 4 || mm->msgtype == 16 || mm->msgtype == 20) { - mm->altitude = decodeAC13Field(msg, &mm->unit); + mm->altitude = decodeAC13Field(((msg[2] << 8) | msg[3]), &mm->unit); } /* Decode extended squitter specific stuff. */ @@ -1598,7 +1568,7 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) { /* Airborne position Message */ mm->fflag = msg[6] & (1<<2); mm->tflag = msg[6] & (1<<3); - mm->altitude = decodeAC12Field(msg,&mm->unit); + mm->altitude = decodeAC12Field(((msg[5] << 4) | (msg[6] >> 4)), &mm->unit); mm->raw_latitude = ((msg[6] & 3) << 15) | (msg[7] << 7) | (msg[8] >> 1); @@ -2083,8 +2053,9 @@ void detectModeS(uint16_t *m, uint32_t mlen) { } } - // Don't forget to add 4 for the preamble samples. This also removes any risk of dividing by zero. - sigStrength /= 60; + // We measured signal strength over the first 56 bits. Don't forget to add 4 + // for the preamble samples, so round up and divide by 60. + sigStrength = (sigStrength + 29) / 60; /* If we reached this point, and error is zero, we are very likely * with a Mode S message in our hands, but it may still be broken