Aerospace and Electronic Systems Magazine November 2017 - 55

Kim, Jo, and Lee
these methods require sophisticated devices and higher manufacturing cost, and have a nonzero false positive ratio due to their
statistical approach.

SECURE LOCATION VERIFICATION
These techniques include multilateration, group verification, distance bounding, Kalman filtering, data fusion, traffic modeling,
using received signal strength or angle of arrival of signal. Multilateration technique can geometrically calculate an unknown
location from a precise distance between four or more known locations [20]. The time difference of arrival can be obtained from
several antennas in different locations that receive the same signal
at different times. Another way of utilizing multilateration is group
verification. It verifies the location claimed by a nongroup aircraft
using multilateration by a group of aircraft [7]. Distance bounding
finds the upper bound of locations by sending a challenge to the
receiver and getting a response [7]. The upper bound is calculated
based on the speed of radio wave. The actual location can be calculated using the differences in distance among the measurements
from the various ground stations. Other methods use received signal strengths and calculate the source location or use directional
antenna to triangulate [7]. However, most of these methods require
real-time communication and additional bandwidth. The communication link itself can be attacked, and more importantly, they cannot function as a standalone unit. Increased manufacturing cost and
lack of backward compatibility make them less practical. Another
method based on Doppler frequency shift compares the predicted
Doppler shift amount with the actually observed one, and identifies spoofed ADS-B frames from the discrepancy [21], [22]. This
method can work alone without a third party assistance, but will
require a hardware modification.
Kalman filtering is already used to filter and smoothen GPS
position data in messages in ADS-B [23]. It is an important tool
for filtering out noisy signals and smoothing over missing data for
multilateration approach. Data fusion verifies the data obtained
within the system by comparing it with the data coming from other
independent sources, e.g., the fusion of ADS-B and radar. Traffic
modeling can be created from historical data and machine learning methods to detect deviations from normal ADS-B behavior
[7]. The technique can also be applied to establish red flags for
intrusion detection system so that the technically and physically
impossible data are reasonably dropped to reduce the strain on the
ADS-B system and prevent spoofing or DoS attacks. While these
algorithms offer cost-effective screening method, they cannot be
used by themselves to verify ADS-B signal completely.

CURRENT PRACTICE AND LIMITATIONS
FAA APPROACHES FOR ADS-B SECURITY
FAA currently uses the ground infrastructure to evaluate the integrity of the received ADS-B messages using three methods: 1)
Comparison with the radar tracking data, 2) Multilateration with
Time Difference Of Arrival (TDOA) at multiple ground stations,
and 3) Ranging to a ground station based on the UAT signal charNOVEMBER 2017

acteristics. While they offer ADS-B data filtering capability to
some degree, it is not complete for a number of reasons.
1. Radar system data: The location accuracy of radar is much
coarser (5 to 300 meters) than ADS-B (up to 5.1 meters) and it
is infrequently updated (6 to 12 seconds) due to its mechanical rotation. This introduces a significant delay and inaccuracy.
Therefore, not all ADS-B data can be verified effectively using radar surveillance data. Furthermore, it is envisioned that
the ADS-B system can eliminate the use of radar in the future.
Continued use of radar systems can diminish the benefit of
ADS-B.
2. There are about 650 ADS-B ground stations in the U.S., and
they are equipped with TDOA measurement features [24].
They are very accurate down to a few nanoseconds (roughly
equivalent to a few feet in distance), so in theory, the multilateration should be able to filter out all invalid packets. However,
as explained before, ADS-B requires an even and odd packet
pair to decode the CPR data. During this time, the aircraft
could have moved significantly. In a crowded airspace with
a lot of ADS-B transmission, packet losses due to collisions
among ADS-B packets are quite common. If one of the pair
is lost, the receiver must get another from the next 10 packets.
This process can introduce a lot of error, possibly hundreds of
meters of error. If there are bogus ADS-B messages within the
error range, it is difficult to verify their accuracy.
3. UAT signals offer some verification capability with timing of
the signal, but it still has some limitations: 1) it only allows for
a roughly 1 Hz range update rate; 2) it only allows transmission
timing variations of up to 500 nanoseconds (ns) off Coordinated Universal Time; and 3) it may have significant multipath errors [25]. Besides, it can cover only the systems using UAT, but
not the more common ones based on Mode S ES 1090 MHz.
Finally, the verified signal may be safely used by the ATC, but
not by the aircraft in the air. To make the filtered ADS-B data available to them, it should be rebroadcast over Automatic Dependent
Surveillance-Rebroadcast (ADS-R) or Traffic Information System-Broadcast (TIS-B), which consumes significant bandwidth
and also causes a delay. As these protocols do not have any security
measure either, they could also be attacked, which leaves the victim aircraft to sustain an attack continuously. So, a reliable onboard
ADS-B security system is still needed.

NEW CHALLENGES FOR UTM
The proliferation of UAS introduces a new problem for ADS-B
signal verification. Although Beyond Visual Line of Sight (BVLOS) flights are not approved yet, it will bring a lot of benefit to
UAS community once approved. In the UAS Traffic Management
(UTM) for BVLOS operation, ADS-B will be essential for tracking purposes.
However, the current methods are not effective in UTM environment. Small UASs generally fly at lower altitudes than large manned
aircraft and their transmission power is very limited (e.g., less than
1 watt), so it is difficult for ground stations to receive their ADS-B

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Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine November 2017