mode_ac.c

// dump1090, a Mode S messages decoder for RTLSDR devices.
//
// 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,
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// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//

#include "dump1090.h"
//
// ===================== Mode A/C detection and decoding  ===================
//
//
// This table is used to build the Mode A/C variable called ModeABits.Each 
// bit period is inspected, and if it's value exceeds the threshold limit, 
// then the value in this table is or-ed into ModeABits.
//
// At the end of message processing, ModeABits will be the decoded ModeA value.
//
// We can also flag noise in bits that should be zeros - the xx bits. Noise in
// these bits cause bits (31-16) in ModeABits to be set. Then at the end of message
// processing we can test for errors by looking at these bits.
//
uint32_t ModeABitTable[24] = {
0x00000000, // F1 = 1
0x00000010, // C1
0x00001000, // A1
0x00000020, // C2 
0x00002000, // A2
0x00000040, // C4
0x00004000, // A4
0x40000000, // xx = 0  Set bit 30 if we see this high
0x00000100, // B1 
0x00000001, // D1
0x00000200, // B2
0x00000002, // D2
0x00000400, // B4
0x00000004, // D4
0x00000000, // F2 = 1
0x08000000, // xx = 0  Set bit 27 if we see this high
0x04000000, // xx = 0  Set bit 26 if we see this high
0x00000080, // SPI
0x02000000, // xx = 0  Set bit 25 if we see this high
0x01000000, // xx = 0  Set bit 24 if we see this high
0x00800000, // xx = 0  Set bit 23 if we see this high
0x00400000, // xx = 0  Set bit 22 if we see this high
0x00200000, // xx = 0  Set bit 21 if we see this high
0x00100000, // xx = 0  Set bit 20 if we see this high
};
//
// This table is used to produce an error variable called ModeAErrs.Each 
// inter-bit period is inspected, and if it's value falls outside of the 
// expected range, then the value in this table is or-ed into ModeAErrs.
//
// At the end of message processing, ModeAErrs will indicate if we saw 
// any inter-bit anomolies, and the bits that are set will show which 
// bits had them.
//
uint32_t ModeAMidTable[24] = {
0x80000000, // F1 = 1  Set bit 31 if we see F1_C1  error
0x00000010, // C1      Set bit  4 if we see C1_A1  error
0x00001000, // A1      Set bit 12 if we see A1_C2  error
0x00000020, // C2      Set bit  5 if we see C2_A2  error
0x00002000, // A2      Set bit 13 if we see A2_C4  error
0x00000040, // C4      Set bit  6 if we see C3_A4  error
0x00004000, // A4      Set bit 14 if we see A4_xx  error
0x40000000, // xx = 0  Set bit 30 if we see xx_B1  error
0x00000100, // B1      Set bit  8 if we see B1_D1  error
0x00000001, // D1      Set bit  0 if we see D1_B2  error
0x00000200, // B2      Set bit  9 if we see B2_D2  error
0x00000002, // D2      Set bit  1 if we see D2_B4  error
0x00000400, // B4      Set bit 10 if we see B4_D4  error
0x00000004, // D4      Set bit  2 if we see D4_F2  error
0x20000000, // F2 = 1  Set bit 29 if we see F2_xx  error
0x08000000, // xx = 0  Set bit 27 if we see xx_xx  error
0x04000000, // xx = 0  Set bit 26 if we see xx_SPI error
0x00000080, // SPI     Set bit 15 if we see SPI_xx error
0x02000000, // xx = 0  Set bit 25 if we see xx_xx  error
0x01000000, // xx = 0  Set bit 24 if we see xx_xx  error
0x00800000, // xx = 0  Set bit 23 if we see xx_xx  error
0x00400000, // xx = 0  Set bit 22 if we see xx_xx  error
0x00200000, // xx = 0  Set bit 21 if we see xx_xx  error
0x00100000, // xx = 0  Set bit 20 if we see xx_xx  error
};
//
// The "off air" format is,,
// _F1_C1_A1_C2_A2_C4_A4_xx_B1_D1_B2_D2_B4_D4_F2_xx_xx_SPI_
//
// Bit spacing is 1.45uS, with 0.45uS high, and 1.00us low. This is a problem
// because we ase sampling at 2Mhz (500nS) so we are below Nyquist. 
//
// The bit spacings are..
// F1 :  0.00,   
//       1.45,  2.90,  4.35,  5.80,  7.25,  8.70, 
// X  : 10.15, 
//    : 11.60, 13.05, 14.50, 15.95, 17.40, 18.85, 
// F2 : 20.30, 
// X  : 21.75, 23.20, 24.65 
//
// This equates to the following sample point centers at 2Mhz.
// [ 0.0], 
// [ 2.9], [ 5.8], [ 8.7], [11.6], [14.5], [17.4], 
// [20.3], 
// [23.2], [26.1], [29.0], [31.9], [34.8], [37.7]
// [40.6]
// [43.5], [46.4], [49.3]
//
// We know that this is a supposed to be a binary stream, so the signal
// should either be a 1 or a 0. Therefore, any energy above the noise level 
// in two adjacent samples must be from the same pulse, so we can simply 
// add the values together.. 
// 
int detectModeA(uint16_t *m, struct modesMessage *mm)
  {
  int j, lastBitWasOne;
  int ModeABits = 0;
  int ModeAErrs = 0;
  int byte, bit;
  int thisSample, lastBit, lastSpace = 0; 
  int m0, m1, m2, m3, mPhase;
  int n0, n1, n2 ,n3;
  int F1_sig, F1_noise;
  int F2_sig, F2_noise;
  int fSig, fNoise, fLevel, fLoLo;

