Simplification of the GPS L1 CA telemetry decoder and synchronizer. Improving preamble detection reliability

src/algorithms/telemetry_decoder/gnuradio_blocks/gps_l1_ca_telemetry_decoder_cc.cc

@@ -83,26 +83,20 @@ gps_l1_ca_telemetry_decoder_cc::gps_l1_ca_telemetry_decoder_cc(
                 }
         }
     d_stat = 0;
-    d_symbol_accumulator = 0;
-    d_symbol_accumulator_counter = 0;
-    d_frame_bit_index = 0;
     d_flag_frame_sync = false;
-    d_GPS_frame_4bytes = 0;
     d_prev_GPS_frame_4bytes = 0;
-    d_flag_parity = false;
     d_TOW_at_Preamble_ms = 0;
     flag_TOW_set = false;
     d_flag_preamble = false;
     d_flag_new_tow_available = false;
-    d_word_number = 0;
     d_channel = 0;
     flag_PLL_180_deg_phase_locked = false;
     d_preamble_time_samples = 0;
     d_TOW_at_current_symbol_ms = 0;
-    d_symbol_history.resize(GPS_CA_PREAMBLE_LENGTH_SYMBOLS + 1);  // Change fixed buffer size
+    d_symbol_history.resize(GPS_CA_PREAMBLE_LENGTH_SYMBOLS);  // Change fixed buffer size
-    d_symbol_history.clear();                                     // Clear all the elements in the buffer
+    d_symbol_history.clear();                                 // Clear all the elements in the buffer
-    d_make_correlation = true;
+    d_crc_error_synchronization_counter = 0;
-    d_symbol_counter_corr = 0;
+    d_current_subframe_symbol = 0;
 }
 
 
@@ -150,9 +144,10 @@ bool gps_l1_ca_telemetry_decoder_cc::gps_word_parityCheck(unsigned int gpsword)
 
 void gps_l1_ca_telemetry_decoder_cc::set_satellite(const Gnss_Satellite &satellite)
 {
+    d_nav.reset();
     d_satellite = Gnss_Satellite(satellite.get_system(), satellite.get_PRN());
     DLOG(INFO) << "Setting decoder Finite State Machine to satellite " << d_satellite;
-    d_GPS_FSM.i_satellite_PRN = d_satellite.get_PRN();
+    d_nav.i_satellite_PRN = d_satellite.get_PRN();
     DLOG(INFO) << "Navigation Satellite set to " << d_satellite;
 }
 
@@ -160,7 +155,7 @@ void gps_l1_ca_telemetry_decoder_cc::set_satellite(const Gnss_Satellite &satelli
 void gps_l1_ca_telemetry_decoder_cc::set_channel(int channel)
 {
     d_channel = channel;
-    d_GPS_FSM.i_channel_ID = channel;
+    d_nav.i_channel_ID = channel;
     DLOG(INFO) << "Navigation channel set to " << channel;
     // ############# ENABLE DATA FILE LOG #################
     if (d_dump == true)
@@ -186,10 +181,128 @@ void gps_l1_ca_telemetry_decoder_cc::set_channel(int channel)
 }
 
