394 lines
14 KiB
C
394 lines
14 KiB
C
// dump1090, a Mode S messages decoder for RTLSDR devices.
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//
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// Copyright (C) 2012 by Salvatore Sanfilippo <antirez@gmail.com>
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//
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// All rights reserved.
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//
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// Redistribution and use in source and binary forms, with or without
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// modification, are permitted provided that the following conditions are
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// met:
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//
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// * Redistributions of source code must retain the above copyright
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// notice, this list of conditions and the following disclaimer.
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//
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// * Redistributions in binary form must reproduce the above copyright
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// notice, this list of conditions and the following disclaimer in the
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// documentation and/or other materials provided with the distribution.
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//
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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//
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#include "dump1090.h"
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//
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// ===================== Mode A/C detection and decoding ===================
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//
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//
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// This table is used to build the Mode A/C variable called ModeABits.Each
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// bit period is inspected, and if it's value exceeds the threshold limit,
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// then the value in this table is or-ed into ModeABits.
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//
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// At the end of message processing, ModeABits will be the decoded ModeA value.
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//
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// We can also flag noise in bits that should be zeros - the xx bits. Noise in
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// these bits cause bits (31-16) in ModeABits to be set. Then at the end of message
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// processing we can test for errors by looking at these bits.
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//
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uint32_t ModeABitTable[24] = {
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0x00000000, // F1 = 1
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0x00000010, // C1
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0x00001000, // A1
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0x00000020, // C2
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0x00002000, // A2
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0x00000040, // C4
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0x00004000, // A4
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0x40000000, // xx = 0 Set bit 30 if we see this high
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0x00000100, // B1
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0x00000001, // D1
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0x00000200, // B2
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0x00000002, // D2
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0x00000400, // B4
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0x00000004, // D4
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0x00000000, // F2 = 1
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0x08000000, // xx = 0 Set bit 27 if we see this high
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0x04000000, // xx = 0 Set bit 26 if we see this high
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0x00000080, // SPI
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0x02000000, // xx = 0 Set bit 25 if we see this high
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0x01000000, // xx = 0 Set bit 24 if we see this high
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0x00800000, // xx = 0 Set bit 23 if we see this high
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0x00400000, // xx = 0 Set bit 22 if we see this high
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0x00200000, // xx = 0 Set bit 21 if we see this high
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0x00100000, // xx = 0 Set bit 20 if we see this high
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};
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//
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// This table is used to produce an error variable called ModeAErrs.Each
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// inter-bit period is inspected, and if it's value falls outside of the
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// expected range, then the value in this table is or-ed into ModeAErrs.
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//
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// At the end of message processing, ModeAErrs will indicate if we saw
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// any inter-bit anomolies, and the bits that are set will show which
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// bits had them.
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//
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uint32_t ModeAMidTable[24] = {
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0x80000000, // F1 = 1 Set bit 31 if we see F1_C1 error
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0x00000010, // C1 Set bit 4 if we see C1_A1 error
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0x00001000, // A1 Set bit 12 if we see A1_C2 error
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0x00000020, // C2 Set bit 5 if we see C2_A2 error
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0x00002000, // A2 Set bit 13 if we see A2_C4 error
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0x00000040, // C4 Set bit 6 if we see C3_A4 error
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0x00004000, // A4 Set bit 14 if we see A4_xx error
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0x40000000, // xx = 0 Set bit 30 if we see xx_B1 error
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0x00000100, // B1 Set bit 8 if we see B1_D1 error
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0x00000001, // D1 Set bit 0 if we see D1_B2 error
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0x00000200, // B2 Set bit 9 if we see B2_D2 error
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0x00000002, // D2 Set bit 1 if we see D2_B4 error
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0x00000400, // B4 Set bit 10 if we see B4_D4 error
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0x00000004, // D4 Set bit 2 if we see D4_F2 error
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0x20000000, // F2 = 1 Set bit 29 if we see F2_xx error
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0x08000000, // xx = 0 Set bit 27 if we see xx_xx error
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0x04000000, // xx = 0 Set bit 26 if we see xx_SPI error
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0x00000080, // SPI Set bit 15 if we see SPI_xx error
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0x02000000, // xx = 0 Set bit 25 if we see xx_xx error
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0x01000000, // xx = 0 Set bit 24 if we see xx_xx error
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0x00800000, // xx = 0 Set bit 23 if we see xx_xx error
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0x00400000, // xx = 0 Set bit 22 if we see xx_xx error
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0x00200000, // xx = 0 Set bit 21 if we see xx_xx error
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0x00100000, // xx = 0 Set bit 20 if we see xx_xx error
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};
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//
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// The "off air" format is,,
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// _F1_C1_A1_C2_A2_C4_A4_xx_B1_D1_B2_D2_B4_D4_F2_xx_xx_SPI_
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//
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// Bit spacing is 1.45uS, with 0.45uS high, and 1.00us low. This is a problem
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// because we ase sampling at 2Mhz (500nS) so we are below Nyquist.
