The 1090 Megahertz Riddle

Front matter

The 1090 Megahertz Riddle A Guide to Decoding Mode S and ADS-B Signals Junzi SUN Faculty of Aerospace Engineering Delft University of Technology

Title: The 1090 Megahertz Riddle Subtitle: A Guide to Decoding Mode S and ADS-B Signals Author: Junzi Sun Edition: 2 Keywords: Aircraft surveillance, ADS-B, Decoding, Mode S, Open data, pyModeS. Publisher: TU Delft OPEN Publishing Cover design: Junzi Sun ISBN: 978-94-6366-402-8 Copyright © 2021 by Junzi Sun Published under CC BY-NC-SA 4.0 license. https://creativecommons.org/licenses/by-nc-sa/4.0/ Electronic versions of this book are available at: https://doi.org/10.34641/mg.11 https://mode-s.org

This book is dedicated to my sons: William and Vincent

Part I Getting Started

1 | Introduction 1.1 Background: The “death ray” that saves lives Like many modern technologies such as computers, the internet, and GPS whose origins can be traced back to the military , aviation radar is no exception. The fundamental theory of radar started in late 19th century . Since the 1860s, when the electromagnetic theory was discovered by James Clerk Maxwell, the foundation for many science and technology fields was laid out. In the late 19th century , Heinrich Hertz, who proved the existence of electromagnetic waves, also confirmed that metals could reflect radio waves. In the first decades of the 20th century , several systems for using radio waves to provide short-range directional information of objects were developed. German inventor Christian Hülsmeyer is often considered as the first person to use radio waves to detect metal objects in 1904. However, not until the Second World War, was the concept of RAdio Detection And Ranging (RADAR) developed. The technology was simultaneously researched by both major Allies and Axis countries. However, the United Kingdom led the race in developing a functional radar system.

Originally , British Air Ministry officials, who were concerned about falling behind the technology race with the Germans, advised physicist Robert Watson Watt to propose a set of abstract technical challenges to his colleague Skip Wilkins: "Suppose, just suppose, that you had eight pints [17kg] of water, 3,000ft [1km] above the ground. And suppose that water was at 98 ◦F [37◦C], and you wanted to heat it to 105◦F [41◦C]. How much radio frequency power would you require, from a distance of 5km?"

Both scientists understood very well that the average adult human has 17 kg blood and a critical body temperature of 41◦C. This represented an idea of the death ray that could be used to disable enemy aircraft pilots. Without even developing such a system, they figured out that the power requirement was far beyond practice at that time. However, they realized an opportunity for funding a different project. Given the amount of power that can be transmitted, it was possible to detect the reflection of aircraft, and, thus calculate the position of aircraft. This proposal was approved by the air ministry . Later on, this technology was developed and shared with Americans during the war. After that, the widespread usage of RADAR (which later became radar) had begun. After the war ended, radar became more and more important for civil aviation. With the rapid growth of commercial flights, it developed into a prominent technology for aircraft surveillance in the aviation industry .

14 Chapter 1. Introduction 1.2 The primary radar Aircraft surveillance in the early days relied only on, what is known today as, the primary surveillance radar (PSR). The concept of PSR is fairly simple. It is a rotating radio transponder with an omnidirectional antenna. Commonly , the radar transmits a one-microsecond pulse for every one millisecond and listens to the reflections from the airplanes. The position of the aircraft is measured by the distance and angle to the radar. The distance is known as the slant distance, which is the line-ofsight distance between an aircraft and the radar. It can be calculated by measuring the time difference between the original signal and the reflection received, since the speed of the radio wave (speed of light) is known. The azimuth angle 1 of the aircraft is determined by the rotation angle of the radar. By processing radar signals, technologies like phase filter and Doppler filter can be used to filter out moving targets like aircraft and remove static objects, such as mountains, buildings, and other obstacles.