 
+bool gps_l1_ca_telemetry_decoder_cc::decode_subframe()
+{
+    char subframe[GPS_SUBFRAME_LENGTH];
+
+    int symbol_accumulator_counter = 0;
+    int frame_bit_index = 0;
+    int word_index = 0;
+    uint32_t GPS_frame_4bytes = 0;
+    float symbol_accumulator = 0;
+    bool subframe_synchro_confirmation = false;
+    bool CRC_ok = true;
+
+    for (int n = 0; n < GPS_SUBFRAME_MS; n++)
+        {
+            //******* SYMBOL TO BIT *******
+            // extended correlation to bit period is enabled in tracking!
+            symbol_accumulator += d_subframe_symbols[n];  // accumulate the input value in d_symbol_accumulator
+            symbol_accumulator_counter++;
+            if (symbol_accumulator_counter == 20)
+                {
+                    //symbol to bit
+                    if (symbol_accumulator > 0) GPS_frame_4bytes += 1;  //insert the telemetry bit in LSB
+                    symbol_accumulator = 0;
+                    symbol_accumulator_counter = 0;
+
+                    //******* bits to words ******
+                    frame_bit_index++;
+                    if (frame_bit_index == 30)
+                        {
+                            frame_bit_index = 0;
+                            // parity check
+                            // Each word in wordbuff is composed of:
+                            //      Bits 0 to 29 = the GPS data word
+                            //      Bits 30 to 31 = 2 LSBs of the GPS word ahead.
+                            // prepare the extended frame [-2 -1 0 ... 30]
+                            if (d_prev_GPS_frame_4bytes & 0x00000001)
+                                {
+                                    GPS_frame_4bytes = GPS_frame_4bytes | 0x40000000;
+                                }
+                            if (d_prev_GPS_frame_4bytes & 0x00000002)
+                                {
+                                    GPS_frame_4bytes = GPS_frame_4bytes | 0x80000000;
+                                }
+                            /* Check that the 2 most recently logged words pass parity. Have to first
+                      invert the data bits according to bit 30 of the previous word. */
+                            if (GPS_frame_4bytes & 0x40000000)
+                                {
+                                    GPS_frame_4bytes ^= 0x3FFFFFC0;  // invert the data bits (using XOR)
+                                }
+                            if (gps_l1_ca_telemetry_decoder_cc::gps_word_parityCheck(GPS_frame_4bytes))
+                                {
+                                    subframe_synchro_confirmation = true;
+                                }
+                            else
+                                {
+                                    //std::cout << "word invalid sat " << this->d_satellite << std::endl;
+                                    CRC_ok = false;
+                                    //break;
+                                }
+                            //add word to subframe
+                            // insert the word in the correct position of the subframe
+                            std::memcpy(&subframe[word_index * GPS_WORD_LENGTH], &GPS_frame_4bytes, sizeof(uint32_t));
+                            word_index++;
+                            d_prev_GPS_frame_4bytes = GPS_frame_4bytes;  // save the actual frame
+                            GPS_frame_4bytes = 0;
+                        }
+                    else
+                        {
+                            GPS_frame_4bytes <<= 1;  //shift 1 bit left the telemetry word
+                        }
+                }
+        }
+
+
+    //decode subframe
+    // NEW GPS SUBFRAME HAS ARRIVED!
+    if (CRC_ok)
+        {
+            int subframe_ID = d_nav.subframe_decoder(subframe);  //decode the subframe
+            std::cout << "New GPS NAV message received in channel " << this->d_channel << ": "
+                      << "subframe "
+                      << subframe_ID << " from satellite "
+                      << Gnss_Satellite(std::string("GPS"), d_nav.i_satellite_PRN) << std::endl;
+
+            switch (subframe_ID)
+                {
+                case 3:  //we have a new set of ephemeris data for the current SV
+                    if (d_nav.satellite_validation() == true)
+                        {
+                            // get ephemeris object for this SV (mandatory)
+                            std::shared_ptr<Gps_Ephemeris> tmp_obj = std::make_shared<Gps_Ephemeris>(d_nav.get_ephemeris());
+                            this->message_port_pub(pmt::mp("telemetry"), pmt::make_any(tmp_obj));
+                        }
+                    break;
+                case 4:  // Possible IONOSPHERE and UTC model update (page 18)
+                    if (d_nav.flag_iono_valid == true)
+                        {
+                            std::shared_ptr<Gps_Iono> tmp_obj = std::make_shared<Gps_Iono>(d_nav.get_iono());
+                            this->message_port_pub(pmt::mp("telemetry"), pmt::make_any(tmp_obj));
+                        }
+                    if (d_nav.flag_utc_model_valid == true)
+                        {
+                            std::shared_ptr<Gps_Utc_Model> tmp_obj = std::make_shared<Gps_Utc_Model>(d_nav.get_utc_model());
+                            this->message_port_pub(pmt::mp("telemetry"), pmt::make_any(tmp_obj));
+                        }
+                    break;
+                case 5:
+                    // get almanac (if available)
+                    //TODO: implement almanac reader in navigation_message
+                    break;
+                default:
+                    break;
+                }
+            d_flag_new_tow_available = true;
+        }
+
+    return subframe_synchro_confirmation;
+}
+
 int gps_l1_ca_telemetry_decoder_cc::general_work(int noutput_items __attribute__((unused)), gr_vector_int &ninput_items __attribute__((unused)),
     gr_vector_const_void_star &input_items, gr_vector_void_star &output_items)
 {
-    int corr_value = 0;
     int preamble_diff_ms = 0;
 
