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Airplane Tracking Using ADS-B Signals

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This example shows how to track planes by processing automatic dependent surveillance-broadcast (ADS-B) signals. You can use previously captured signals or receive signals in real time using an RTL-SDR radio, ADALM-PLUTO radio or USRP™ radio. You can also visualize the tracked planes on a map using Mapping Toolbox™.

Required Hardware and Software

By default, this example runs using previously captured data. Optionally, you can receive signals over-the-air. For this, you also need one of the following:

Introduction

ADS-B is a cooperative surveillance technology for tracking aircraft. This technology enables an aircraft to periodically broadcast its position information such as altitude, GPS coordinates, and heading, using the Mode-S signaling scheme.

Mode-S is a type of aviation transponder interrogation mode. When an aircraft receives an interrogation request, it sends back the squawk code of the transponder. This is referred to as Mode 3A. Mode-S (Select) is another type of interrogation mode that is designed to help avoid interrogating the transponder too often. More details about Mode-S can be found in [ 1 ]. This mode is widely adopted in Europe and is being phased in for North America.

Mode-S signaling scheme uses squitter messages, which are defined as a non-solicited messages used in aviation radio systems. Mode-S has these attributes:

  • A transmit frequency of 1090 MHz

  • Pulse Position Modulation (PPM)

  • A data rate of 1 Mbit/s

  • A short squitter length of 56 microseconds

  • An extended squitter length of 112 microseconds

Short squitter messages contain these fields:

  • Downlink Format (DF)

  • Capability (CA)

  • Aircraft ID, which comprises of a unique 24-bit sequence

  • CRC Checksum

Extended squitter (ADS-B) messages contain all the information in a short squitter and one of these values:

  • Altitude

  • Position

  • Heading

  • Horizontal and Vertical Velocity

The signal format of Mode-S has a sync pulse that is 8 microseconds long followed by 56 or 112 microseconds of data, as this figure shows.

Receiver Structure

This block diagram summarizes the receiver code structure. The processing has four main parts: signal source, physical layer, message parser, and data viewer.

Signal Source

You can specify one of these signal sources:

  • ''Captured Signal'' - Over-the-air signals written to a file and sourced from a Baseband File Reader object at 2.4 Msps

  • ''RTL-SDR Radio'' - RTL-SDR radio at 2.4 Msps

  • ''ADALM-PLUTO Radio'' - ADALM-PLUTO radio at 12 Msps

  • ''USRP Radio'' - USRP radio at 20 Msps for all radios, except N310/N300 series that uses 2.4 Msps

If you set ''RTL-SDR'' or ''ADALM-PLUTO'' or ''USRP Radio'' as the signal source, the example searches your computer for the radio an RTL-SDR radio at radio address '0' or an ADALM-PLUTO radio at radio address 'usb:0' and uses it as the signal source.

The extended squitter message is 120 micro seconds long, so the signal source is configured to process enough samples to contain 180 extended squitter messages simultaneously, and set SamplesPerFrame of the signal property accordingly. The rest of the algorithm searches for Mode-S packets in this frame of data and returns all correctly identified packets. This type of processing is reffered to as batch processing. An alternative approach is to process one extended squitter message at a time. This single packet processing approach incurs 180 times more overhead than the batch processing, while it has 180 times less delay. Since the ADS-B receiver is delay tolerant, you use batch processing in this example.

Physical Layer

The physical layer (PHY) processes the baseband samples from the signal source to produce packets that contain the PHY layer header information and raw message bits. This diagram shows the physical layer structure.

The RTL-SDR radio can use a sampling rate in the range [200e3, 2.8e6] Hz. When the source is an RTL-SDR radio, the example uses a sampling rate of 2.4 MHz and interpolates by a factor of 5 to a practical sampling rate of 12 MHz.

The ADALM-PLUTO radio can use a sampling rate in the range [520e3, 61.44e6] Hz. When the source is an ADALM-PLUTO, the example samples the input directly at 12 MHz.

The USRP radios are capable of using different sampling rates. When the USRP radio is the source, the example samples the input directly at 20 MHz sample rate for most of the radios. For the N310/N300 radio the data is received at 2.4 MHz sample rate and interpolates by a factor of 5 to a practical sampling rate of 12e6.

For example, if the data rate is 1 Mbit/s and the effective sampling rate is 12 MHz, the signal contains 12 samples per symbol. The receive processing chain uses the magnitude of the complex symbols.

