Aerospace and Electronic Systems Magazine November 2017 - 38

Addressing Vulnerabilities of the CNS Infrastructure to Targeted Radio Interference
This yields the radius dmax, outside of which the received signal to
interference ratio drops below 12 dB, causing a denial of service.
Figure 4 displays dmax as a function of dG and PT( A) G(A) indicating
that significant attacks can be performed with reasonable attack
transmit power and antenna gain.

Lockout and Flooding and Overinterrogation
With an adequately designed transmit antenna, an unauthorized
user can "lock out" or overinterrogate one or more transponders.
An SSR interrogator has the ability to lock out transponders, which
will subsequently not reply to interrogations from that particular
station for the following 18 seconds, unless an appropriate lockout
override is issued. Clearly, an adequately designed intervention
could lead to a transponder being permanently locked out, which,
in turn, would make the aircraft invisible to SSR. Note, however,
that PSR will still detect the aircraft.

Ghost Aircraft
One of the more sophisticated actions an unauthorized user of SSR
could take is to spoof the interrogator into detecting aircraft that
are not actually in the airspace. If the attacker is able to generate
signals that are compliant with SSR requirements, it may choose to
reply to interrogations as if it were airborne. This form of event is
more challenging, as the interrogator may monitor the consistency
of the time to reply, the power level, and the reported transponder position. The unauthorized user needs to be able to adequately
modulate its power level and time its signals to within a few microseconds. The injection of ghost aircraft has been investigated
by [1], [6].

ρ k =  ( xu − x k ) ⋅ 1k + bu + I k + T k + ε k ,

(5)

where the random errors are modeled as εk ~ N(0,σk2). The term
Ik accounts for the ionospheric delay of the satellite signal and Tk
models tropospheric propagation. The vector 1k represents the unit
vector pointing along the line of sight between the user and satellite k.
The user position is solved iteratively by grouping all pseudorange measurements ρk into a measurement vector ρ, the direction
vectors 1k into a geometry matrix G, and minimizing the position
residuals. For details on the estimation process, see [16], but for
this argument, let it suffice to say that the iterative process converges at a point in which
x 
ρ = G  u  + εk .
b

(6)

The iteration makes use of a weighted pseudoinverse of G to compute position hypotheses. The position hypotheses produce pseudorange estimates that approximate the measured pseudoranges
more with every step:



δxu  G TWG



1

G TWδρ.

(7)

The weighting matrix W is the inverse of the measurement covariance matrix and compensates for the random errors in the pseudorange observables. A weighted least-squares approach uses a model
of the measurement error and thus provides a more accurate posi-

GROUND-BASED AUGMENTATION
SYSTEM
The development of GBAS addresses
the need to monitor GNSS performance
in real time, as a way of improving accuracy and adding integrity to satellite
navigation. In a GBAS setup, a ground
station is installed at a precisely surveyed position, which then observes the
signals broadcast by GNSS satellites.
In a conventional GNSS receiver,
the user measures the time of reception
of the signals it receives from GNSS satellites. The time of reception depends on
the position of each satellite xk, the actual position of the user receiver xu, and
the clock offset of the user receiver with
respect to the GNSS system time bu. The
fundamental observable quantity, for
the user, is the pseudorange ρk, which
includes the actual distance rk from the
user position xu to the position xk of satellite k, and the user clock bu
38

Figure 4.

Jamming attack on interrogator: Radius outside of which the received signal to interference ratio drops
( A)
( A)
below 12 dB as a function of dG and PT + G . Beyond this radius, all interrogations will be lost for
the given ground station.

IEEE A&E SYSTEMS MAGAZINE

NOVEMBER 2017



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