     Gnss_Synchro **out = reinterpret_cast<Gnss_Synchro **>(&output_items[0]);            // Get the output buffer pointer
@@ -198,15 +311,25 @@ int gps_l1_ca_telemetry_decoder_cc::general_work(int noutput_items __attribute__
     Gnss_Synchro current_symbol;  //structure to save the synchronization information and send the output object to the next block
     //1. Copy the current tracking output
     current_symbol = in[0][0];
+
+    //record the oldest subframe symbol before inserting a new symbol into the circular buffer
+    if (d_current_subframe_symbol < GPS_SUBFRAME_MS and d_symbol_history.size() > 0)
+        {
+            d_subframe_symbols[d_current_subframe_symbol] = d_symbol_history.at(0).Prompt_I;
+            d_current_subframe_symbol++;
+        }
+
     d_symbol_history.push_back(current_symbol);  //add new symbol to the symbol queue
     consume_each(1);
 
-    unsigned int required_symbols = GPS_CA_PREAMBLE_LENGTH_SYMBOLS;
     d_flag_preamble = false;
 
-    if ((d_symbol_history.size() > required_symbols) and (d_make_correlation or !d_flag_frame_sync))
+
+    //******* preamble correlation ********
+    int corr_value = 0;
+    if ((d_symbol_history.size() == GPS_CA_PREAMBLE_LENGTH_SYMBOLS))  // and (d_make_correlation or !d_flag_frame_sync))
         {
-            //******* preamble correlation ********
+            //            std::cout << "-------\n";
             for (unsigned int i = 0; i < GPS_CA_PREAMBLE_LENGTH_SYMBOLS; i++)
                 {
                     if (d_symbol_history.at(i).Flag_valid_symbol_output == true)
@@ -221,38 +344,27 @@ int gps_l1_ca_telemetry_decoder_cc::general_work(int noutput_items __attribute__
                                 }
                         }
                 }
-            if (std::abs(corr_value) >= GPS_CA_PREAMBLE_LENGTH_SYMBOLS)
-                {
-                    d_symbol_counter_corr++;
-                }
         }
 
+
     //******* frame sync ******************
     if (std::abs(corr_value) == GPS_CA_PREAMBLE_LENGTH_SYMBOLS)
         {
             //TODO: Rewrite with state machine
             if (d_stat == 0)
                 {
-                    d_GPS_FSM.Event_gps_word_preamble();
                     //record the preamble sample stamp
                     d_preamble_time_samples = d_symbol_history.at(0).Tracking_sample_counter;  // record the preamble sample stamp
                     DLOG(INFO) << "Preamble detection for SAT " << this->d_satellite << "d_symbol_history.at(0).Tracking_sample_counter=" << d_symbol_history.at(0).Tracking_sample_counter;
-                    //sync the symbol to bits integrator
-                    d_symbol_accumulator = 0;
-                    d_symbol_accumulator_counter = 0;
-                    d_frame_bit_index = 0;
                     d_stat = 1;  // enter into frame pre-detection status
                 }
             else if (d_stat == 1)  //check 6 seconds of preamble separation
                 {
                     preamble_diff_ms = std::round(((static_cast<double>(d_symbol_history.at(0).Tracking_sample_counter) - d_preamble_time_samples) / static_cast<double>(d_symbol_history.at(0).fs)) * 1000.0);
-                    if (std::abs(preamble_diff_ms - GPS_SUBFRAME_MS) < 1)
+                    if (std::abs(preamble_diff_ms - GPS_SUBFRAME_MS) % GPS_SUBFRAME_MS == 0)
                         {
                             DLOG(INFO) << "Preamble confirmation for SAT " << this->d_satellite;
-                            d_GPS_FSM.Event_gps_word_preamble();
                             d_flag_preamble = true;
-                            d_make_correlation = false;
-                            d_symbol_counter_corr = 0;
                             d_preamble_time_samples = d_symbol_history.at(0).Tracking_sample_counter;  // record the PRN start sample index associated to the preamble
                             if (!d_flag_frame_sync)
                                 {
@@ -269,131 +381,46 @@ int gps_l1_ca_telemetry_decoder_cc::general_work(int noutput_items __attribute__
                                     DLOG(INFO) << " Frame sync SAT " << this->d_satellite << " with preamble start at "
                                                << static_cast<double>(d_preamble_time_samples) / static_cast<double>(d_symbol_history.at(0).fs) << " [s]";
                                 }
+
+                            //try to decode the subframe:
+                            if (decode_subframe() == false)
+                                {
+                                    d_crc_error_synchronization_counter++;
+                                    if (d_crc_error_synchronization_counter > 3)
+                                        {
+                                            DLOG(INFO) << "TOO MANY CRC ERRORS: Lost of frame sync SAT " << this->d_satellite << std::endl;
+                                            d_stat = 0;  //lost of frame sync
+                                            d_flag_frame_sync = false;
+                                            flag_TOW_set = false;
+                                            d_crc_error_synchronization_counter = 0;
+                                        }
+                                }
+                            d_current_subframe_symbol = 0;
                         }
                 }
         }
     else
         {
-            d_symbol_counter_corr++;
-            if (d_symbol_counter_corr > (GPS_SUBFRAME_MS - GPS_CA_TELEMETRY_SYMBOLS_PER_BIT))
-                {
-                    d_make_correlation = true;
-                }
             if (d_stat == 1)
                 {
                     preamble_diff_ms = round(((static_cast<double>(d_symbol_history.at(0).Tracking_sample_counter) - static_cast<double>(d_preamble_time_samples)) / static_cast<double>(d_symbol_history.at(0).fs)) * 1000.0);
-                    if (preamble_diff_ms > GPS_SUBFRAME_MS + 1)
+                    if (preamble_diff_ms > GPS_SUBFRAME_MS)
                         {
                             DLOG(INFO) << "Lost of frame sync SAT " << this->d_satellite << " preamble_diff= " << preamble_diff_ms;
+                            //                            std::cout << "Lost of frame sync SAT " << this->d_satellite << " preamble_diff= " << preamble_diff_ms << std::endl;
                             d_stat = 0;  //lost of frame sync
                             d_flag_frame_sync = false;
                             flag_TOW_set = false;
-                            d_make_correlation = true;
+                            d_current_subframe_symbol = 0;
-                            d_symbol_counter_corr = 0;
+                            d_crc_error_synchronization_counter = 0;
                         }
                 }
         }
 
