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mode_ac.c
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mode_ac.c
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// Part of readsb, a Mode-S/ADSB/TIS message decoder.
//
// mode_ac.c: Mode A/C decoder.
//
// Copyright (c) 2020 Michael Wolf <michael@mictronics.de>
//
// This code is based on a detached fork of dump1090-fa.
//
// This file is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// any later version.
//
// This file is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.
//
// This file incorporates work covered by the following copyright and
// license:
// Copyright (C) 2012 by Salvatore Sanfilippo <antirez@gmail.com>
//
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//
// * Redistributions in binary form must reproduce the above copyright
// notice, this list of conditions and the following disclaimer in the
// documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
#include "readsb.h"
#include <assert.h>
//
//=========================================================================
//
// Input format is : 00:A4:A2:A1:00:B4:B2:B1:00:C4:C2:C1:00:D4:D2:D1
//
static int modeAToCTable[4096];
static unsigned modeCToATable[4096];
static int internalModeAToModeC(unsigned int ModeA);
void modeACInit() {
for (unsigned i = 0; i < 4096; ++i) {
unsigned modeA = indexToModeA(i);
int modeC = internalModeAToModeC(modeA);
modeAToCTable[i] = modeC;
modeC += 13;
if (modeC >= 0 && modeC < 4096) {
assert(modeCToATable[modeC] == 0);
modeCToATable[modeC] = modeA;
}
}
}
// Given a mode A value (hex-encoded, see above)
// return the mode C value (signed multiple of 100s of feet)
// or INVALID_ALITITUDE if not a valid mode C value
int modeAToModeC(unsigned modeA) {
unsigned i = modeAToIndex(modeA);
if (i >= 4096)
return INVALID_ALTITUDE;
return modeAToCTable[i];
}
// Given a mode C value (signed multiple of 100s of feet)
// return the mode A value, or 0 if not a valid mode C value
unsigned modeCToModeA(int modeC) {
modeC += 13;
if (modeC < 0 || modeC >= 4096)
return 0;
return modeCToATable[modeC];
}
static int internalModeAToModeC(unsigned int ModeA) {
unsigned int FiveHundreds = 0;
unsigned int OneHundreds = 0;
if ((ModeA & 0xFFFF8889) != 0 || // check zero bits are zero, D1 set is illegal
(ModeA & 0x000000F0) == 0) { // C1,,C4 cannot be Zero
return INVALID_ALTITUDE;
}
if (ModeA & 0x0010) {
OneHundreds ^= 0x007;
} // C1
if (ModeA & 0x0020) {
OneHundreds ^= 0x003;
} // C2
if (ModeA & 0x0040) {
OneHundreds ^= 0x001;
} // C4
// Remove 7s from OneHundreds (Make 7->5, snd 5->7).
if ((OneHundreds & 5) == 5) {
OneHundreds ^= 2;
}
// Check for invalid codes, only 1 to 5 are valid
if (OneHundreds > 5) {
return INVALID_ALTITUDE;
}
//if (ModeA & 0x0001) {FiveHundreds ^= 0x1FF;} // D1 never used for altitude
if (ModeA & 0x0002) {
FiveHundreds ^= 0x0FF;
} // D2
if (ModeA & 0x0004) {
FiveHundreds ^= 0x07F;
} // D4
if (ModeA & 0x1000) {
FiveHundreds ^= 0x03F;
} // A1
if (ModeA & 0x2000) {
FiveHundreds ^= 0x01F;
} // A2
if (ModeA & 0x4000) {
FiveHundreds ^= 0x00F;
} // A4
if (ModeA & 0x0100) {
FiveHundreds ^= 0x007;
} // B1
if (ModeA & 0x0200) {
FiveHundreds ^= 0x003;
} // B2
if (ModeA & 0x0400) {
FiveHundreds ^= 0x001;
} // B4
// Correct order of OneHundreds.
if (FiveHundreds & 1) {
OneHundreds = 6 - OneHundreds;
}
return ((FiveHundreds * 5) + OneHundreds - 13);
}
//
//=========================================================================
//
void decodeModeAMessage(struct modesMessage *mm, int ModeA) {
mm->source = SOURCE_MODE_AC;
mm->addrtype = AIRCRAFT_META__ADDR_TYPE__ADDR_MODE_A;
mm->msgtype = 32; // Valid Mode S DF's are DF-00 to DF-31.
// so use 32 to indicate Mode A/C
mm->msgbits = 16; // Fudge up a Mode S style data stream
mm->msg[0] = mm->verbatim[0] = (ModeA >> 8);
mm->msg[1] = mm->verbatim[1] = (ModeA);
// Fudge an address based on Mode A (remove the Ident bit)
mm->addr = (ModeA & 0x0000FF7F) | MODES_NON_ICAO_ADDRESS;
// Set the Identity field to ModeA
mm->squawk = ModeA & 0x7777;
mm->squawk_valid = 1;
// Flag ident in flight status
mm->spi = (ModeA & 0x0080) ? 1 : 0;
mm->spi_valid = 1;
// Decode an altitude if this looks like a possible mode C
if (!mm->spi) {
int modeC = modeAToModeC(ModeA);
if (modeC != INVALID_ALTITUDE) {
mm->altitude_baro = modeC * 100;
mm->altitude_baro_unit = UNIT_FEET;
mm->altitude_baro_valid = 1;
}
}
// Not much else we can tell from a Mode A/C reply.
// Just fudge up a few bits to keep other code happy
mm->correctedbits = 0;
}
//
// ===================== Mode A/C detection and decoding ===================
//