  // m[0] contains the energy from    0 ->  499 nS
  // m[1] contains the energy from  500 ->  999 nS
  // m[2] contains the energy from 1000 -> 1499 nS
  // m[3] contains the energy from 1500 -> 1999 nS
  //
  // We are looking for a Frame bit (F1) whose width is 450nS, followed by
  // 1000nS of quiet.
  //
  // The width of the frame bit is 450nS, which is 90% of our sample rate.
  // Therefore, in an ideal world, all the energy for the frame bit will be
  // in a single sample, preceeded by (at least) one zero, and followed by 
  // two zeros, Best case we can look for ...
  //
  // 0 - 1 - 0 - 0
  //
  // However, our samples are not phase aligned, so some of the energy from 
  // each bit could be spread over two consecutive samples. Worst case is
  // that we sample half in one bit, and half in the next. In that case, 
  // we're looking for 
  //
  // 0 - 0.5 - 0.5 - 0.

  m0 = m[0]; m1 = m[1];

  if (m0 >= m1)   // m1 *must* be bigger than m0 for this to be F1
    {return (0);}

  m2 = m[2]; m3 = m[3];

  // 
  // if (m2 <= m0), then assume the sample bob on (Phase == 0), so don't look at m3 
  if ((m2 <= m0) || (m2 < m3))
    {m3 = m2; m2 = m0;}

  if (  (m3 >= m1)   // m1 must be bigger than m3
     || (m0 >  m2)   // m2 can be equal to m0 if ( 0,1,0,0 )
     || (m3 >  m2) ) // m2 can be equal to m3 if ( 0,1,0,0 )
    {return (0);}

  // m0 = noise
  // m1 = noise + (signal *    X))
  // m2 = noise + (signal * (1-X))
  // m3 = noise
  //
  // Hence, assuming all 4 samples have similar amounts of noise in them 
  //      signal = (m1 + m2) - ((m0 + m3) * 2)
  //      noise  = (m0 + m3) / 2
  //
  F1_sig   = (m1 + m2) - ((m0 + m3) << 1);
  F1_noise = (m0 + m3) >> 1;

  if ( (F1_sig < MODEAC_MSG_SQUELCH_LEVEL) // minimum required  F1 signal amplitude
    || (F1_sig < (F1_noise << 2)) )        // minimum allowable Sig/Noise ratio 4:1
    {return (0);}

  // If we get here then we have a potential F1, so look for an equally valid F2 20.3uS later
  //
  // Our F1 is centered somewhere between samples m[1] and m[2]. We can guestimate where F2 is 
  // by comparing the ratio of m1 and m2, and adding on 20.3 uS (40.6 samples)
  //
  mPhase = ((m2 * 20) / (m1 + m2));
  byte   = (mPhase + 812) / 20; 
  n0     = m[byte++]; n1 = m[byte++]; 

  if (n0 >= n1)   // n1 *must* be bigger than n0 for this to be F2
    {return (0);}

  n2 = m[byte++];
  // 
  // if the sample bob on (Phase == 0), don't look at n3 
  //
  if ((mPhase + 812) % 20)
    {n3 = m[byte++];}
  else
    {n3 = n2; n2 = n0;}

  if (  (n3 >= n1)   // n1 must be bigger than n3
     || (n0 >  n2)   // n2 can be equal to n0 ( 0,1,0,0 )
     || (n3 >  n2) ) // n2 can be equal to n3 ( 0,1,0,0 )
    {return (0);}

