ADS-B TX works well enough for dump1090 and gr-air-modes

Hooked ADS-B RX to baseband instead of debug IQ file, not tested
2025-08-01 11:06:30 -04:00 · 2017-07-23 12:20:32 +01:00 · 2017-07-23 12:20:32 +01:00 · 5a67a7080a
commit 5a67a7080a
parent b57b41753f
23 changed files with 949 additions and 437 deletions
--- a/firmware/baseband/proc_adsbrx.cpp
+++ b/firmware/baseband/proc_adsbrx.cpp
@ -0,0 +1,158 @@
+/*
+ * Copyright (C) 2014 Jared Boone, ShareBrained Technology, Inc.
+ * Copyright (C) 2017 Furrtek
+ *
+ * This file is part of PortaPack.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#include "proc_adsbrx.hpp"
+#include "portapack_shared_memory.hpp"
+#include "sine_table_int8.hpp"
+#include "event_m4.hpp"
+
+#include <cstdint>
+#include <cstddef>
+
+using namespace adsb;
+	
+void ADSBRXProcessor::execute(const buffer_c8_t& buffer) {
+	int8_t re, im;
+	float mag;
+	uint32_t c;
+	uint8_t level, bit, byte;
+	bool confidence, first_in_window, last_in_window;
+	
+	// This is called at 2M/2048 = 977Hz
+	// One pulse = 500ns = 2 samples
+	// One bit = 2 pulses = 1us = 4 samples
+	
+	if (!configured) return;
+	
+	for (size_t i = 0; i < buffer.count; i++) {
+		
+		// Compute sample's magnitude
+		re = buffer.p[i].real();
+		im = buffer.p[i].imag();
+		mag = __builtin_sqrtf((re * re) + (im * im)) * k;
+		
+		// Only used for preamble detection and visualisation
+		level = (mag < 0.3) ? 0 :		// Blank weak signals
+					(mag > prev_mag) ? 1 : 0;
+		
+		if (decoding) {
+			// Decode
+			
+			// 1 bit lasts 2 samples
+			if (sample_count & 1) {
+				if ((prev_mag < threshold_low) && (mag < threshold_low)) {
+					// Both under window, silence.
+					if (null_count > 3) {
+						//text_debug_b.set("Bits:" + bits.substr(0, 25));
+						//text_debug_c.set("Hex:" + hex_str.substr(0, 26));
+						//text_debug_d.set("DF=" + to_string_dec_uint(frame.get_DF()) + " ICAO=" + to_string_hex(frame.get_ICAO_address(), 6));
+						
+						const ADSBFrameMessage message(frame);
+						shared_memory.application_queue.push(message);
+							
+						decoding = false;
+					} else
+						null_count++;
+						
+					confidence = false;
+					if (prev_mag > mag)
+						bit = 1;
+					else
+						bit = 0;
+					
+				} else {
+					
+					null_count = 0;
+				
+					first_in_window = ((prev_mag >= threshold_low) && (prev_mag <= threshold_high));
+					last_in_window = ((mag >= threshold_low) && (mag <= threshold_high));
+					
+					if ((first_in_window && !last_in_window) || (!first_in_window && last_in_window)) {
+						confidence = true;
+						if (prev_mag > mag)
+							bit = 1;
+						else
+							bit = 0;
+					} else {
+						confidence = false;
+						if (prev_mag > mag)
+							bit = 1;
+						else
+							bit = 0;
+					}
+				}
+				
+				byte = bit | (byte << 1);
+				bit_count++;
+				if (!(bit_count & 7)) {
+					// Got one byte
+					frame.push_byte(byte);
+				}
+			}
+			sample_count++;
+		} else {
+			// Look for preamble
+			
+			// Shift
+			for (c = 0; c < (ADSB_PREAMBLE_LENGTH - 1); c++)
+				shifter[c] = shifter[c + 1];
+			shifter[15] = std::make_pair(mag, level);
+			
+			// Compare
+			for (c = 0; c < ADSB_PREAMBLE_LENGTH; c++) {
+				if (shifter[c].second != adsb_preamble[c])
+					break;
+			}
+				
+			if (c == ADSB_PREAMBLE_LENGTH) {
+				decoding = true;
+				sample_count = 0;
+				null_count = 0;
+				bit_count = 0;
+				frame.clear();
+				
+				// Compute preamble pulses power to set thresholds
+				threshold = (shifter[0].first + shifter[2].first + shifter[7].first + shifter[9].first) / 4;
+				threshold_high = threshold * 1.414;		// +3dB
+				threshold_low = threshold * 0.707;		// -3dB
+			}
+		}
+		
+		prev_mag = mag;
+	}
+}
+
+void ADSBRXProcessor::on_message(const Message* const message) {
+	if (message->id == Message::ID::ADSBConfigure) {
+		null_count = 0;
+		bit_count = 0;
+		sample_count = 0;
+		decoding = false;
+		configured = true;
+	}
+}
+
+int main() {
+	EventDispatcher event_dispatcher { std::make_unique<ADSBRXProcessor>() };
+	event_dispatcher.run();
+	return 0;
+}