a combination FLL and PLL.  It will attempt to track for xxx ms, the last xxx ms of data will be  gathered to determine if we have both code and carrier lock.  If so we will transition to state 'tracking'.  Determine start of data bit transition (bit synchronization) using   histogram method.      REFERENCES  Tsui, Fundamentals of GPS receivers: A software approach, Wiley  Krunvieda et al., A complete IF software GPS receiver: a tutorial about    the detail, Data Fusion Corporation  Kaplan, Understanding GPS: Principles and ApplicationsWRITTEN BY  Clifford Kelley  Modifications by G. Beyerle  TO DO  02-05-01: Replace floating point with integer arithmeticNOTES  02-03-20:  *******************************************************************************/static void ch_pull_in( char ch){  int i, idx, bitsign;  long Ip, Qp, Id, Qd;  long ddf, ddcar, q_sum, i_sum;  double dsc_car_phs_old,          dsc_car_phs,          car_frq_ofs,         dsc_cod_phs_old,          dsc_cod_phs,          cod_frq_ofs;  double sum_IQd, sum_IQp;   double Ip2, Qp2, IQp2;  double curphs;  if ( chan[ch].ch_time < pull_in_time)  {////  if current phase deviated more than 90 deg from last value//  we probably have a nav bit flip//    curphs = atan2( chan[ch].bitsign*chan[ch].q_prmt,                     chan[ch].bitsign*chan[ch].i_prmt) / (2*M_PI);    if ( fabs( chan[ch].dsc_car_phs - curphs) > 0.25)    {      chan[ch].bitsign *= -1;      chan[ch].databithist[chan[ch].ms_count] += 1;    }  //    printf( "chan[ch].dsc_car_phs = %e, curphs = %e\n", //      chan[ch].dsc_car_phs, curphs);    bitsign = chan[ch].bitsign;        Ip = bitsign * chan[ch].i_prmt;    Qp = bitsign * chan[ch].q_prmt;    Id = bitsign * chan[ch].i_dith;    Qd = bitsign * chan[ch].q_dith;////  for efficiency we implement summation over NOFCOHINT values by //  keeping track of individual additions in arrays 'chan[ch].i/q_prmt/dith_sav[]'//// --- add latest value ...    chan[ch].i_prmt_sum   += Ip;    chan[ch].q_prmt_sum   += Qp;    chan[ch].i_dith_sum   += Id;    chan[ch].q_dith_sum   += Qd;    chan[ch].iq2_prmt_sum += Ip * Ip + Qp * Qp;// ... subtract first value ...    idx = (chan[ch].iq_sav_idx + 1) % NOFCOHINT;    chan[ch].iq_sav_idx = idx;    chan[ch].i_prmt_sum   -= chan[ch].i_prmt_sav[idx];    chan[ch].q_prmt_sum   -= chan[ch].q_prmt_sav[idx];    chan[ch].i_dith_sum   -= chan[ch].i_dith_sav[idx];    chan[ch].q_dith_sum   -= chan[ch].q_dith_sav[idx];    chan[ch].iq2_prmt_sum -= chan[ch].iq2_prmt_sav[idx];// ... and replace by latest value in "store array"    chan[ch].i_prmt_sav[idx]   = Ip;    chan[ch].q_prmt_sav[idx]   = Qp;    chan[ch].i_dith_sav[idx]   = Id;    chan[ch].q_dith_sav[idx]   = Qd;    chan[ch].iq2_prmt_sav[idx] = Ip * Ip + Qp * Qp;// --- carrier tracking loop ---    dsc_car_phs_old      = chan[ch].dsc_car_phs;//    printf( "chan[%d].q_prmt_sum = %d\n", ch, chan[ch].q_prmt_sum);//    printf( "chan[%d].i_prmt_sum = %d\n", ch, chan[ch].i_prmt_sum);    dsc_car_phs          = atan2( chan[ch].q_prmt_sum, chan[ch].i_prmt_sum) / (2*M_PI);  // optimal carrier phase discriminator    chan[ch].dsc_car_phs = dsc_car_phs;    car_frq_ofs = car_PLL_pullin_p1 * dsc_car_phs - car_PLL_pullin_p2 * dsc_car_phs_old;//    if (ch==0)//    {//      printf( "car_PLL_pullin_p1    = %e\n", car_PLL_pullin_p1);//      printf( "car_PLL_pullin_p2    = %e\n", car_PLL_pullin_p2);//      printf( "dsc_car_phs          = %e\n", dsc_car_phs);//      printf( "dsc_car_phs_old      = %e\n", dsc_car_phs_old);//      printf( "chan[ch].dsc_car_phs = %e\n", chan[ch].dsc_car_phs);//      printf( "car_frq_ofs          = %e\n", car_frq_ofs);//    }  //    getchar();    car_frq_ofs /= FRQUNIT;   // INCR counter runs in units of 42 mHz  //  printf( "d codfrq = %.6f Hz, d carfrq = %.6f Hz\n",   //    (chan[ch].code_freq    - 24028328) * 42.57475e-3,   //    (chan[ch].carrier_freq - 33010105) * 42.57475e-3);// --- update INCR register ---    chan[ch].carrier_freq += (long)( car_frq_ofs);    ch_carrier( ch, chan[ch].carrier_freq);// --- code tracking loop ---//    printf("vor sqrt() *** 1 ***\n");//    printf("iDith = %e \n", (double)( chan[ch].i_dith_sum * chan[ch].i_dith_sum));//    printf("qDith = %e \n", (double)( chan[ch].q_dith_sum * chan[ch].q_dith_sum));//    printf("iPrmt = %e \n", (double)( chan[ch].i_prmt_sum * chan[ch].i_prmt_sum));//    printf("qPrmt = %e \n", (double)( chan[ch].q_prmt_sum * chan[ch].q_prmt_sum));    sum_IQd = sqrt( (double)( chan[ch].i_dith_sum) * (double)( chan[ch].i_dith_sum) +                     (double)( chan[ch].q_dith_sum) * (double)( chan[ch].q_dith_sum));    sum_IQp = sqrt( (double)( chan[ch].i_prmt_sum) * (double)( chan[ch].i_prmt_sum) +                     (double)( chan[ch].q_prmt_sum) * (double)( chan[ch].q_prmt_sum));//    printf("nach sqrt() *** 1 ***\n");                      dsc_cod_phs_old = chan[ch].dsc_cod_phs;    if ( sum_IQp)      dsc_cod_phs = (sum_IQd / sum_IQp - 0.5);     else        dsc_cod_phs = 0.0;     chan[ch].dsc_cod_phs = dsc_cod_phs;    cod_frq_ofs = cod_PLL_pullin_p1 * dsc_cod_phs - cod_PLL_pullin_p2 * dsc_cod_phs_old;    cod_frq_ofs /= 2*FRQUNIT;   // INCR counter runs in units of 85 mHz// --- update INCR register ---    chan[ch].code_freq += (long)( cod_frq_ofs);    ch_code( ch, chan[ch].code_freq);// --- carrier and code lock detector ---    Ip2 = (double)( chan[ch].i_prmt_sum) * (double)( chan[ch].i_prmt_sum);    Qp2 = (double)( chan[ch].q_prmt_sum) * (double)( chan[ch].q_prmt_sum);// --- carrier lock detector ---    chan[ch].car_lock_det = (Ip2 - Qp2) / (Ip2 + Qp2);//    if (chan[ch].car_lock_det > 1.0)//    {//      printf("** r/lk = %e, Ip2 = %e, Qp2 = %e\n", chan[ch].car_lock_det, Ip2, Qp2);//      getchar();//    }// --- code lock detector ---    chan[ch].cod_lock_det = (Ip2 + Qp2) / chan[ch].iq2_prmt_sum / NOFCOHINT;// ************** debug *******************//    if ( ch==0)//    {//      extern unsigned long Sample_Counter_Start[], Sample_Counter_End[];////      printf( "pll: (%d-%d) Ip=%4d, Qp=%4d, Id=%4d, Qd=%4d, r/lk:%.2f, d/lk:%.2f", //        Sample_Counter_Start[0], Sample_Counter_End[0], Ip, Qp, Id, Qd, //        chan[ch].car_lock_det, chan[ch].cod_lock_det);//      printf( "pll: Ip=%4d, Qp=%4d, Id=%4d, Qd=%4d, r/lk:%.2f, d/lk:%.2f\n", //        Ip, Qp, Id, Qd, chan[ch].car_lock_det, chan[ch].cod_lock_det);//      getchar();  //    }  // ************** debug *******************    if ( chan[ch].ms_count >= 19)    {      chan[ch].tr_bit_time++;      chan[ch].prompt_mag = rss( chan[ch].i_prmt_sum, chan[ch].q_prmt_sum);      chan[ch].dith_mag   = rss( chan[ch].i_dith_sum, chan[ch].q_dith_sum);      chan[ch].sum        += chan[ch].prompt_mag + chan[ch].dith_mag;      chan[ch].t_count++;      if ( chan[ch].t_count % 5 == 0)      {        chan[ch].avg = chan[ch].sum / 10;        chan[ch].sum = 0;      }    }    if ( chan[ch].ch_time > pull_in_time - phase_test)     {      chan[ch].th_rms += (long)( dsc_car_phs * dsc_car_phs * 268435456);   // (2^14)^2//      printf( "pull_in: chan[ch].th_rms = %d\n", chan[ch].th_rms); //      getchar();       }  //    if ( chan[ch].cod_lock_det > cod_lock_det_thresh)//    {//      i_sum = chan[ch].i_dith + chan[ch].i_prmt;////// --- sign change? if yes mark in histogram ---//      if ( sign( chan[ch].old_i_sum) == -sign( i_sum))//        chan[ch].databithist[chan[ch].ms_count] += 1;//    }    chan[ch].old_i_sum = i_sum;    chan[ch].ch_time++;  } // --- if ( chan[ch].ch_time < pull_in_time) ---  else  {    if ( chan[ch].cod_lock_det < cod_lock_det_thresh)    {// --- go back to square one and start with acq ---      chan[ch].state = acquisition;    }      else    {// --- analyze data bit histogram (bit synchronization)      int ret;          ret = analyze_databit_histogram( ch);//      printf("pll: ret = %d\n", ret);//      getchar();      if ( ret >= 0)      {//        chan[ch].ms_count  = (20-ret+chan[ch].ms_count) % 20;  // reset msec counter        chan[ch].ms_count  = (chan[ch].ms_count-ret+20) % 20;  // reset msec counter//      chan[ch].databit_start = ret;// --- transition to tracking state ---        chan[ch].state     = track;        chan[ch].t_count   = 0;//      chan[ch].avg       = chan[ch].avg * 20;//      chan[ch].sum       = 0;      }        else        chan[ch].state = acquisition;    }    }  // --- if ( chan[ch].ch_time == pull_in_time) ---  // --- if back to acq, reset frq ---  if ( chan[ch].state == acquisition)  {    chan[ch].codes     = 0;    chan[ch].code_freq = code_ref + code_corr;    ch_code( ch, chan[ch].code_freq);        // 1.023 MHz chipping rate  }  chan[ch].ms_count = (++chan[ch].ms_count) % NOFMSECPERDATABIT;  return;}/*******************************************************************************FUNCTION ch_track()RETURNS  None.PARAMETERS None.PURPOSE  Track signal with a combination FLL and PLL.  Identify navigation data frame (frame synchronization).REFERENCES  Kaplan, Understanding GPS: Principles and ApplicationsWRITTEN BY  Clifford Kelley  Modifications by G. BeyerleTO DO  02-05-01: Replace floating point with integer arithmeticNOTES  02-03-20: *******************************************************************************/static void ch_track( char ch){  int i, idx, bitsign;  long Ip, Qp, Id, Qd;  long ddf, ddcar, q_sum, i_sum;  float  dsc_car_phs_old,          dsc_car_phs,          car_frq_ofs,         dsc_cod_phs_old,          dsc_cod_phs,          cod_frq_ofs;  float sum_IQd, sum_IQp;   float Ip2, Qp2, IQp2, curphs;////  if current phase deviated more than 90 deg from last value//  we probably have a nav bit flip//  curphs = atan2( chan[ch].bitsign*chan[ch].q_prmt,                   chan[ch].bitsign*chan[ch].i_prmt) / (2*M_PI);//  if ( fabs( chan[ch].dsc_car_phs - curphs) > 0.25 &&//       chan[ch].ms_count == 0)  if ( fabs( chan[ch].dsc_car_phs - curphs) > 0.25)  {//    printf("trk: chan[%d].ms_count = %d\n", ch, chan[ch].ms_count);//    getchar();    chan[ch].bitsign *= -1;//    printf("trk: chan[%d].ms_count = %d\n", ch, chan[ch].ms_count); //    getchar();//    chan[ch].databithist[chan[ch].ms_count] += 1;  }  ////  implement summation over NOFCOHINT values by keeping track//  of individual additions//  bitsign = chan[ch].bitsign;  Ip = bitsign * chan[ch].i_prmt;  Qp = bitsign * chan[ch].q_prmt;  Id = bitsign * chan[ch].i_dith;  Qd = bitsign * chan[ch].q_dith;  if ( OutputIQ)    write_to_file_IQ( chan[ch].i_prmt, chan[ch].q_prmt, chan[ch].prn);  // with nav msg//    write_to_file_IQ( Ip, Qp, chan[ch].prn);  // nav msg wiped off// add latest value ...  