Parallel-frequency partially-coherent reception of pulse-position modulated ads-b messages

A method of receiving a pulse-position modulated signal with unknown center frequency offset and preceded by a preamble consisting of at least 4 pulses with known pulse spacing, the method comprising:

• generating a digital sample stream from a signal sensed at an antenna, wherein the digital sample stream has a sample rate greater than or equal to twice a bandwidth of an expected pulse plus maximum center frequency uncertainty;
• performing multiple preamble sequence correlations in parallel, wherein each of the multiple preamble sequence correlations operates on a version of the digital sample stream having a different frequency offset, wherein spacing between frequency offsets is a fraction of the bandwidth of an expected pulse, wherein a total of all the frequency offsets span a range for the center frequency uncertainty, wherein each preamble sequence correlation includes:
• filtering the digital sample stream with a filter having an impulse response matched to a duration and sequence of pulses in an earlier half of an expected preamble sequence to produce a first matched filter output sample stream;
• filter the digital sample stream with a filter having an impulse response matched to a duration and sequence of pulses in a latter half of the expected preamble sequence to produce a second matched filter output sample stream;
• and
• determine that a sequence of pulses in the received signal match the expected preamble sequence based on when the first matched filter output sample stream and the second matched filter output sample stream are above a minimum trigger level at the same time.

An ADS-B radio frequency (RF) receiver for a pulse-position modulated signal transmitted with a 1090 MHz ADS-B transmitter, the pulse-position modulated signal preceded by a preamble consisting of 4 pulses having spacing that conforms to an ADS-B protocol, the receiver comprising:

• a linear RF front end configured to sense a pulse-position modulated signal and output a digital sample stream based thereon;
• one or more processing devices;
• and
• one or more storage devices coupled to the one or more processing devices and including instructions which, when executed by the one or more processing devices, cause the one or more processing devices to detect the preamble by performing multiple preamble sequence correlations in parallel, wherein each of the multiple preamble sequence correlations operates on a version of the digital sample stream having a different frequency offset, wherein to perform each preamble sequence correlation the one or more processing devices are configured to:
• filter the digital sample stream with a filter having an impulse response matched to a duration and sequence of pulses in an earlier half of an expected preamble sequence to produce a first matched filter output sample stream;
• filter the digital sample stream with a filter having an impulse response matched to a duration and sequence of pulses in a latter half of the expected preamble sequence to produce a second matched filter output sample stream;
• and
• determine that a sequence of pulses in the signal match the expected preamble when, at the same time, the first matched filter output sample stream and the second matched filter output stream are above a minimum trigger level and the first matched filter output sample stream and the second matched filter output sample stream have near equal energy levels.