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Mode S Gillham altitude changes

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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 2013-04-29 18:33:59 +01:00
parent e161b7662c
commit 19c11509e7
1 changed files with 55 additions and 84 deletions
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139
dump1090.c
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@ -56,7 +56,7 @@
// MinorVer changes when additional features are added, but not for bug fixes (range 00-99) // 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 // 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_RATE 2000000
#define MODES_DEFAULT_FREQ 1090000000 #define MODES_DEFAULT_FREQ 1090000000
@ -1316,41 +1316,58 @@ int bruteForceAP(unsigned char *msg, struct modesMessage *mm) {
return (0); 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). // 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. // Returns the altitude, and set 'unit' to either MODES_UNIT_METERS or MDOES_UNIT_FEETS.
// //
int decodeAC13Field(unsigned char *msg, int *unit) { int decodeAC13Field(int AC13Field, int *unit) {
int msg2 = msg[2]; int m_bit = AC13Field & 0x0040; // set = meters, clear = feet
int msg3 = msg[3]; int q_bit = AC13Field & 0x0010; // set = 25 ft encoding, clear = Gillham Mode C encoding
int m_bit = msg3 & 0x40; // set = meters, clear = feet AC13Field &= 0x1FFF; // limit the field to 13 bits
int q_bit = msg3 & 0x10; // set = 25 ft encoding, clear = Gillham Mode C encoding
if (!m_bit) { if (!m_bit) {
*unit = MODES_UNIT_FEET; *unit = MODES_UNIT_FEET;
if (q_bit) { if (q_bit) {
// N is the 11 bit integer resulting from the removal of bit Q and M // N is the 11 bit integer resulting from the removal of bit Q and M
int n = ((msg2 & 0x1F) << 6) | int n = ((AC13Field & 0x1F80) >> 2) |
((msg3 & 0x80) >> 2) | ((AC13Field & 0x0020) >> 1) |
((msg3 & 0x20) >> 1) | (AC13Field & 0x000F);
(msg3 & 0x0F);
// The final altitude is resulting number multiplied by 25, minus 1000. // The final altitude is resulting number multiplied by 25, minus 1000.
return ((n * 25) - 1000); return ((n * 25) - 1000);
} else { } else {
// N is an 11 bit Gillham coded altitude // N is an 11 bit Gillham coded altitude
int n = 0; int n = ModeAToModeC(decodeGillhamField(AC13Field));
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);
if (n < -12) {n = 0;} if (n < -12) {n = 0;}
return (100 * n); 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). // Decode the 12 bit AC altitude field (in DF 17 and others).
// //
int decodeAC12Field(unsigned char *msg, int *unit) { int decodeAC12Field(int AC12Field, int *unit) {
int msg5 = msg[5]; int q_bit = AC12Field & 0x10; // Bit 48 = Q
int msg6 = msg[6]; AC12Field &= 0x0FFF; // limit the field to 12 bits
int q_bit = msg5 & 1; // Bit 48 = Q
*unit = MODES_UNIT_FEET; *unit = MODES_UNIT_FEET;
if (q_bit) { if (q_bit) {
/// N is the 11 bit integer resulting from the removal of bit Q /// 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. // The final altitude is the resulting number multiplied by 25, minus 1000.
return ((n * 25) - 1000); return ((n * 25) - 1000);
} else { } else {
// N is an 11 bit Gillham coded altitude // N is an 11 bit Gillham coded altitude
int n = 0; int n = n = ModeAToModeC(decodeGillhamField(AC12Field));
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);
if (n < -12) {n = 0;} if (n < -12) {n = 0;}
return (100 * n); 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. */ mm->um = ((msg[1] & 7)<<3)| /* Request extraction of downlink request. */
msg[2]>>5; msg[2]>>5;
/* In the squawk (identity) field bits are interleaved like that mm->modeA = decodeGillhamField((msg[2] << 8) | msg[3]);
* (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;
}
/* DF 11 & 17: try to populate our ICAO addresses whitelist. /* DF 11 & 17: try to populate our ICAO addresses whitelist.
* DFs with an AP field (xored addr and crc), try to decode it. */ * 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 */ // Fields for DF0, DF4, DF16, DF20 13 bit altitude
if (mm->msgtype == 0 || mm->msgtype == 4 || if (mm->msgtype == 0 || mm->msgtype == 4 ||
mm->msgtype == 16 || mm->msgtype == 20) { 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. */ /* Decode extended squitter specific stuff. */
@ -1598,7 +1568,7 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
/* Airborne position Message */ /* Airborne position Message */
mm->fflag = msg[6] & (1<<2); mm->fflag = msg[6] & (1<<2);
mm->tflag = msg[6] & (1<<3); 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) | mm->raw_latitude = ((msg[6] & 3) << 15) |
(msg[7] << 7) | (msg[7] << 7) |
(msg[8] >> 1); (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. // We measured signal strength over the first 56 bits. Don't forget to add 4
sigStrength /= 60; // 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 /* 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 * with a Mode S message in our hands, but it may still be broken