The slant distance of an aircraft does not always correspond to the horizontal distance to the radar. This is because all points having the same slant distance are located on a sphere that has the radius equal to the distance. Since the civil radar usually does not provide elevation information on the target, it is not possible to accurately convert the slant distance to the horizontal distance. Historically , it is sufficient to use primary radar for separating airplanes without considering these altitude differences. However, other systems have to be in place to provide air traffic controllers more accurate positions of the aircraft. 1.3 The secondary radar The secondary surveillance radar (SSR), also known as the air traffic control radar beacon system (ATCRBS), was designed to provide air traffic controllers more information than what is provided by the primary radar. The secondary radar can be installed separately or installed on top of the primary radar. It uses a different radio frequency to actively interrogate the aircraft and receive information transmitted by the aircraft.

The SSR transmits interrogations using the 1030 MHz radio frequency and the aircraft transponder transmits replies using the 1090 MHz radio frequency . In the early design of SSR, two civilian communication protocols (Mode A and Mode C) were introduced. Mode A and Mode C respectively allow the SSR to continuously interrogate the identity (squawk code) and the altitude of an aircraft. The squawk code in Mode A is a unique 4-octal digit code given by air traffic controllers to aircraft in their flight information region for identification. The altitude in Mode C refers to the barometric altitude obtained from the aircraft’s air data system. Although Mode A and Mode C provide additional information to air traffic con- 1 Azimuth angle is the horizontal angle measured clockwise to the north of the observer.

1.3. The secondary radar 15 trollers, they present several design challenges [12]. The secondary surveillance radar initiates Mode A and C interrogations with two different pulse patterns, which are shown in Figure 1.1.

P1 P3 P2 Mode A P1 P3 P2 Mode C 0.8 Figure 1.1: Mode A/C interrogation pulses The pulses are about 0.8 µs wide. P1 and P3 are the two main pulses sent by the directional antenna. They are separated by 8 µs and 21 µs, respectively for Mode A and C. P2 is a pulse emitted through the omnidirectional antenna right after P1. Pulse P2 is introduced for sidelobe suppression [20]. When the power of P2 is higher than P1, the interrogation is likely from the side lobes of the directional antenna and should be ignored by the aircraft. This is can happen when the aircraft is close to the radar.

Figure 1.2 shows an example of a Mode A/C reply . Each reply consists of two persistent pulses, F1 and F2, separated by 20.3 µs. Within this period, either the identity code or the altitude code is encoded using 13 pulses of 0.45 µs. The pulses are separated by gaps of 1 µs. The pulse at the center serves as a verification pulse and is always absent. The presence or absence of any of the other 12 pulses represents a 1 or 0 bit. When required by air traffic controllers for identification purposes, a special purpose identification (SPI) may follow F2 after two absent pulses. F1 F2 SPI X X X 0.45 1 20.75 4.35 Figure 1.2: Mode A/C reply pulses The response type (A or C) cannot be identified based on the reply signal itself. The SSR determines the content based on the synchronization with interrogations. This

16 Chapter 1. Introduction design of ATCRBS works sufficiently well with low-density air traffic, but it cannot efficiently cope with higher flight densities since all aircraft replies are transmitted on the same frequency . When several aircraft are in the same direction of the radar beam, reply signals can overlap and introduce errors for decoding. This is known as synchronous garbling.

When there are multiple secondary radars in the vicinity , replies originated by other radars may be considered as valid responses of one radar, which in turn, causes errors and confusion. This syndrome is called FRUIT (False Replies Unsynchronized In Time).

Though some of the garbling and the FRUIT problems can be mitigated with the reduction of interrogation frequency and improvements in signal processing ability , the information transmitted in Mode A and Mode C is still very limited. The number of identity codes available in Mode A communication is limited to a maximum of 4096 (84 given the 4 octal digits) unique codes, which poses another clear limitation. Hence, more advanced communication protocols needed to be developed to acquire more information from a large number of aircraft. 1.4 Mode S Mode S (Mode Select Beacon System) was designed by Lincoln Laboratory at Massachusetts Institute of Technology in the 1970s. Based on different iterations of hardware and software design in the 1980s, the implementation of Mode S in air traffic control began in the 1990s. Since then, Mode S has become one of the main sources for aircraft surveillance.