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//
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// The bit spacings are..
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// F1 : 0.00,
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// 1.45, 2.90, 4.35, 5.80, 7.25, 8.70,
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// X : 10.15,
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// : 11.60, 13.05, 14.50, 15.95, 17.40, 18.85,
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// F2 : 20.30,
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// X : 21.75, 23.20, 24.65
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//
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// This equates to the following sample point centers at 2Mhz.
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// [ 0.0],
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// [ 2.9], [ 5.8], [ 8.7], [11.6], [14.5], [17.4],
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// [20.3],
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// [23.2], [26.1], [29.0], [31.9], [34.8], [37.7]
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// [40.6]
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// [43.5], [46.4], [49.3]
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//
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// We know that this is a supposed to be a binary stream, so the signal
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// should either be a 1 or a 0. Therefore, any energy above the noise level
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// in two adjacent samples must be from the same pulse, so we can simply
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// add the values together..
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//
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int detectModeA(uint16_t *m, struct modesMessage *mm)
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{
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int j, lastBitWasOne;
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int ModeABits = 0;
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int ModeAErrs = 0;
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int byte, bit;
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int thisSample, lastBit, lastSpace = 0;
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int m0, m1, m2, m3, mPhase;
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int n0, n1, n2 ,n3;
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int F1_sig, F1_noise;
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int F2_sig, F2_noise;
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int fSig, fNoise, fLevel, fLoLo;
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int snr;
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// m[0] contains the energy from 0 -> 499 nS
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// m[1] contains the energy from 500 -> 999 nS
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// m[2] contains the energy from 1000 -> 1499 nS
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// m[3] contains the energy from 1500 -> 1999 nS
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//
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// We are looking for a Frame bit (F1) whose width is 450nS, followed by
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// 1000nS of quiet.
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//
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// The width of the frame bit is 450nS, which is 90% of our sample rate.
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// Therefore, in an ideal world, all the energy for the frame bit will be
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// in a single sample, preceeded by (at least) one zero, and followed by
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// two zeros, Best case we can look for ...
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//
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// 0 - 1 - 0 - 0
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//
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// However, our samples are not phase aligned, so some of the energy from
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// each bit could be spread over two consecutive samples. Worst case is
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// that we sample half in one bit, and half in the next. In that case,
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// we're looking for
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//
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// 0 - 0.5 - 0.5 - 0.
