In the first part, we briefly introduced the ADS-B concept and the requirement for securing this type of communications. In this second part, we provide more details about the ADS-B message format, to justify the need for very efficient cryptographic techniques.

ADS-B consists of three distinct categories: ADS-B Out, ADS-B In, and ADS-Rebroadcast (ADS-R). With ADS-B Out every participant retrieves its own position and velocity by using an onboard GPS receiver and each aircraft broadcasts its own position and velocity. Messages can integrate further fields such as ID, intent, urgency code, and uncertainty level. Unlike current transponders, these broadcasts are not sent in response to interrogations, but blindly at periodic intervals (i.e., twice per second). They are received by air traffic control (ATC) stations on the ground, as well as by nearby aircraft if they are equipped with the receiving subsystem (ADS-B In). While ADS-B Out is primarily for ground-based surveillance, ADS-B In provides airborne surveillance capability, allowing the aircraft to use these position messages to monitor neighboring aircraft in airspace and airports, thus improving pilot situational awareness and alertness. Lastly, ADS-R is the rebroadcast by ground stations of an ADS-B message from one format to another when two aircraft with different data link modes are sensed in the same area. This is required because the two ADS-B message formats currently in use (1090 ES and UAT) are inherently different. The one that is mandatory (by current FAA regulations) by Jan 1, 2020 is the ADS-B Out.

Two competing ADS-B data link standards have been proposed: Universal Access Transceiver (UAT) and 1090 MHz Extended Squitter (1090ES). UAT has the advantage of operating in a less congested frequency (978 MHz) with a bandwidth of 1Mbps. It is currently used only for general aviation, and allows for additional services such as graphical weather uploads. Commercial aircraft employ SSR Mode-S with Extended Squitter (1090ES). From Figure 1, the ADS-B function could be a normal upgrade path to the traditional Mode-S transponders.

Figure 1 - ADS-B hierarchy

Before ADS-B, all secondary surveillance radar (SSR) systems in ATC were interrogation based. The so-called modes used in civil aviation to query the identification and altitude of the aircraft are shown in Figure 2.

Figure 2 - Civil aviation transponder modes

The 1090ES data link uses the 1090MHz frequency for the communication sent out by aircraft to other aircraft as well as to ground stations. Mode-S uses ground-to-air communication at 1030MHz for interrogations and information services. 1090ES transmission starts with a preamble of two synchronization pulses. The data bits are transmitted with Pulse Position Modulation (PPM). Because every time slot is 1 µs long, a bit is indicated by either sending a 0.5 µs pulse in the first half of the slot (1-bit) or in the second half (0-bit). Unfortunately, PPM is very sensitive to reflected signals and multipath dispersion, which plays an important role in the crypto protocol selection. There are two different possible message lengths specified in Mode-S, 56 bit and 112 bit, whereas ADS-B uses only the longer format (Figure 3):

• The downlink format field DF (5 bits) assigns the type of the message. 1090ES uses a multipurpose format. When set to 17, it indicates that the message is an extended squitter, enabling the transmission of 56 arbitrary bits in the ME field.
• The CA field (3 bits) indicates information about the capabilities of the employed transponder.
• The 24-bit AA field carries the unique ICAO aircraft address that enables aircraft identification.
• The PI field provides a 24-bit CRC, to detect and correct possible transmission errors. It is possible for recipients to correct up to 5-bit errors in 1090ES messages using a fixed generator polynomial of degree 24.

Figure 3 - 1090ES Data Link

Figure 3 shows an important design aspect – the 56-bit ME field is the only one that can be used to transmit arbitrary data. As mentioned in part one, ADS-B is very much bandwidth and interference constrained. Therefore, we have to choose carefully the cryptographic techniques that do not extend the ADS-B message, to allow compatibility with the current message processing and a seamless integration with current transmission equipment.

Stay tuned for part three of this series in which we introduce our options for securing the ADS-B transmissions.