-    //******* SYMBOL TO BIT *******
-    if (d_symbol_history.at(0).Flag_valid_symbol_output == true)
-        {
-            // extended correlation to bit period is enabled in tracking!
-            d_symbol_accumulator += d_symbol_history.at(0).Prompt_I;  // accumulate the input value in d_symbol_accumulator
-            d_symbol_accumulator_counter += d_symbol_history.at(0).correlation_length_ms;
-        }
-    if (d_symbol_accumulator_counter >= 20)
-        {
-            if (d_symbol_accumulator > 0)
-                {                             //symbol to bit
-                    d_GPS_frame_4bytes += 1;  //insert the telemetry bit in LSB
-                }
-            d_symbol_accumulator = 0;
-            d_symbol_accumulator_counter = 0;
-            //******* bits to words ******
-            d_frame_bit_index++;
-            if (d_frame_bit_index == 30)
-                {
-                    d_frame_bit_index = 0;
-                    // parity check
-                    // Each word in wordbuff is composed of:
-                    //      Bits 0 to 29 = the GPS data word
-                    //      Bits 30 to 31 = 2 LSBs of the GPS word ahead.
-                    // prepare the extended frame [-2 -1 0 ... 30]
-                    if (d_prev_GPS_frame_4bytes & 0x00000001)
-                        {
-                            d_GPS_frame_4bytes = d_GPS_frame_4bytes | 0x40000000;
-                        }
-                    if (d_prev_GPS_frame_4bytes & 0x00000002)
-                        {
-                            d_GPS_frame_4bytes = d_GPS_frame_4bytes | 0x80000000;
-                        }
-                    /* Check that the 2 most recently logged words pass parity. Have to first
-                      invert the data bits according to bit 30 of the previous word. */
-                    if (d_GPS_frame_4bytes & 0x40000000)
-                        {
-                            d_GPS_frame_4bytes ^= 0x3FFFFFC0;  // invert the data bits (using XOR)
-                        }
-                    if (gps_l1_ca_telemetry_decoder_cc::gps_word_parityCheck(d_GPS_frame_4bytes))
-                        {
-                            memcpy(&d_GPS_FSM.d_GPS_frame_4bytes, &d_GPS_frame_4bytes, sizeof(char) * 4);
-                            //d_GPS_FSM.d_preamble_time_ms = d_preamble_time_seconds * 1000.0;
-                            d_GPS_FSM.Event_gps_word_valid();
-                            // send TLM data to PVT using asynchronous message queues
-                            if (d_GPS_FSM.d_flag_new_subframe == true)
-                                {
-                                    switch (d_GPS_FSM.d_subframe_ID)
-                                        {
-                                        case 3:  //we have a new set of ephemeris data for the current SV
-                                            if (d_GPS_FSM.d_nav.satellite_validation() == true)
-                                                {
-                                                    // get ephemeris object for this SV (mandatory)
-                                                    std::shared_ptr<Gps_Ephemeris> tmp_obj = std::make_shared<Gps_Ephemeris>(d_GPS_FSM.d_nav.get_ephemeris());
-                                                    this->message_port_pub(pmt::mp("telemetry"), pmt::make_any(tmp_obj));
-                                                }
-                                            break;
-                                        case 4:  // Possible IONOSPHERE and UTC model update (page 18)
-                                            if (d_GPS_FSM.d_nav.flag_iono_valid == true)
-                                                {
-                                                    std::shared_ptr<Gps_Iono> tmp_obj = std::make_shared<Gps_Iono>(d_GPS_FSM.d_nav.get_iono());
-                                                    this->message_port_pub(pmt::mp("telemetry"), pmt::make_any(tmp_obj));
-                                                }
-                                            if (d_GPS_FSM.d_nav.flag_utc_model_valid == true)
-                                                {
-                                                    std::shared_ptr<Gps_Utc_Model> tmp_obj = std::make_shared<Gps_Utc_Model>(d_GPS_FSM.d_nav.get_utc_model());
-                                                    this->message_port_pub(pmt::mp("telemetry"), pmt::make_any(tmp_obj));
-                                                }
-                                            break;
-                                        case 5:
-                                            // get almanac (if available)
-                                            //TODO: implement almanac reader in navigation_message
-                                            break;
-                                        default:
-                                            break;
-                                        }
-                                    d_GPS_FSM.clear_flag_new_subframe();
-                                    d_flag_new_tow_available = true;
-                                }
-
-                            d_flag_parity = true;
-                        }
-                    else
-                        {
-                            d_GPS_FSM.Event_gps_word_invalid();
-                            d_flag_parity = false;
-                        }
-                    d_prev_GPS_frame_4bytes = d_GPS_frame_4bytes;  // save the actual frame
-                    d_GPS_frame_4bytes = d_GPS_frame_4bytes & 0;
-                }
-            else
-                {
-                    d_GPS_frame_4bytes <<= 1;  //shift 1 bit left the telemetry word
-                }
-        }
-
     //2. Add the telemetry decoder information
     if (this->d_flag_preamble == true and d_flag_new_tow_available == true)
         {
-            d_TOW_at_current_symbol_ms = static_cast<unsigned int>(d_GPS_FSM.d_nav.d_TOW) * 1000 + GPS_L1_CA_CODE_PERIOD_MS + GPS_CA_PREAMBLE_DURATION_MS;
+            d_TOW_at_current_symbol_ms = static_cast<unsigned int>(d_nav.d_TOW) * 1000 + GPS_L1_CA_CODE_PERIOD_MS + GPS_CA_PREAMBLE_DURATION_MS;
             d_TOW_at_Preamble_ms = d_TOW_at_current_symbol_ms;
             flag_TOW_set = true;
             d_flag_new_tow_available = false;

