mirror of https://github.com/gnss-sdr/gnss-sdr
134 lines
4.3 KiB
C++
134 lines
4.3 KiB
C++
/*!
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* \file glonass_l1_signal_processing.cc
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* \brief This class implements various functions for GLONASS L1 CA signals
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* \author Javier Arribas, 2011. jarribas(at)cttc.es
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*
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*
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* -----------------------------------------------------------------------------
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*
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* Copyright (C) 2010-2020 (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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* SPDX-License-Identifier: GPL-3.0-or-later
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*
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* -----------------------------------------------------------------------------
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*/
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#include "glonass_l1_signal_processing.h"
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#include <array>
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#include <bitset>
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const auto AUX_CEIL = [](float x) { return static_cast<int32_t>(static_cast<int64_t>((x) + 1)); };
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void glonass_l1_ca_code_gen_complex(own::span<std::complex<float>> _dest, uint32_t _chip_shift)
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{
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const uint32_t _code_length = 511;
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std::bitset<_code_length> G1{};
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auto G1_register = std::bitset<9>{}.set(); // All true
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bool feedback1;
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bool aux;
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uint32_t delay;
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uint32_t lcv;
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uint32_t lcv2;
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/* Generate G1 Register */
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for (lcv = 0; lcv < _code_length; lcv++)
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{
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G1[lcv] = G1_register[2];
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feedback1 = G1_register[4] xor G1_register[0];
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for (lcv2 = 0; lcv2 < 8; lcv2++)
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{
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G1_register[lcv2] = G1_register[lcv2 + 1];
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}
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G1_register[8] = feedback1;
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}
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/* Generate PRN from G1 Register */
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for (lcv = 0; lcv < _code_length; lcv++)
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{
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aux = G1[lcv];
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if (aux == true)
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{
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_dest[lcv] = std::complex<float>(1, 0);
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}
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else
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{
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_dest[lcv] = std::complex<float>(-1, 0);
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}
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}
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/* Set the delay */
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delay = _code_length;
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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];
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if (aux == true)
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{
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_dest[lcv] = std::complex<float>(1, 0);
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}
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else
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{
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_dest[lcv] = std::complex<float>(-1, 0);
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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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/*
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* Generates complex GLONASS L1 C/A code for the desired SV ID and sampled to specific sampling frequency
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*/
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void glonass_l1_ca_code_gen_complex_sampled(own::span<std::complex<float>> _dest, int32_t _fs, uint32_t _chip_shift)
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{
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constexpr int32_t _codeFreqBasis = 511000; // Hz
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constexpr int32_t _codeLength = 511;
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constexpr float _tc = 1.0 / static_cast<float>(_codeFreqBasis); // C/A chip period in sec
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const float _ts = 1.0F / static_cast<float>(_fs); // Sampling period in sec
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const auto _samplesPerCode = static_cast<int32_t>(static_cast<double>(_fs) / (static_cast<double>(_codeFreqBasis) / static_cast<double>(_codeLength)));
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std::array<std::complex<float>, 511> _code{};
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int32_t _codeValueIndex;
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float aux;
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glonass_l1_ca_code_gen_complex(_code, _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 * (static_cast<float>(i) + 1)) / _tc;
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_codeValueIndex = AUX_CEIL(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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