8,327 research outputs found
Authentication of Satellite Navigation Signals by Wiretap Coding and Artificial Noise
In order to combat the spoofing of global navigation satellite system (GNSS)
signals we propose a novel approach for satellite signal authentication based
on information-theoretic security. In particular we superimpose to the
navigation signal an authentication signal containing a secret message
corrupted by artificial noise (AN), still transmitted by the satellite. We
impose the following properties: a) the authentication signal is synchronous
with the navigation signal, b) the authentication signal is orthogonal to the
navigation signal and c) the secret message is undecodable by the attacker due
to the presence of the AN. The legitimate receiver synchronizes with the
navigation signal and stores the samples of the authentication signal with the
same synchronization. After the transmission of the authentication signal,
through a separate public asynchronous authenticated channel (e.g., a secure
Internet connection) additional information is made public allowing the
receiver to a) decode the secret message, thus overcoming the effects of AN,
and b) verify the secret message. We assess the performance of the proposed
scheme by the analysis of both the secrecy capacity of the authentication
message and the attack success probability, under various attack scenarios. A
comparison with existing approaches shows the effectiveness of the proposed
scheme
Fiber Based Multiple-Access Optical Frequency Dissemination
We demonstrate a fiber based multiple-access optical frequency dissemination
scheme. Without using any additional laser sources, we reproduce the stable
disseminated frequency at an arbitrary point of fiber link. Relative frequency
stability of 3E10^{-16}/s and 4E10^{-18}/10^4s is obtained. A branching fiber
network for highly-precision synchronization of optical frequency is made
possible by this method and its applications are discussed.Comment: 5 pages, 3 figure
Navigation system and method
In a global positioning system (GPS), such as the NAVSTAR/GPS system, wherein the position coordinates of user terminals are obtained by processing multiple signals transmitted by a constellation of orbiting satellites, an acquisition-aiding signal generated by an earth-based control station is relayed to user terminals via a geostationary satellite to simplify user equipment. The aiding signal is FSK modulated on a reference channel slightly offset from the standard GPS channel. The aiding signal identifies satellites in view having best geometry and includes Doppler prediction data as well as GPS satellite coordinates and identification data associated with user terminals within an area being served by the control station and relay satellite. The aiding signal significantly reduces user equipment by simplifying spread spectrum signal demodulation and reducing data processing functions previously carried out at the user terminals
Time division multiplex system
Synchronizing apparatus for multi-access satellite time division multiplex syste
Space-based Aperture Array For Ultra-Long Wavelength Radio Astronomy
The past decade has seen the rise of various radio astronomy arrays,
particularly for low-frequency observations below 100MHz. These developments
have been primarily driven by interesting and fundamental scientific questions,
such as studying the dark ages and epoch of re-ionization, by detecting the
highly red-shifted 21cm line emission. However, Earth-based radio astronomy
below frequencies of 30MHz is severely restricted due to man-made interference,
ionospheric distortion and almost complete non-transparency of the ionosphere
below 10MHz. Therefore, this narrow spectral band remains possibly the last
unexplored frequency range in radio astronomy. A straightforward solution to
study the universe at these frequencies is to deploy a space-based antenna
array far away from Earths' ionosphere. Various studies in the past were
principally limited by technology and computing resources, however current
processing and communication trends indicate otherwise. We briefly present the
achievable science cases, and discuss the system design for selected scenarios,
such as extra-galactic surveys. An extensive discussion is presented on various
sub-systems of the potential satellite array, such as radio astronomical
antenna design, the on-board signal processing, communication architectures and
joint space-time estimation of the satellite network. In light of a scalable
array and to avert single point of failure, we propose both centralized and
distributed solutions for the ULW space-based array. We highlight the benefits
of various deployment locations and summarize the technological challenges for
future space-based radio arrays.Comment: Submitte
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