The main characteristic of Mode S is its selective interrogation, which allows the SSR to interrogate different information from different aircraft separately . By using selective interrogation, it largely mitigated the problem of garbling in Mode A/C and thus greatly improved the capacity of the communication channel. Unlike the limited number (4096) of unique identification codes in Mode A communication, the Mode S transponder is identified by a 24-bit transponder code, which can support up to 16,777,216 (224) unique addresses. In addition to these two major advantages, the Mode S protocol introduced many different types of information that could be interrogated and downlinked. 1.4.1 Mode S interrogations The Mode S uplink signal contains parameters that indicate which information is desired by the air traffic controller. There are two types of Mode S interrogations, which are shown in Figure 1.3. The short interrogation has 56 bits of information contained in the data block, while the long interrogation contains 112 bits of information. The P2 pulse acts as the sidelobe suppression for Mode A/C transponders so that they will ignore the rest of the interrogation pulses. Information in the Mode S

1.4. Mode S 17 interrogation data block uses the Differential Phase-Shift Keying (DPSK) modulation [17].

P1 P2 Data block (P5) 16.25 P1 P2 Data block (P5) 30.25 ShortLong Figure 1.3: Mode S uplink pulses 1.4.2 Mode S replies There are two types of Mode S downlink signals, the short reply and the long reply , which correspond to the short and long interrogations from the SSR. For each microsecond, one bit is transmitted. All Mode S replies start with an 8 µs fixed preamble and continue with 56- or 112 µs data block. The structure of the downlink message is shown in Figure 1.4.

Preamble (8 ) Data (56 or 112 ) 10 01 10 10 01 01 10 01 1 0 1 1 0 0 1 0 10 10 00 01 01 00 00 00 Figure 1.4: An example of Mode S reply message The 16-bit fix preamble can be represented as 1010000101000000 in binary . The information contained in the data block is modulated using the Pulse Position Modulation (PPM), which is a type of amplitude modulation. In PPM, the 1 bit is represented by a 0.5 µs of pulse followed by a 0.5 µs flat signal. The 0 bit is reversed compared to the 1 bit, which is represented by a 0.5 µs flat signal and followed by a 0.5 µs pulse.

18 Chapter 1. Introduction Note Mode S uplink and downlink signals use different modulation methods. The uplink uses phase modulation, while the downlink uses amplitude modulation. Phase modulation requires slightly more complicated hardware implementation but has a better performance in terms of error tolerance. 1.4.3 Compatibility with Mode A/C Similar to Mode A/C, Mode S SSR can send All-Call interrogations to all aircraft in the vicinity . The All-Call interrogation is designed to be compatible with Mode A/C transponders. Figure 1.5 shows the pulses of an All-Call interrogation. There are 8 or 21 µs between P1 and P3, which is the same as Mode A/C. P4 is a wider pulse lasting for 1.6 µs.

P1 P3 All-Call 0.8 P4 1.6 8 or 21 Figure 1.5: Mode A/C/S All-Call interrogation When a Mode A/C transponder receives this interrogation, the last pulse P4 is ignored. Hence, a normal Mode A or Mode C reply is transmitted. A Mode S transponder would detect P4, and thus, produce a Mode S All-Call reply . The Mode A/C/S All-Call reply is designed for SSR to identify which aircraft are in the vicinity , including Mode A/C only aircraft.

This is one of the several different interrogation pulse patterns from a Mode A/C compatible Mode S radar. In addition, Mode S SSRs can perform both Mode A/C and Mode S interrogations.

Note The Mode A/C/S All-Call is being phased out, as new transponders stop replying to this type of interrogations. It is common for Mode S SSR makes use of Mode S only All-Call interrogation with the standard Mode S uplink (format 11) as shown in Figure 1.3. 1.4.4 Mode S format The Mode S communication protocol is designed to handle different types of uplink and downlink message formats. The first five bits of the message define the uplink format (UF) or downlink format (DF) number of the message. Based on the UF/DF

1.5. ADS-B 19 number, different structures of the data block are presented. Table 1.1 shows all available Mode S formats. Currently , 11 Mode S formats are being used. Numbers not in this table are reserved for future use.

