Electronic countermeasures (ECM) against a radar are actions taken by an adversarial jammer to mitigate effective utilization of the electromagnetic spectrum by the radar. On the other hand, electronic counter-countermeasures (ECCM) are actions taken
by the radar to mitigate the impact of electronic countermeasures (ECM) so that the radar can continue to operate effectively. The main idea of this paper is to show that ECCM involving a radar and a jammer can be formulated as a principal-agent problem (PAP) - a problem widely studied in microeconomics. With the radar as the principal and the jammer as the agent, we design a PAP to optimize the radars ECCM strategy in the presence of a jammer. The radar seeks to optimally trade-off signal-to-noise ratio (SNR) of the target measurement with the measurement cost: cost for generating radiation power for the pulse to probe the target. We show that for a suitable choice of utility functions, PAP is a convex optimization problem. Further, we analyze the structure of the PAP and provide sufficient conditions under which the optimal solution is an increasing function of the jamming power observed by the radar; this enables computation of the radars optimal ECCM within the class of increasing affine functions at a low computation cost. Finally, we illustrate the PAP formulation of the radars ECCM problem via numerical simulations. We also use simulations to study a radars ECCM problem wherein the radar and the jammer have mismatched information.
We report synthesis of Co substitution at Fe site in Fe1-xCoxSe0.5Te0.5 (x=0.0 to 0.10) single crystals via vacuum shield solid state reaction route using flux free method. Single crystal XRD results showed that these crystals grow in (00l) plane i.e
., orientation in c-direction. All the crystals possess tetragonal structure having P4/nmm space group. Detailed scanning electron microscopy (SEM) images show that the crystals are grown in slab-like morphology. The EDAX results revealed the final elemental composition to be near stoichiometric. Powder X-Ray diffraction (PXRD) Rietveld analysis results show that (00l) peaks are shifted towards higher angle with increasing Co concentration. Both a and c lattice parameters decrease with increasing Co concentration in Fe1-xCoxSe0.5Te0.5 (x=0.0 to 0.10) single crystals. Low temperature transport and magnetic measurements show that the superconducting transition temperature (Tc), decreases from around 12K to 10K and 4K for x=0.03 and x=0.05 respectively. For x=0.10 crystal superconductivity is not observed down to 2K.
The influence of YBa2Cu3O4 (YBCO) superconductor layer (S-layer) with varying thickness d-YBCO = 20 to 50 nm on the magnetic coupling between two La0.67Ca0.33MnO3 (LCMO) ferromagnet layers (F-layer, thickness d-LCMO = 50 nm) in F/S/F heterostructures
(HSs) was investigated by measuring global magnetization (M) in a temperature (T) range = 2 - 300 K and magnetic field (H) range = 0 - 10 kOe. All the HSs were superconducting with critical temperature (Tc) decreasing from = 78 to 36 K with decrease in d-YBCO, whereas the ferromagnetic ordering temperature Tm = 250 K did not change much. Systematically measured M-H loops of all HSs at both T > Tc and T < Tc show three main results- (a) the two step magnetic reversal above Tc converts into a four step reversal below Tc in HSs with d-YBCO >= 30 nm, (b) the magnetic field corresponding to the additional two switching steps and their magnitude show characteristic evolution with T and d-YBCO and (c) the HS with d-YBCO = 20 nm shows radically different behaviour, where the two step magnetic reversal above Tc continues to persist below Tc and converts into a single step reversal at T << Tc. The first two results indicate magnetostatic coupling between the magnetic domains and the vortices across the two F/S interfaces resulting in reversal dynamics different from that deep within the LCMO layers. Whereas, the result c reveals indirect exchange coupling between LCMO layers through the superconducting YBCO layer, which is a clear experimental evidence of coexistence of ferromagnetism and superconductivity in nm scale F/S/F HSs expected theoretically by C.A.R. Sa de Melo (Physica C 387, 17-25 (2003)).
