Achieving sub-arcsecond co-registration across varying time-lines of multi-wavelength and instrument images is not trivial, and requires accurate characterization of instrument pointing jitter. In this work we have investigated internal pointing errors, on daily and yearly time-scales, occurring across the Solar Dynamics Observatory's (SDO) Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager (HMI). Using cross-correlation techniques on AIA 1700Å passband and HMI line-of-sight (LOS) magnetograms, from three years of observational image pairs at approximately three day intervals, internal pointing errors are quantified. Pointing variations of ± 0.26 (jitter limited) and ± 0.50 in the solar East-West (x) and North-South (y) directions, respectively, are measured. AIA observations of the Venus June 2012 transit are used to measure existing coalignment offsets in all passbands. We find AIA passband pointing variations are ∆X CO = 1.10 ± 1.41 and ∆Y CO = 1.25 ± 1.24 , when aligned to HMI's nominal image center, referred to herein as the CutOut technique (CO). Minimal long-term pointing variations found between limb and correlation derived pointings provide evidence that image center positions provided by the instrument teams achieve single pixel accuracy on time-scales below their characterization. However, daily AIA passband pointing variations of 1.18 indicate autonomous sub-arcsecond co-registration is not yet fully achievable.

No abstract

Physical-layer key generation methods utilize the variations of the communication channel to achieve a secure key agreement between two parties with no prior security association. Their secrecy rate (bit generation rate) depends heavily on the randomness of the channel, which may reduce significantly in a stable environment. Existing methods seek to improve the secrecy rate by injecting artificial noise into the channel. Unfortunately, noise injection cannot alter the underlying channel state, which depends on the multipath environment between the transmitter and receiver. Consequently, these methods are known to leak key bits toward multi-antenna eavesdroppers, which is capable of filtering the noise through the differential of multiple signal receptions. This work demonstrates an improved approach to reinforce physical-layer key generation schemes, e.g., channel randomization. The channel randomization approach leverages a reconfigurable antenna to rapidly change the channel state during transmission, and an angle-of-departure (AoD) based channel estimation algorithm to cancel the changing effects for the intended receiver. The combined result is a communication channel stable in the eyes of the intended receiver but randomly changing from the viewpoint of the eavesdropper. We augmented an existing physical-layer key generation protocol, iJam, with the proposed approach and developed a full-fledged remote instrumentation platform to demonstrate its performance. Our evaluations show that augmentation does not affect the bit error rate (BER) of the intended receiver during key establishment but reduces the eavesdropper's BER to the level of random guessing, regardless of the number of antennas it equips. CCS CONCEPTS • Security and privacy → Key management; Mobile and wireless security.