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Implement 100 ft Altitudes

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The DF's for altitude include a Q bit to distinguish either 100ft or
25ft altitude encoding. This version implements the 100ft increments
used by older aircraft. These codes are basically ModeC altitudes
transmitted over ModeS.
This commit is contained in:
Malcolm Robb 2013-04-28 02:17:57 +01:00
parent 05187f76e7
commit 8ed0574ac8
1 changed files with 64 additions and 30 deletions
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94
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 "1.03.2604.13" #define MODES_DUMP1090_VERSION "1.04.2704.13"
#define MODES_DEFAULT_RATE 2000000 #define MODES_DEFAULT_RATE 2000000
#define MODES_DEFAULT_FREQ 1090000000 #define MODES_DEFAULT_FREQ 1090000000
@ -1314,51 +1314,85 @@ int bruteForceAP(unsigned char *msg, struct modesMessage *mm) {
} }
return (0); return (0);
} }
//
/* 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 // Returns the altitude, and set 'unit' to either MODES_UNIT_METERS or MDOES_UNIT_FEETS.
* or MDOES_UNIT_FEETS. */ //
int decodeAC13Field(unsigned char *msg, int *unit) { int decodeAC13Field(unsigned char *msg, int *unit) {
int m_bit = msg[3] & (1<<6); int msg2 = msg[2];
int q_bit = msg[3] & (1<<4); 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
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 // N is the 11 bit integer resulting from the removal of bit Q and M
* Q and M */ int n = ((msg2 & 0x1F) << 6) |
int n = ((msg[2]&31)<<6) | ((msg3 & 0x80) >> 2) |
((msg[3]&0x80)>>2) | ((msg3 & 0x20) >> 1) |
((msg[3]&0x20)>>1) | (msg3 & 0x0F);
(msg[3]&15); // The final altitude is resulting number multiplied by 25, minus 1000.
/* The final altitude is due to the resulting number multiplied return ((n * 25) - 1000);
* by 25, minus 1000. */
return n*25-1000;
} else { } else {
/* TODO: Implement altitude where Q=0 and M=0 */ // 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);
if (n < -12) {n = 0;}
return (100 * n);
} }
} else { } else {
*unit = MODES_UNIT_METERS; *unit = MODES_UNIT_METERS;
/* TODO: Implement altitude when meter unit is selected. */ // TODO: Implement altitude when meter unit is selected
} }
return 0; return 0;
} }
//
/* Decode the 12 bit AC altitude field (in DF 17 and others). // Decode the 12 bit AC altitude field (in DF 17 and others).
* Returns the altitude or 0 if it can't be decoded. */ //
int decodeAC12Field(unsigned char *msg, int *unit) { int decodeAC12Field(unsigned char *msg, int *unit) {
int q_bit = msg[5] & 1; int msg5 = msg[5];
int msg6 = msg[6];
int q_bit = msg5 & 1; // Bit 48 = Q
if (q_bit) {
/* N is the 11 bit integer resulting from the removal of bit
* Q */
int n = ((msg[5]>>1)<<4) | ((msg[6]&0xF0) >> 4);
*unit = MODES_UNIT_FEET; *unit = MODES_UNIT_FEET;
/* The final altitude is due to the resulting number multiplied if (q_bit) {
* by 25, minus 1000. */ /// N is the 11 bit integer resulting from the removal of bit Q
return n*25-1000; int n = ((msg5 & 0xFE) << 3) | ((msg6 & 0xF0) >> 4);
// The final altitude is the resulting number multiplied by 25, minus 1000.
return ((n * 25) - 1000);
} else { } else {
return 0; // 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);
if (n < -12) {n = 0;}
return (100 * n);
} }
} }