The ADS-B Framer (adsb_framer) is the first block in the gr-adsb pipeline. It consumes a magnitude-squared power signal, searches for the 8-symbol ADS-B preamble pattern, and annotates the stream with "burst" stream tags at each detected preamble position. Downstream blocks use these tags to locate and demodulate individual ADS-B frames.
Block info
| Property | Value |
|---|
| GRC block ID | adsb_framer |
| GRC label | ADS-B Framer |
| GRC category | [ADS-B] |
| Python class | adsb.framer |
| Constructor | adsb.framer(fs, threshold) |
| Block type | sync block |
Parameters
Sample rate in samples per second. The block computes sps = int(fs // 1e6) to determine the number of samples per symbol. The sample rate must be an integer multiple of 1 MHz (fs % 1e6 == 0); the block raises an AssertionError otherwise. The minimum usable sample rate is 2 Msps (2 samples per symbol at the 1 Msym/s ADS-B symbol rate).
Detection threshold applied to the magnitude-squared input signal. Any sample at or above this value is treated as a high pulse; samples below it are treated as low. Lower values increase sensitivity but also increase false-positive preamble detections. This parameter can be updated at runtime by calling set_threshold(threshold).
Ports
| Label | Domain | Data type | Vector length |
|---|
in | stream | float32 | 1 |
|I|² + |Q|². Use a blocks.complex_to_mag_squared block upstream to compute this from a complex IQ source.
Outputs
| Label | Domain | Data type | Vector length |
|---|
out | stream | float32 | 1 |
| Field | Value |
|---|
| Key | "burst" |
| Value | ("SOB", snr) — a tuple of the string "SOB" (Start of Burst) and the SNR in dB |
| Source | "framer" |
SNR (dB) = 10 * log10(pulse_amplitude / median_noise) + 1.6
Preamble detection
ADS-B uses Pulse Position Modulation (PPM) at 1 Msym/s. The 8-symbol preamble produces the following half-symbol pulse pattern (sampled at 2× the symbol rate):
[1, 0, 1, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0]
History buffer
The Framer sets a history buffer of N_hist = NUM_PREAMBLE_BITS * sps samples (8 × sps). This history allows the work() function to detect preambles whose first pulse falls at the very end of the previous work() call’s input buffer. At 2 Msps, N_hist is 8 samples; at higher sample rates it scales proportionally.
Runtime callback
The detection threshold can be changed while the flowgraph is running:
framer.set_threshold(0.005)
self.threshold value. The next work() call will use the new threshold.
End-of-burst guard
After confirming a preamble, the Framer records the estimated end-of-burst index (prev_eob_idx) based on the shorter 56-bit (Short Squitter) packet length. Subsequent rising edges that fall before this index are skipped, avoiding spurious preamble detections on data pulses within the same packet.
Instantiation example
import gnuradio.adsb as adsb
framer = adsb.framer(fs=2e6, threshold=0.01)
adsb_framer.