The packet synchronizer works on subframes of data equivalent to two extended squitter packets, that is, 1440 samples at 12 MHz or 120 micro seconds. This subframe length ensures that the subframe contains whole extended squitter. The packet synchronizer first correlates the received signal with the 8 microsecond preamble and finds the peak value. The synchronizer then validates the synchronization point by checking if it matches the preamble sequence, [1 0 0 0 0 0 1 0 1 0 0 0 0 0 0], where a value of 1 represents a high value and a value of 0 represents a low value.

The Mode-S PPM scheme defines two symbols. Each symbol has two chips, where one has a high value and the other has a low value. If the first chip is high and the subsequent chip is low, the symbol is 1. Alternatively, if the first chip is low and the subsequent chip is high chip, then the symbol is 0. The bit parser demodulates the received chips and creates a binary message. A CRC checker then validates the binary message. The output of the bit parser is a vector of Mode-S physical layer header packets that contain these fields:

  • RawBits - Raw message bits

  • CRCError - FALSE if CRC passes, TRUE if CRC fails

  • Time - Time of reception in seconds, from the start of reception

  • DF - Downlink format (packet type)

  • CA - Capability

Message Parser

The message parser extracts data from the raw bits based on the packet type described in [ 2 ]. This example can parse short squitter packets and extended squitter packets that contain airborne velocity, identification, and airborne position data.

Data Viewer

The data viewer shows the received messages on a graphical user interface (GUI). For each packet type, the data viewer shows the number of detected packets, the number of correctly decoded packets, and the packet error rate (PER). As the radio captures data, the application lists information decoded from these messages in a table.

Track Airplanes Using ADS-B Signals

The receiver prompts you for user input and initializes variables. After you set the input values call the signal source, physical layer, message parser, and data viewer in a loop. The loop keeps track of the radio time using the frame duration.

% The default configuration runs using captured data. You can set
% |cmdlineInput| to |1|, then run the example to optionally change these
% configuration settings:
% # Reception duration in seconds,
% # Signal source (captured data or RTL-SDR radio or ADALM-PLUTO radio or USRP radio),
% # Optional output methods (map and/or text file).

% For the option to change default settings, set |cmdlineInput| to 1.
cmdlineInput = 0;
if cmdlineInput
    % Request user input from the command-line for application parameters
    userInput = helperAdsbUserInput;
else
    load('defaultinputsADSB.mat');
end

% Calculate ADS-B system parameters based on the user input
[adsbParam,sigSrc] = helperAdsbConfig(userInput);

% Create the data viewer object and configure based on user input
viewer = helperAdsbViewer('LogFileName',userInput.LogFilename, ...
    'SignalSourceType',userInput.SignalSourceType);
if userInput.LogData
    startDataLog(viewer);
end
if userInput.LaunchMap
    startMapUpdate(viewer);
end

% Create message parser object
msgParser = helperAdsbRxMsgParser(adsbParam);

% Start the viewer and initialize radio time
start(viewer)
radioTime = 0;

% Main loop
while radioTime < userInput.Duration


    if adsbParam.isSourceRadio
        if adsbParam.isSourcePlutoSDR
            [rcv,~,lostFlag] = sigSrc();
        else
            [rcv,~,lost] = sigSrc();
            lostFlag = logical(lost);
        end
    else
        rcv = sigSrc();
        lostFlag = false;
    end

    % Process physical layer information (Physical Layer)
    [pkt,pktCnt] = helperAdsbRxPhy(rcv,radioTime,adsbParam);

    % Parse message bits (Message Parser)
    [msg,msgCnt] = msgParser(pkt,pktCnt);

    % View results packet contents (Data Viewer)
    update(viewer,msg,msgCnt,lostFlag);

    % Update radio time
    radioTime = radioTime + adsbParam.FrameDuration;
end

% Stop the viewer and release the signal source
stop(viewer)
release(sigSrc)

Downloading captured ADSB data set.
Done.

This figure shows the information about the detected airplanes.

You can also observe the airplanes on a map if you have a Mapping Toolbox license.

Further Exploration

You can investigate ADS-B signals using the ADSBExampleApp app. Use this app to select the signal source and change the duration. To launch the app, enter ADSBExampleApp in the MATLAB Command Window.

References

  1. International Civil Aviation Organization, Annex 10, Volume 4. Surveillance and Collision Avoidance Systems.

  2. Technical Provisions For Mode S Services and Extended Squitter (Doc 9871)