Required Hardware and Software

To run this example using captured signals, you need the following software:

To receive signals in real time, you also need one of the following SDR devices and the corresponding support package Add-On:

For a full list of Communications Toolbox supported SDR platforms, refer to Supported Hardware section ofSoftware Defined Radio (SDR) discovery page.

Background

ADS-B is a cooperative surveillance technology for tracking aircraft. This technology enables an aircraft to periodically broadcast its position information (altitude, GPS coordinates, heading, etc.) 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 thesquawk codeof 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 the following properties:

Short squitter messages contain the following information:

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

The signal format of Mode-S has a sync pulse that is 8 microseconds long followed by either 56 or 112 microseconds of data as illustrated in the following figure.

Run the Example

The default configuration runs using captured data. Later in the example, you can setcmdlineInputto1, then run the example to optionally change these configuration settings:

The example shows the information on the detected airplanes in a tabular form as shown in the following figure.

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

Receiver Structure

The following block diagram summarizes the receiver code structure. The processing has four main parts: Signal Source, Physical Layer, Message Parser, and Data Viewer.

Signal Source

This example can use three signal sources:

If you assign ''RTL-SDR'' or ''ADALM-PLUTO'' as the signal source, the example searches your computer for the radio you specified, either 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.

在这里,120微se扩展唠叨消息conds long, so the signal source is configured to process enough samples to contain 180 extended squitter messages at once, and setSamplesPerFrameof the signal property accordingly. The rest of the algorithm searches for Mode-S packets in this frame of data and outputs all correctly identified packets. This type of processing is defined 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, batch processing was used.

Physical Layer

The baseband samples received from the signal source are processed by the physical (PHY) layer to produce packets that contain the PHY layer header information and raw message bits. The following diagram shows the physical layer structure.

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

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

With the data rate of 1 Mbit/s and a practical sampling rate of 12 MHz, there are 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 a whole extended squitter packet is contained in the subframe. The Packet synchronizer first correlates the received signal with the 8 microsecond preamble and finds the peak value. Then, it 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 '1' represents a high value and a '0' represents a low value.

The Mode-S PPM modulation 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 followed by low chip, this corresponds to the symbol being a 1. Alternatively, if the first chip is low followed by high chip, then the symbol is 0. The bit parser demodulates the received chips and creates a binary message. The binary message is validated using a CRC checker. The output of bit parser is a vector of Mode-S physical layer header packets that contains the following fields:

Message Parser

The message parser extracts data from the raw bits based on the packet type as 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 number of detected packets, the number of correctly decoded packets, and the packet error rate (PER) is shown. As data is captured, the application lists information decoded from these messages in a tabular form.

Example Code

The receiver asks for user input and initializes variables. Then it calls 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.

%For the option to change default settings, set |cmdlineInput| to 1.cmdlineInput = 0;ifcmdlineInput% Request user input from the command-line for application parametersuserInput = helperAdsbUserInput;elseload('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 inputviewer = helperAdsbViewer('LogFileName',userInput.LogFilename,...'SignalSourceType',userInput.SignalSourceType);ifuserInput.LogData startDataLog(viewer);endifuserInput.LaunchMap startMapUpdate(viewer);end% Create message parser objectmsgParser = helperAdsbRxMsgParser(adsbParam);% Start the viewer and initialize radio timestart(viewer) radioTime = 0;% Main loopwhileradioTime < userInput.DurationifadsbParam.isSourceRadioifadsbParam.isSourcePlutoSDR [rcv,~,lostFlag] = sigSrc();else[rcv,~,lost] = sigSrc(); lostFlag = logical(lost);endelsercv = sigSrc(); lostFlag = false;end%的物理过程layer information (Physical Layer)[pkt,pktCnt] = helperAdsbRxPhy(rcv,radioTime,adsbParam);% Parse message bits (Message Parser)[msg,msgCnt] = msgParser(pkt,pktCnt);%查看结果包内容(数据查看器)update(viewer,msg,msgCnt,lostFlag);% Update radio timeradioTime = radioTime + adsbParam.FrameDuration;end% Stop the viewer and release the signal sourcestop(viewer) release(sigSrc)

Further Exploration

You can further explore ADS-B signals using the ADSBExampleApp app. This app allows you to select the signal source and change the duration. To launch the app, typeDSBExampleApp在e MATLAB command window or click the link.

You can explore following helper functions for details of the physical layer implementation:

*helperAdsbRxPhy.m*helperAdsbRxPhySync.m*helperAdsbRxPhyBitParser.m*helperAdsbRxMsgParser.m

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