  F2_sig   = (n1 + n2) - ((n0 + n3) << 1);
  F2_noise = (n0 + n3) >> 1;

  if ( (F2_sig < MODEAC_MSG_SQUELCH_LEVEL) // minimum required  F2 signal amplitude
    || (F2_sig < (F2_noise << 2)) )       // maximum allowable Sig/Noise ratio 4:1
    {return (0);}

  fSig          = (F1_sig   + F2_sig)   >> 1;
  fNoise        = (F1_noise + F2_noise) >> 1;
  fLoLo         = fNoise    + (fSig >> 2);       // 1/2
  fLevel        = fNoise    + (fSig >> 1);
  lastBitWasOne = 1;
  lastBit       = F1_sig;
  //
  // Now step by a half ModeA bit, 0.725nS, which is 1.45 samples, which is 29/20
  // No need to do bit 0 because we've already selected it as a valid F1
  // Do several bits past the SPI to increase error rejection
  //
  for (j = 1, mPhase += 29; j < 48; mPhase += 29, j ++)
    {
    byte  = 1 + (mPhase / 20);
    
    thisSample = m[byte] - fNoise;
    if (mPhase % 20)                     // If the bit is split over two samples...
      {thisSample += (m[byte+1] - fNoise);}  //    add in the second sample's energy

     // If we're calculating a space value
    if (j & 1)               
      {lastSpace = thisSample;}

    else 
      {// We're calculating a new bit value
      bit = j >> 1;
      if (thisSample >= fLevel)
        {// We're calculating a new bit value, and its a one
        ModeABits |= ModeABitTable[bit--];  // or in the correct bit

        if (lastBitWasOne)
          { // This bit is one, last bit was one, so check the last space is somewhere less than one
          if ( (lastSpace >= (thisSample>>1)) || (lastSpace >= lastBit) )
            {ModeAErrs |= ModeAMidTable[bit];}
          }

        else              
          {// This bit,is one, last bit was zero, so check the last space is somewhere less than one
          if (lastSpace >= (thisSample >> 1))
            {ModeAErrs |= ModeAMidTable[bit];}
          }

        lastBitWasOne = 1;
        }

      
      else 
        {// We're calculating a new bit value, and its a zero
        if (lastBitWasOne)
          { // This bit is zero, last bit was one, so check the last space is somewhere in between
          if (lastSpace >= lastBit)
            {ModeAErrs |= ModeAMidTable[bit];}
          }

        else              
          {// This bit,is zero, last bit was zero, so check the last space is zero too
          if (lastSpace >= fLoLo)
            {ModeAErrs |= ModeAMidTable[bit];}
          }

        lastBitWasOne = 0;   
        }

      lastBit = (thisSample >> 1); 
      }
    }

  //
  // Output format is : 00:A4:A2:A1:00:B4:B2:B1:00:C4:C2:C1:00:D4:D2:D1
  //
  if ((ModeABits < 3) || (ModeABits & 0xFFFF8808) || (ModeAErrs) )
    {return (ModeABits = 0);}

  fSig            = (fSig + 0x7F) >> 8;
  mm->signalLevel = ((fSig < 255) ? fSig : 255);

  return ModeABits;
  }
//
//=========================================================================
//
// Input format is : 00:A4:A2:A1:00:B4:B2:B1:00:C4:C2:C1:00:D4:D2:D1
//
int ModeAToModeC(unsigned int ModeA) 
  { 
  unsigned int FiveHundreds = 0;
  unsigned int OneHundreds  = 0;

  if (  (ModeA & 0xFFFF888B)         // D1 set is illegal. D2 set is > 62700ft which is unlikely
    || ((ModeA & 0x000000F0) == 0) ) // C1,,C4 cannot be Zero
    {return -9999;}

  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 -9999;} 

//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->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] = (ModeA >> 8);
  mm->msg[1] = (ModeA);

  // Fudge an ICAO address based on Mode A (remove the Ident bit)
  // Use an upper address byte of FF, since this is ICAO unallocated
  mm->addr = 0x00FF0000 | (ModeA & 0x0000FF7F);

  // Set the Identity field to ModeA
  mm->modeA   = ModeA & 0x7777;
  mm->bFlags |= MODES_ACFLAGS_SQUAWK_VALID;

  // Flag ident in flight status
  mm->fs = ModeA & 0x0080;

  // Not much else we can tell from a Mode A/C reply.
  // Just fudge up a few bits to keep other code happy
  mm->crcok = 1;
  mm->correctedbits = 0;
  }
//
// ===================== Mode A/C detection and decoding  ===================
//

// vim: set ts=4 sw=4 sts=4 expandtab :