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m0 = m[0]; m1 = m[1];
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if (m0 >= m1) // m1 *must* be bigger than m0 for this to be F1
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{return (0);}
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m2 = m[2]; m3 = m[3];
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//
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// if (m2 <= m0), then assume the sample bob on (Phase == 0), so don't look at m3
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if ((m2 <= m0) || (m2 < m3))
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{m3 = m2; m2 = m0;}
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if ( (m3 >= m1) // m1 must be bigger than m3
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|| (m0 > m2) // m2 can be equal to m0 if ( 0,1,0,0 )
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|| (m3 > m2) ) // m2 can be equal to m3 if ( 0,1,0,0 )
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{return (0);}
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// m0 = noise
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// m1 = noise + (signal * X))
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// m2 = noise + (signal * (1-X))
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// m3 = noise
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//
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// Hence, assuming all 4 samples have similar amounts of noise in them
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// signal = (m1 + m2) - ((m0 + m3) * 2)
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// noise = (m0 + m3) / 2
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//
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F1_sig = (m1 + m2) - ((m0 + m3) << 1);
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F1_noise = (m0 + m3) >> 1;
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if ( (F1_sig < MODEAC_MSG_SQUELCH_LEVEL) // minimum required F1 signal amplitude
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|| (F1_sig < (F1_noise << 2)) ) // minimum allowable Sig/Noise ratio 4:1
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{return (0);}
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// If we get here then we have a potential F1, so look for an equally valid F2 20.3uS later
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//
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// Our F1 is centered somewhere between samples m[1] and m[2]. We can guestimate where F2 is
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// by comparing the ratio of m1 and m2, and adding on 20.3 uS (40.6 samples)
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//
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mPhase = ((m2 * 20) / (m1 + m2));
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byte = (mPhase + 812) / 20;
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n0 = m[byte++]; n1 = m[byte++];
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if (n0 >= n1) // n1 *must* be bigger than n0 for this to be F2
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{return (0);}
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n2 = m[byte++];
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//
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// if the sample bob on (Phase == 0), don't look at n3
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//
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if ((mPhase + 812) % 20)
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{n3 = m[byte++];}
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else
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{n3 = n2; n2 = n0;}
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if ( (n3 >= n1) // n1 must be bigger than n3
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|| (n0 > n2) // n2 can be equal to n0 ( 0,1,0,0 )
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|| (n3 > n2) ) // n2 can be equal to n3 ( 0,1,0,0 )
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{return (0);}
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F2_sig = (n1 + n2) - ((n0 + n3) << 1);
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F2_noise = (n0 + n3) >> 1;
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if ( (F2_sig < MODEAC_MSG_SQUELCH_LEVEL) // minimum required F2 signal amplitude
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|| (F2_sig < (F2_noise << 2)) ) // maximum allowable Sig/Noise ratio 4:1
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{return (0);}
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fSig = (F1_sig + F2_sig) >> 1;
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fNoise = (F1_noise + F2_noise) >> 1;
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fLoLo = fNoise + (fSig >> 2); // 1/2
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fLevel = fNoise + (fSig >> 1);
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lastBitWasOne = 1;
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lastBit = F1_sig;
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//
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// Now step by a half ModeA bit, 0.725nS, which is 1.45 samples, which is 29/20
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// No need to do bit 0 because we've already selected it as a valid F1
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// Do several bits past the SPI to increase error rejection
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//
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for (j = 1, mPhase += 29; j < 48; mPhase += 29, j ++)
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{
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byte = 1 + (mPhase / 20);
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thisSample = m[byte] - fNoise;
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if (mPhase % 20) // If the bit is split over two samples...
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{thisSample += (m[byte+1] - fNoise);} // add in the second sample's energy
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// If we're calculating a space value
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if (j & 1)
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{lastSpace = thisSample;}
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else
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{// We're calculating a new bit value
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bit = j >> 1;
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if (thisSample >= fLevel)
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{// We're calculating a new bit value, and its a one
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ModeABits |= ModeABitTable[bit--]; // or in the correct bit
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if (lastBitWasOne)
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{ // This bit is one, last bit was one, so check the last space is somewhere less than one
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if ( (lastSpace >= (thisSample>>1)) || (lastSpace >= lastBit) )
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{ModeAErrs |= ModeAMidTable[bit];}
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}
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else
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{// This bit,is one, last bit was zero, so check the last space is somewhere less than one
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if (lastSpace >= (thisSample >> 1))
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{ModeAErrs |= ModeAMidTable[bit];}
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}
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lastBitWasOne = 1;
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}
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else