chan[ch].i_prmt_sum   += Ip;  chan[ch].q_prmt_sum   += Qp;  chan[ch].i_dith_sum   += Id;  chan[ch].q_dith_sum   += Qd;  chan[ch].iq2_prmt_sum += Ip * Ip + Qp * Qp;// ... subtract first value ...  idx = (chan[ch].iq_sav_idx + 1) % NOFCOHINT;  chan[ch].iq_sav_idx = idx;  chan[ch].i_prmt_sum   -= chan[ch].i_prmt_sav[idx];  chan[ch].q_prmt_sum   -= chan[ch].q_prmt_sav[idx];  chan[ch].i_dith_sum   -= chan[ch].i_dith_sav[idx];  chan[ch].q_dith_sum   -= chan[ch].q_dith_sav[idx];  chan[ch].iq2_prmt_sum -= chan[ch].iq2_prmt_sav[idx];// ... and replace by latest value in "store array"  chan[ch].i_prmt_sav[idx] = Ip;  chan[ch].q_prmt_sav[idx] = Qp;  chan[ch].i_dith_sav[idx] = Id;  chan[ch].q_dith_sav[idx] = Qd;  chan[ch].iq2_prmt_sav[idx] = Ip * Ip + Qp * Qp;//  carrier tracking loop  dsc_car_phs_old      = chan[ch].dsc_car_phs;  dsc_car_phs          = atan2( chan[ch].q_prmt_sum, chan[ch].i_prmt_sum) / (2*M_PI);  // optimal carrier phase discriminator  chan[ch].dsc_car_phs = dsc_car_phs;  car_frq_ofs = car_PLL_track_p1 * dsc_car_phs - car_PLL_track_p2 * dsc_car_phs_old;  car_frq_ofs /= FRQUNIT;   // INCR counter runs in units of 42.57 mHz//  update INCR register  chan[ch].carrier_freq += (long)( car_frq_ofs);  ch_carrier( ch, chan[ch].carrier_freq);  if ( OutputCarFrq)    write_to_file_carfrq( (long)( car_frq_ofs), chan[ch].prn);//  code tracking loop  sum_IQd = sqrt( (double)( chan[ch].i_dith_sum) * (double)( chan[ch].i_dith_sum) +                   (double)( chan[ch].q_dith_sum) * (double)( chan[ch].q_dith_sum));  sum_IQp = sqrt( (double)( chan[ch].i_prmt_sum) * (double)( chan[ch].i_prmt_sum) +                   (double)( chan[ch].q_prmt_sum) * (double)( chan[ch].q_prmt_sum));                    dsc_cod_phs_old = chan[ch].dsc_cod_phs;  if ( sum_IQp)    dsc_cod_phs = (sum_IQd / sum_IQp - 0.5);   else      dsc_cod_phs = 0.0;   chan[ch].dsc_cod_phs = dsc_cod_phs;  cod_frq_ofs = cod_PLL_track_p1 * dsc_cod_phs - cod_PLL_track_p2 * dsc_cod_phs_old;//  cod_frq_ofs *= NOFCHIPS;  cod_frq_ofs /= 2*FRQUNIT;   // INCR counter runs in units of 85 mHz//  update INCR register  chan[ch].code_freq += (long)( cod_frq_ofs);  ch_code( ch, chan[ch].code_freq);//  carrier and code lock detector  Ip2 = (double)( chan[ch].i_prmt_sum) * (double)( chan[ch].i_prmt_sum);  Qp2 = (double)( chan[ch].q_prmt_sum) * (double)( chan[ch].q_prmt_sum);  chan[ch].car_lock_det = (Ip2 - Qp2) / (Ip2 + Qp2);    chan[ch].cod_lock_det = (Ip2 + Qp2) / chan[ch].iq2_prmt_sum / NOFCOHINT;  if ( chan[ch].cod_lock_det < cod_lock_det_thresh)  {    chan[ch].state = acquisition;//    printf("trk: chan[%d].cod_lock_det = %e, cod_lock_det_thresh = %e\n", //      ch, chan[ch].cod_lock_det, cod_lock_det_thresh);//    getchar();  }// ----------------------//  printf( "Ip=%5d, Qp=%5d, Id=%5d, Qd=%5d, r/lk:%.2f, d/lk:%.2f\n", //    Ip, Qp, Id, Qd, chan[ch].car_lock_det, chan[ch].cod_lock_det);//  getchar();    // ----------------------//  printf( "d codfrq = %.6f Hz, d carfrq = %.6f Hz\n", //    (chan[ch].code_freq    - 24028328) * 42.57475e-3, //    (chan[ch].carrier_freq - 33010105) * 42.57475e-3);//  printf( "DrPhs = %e, DdPhs = %e, drFrq = %e, ddFrq = %e\n", //    dsc_car_phs, dsc_cod_phs, car_frq_ofs, cod_frq_ofs);//  LastSmplIdx = BUFFERPTR;//  if ( abs( chan[ch].i_prmtpt) < abs( chan[ch].q_prmtpt))//    getchar();