src/algorithms/telemetry_decoder/gnuradio_blocks/gps_l1_ca_telemetry_decoder_cc.h

@@ -72,7 +72,9 @@ private:
 
     bool gps_word_parityCheck(unsigned int gpsword);
 
-    // class private vars
+    bool decode_subframe();
+    bool new_decoder();
+    int d_crc_error_synchronization_counter;
 
     int *d_preambles_symbols;
     unsigned int d_stat;
@@ -80,26 +82,22 @@ private:
 
     // symbols
     boost::circular_buffer<Gnss_Synchro> d_symbol_history;
-
+    float d_subframe_symbols[GPS_SUBFRAME_MS];  //symbols per subframe
-    double d_symbol_accumulator;
+    int d_current_subframe_symbol;
-    short int d_symbol_accumulator_counter;
+    //double d_symbol_accumulator;
-
+    //short int d_symbol_accumulator_counter;
-    // symbol counting
-    bool d_make_correlation;
-    unsigned int d_symbol_counter_corr;
 
     //bits and frame
-    unsigned short int d_frame_bit_index;
+    //unsigned short int d_frame_bit_index;
-    unsigned int d_GPS_frame_4bytes;
+    //unsigned int d_GPS_frame_4bytes;
     unsigned int d_prev_GPS_frame_4bytes;
-    bool d_flag_parity;
+    //bool d_flag_parity;
     bool d_flag_preamble;
     bool d_flag_new_tow_available;
-    int d_word_number;
+    //int d_word_number;
 
     // navigation message vars
     Gps_Navigation_Message d_nav;
-    GpsL1CaSubframeFsm d_GPS_FSM;
 
     bool d_dump;
     Gnss_Satellite d_satellite;