mirror of https://github.com/gnss-sdr/gnss-sdr
199 lines
6.9 KiB
C++
199 lines
6.9 KiB
C++
/*!
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* \file gps_sdr_signal_processing.cc
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* \brief This class implements various functions for GPS L1 CA signals
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* \author Javier Arribas, 2011. jarribas(at)cttc.es
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*
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* Detailed description of the file here if needed.
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*
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* -------------------------------------------------------------------------
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*
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* Copyright (C) 2010-2019 (see AUTHORS file for a list of contributors)
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*
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* GNSS-SDR is a software defined Global Navigation
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* Satellite Systems receiver
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*
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* This file is part of GNSS-SDR.
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*
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* GNSS-SDR is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* GNSS-SDR is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with GNSS-SDR. If not, see <https://www.gnu.org/licenses/>.
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*
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* -------------------------------------------------------------------------
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*/
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#include "gps_sdr_signal_processing.h"
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#include <array>
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#include <bitset>
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auto auxCeil = [](float x) { return static_cast<int32_t>(static_cast<int64_t>((x) + 1)); };
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void gps_l1_ca_code_gen_int(gsl::span<int32_t> _dest, int32_t _prn, uint32_t _chip_shift)
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{
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const uint32_t _code_length = 1023;
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std::bitset<_code_length> G1{};
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std::bitset<_code_length> G2{};
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std::bitset<10> G1_register{};
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std::bitset<10> G2_register{};
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bool feedback1, feedback2;
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bool aux;
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uint32_t lcv, lcv2;
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uint32_t delay;
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int32_t prn_idx;
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// G2 Delays as defined in GPS-ISD-200D
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const std::array<int32_t, 51> delays = {5 /*PRN1*/, 6, 7, 8, 17, 18, 139, 140, 141, 251, 252, 254, 255, 256, 257, 258, 469, 470, 471, 472,
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473, 474, 509, 512, 513, 514, 515, 516, 859, 860, 861, 862 /*PRN32*/,
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145 /*PRN120*/, 175, 52, 21, 237, 235, 886, 657, 634, 762,
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355, 1012, 176, 603, 130, 359, 595, 68, 386 /*PRN138*/};
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// compute delay array index for given PRN number
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if (120 <= _prn && _prn <= 138)
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{
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prn_idx = _prn - 88; // SBAS PRNs are at array indices 31 to 50 (offset: -120+33-1 =-88)
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}
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else
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{
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prn_idx = _prn - 1;
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}
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// A simple error check
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if ((prn_idx < 0) || (prn_idx > 51))
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{
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return;
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}
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for (lcv = 0; lcv < 10; lcv++)
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{
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G1_register[lcv] = true;
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G2_register[lcv] = true;
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}
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// Generate G1 & G2 Register
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for (lcv = 0; lcv < _code_length; lcv++)
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{
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G1[lcv] = G1_register[0];
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G2[lcv] = G2_register[0];
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feedback1 = G1_register[7] xor G1_register[0];
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feedback2 = G2_register[8] xor G2_register[7] xor G2_register[4] xor G2_register[2] xor G2_register[1] xor G2_register[0];
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for (lcv2 = 0; lcv2 < 9; lcv2++)
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{
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G1_register[lcv2] = G1_register[lcv2 + 1];
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G2_register[lcv2] = G2_register[lcv2 + 1];
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}
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G1_register[9] = feedback1;
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G2_register[9] = feedback2;
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}
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// Set the delay
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delay = _code_length - delays[prn_idx];
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delay += _chip_shift;
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delay %= _code_length;
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// Generate PRN from G1 and G2 Registers
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for (lcv = 0; lcv < _code_length; lcv++)
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{
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aux = G1[(lcv + _chip_shift) % _code_length] xor G2[delay];
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if (aux == true)
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{
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_dest[lcv] = 1;
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}
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else
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{
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_dest[lcv] = -1;
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}
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delay++;
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delay %= _code_length;
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}
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}
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void gps_l1_ca_code_gen_float(gsl::span<float> _dest, int32_t _prn, uint32_t _chip_shift)
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{
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const uint32_t _code_length = 1023;
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std::array<int32_t, _code_length> ca_code_int{};
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gps_l1_ca_code_gen_int(ca_code_int, _prn, _chip_shift);
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for (uint32_t ii = 0; ii < _code_length; ++ii)
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{
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_dest[ii] = static_cast<float>(ca_code_int[ii]);
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}
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}
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void gps_l1_ca_code_gen_complex(gsl::span<std::complex<float>> _dest, int32_t _prn, uint32_t _chip_shift)
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{
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const uint32_t _code_length = 1023;
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std::array<int32_t, _code_length> ca_code_int{};
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gps_l1_ca_code_gen_int(ca_code_int, _prn, _chip_shift);
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for (uint32_t ii = 0; ii < _code_length; ++ii)
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{
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_dest[ii] = std::complex<float>(static_cast<float>(ca_code_int[ii]), 0.0F);
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}
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}
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/*
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* Generates complex GPS L1 C/A code for the desired SV ID and sampled to specific sampling frequency
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* NOTICE: the number of samples is rounded towards zero (integer truncation)
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*/
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void gps_l1_ca_code_gen_complex_sampled(gsl::span<std::complex<float>> _dest, uint32_t _prn, int32_t _fs, uint32_t _chip_shift)
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{
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// This function is based on the GNU software GPS for MATLAB in the Kay Borre book
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std::array<std::complex<float>, 1023> _code{};
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int32_t _samplesPerCode, _codeValueIndex;
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float _ts;
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float _tc;
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float aux;
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const int32_t _codeFreqBasis = 1023000; // Hz
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const int32_t _codeLength = 1023;
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// --- Find number of samples per spreading code ---------------------------
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_samplesPerCode = static_cast<int32_t>(static_cast<double>(_fs) / static_cast<double>(_codeFreqBasis / _codeLength));
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// --- Find time constants -------------------------------------------------
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_ts = 1.0 / static_cast<float>(_fs); // Sampling period in sec
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_tc = 1.0 / static_cast<float>(_codeFreqBasis); // C/A chip period in sec
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gps_l1_ca_code_gen_complex(_code, _prn, _chip_shift); // generate C/A code 1 sample per chip
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for (int32_t i = 0; i < _samplesPerCode; i++)
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{
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// === Digitizing ==================================================
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// --- Make index array to read C/A code values --------------------
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// The length of the index array depends on the sampling frequency -
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// number of samples per millisecond (because one C/A code period is one
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// millisecond).
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aux = (_ts * (i + 1)) / _tc;
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_codeValueIndex = auxCeil(aux) - 1;
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// --- Make the digitized version of the C/A code -------------------
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// The "upsampled" code is made by selecting values form the CA code
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// chip array (caCode) for the time instances of each sample.
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if (i == _samplesPerCode - 1)
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{
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// --- Correct the last index (due to number rounding issues)
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_dest[i] = _code[_codeLength - 1];
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}
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else
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{
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_dest[i] = _code[_codeValueIndex]; // repeat the chip -> upsample
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}
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}
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}
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