Aerospace and Electronic Systems Magazine November 2017 - 60

ADS-B Vulnerabilities and a Security Solution with a Timestamp

REFERENCES
[1]
[2]

[3]

Figure 9.

ADS-B vs. ADS-BT packet format. (a) Current ADS-B format. (b)
Proposed ADS-BT format.

[5]

TRANSITION TO ADS-BT
We find that 8-bit is enough for timestamp size. This value can
be integrated into the current ADS-B structure without increasing
the packet size. The current ADS-B uses the 112-bit mode-S extended squitter message format (Figure 9a). The last 24 bits are
used for Cyclic Redundancy Check (CRC) checksum, which can
be reduced to 16 bits without any noticeable degradation of the
error detection performance. It is known that the error checking
performances of CRC-16 and CRC-24 are virtually identical for
data smaller than 256 bits [26].
Thus the 8-bit timestamp value in ADS-BT can be safely embedded in the first 8 bits of the CRC checksum (Figure 9b), which
keeps the ADS-BT data size same as ADS-B. Its first 5 bits are for
downlink format (DF), and the value of 17 is assigned to ADS-B.
As some of the DF values are not used currently, a new value (e.g.,
15) can be assigned to ADS-BT. This allows coexistence of ADSB and ADS-BT without any disruption. The devices that do not
understand the new downlink code can simply ignore the packet. A
device with a firmware upgrade can decode the ADS-BT message
using the new 16-bit CRC.

CONCLUSIONS

[6]
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[8]
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[11]

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[14]

ADS-B is the foundation of NextGen ATC and a trustworthy
ADS-B system will make the NextGen system more reliable. In
this research we have developed the ADS-BT method that measures the time of flight for the message between the aircraft and
compares that with the time of flight derived from the distance
between the aircraft. To enable time measurements, the ADSB messages will include a timestamp value. In spoofed ADS-B
messages, there is a discrepancy between the times of flight,
which grows or shrinks over time continuously. By observing the
change, we can detect all spoofed ADS-B frames with very high
accuracy. Although ADS-BT requires some change in the ADS-B
packet format, it can coexist with the existing ADS-B. ADS-BT
can especially offer a practical location verification capability for
UAS traffic management. Although some upgrade effort is necessary, the assurance attained by the adoption of ADS-BT may
outweigh the cost.
60

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[16]

[17]

[18]

IEEE A&E SYSTEMS MAGAZINE

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NOVEMBER 2017

http://www.flyingmag.com/getting-compliant-with-looming-ads-b-mandate http://www.flyingmag.com/getting-compliant-with-looming-ads-b-mandate http://diydrones.com/profiles/blogs/google-producing-low-cost-adsb-transponders-fordrones http://diydrones.com/profiles/blogs/google-producing-low-cost-adsb-transponders-fordrones http://diydrones.com/profiles/blogs/google-producing-low-cost-adsb-transponders-fordrones http://www.nexairavionics.com/hot-topics/ads-b http://www.nexairavionics.com/hot-topics/ads-b https://www.flightradar24.com/how-it-works https://www.flightradar24.com/how-it-works http://www.flightradar24.com http://www.theairlinepilots.com/ forumarchive/flightsafety/tcastrafficdisplay.php http://www.theairlinepilots.com/ forumarchive/flightsafety/tcastrafficdisplay.php https://www.atsb.gov.au/media/5214362/AO-2013-161%20final.pdf https://www.atsb.gov.au/media/5214362/AO-2013-161%20final.pdf http://http:// http://www.aahs-online.org/bk_review.php?ibook=45 https://www.aopa.org/news-and-media/all-news/2016/may/04/nextgen-for-drones https://www.aopa.org/news-and-media/all-news/2016/may/04/nextgen-for-drones http://www. suasnews.com/2015/11/new-research-ads-b-and-its-use-for-small-dronetraffic-management http://www. suasnews.com/2015/11/new-research-ads-b-and-its-use-for-small-dronetraffic-management http://www. suasnews.com/2015/11/new-research-ads-b-and-its-use-for-small-dronetraffic-management

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