Table 1.1: Mode S uplink and downlink formats UF/DF Bits Uplink type Downlink type 0 56 Short air-air surveillance (ACAS) Short air-air surveillance (ACAS) 4 56 Surveillance, altitude request Surveillance, altitude reply 5 56 Surveillance, identity request Surveillance, identity reply 11 56 Mode S All-Call All-Call reply 16 112 Long air-air surveillance (ACAS) Long air-air surveillance (ACAS) 17 112 - Extended squitter 18 112 - Extended squitter/non transponder 19 112 - Military extended squitter 20 112 Comm-A, altitude request Comm-B, altitude reply 21 112 Comm-A, identity request Comm-B, identity reply 24 112 Comm-C (ELM) Comm-D (ELM) Note Format number 24 is an exception. It is identified using only the first two bits, which must be 11 in binary . All following bits are used for encoding other information. We can see that the short 56-bit data block is used to encode messages with format numbers from 0 to 11. Messages with format numbers above 16 are encoded with the long 112-bit data block. Among all uplink formats, UF 17, 18, and 19 are not used. This is because the corresponding downlink messages (extended squitter messages) are designed to be broadcast automatically without the need for SSR interrogations.

One of the most common applications for the extended squitter is the Automatic Dependent Surveillance-Broadcast service, which is commonly known as ADS-B. 1.5 ADS-B Automatic Dependent Surveillance-Broadcast (ADS-B) is a surveillance technology designed to allow aircraft to broadcast their flight state periodically without the need for interrogation. The word automatic refers to the fact that no inputs from controllers or pilots are required. The word dependent indicates this technology depends on information from other onboard systems, such as air data systems and navigation systems. ADS-B only become possible because of Global Navigation Satellite Systems (GNSS), such as Global Positioning System (GPS), which is the first GNSS system made available for civil usage by the USA after the Korean Air Lines Flight 007 accident in 1983.

Common aircraft state parameters included in ADS-B are position, altitude, and

20 Chapter 1. Introduction speed. The position is determined by GNSS and air data systems. The velocity is derived from the GNSS position and the inertial measurement system. The altitude information includes both barometric altitude and GNSS altitude. The barometric altitude is provided by the air data system. In addition to these primary state parameters, ADS-B also allows other information to be broadcast, for example, aircraft call-sign, accuracy indicators, integrity indicators, and operational status. It is worth emphasizing that ADS-B is a broadcast-based surveillance system. Unlike other types of Mode S surveillance, no surveillance interrogation is required. The decoding of ADS-B messages is also easier than other Mode S messages because the message type and structure are clear for each individual message. Different types of ADS-B messages are identified by Type Codes. The structures of these messages are all defined in ICAO documents, such as in [13] and [21]. In Part I of this book, we will explain how these different types of ADS-B messages can be decoded.

Note In this book, ADS-B only refers to the ADS-B standard implemented through Mode S extended squitter. Originally , there were three candidates to build ADS-B: Mode S Extended Squitter, VHF Data Link - Mode 4 (VDL4), and Universal Access Transceiver (UAT). Different specifications are designed for these frequency channels [21, 22]. However, the Mode S extended squitter implementation of ADS-B is the most adopted one among all candidates. UAT is only partially implemented in some countries like the USA, while VDL4 did not go beyond the testing phase. 1.6 Other Mode S services The most common data format number used in Mode S for information downlink are 4, 5, 20, and 21. Format 4 and 5 are short messages designed to acquire aircraft altitude and identity (similar to Mode A/C messages). Downlink messages with format 20 or 21, as known as Comm-B messages, contain other information desired by air traffic controllers, in addition to altitude and identity . In theory , up to 255 different message types are supported by the Mode S Comm- B. A new parameter, Comm-B Data Selector (BDS), is designed to identify which additional information is included in Mode S messages. It functions similarly to the Type Code of ADS-B message. In practice, not all 255 BDS codes are defined or used. A subgroup of these codes is commonly interrogated by surveillance radars. By combining different BDS codes into groups, several Mode S services are defined. The two most used services are Mode S Elementary Surveillance (ELS) and Mode S Enhanced Surveillance (EHS) [8]. For example, in the European airspace, aircraft above a certain takeoff weight category are required to have these capabilities enabled. The Mode S Elementary Surveillance consists of four BDS codes (10, 17, 20, and

