ADS-B Cape

ADS-B Cape Assembled

Introduction

The ADS-B Cape is a receiver which captures ADS-B messages broadcast by many commercial and general aviation aircraft for air traffic control purposes. It consists of a tuned RF (TRF) front-end with narrowband filtering at 1090MHz, a logarithmic response detector/amplifier (logamp) and a 20MSPS 10-bit ADC coupled to an FPGA which demodulates the log-video data into standard-format message frames compatible with many software applications. Because it's a fully-compatible Beaglebone Cape it can be used with any type of Beaglebone embedded computer - White, Black, Bluesteel, etc.

Features:

Architecture

The ADS-B Cape follows a common architecture which uses no heterodyne processing. Instead, the 1090MHz signals are amplified, filtered and detected at the carrier frequency. The detected signal with a slope of 20mV/dB is DC-coupled into a 10-bit ADC with a 2V reference, yeilding approximately 10 lsb counts/dB of signal power with a usable power range of about -100dBm to -20dBm.

The ADC runs at 20MSPS which provides 20 samples per data bit in the ADS-B frame, sufficient to provide accurate sampling phase within the bit window which improves overall sensitivity beyond that possible with systems that sample at only 2x the bit rate. The 20MSPS 10-bit data is processed within the FPGA to trigger when the detected signal crosses an adaptive minimum trigger level (MTL), at which point a correlator searches for the Mode S reply preamble and then a PPM demod recovers the data. Retriggering is supported, which provides usable data when a stronger signal collides in time with a weaker signal.

Recovered data and confidence bits are queued in a FIFO within the FPGA which can hold up to 32 full frames of data until the host processor fetches them via the SPI bus. Host-side software is responsible for error detection and correction (EDAC) via the 24-bit CRC that all ADS-B messages carry. Frames which do not meet the 56 or 112-bit length requirement are also dropped by the host before processing. Currently the host software sends the raw data packets over the Ethernet connection using a TCP socket and the standard ASCII format used by a wide range of ADS-B tracking applications.

Prototype Testing

The version 0.1 prototype PCBs have been received and one has been assembled. A few schematic and layout issues were found but none that prevent basic operation, although performance isn't quite as good as was seen with the proof-of-concept lashup that was tested prior to the board design. A detailed report of findings is available in the documents section of the github repository.

Design Resources

Status

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Last Updated
:2014-10-21
Comments to:
Eric Brombaugh

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