IEEE - Aerospace and Electronic Systems - October 2022 - 17

Kassas et al.
example, in 3G systems, a 32768-long QPSK
pseudonoise (PN) sequence and Walsh codes are
used to spread the transmitted data. The PN
sequence is shifted by an integer multiple of 64
chips by each cellular tower sector, which allows
a maximum of 512 possible shifts. In this case,
the cross-correlation between the PN sequence
and its shifted version is negligible. As such, for
two towers to significantly interfere at the
receiver, their relative range must be at least
15 km (corresponding to 64-chip offset). However,
in practice, adjacent towers are offset by at
least 4 64 chips, requiring a minimum of 60km
relative range for strong interference to
occur. In addition, the 60-km relative range
implies a 95 dB difference in the path loss
(assuming the free space propagation model),
which means that one signal will be completely
buried in the noise floor of the other. Even for a
relative range of 15 km, the difference in the
path loss is 83 dB. In conclusion, it is very
unlikely for intrachannel interference to be an
impediment for exploiting 3G signals for aerial
navigation. This could explain why interference
from fear and far cells have not been detected in
the case of 3G signals. The same discussion
holds for 4G signals, except that the synchronization
signals were not used in the 4G module in
order to avoid interference, since some of these
sequences are common between different eNodeBs.
Instead, only the CRS was used, which is
unique for each eNodeB and has very low crosscorrelation
properties.
CONCLUSION
This study demonstrated that reliable acquisition and
tracking of cellular 3G CDMA and 4G LTE signals
can be performed by high dynamics aircraft flying at
altitudes up to 23,000 ft AGL and horizontal distances
up to 100 km, making them a reliable source for aircraft
navigation. This finding is further validated by
experimental results showing a USAF C-12 aircraft
navigating for 51 km at around 5000 ft AGL over a 9minute
period exclusively with cellular SOPs, achieving
a 3D position RMSE of 10.5 m.
ACKNOWLEDGMENTS
The authors would like to thank Edwards AFB and
Holloman AFB for inviting the ASPIN Laboratory to
conduct experiments on Air Force aircraft in the
" SNIFFER: Signals of opportunity for Navigation In
Frequency-Forbidden EnviRonments " flight campaign.
OCTOBER 2022
They would also like to thank Joshua Morales, Kimia
Shamaei, Mahdi Maaref, Kyle Semelka, MyLinh
Nguyen, and Trier Mortlock for their help with preparing
for data collection. DISTRIBUTION STATEMENT
A. Approved for public release; Distribution is unlimited
412TW-PA-20146. This work was supported in
part by the Office of Naval Research (ONR) under
Grant N00014-19-1-2511 and Grant N00014-19-12613,
in part by the Sandia National Laboratories
under Grant 1655264, in part by the National Science
Foundation (NSF) under Grant 1929965, and in part by
the U.S. Department of Transportation (USDOT) under
Grant 69A3552047138 for the CARMEN University
Transportation Center (UTC).
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