Aerospace and Electronic Systems Magazine November 2017 - 45

receiver or in a particular geographical area. This type of attack would create denial-of-service problems at any airport;
C

C

Message injection (or spoofing): The intentional transmission of signals in the same band as the ADS-B, with the same
protocol but with erroneous information displaying false
aircraft. Since no authentication measures exist, it is not
particularly difficult to build a transmitter able to produce
ADS-B messages;
Message deletion by SSR reply Garbling: Legitimate messages can be "deleted" or manipulated by the superposition
of false message with higher power. This approach requires
very precise and complex timing, making it extremely challenging. Since Mode S extended squitters' Cyclic Redundancy Check (CRC) can typically correct a maximum of
5 bit errors per message, if a message exceeds this errors
threshold, the receiver will drop it
as corrupted.

Eavesdropping and Message injection are made possible by the particular
vulnerability of the Mode S protocol that
doesn't provide encryption of the data. In
the literature (e.g. in [10]-[14]) it is possible to find proposals to introduce encryption and authentication to overcome
these problems.
Multilateration is also proposed as a
very efficient integrity check for ADS-B
data (see for example in [15], [16], and
[17]), and it is also useful against message injection or spoofing but requires a
large number of receiving stations and a
precise synchronization of all the receiver's clocks [18].
On the other hand, Jamming and
Message deletion vulnerabilities cannot
be reduced with encryption and multilateration and are closely related to the
ADS-B receiver hardware and software
implementations which are developed according to above discussed international
"standards" or "recommendations".
In the following we will introduce
the classical receiver implementations to
NOVEMBER 2017

explain where and how the jammer can reduce the ADS-B system
capability.

THE ADS-B RECEIVER AND ITS VULNERABILITY
Usually, two possible implementations of the ADS-B receiver are
used. The first one is a noncoherent scheme using a logarithmic
receiver to convert the radio frequency (RF) signal directly into
a base band signal, which is digitized and processed according to
the specifications [2]. This implementation is typically made up by
antenna, limiter, RF pass-band filter, low noise amplifier, logarithmic amplifier, and base-band low-pass filter followed by the A/D
converter (ADC). Downstream from the ADC, ADS-B signals are
detected and decoded.
The second implementation uses an intermediate frequency
(IF) coherent receiver and from the samples of the received signal

Figure 1.

Schematic of the ADS-B System and its most vulnerable areas (highlighted).

IEEE A&E SYSTEMS MAGAZINE

45



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