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{// We're calculating a new bit value, and its a zero
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if (lastBitWasOne)
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{ // This bit is zero, last bit was one, so check the last space is somewhere in between
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if (lastSpace >= lastBit)
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{ModeAErrs |= ModeAMidTable[bit];}
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}
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else
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{// This bit,is zero, last bit was zero, so check the last space is zero too
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if (lastSpace >= fLoLo)
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{ModeAErrs |= ModeAMidTable[bit];}
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}
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lastBitWasOne = 0;
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}
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lastBit = (thisSample >> 1);
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}
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}
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//
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// Output format is : 00:A4:A2:A1:00:B4:B2:B1:00:C4:C2:C1:00:D4:D2:D1
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//
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if ((ModeABits < 3) || (ModeABits & 0xFFFF8808) || (ModeAErrs) )
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{return (ModeABits = 0);}
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// snr = 5 * 20log10(fSig / (fSig+fNoise)) (in units of 0.2dB)
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// = 100log10(fSig) - 100log10(fSig+fNoise)
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while (fSig > 65535 || (fSig + fNoise) > 65535) {
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fSig >>= 1;
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fNoise >>= 1;
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}
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snr = Modes.log10lut[fSig] - Modes.log10lut[fSig + fNoise];
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mm->signalLevel = ((snr < 255) ? (uint8_t)snr : 255);
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return ModeABits;
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}
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//
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//=========================================================================
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//
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// Input format is : 00:A4:A2:A1:00:B4:B2:B1:00:C4:C2:C1:00:D4:D2:D1
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//
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int ModeAToModeC(unsigned int ModeA)
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{
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unsigned int FiveHundreds = 0;
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unsigned int OneHundreds = 0;
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if ( (ModeA & 0xFFFF888B) // D1 set is illegal. D2 set is > 62700ft which is unlikely
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|| ((ModeA & 0x000000F0) == 0) ) // C1,,C4 cannot be Zero
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{return -9999;}
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if (ModeA & 0x0010) {OneHundreds ^= 0x007;} // C1
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if (ModeA & 0x0020) {OneHundreds ^= 0x003;} // C2
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if (ModeA & 0x0040) {OneHundreds ^= 0x001;} // C4
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// Remove 7s from OneHundreds (Make 7->5, snd 5->7).
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if ((OneHundreds & 5) == 5) {OneHundreds ^= 2;}
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// Check for invalid codes, only 1 to 5 are valid
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if (OneHundreds > 5)
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{return -9999;}
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//if (ModeA & 0x0001) {FiveHundreds ^= 0x1FF;} // D1 never used for altitude
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if (ModeA & 0x0002) {FiveHundreds ^= 0x0FF;} // D2
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if (ModeA & 0x0004) {FiveHundreds ^= 0x07F;} // D4
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if (ModeA & 0x1000) {FiveHundreds ^= 0x03F;} // A1
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if (ModeA & 0x2000) {FiveHundreds ^= 0x01F;} // A2
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if (ModeA & 0x4000) {FiveHundreds ^= 0x00F;} // A4
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if (ModeA & 0x0100) {FiveHundreds ^= 0x007;} // B1
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if (ModeA & 0x0200) {FiveHundreds ^= 0x003;} // B2
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if (ModeA & 0x0400) {FiveHundreds ^= 0x001;} // B4
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// Correct order of OneHundreds.
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if (FiveHundreds & 1) {OneHundreds = 6 - OneHundreds;}
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return ((FiveHundreds * 5) + OneHundreds - 13);
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}
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//
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//=========================================================================
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//
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void decodeModeAMessage(struct modesMessage *mm, int ModeA)
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{
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mm->msgtype = 32; // Valid Mode S DF's are DF-00 to DF-31.
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// so use 32 to indicate Mode A/C
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mm->msgbits = 16; // Fudge up a Mode S style data stream
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mm->msg[0] = (ModeA >> 8);
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mm->msg[1] = (ModeA);
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// Fudge an ICAO address based on Mode A (remove the Ident bit)
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// Use an upper address byte of FF, since this is ICAO unallocated
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mm->addr = 0x00FF0000 | (ModeA & 0x0000FF7F);
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// Set the Identity field to ModeA
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mm->modeA = ModeA & 0x7777;
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mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID;
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// Flag ident in flight status
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mm->fs = ModeA & 0x0080;
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// Not much else we can tell from a Mode A/C reply.
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// Just fudge up a few bits to keep other code happy
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mm->crcok = 1;
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mm->correctedbits = 0;
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}
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//
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// ===================== Mode A/C detection and decoding ===================
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//
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