1.7. Summary 21 30). It provides basic information such as Mode S capabilities and identification (callsign), in addition to the altitude or identity (squawk code) information. The Mode S Enhanced Surveillance consists of three BDS codes (40, 50, and 60). It provides much more additional information on aircraft states, such as selected altitudes, true airspeed, indicated airspeed, Mach number, bank angle, and turn rate.

In addition to the common ELS and EHS, the meteorological routine air report (MRAR), and the meteorological hazard report (MHR) are also interrogated in some controlled airspaces. These reports provide weather-related information such as wind and temperature that are measured by the aircraft sensors. Another important service, Airborne Collision Avoidance System (ACAS), also uses the Mode S transponder for comminucation. All these services will be explained in details in this book. 1.7 Summary Figure 1.6 illustrates all aforementioned Mode S services and their relationships with different Mode S downlink formats. This book covers all Mode S messages except DF19 and DF24 from this figure.

ADS-B Mode-S Downlink DF 17: Extended squitter DF 0: Short air-air surveillance (ACAS) DF 4: Altitude reply DF 5: Identity reply DF 11: All-call reply DF 16: Long air-air surveillance (ACAS) DF 18: Extended squitter, non transponder DF 19: Military extended squitter DF 20: Comm-B, with altitude reply DF 21: Comm-B, with identity reply DF 24: Comm-D, extended length message BDS 05: Airborne position BDS 06: Surface position BDS 08: Identification and category BDS 09: Airborne velocity BDS 61: Aircraft status BDS 65: Operational status BDS 10: Data link capability BDS 17: Common usage GICB capability BDS 20: Identification BDS 30: ACAS resolution advisory BDS 40: Selected vertical intention BDS 50: Track and turn report BDS 60: Heading and speed report ELS EHS Other services Other BDS codes Comm-B * BDS: Comm-B Data Selector * DF: Downlink format * DF 0-11: Short messages, 56 bits * DF 16-24: Long messages, 112 bits * DF 1-3, 6-10, 11-15, 22-23 not used BDS 44: Meteorological routine air report BDS 45: Meteorological hazard report MRAR MHR Figure 1.6: Relationship between Mode S downlink formats and different services In the rest of this book, the details of these services are discussed in separate chap-

22 Chapter 1. Introduction ters. In Chapter 2 (the rest of part I), common hardware and software setups are shown. In Chapters 3 to 10 (part II), the focus is on ADS-B messages. Detailed guides on how to decode different types of messages are presented. In Chapters 11 to 19 (part III), the focus is on Mode S ELS, EHS, and meteorological services. Accordingly , the decoding and inference of the messages are provided.

2 | Quick Start: Hardware and Software to Receive Mode S Signals Chapter 1 introduced several fundamental concepts about Mode S communication. This chapter focuses on the practical aspects of acquiring Mode S data, specifically , how to receive and obtain messages that are transmitted in Mode S downlink communications. Mode S signals are transmitted using 1090 MHz radio waves. Hence, the receiver and antenna have to be designed to work with this frequency . Nowadays, all necessary components required to obtain the Mode S data can be easily made with low-cost off-the-shelf components. Several open-source software tools are available to support the extraction and decoding of data from Mode S signals. In this chapter, we explain how to step up a functional hardware and software system to receive Mode S data. 2.1 Range Since Mode S uses L band signals that follow the line-of-sight propagation, any obstructions between the transmitter and receiver can cause a significant amount of signals to be blocked. Assuming that no obstacle exists between the aircraft and the receiver, and the transmitter has a sufficient amount of power, the maximum range of the receiver is determined by the curvature of the earth. Figure 2.1 illustrates how the maximum range of the receiver can be obtained. Aircraft Receiver Figure 2.1: Maximum receiver range Knowing the altitude of the receiver antenna, it is possible to determine the maximum range (d) of a Mode S receiver as:

