Aerospace and Electronic Systems Magazine November 2017 - 46
ADS-B Jamming Mitigation
Eavesdropping: A passive
attack that takes the form
of listening to the unsecured
broadcast transmissions of
the ADS-B messages and
usually forms the basis for
a number of sophisticated
active attacks.
the I and Q components are extracted. Typically, it is made up of
antenna, limiter, RF band-pass filter, low noise amplifier, mixer to
convert to IF, band-pass IF filter, and ADC.
Both receivers usually implement digital processing functions
as follows: (1) pulse detection, (2) pulse validation, (3) preamble
detection, (4) preamble validation, (5) data block decoding, (6) reply validation and reply degarbling (when necessary).
Independent of which implementation is used, interference or
jamming can affect many parts of the receiver chain. Some examples follow:
C
C
C
C
High power jammers may cause the saturation of the amplification stages (or of the limiter), which can corrupt the
coded information on the message.
False alarms or missed detections can be caused by the
change of the signal to noise ratio/signal to interference ratio
(SIR) due to the unwanted signals in the 1090 MHz band.
"Smart" jamming, mimicking the Mode S/ADS-B data, may
affect the receiver's logics. For example, the receiver typically is looking for the preamble. If the jammer sends a train
of preambles, the receiver could be overloaded to the saturation of its computational power.
Finally, smart jamming or spoofing may create false targets
or corrupt the data encoded in the ADS-B message.
Against the saturation of the amplification stage nothing can
be done but the other aspects may be mitigated, as shown in the
following sections.
RECEIVER VULNERABILITY EVALUATION AND PROPOSED
SOLUTION
In [19], we have already evaluated the vulnerability of the ADS-B
receiver to the jamming using real ADS-B signals corrupted by a
laboratory implementation of an ADS-B jammer. The jammer was
implemented using a software defined radio (SDR) device with
Rx/Tx capabilities in the 50-2200 MHz band [20] and the receiver
under attack was the Transponder Data Recorder (TDR) which is a
46
multichannel receiver designed and developed at Tor Vergata University [21], [22].
The jammer signal is combined with the signal coming from
the antenna using a combiner device and, varying the output power
level of the SDR, it is possible to emulate a jammer range between
ca. 1 km and 7 km (considering a jammer with a 20 dBm Equivalent Isotropically Radiated Power (EIRP)).
To evaluate the effects of jamming, different interfering waveforms are considered:
a.
an ADS-B message with a random data-block, repeated
with an intermessage time equal to 10 μs;
b.
a stream of International Civil Aviation Organization
(ICAO) standard preambles;
c. a random binary sequence with PPM modulation, similar
to the Data Block.
These waveforms were chosen for their capability to create
false alarms and false messages (spoofing) in addition to the
reduction of the signal to noise ratio and the possible receiver
saturation.
For all the considered jammers, the capability to produce harmful interference is very high. For example, in the worst case, the
coverage was limited from the original value (about 350 km) to
30-40 km emulating a 20 dBm jammer 1.10 km away from the
receiver.
Summing up, the major jammer effects are (excluding the saturation of the receiver chain that implies the complete denial of the
receiver service): (1) the probability of detection of a valid signal
is reduced; (2) the extraction of the information is corrupted and
the CRC test is not passed, (3) the processor is overloaded causing the service interruption or the loss of data (for example, type
(c) jammers produce up to 50,000 detection events in six seconds
while the real detections count without jamming for the same data
is about 2,000).
Here, we propose a solution to be implemented in a multichannel coherent receiver (in particular in its "digital processing"
block) against the ADS-B receiver jamming. In particular, we
propose to use algebraic manipulation (based on singular value
decomposition (SVD)) to generate different signal subspaces corresponding to the different sources, one of which is the jammer to
be counteracted.
ADS-B multichannel receiver with algebraic manipulation
of the signals was already introduced in the past to mitigate the
problem of overlapping signals coming from different airplanes
[23], [24]. In fact, in very dense scenario, with a large number
of targets, the probability of a conflict between two messages
becomes high. In [25]-[28] we have proposed different source
separation methods by algebraic operations exploiting diversity,
i.e. (a) different angle of arrival (AOA) of the impinging signals
or (b) signals frequency shifts due to the transponder transmission frequency tolerance (or the Doppler frequency due to the
airplane velocity).
The idea, here, is to consider the jammer as the other ADSB source but that may have a different AOA from most valid
signals. In this manner, the jamming (or spoofing) problem can
IEEE A&E SYSTEMS MAGAZINE
NOVEMBER 2017
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