24 Chapter 2. Quick Start: Hardware and Software to Receive Mode S Signals d = (αr +αt)R (2.1) = ( arccos RR +hr + arccos RR +ht ) R (2.2) where R is the radius of the earth, while ht and hr are the height of the aircraft and receiver above the sea level. Using this equation, we can calculate the maximum receiving range for aircraft flying at different altitudes. Figure 2.2 shows the maximum range curves calculated using several receiver heights. 0 2 4 6 8 10 12 Aircraft altitude (km) Maximum range (km) Receiver height: 0 m Receiver height: 100 m Receiver height: 200 m Receiver height: 300 m Figure 2.2: Maximum receiver range curve It should be noted that this figure shows the maximum radio range due to the curvature of the Earth. However, in real-life applications, Mode S signal follows the Friis transmission model, which states that the maximum distance also depends on the power of the transmitter, as well as the directivities of the transmission and receiving antennas. When considering these factors, the actual radio range for the receiver is typically lower than the theoretical values shown in the previous figure. 2.2 Antenna In principle, any antenna designed for the radio frequency around 1 GHz can be used for receiving Mode S signals. A large variety of commercial off-the-shelf antennas can be found nowadays.

However, it is not difficult to design your own antenna. The carrier frequency of Mode S is 1090 MHz, which corresponds to the wavelength of 27.5 centimeters. In order to have an antenna that is tuned to this specific frequency , one can design the

2.2. Antenna 25 antenna simply using a piece of a conductor (metal wire) and a coaxial feeder cable. Figure 2.3 shows a few common antenna designs. 13.75 cm 6.875 cm Monopole Antenna Ground Plane Antenna 13.75 cm Dipole Antenna Coaxial cable Figure 2.3: Common antenna designs The monopole antenna and the dipole antenna both are half-wavelength (λ/2) antennas with a total conductor length of 13.75 cm. The ground plane antenna is a quarter-wavelength (λ/4) antenna, where the main pole is 6.875 cm. All these previous antennas are omnidirectional. Some time is also desirable to make use of directional antennas (such a Yagi antenna), for example, to receive messages coming from the airport direction with a higher receiving gain. For a remote installation where the antenna is far from the receiver (i.e., long coaxial cable between the antenna and receiver), the high-frequency signal of Mode S can attenuate quickly . When the signal has arrived at the receiver end, the signal-tonoise ratio can become quite high. To overcome this limitation, an active antenna may be used.

The active antenna (see Figure 2.4) has a low-noise amplifier integrated on the antenna side. Sometimes, it also includes a band-pass filter that is designed for 1090 MHz. The active antenna requires additional powering. Power is commonly supplied through a bias-tee device connected at the receiver end, which is on the left-hand side of the diagram. On the right-hand side, the radio frequency is mixed with the DC component, which allows the electronic circuit to be powered close to the receiver end.

26 Chapter 2. Quick Start: Hardware and Software to Receive Mode S Signals RF RF+DC DC LNA RF Bias Tee Active Antenna BPF Receiver Figure 2.4: Diagram of an active antenna powered through a bias-tee 2.3 Receiver Many modern Mode S receivers are built using software-defined radio (SDR). An SDR usually can be tuned to a wide range of radio frequencies. The user can conveniently select the center frequency of the receiver and the sampling rate, as well as defined the bandwidth of the onboard low-pass filter if it is supported. For receiving ADS-B (and other types of Mode S message), a RTL-SDR is the most common type of low-cost receiver to choose. Many different brands of RTL-SDR receivers exist; they are often built with the Realtek RTL2832U chipset. It can work with radio frequencies from 24 to 1766 MHz. The maximum sampling rate is 2.8 million samples per second (MSPS). Given that a Mode S bit consists of a pulse cycle of 0.5 µs (see Figure 1.3), the minimum SDR sampling rate required for Mode S is 2 MSPS. RLT-SDR devices just barely manage to meet this requirement. Note The maximum frequency range of RTL-SDR can be from 0 to 2220 MHz. However, the sensitivity drops off significantly outside the 24 – 1766 MHz range. The sampling rate can actually go up to a maximum of 3.2 million samples per second, but some samples may be dropped at this sampling rate. So, it is not stable. SDRs that offer a higher performance also exist. They often support a larger frequency range with higher sampling rates. Some offer multiple transmitting (TX) and receiving (RX) channels. Table 2.1 lists a few examples of common SDR devices. The performance of RTL-SDR is listed as a comparison.

2.4. Software tools 27 Table 2.1: Examples of common software defined radio devices RTL-SDR AirSpy R2 HackRF 1 LimeSDR BladeRF 2 Frequency (from) 24 MHz 24 MHz 1 MHz 100 KHz 47 MHz Frequency (to) 1766 MHz 1700 MHz 6000 MHz 3800 MHz 6000 MHz Max sample rate 2.8 MSPS 10 MSPS 20 MSPS 61.44 MSPS 61.44 MSPS RX channels 1 1 1 2 2 TX channels 0 1 1 2 2 Open hardware No Partially Full Full Partially 2.4 Software tools Several software tools are available to work with some of these SDR devices and decode Mode S and ADS-B signals directly . In this section, we explain how to use two open-source tools, which are dump1090 and pyModeS . 2.4.1 dump1090 dump1090 is the most well-known open-source Mode S decoder currently available. The software is written in C programming language and was originally developed by Salvatore Sanfilippo 1 under the BSD-3-Clause license. Since its release in 2013, the repository has been forked and extended intensively on GitHub, creating more than 900 forks so far. Many of the branched versions contain different variations, such as support for additional hardware, additional decoded Mode S messages, and additional output format. Currently , one of the most comprehensive forks is the repository maintained by FlightAware.2 The following examples will be based on the FlightAware version of dump1090 , using a Debian-based Linux system.

Installation of dump1090 The latest version of dump1090 source code can download as follows: $ git checkout https://github.com/flightaware/dump1090.git Before compiling the code, following dependencies must be installed: sudo apt-get install build-essential debhelper librtlsdr-dev \ pkg-config dh-systemd libncurses5-dev libbladerf-dev Next, we can compile the source code as: 1 https://github.com/antirez/dump1090 2 https://github.com/flightaware/dump1090

28 Chapter 2. Quick Start: Hardware and Software to Receive Mode S Signals $ cd dump1090 $ make It is worth noting that the FlightAware version ofdump1090 supports both RTL-SDR and BladeRF devices by default.

Default usage of dump1090 Once it is compiled and the RTL-SDR receiver is connected, we can run the following command to start receiving and decoding signals: $ ./dump1090 With the default option, Mode S messages and decoded information are displayed in the terminal. For example: *8d451dbd9905b5018004005979c5; CRC: 000000 RSSI: -20.6 dBFS Score: 1800 Time: 9214131.08us DF:17 AA:451DBD CA:5 ME:9905B501800400 Extended Squitter Airborne velocity over ground, subsonic (19/1) ICAO Address: 451DBD (Mode~S / ADS-B) Air/Ground: airborne Ground track 271.4 Groundspeed: 436.1 kt Geom rate: 0 ft/min NACv: 0 Raw Mode S messages If we only want to display the raw messages, the ––raw option can be provided, as follows: $ ./dump1090 --raw In this case, the terminal output will only contain the raw messages, for example: *5d4074358ad00c; *8d407435990dbd01900484f66c3c; *8d40743558af828cd326fe0c2fe9; *a80011b1e0da112fe0140060939f; *a00015b8c2680030a80000318667; *5d4074358ad030; *5d4074358ad030;