Adjoint first order sensitivities of transient responses and their applications in the solution of inverse problems

We present a novel algorithm for adjoint sensitivity analysis of time-varying responses that is based on the transmission line modeling (TLM) method. We show that by using at most one extra electromagnetic (EM) simulation, the sensitivities of the complete time domain EM response can be obtained with respect to all designable parameters regardless of their number and regardless of the number of time steps. The complete time domain signature of a certain structure can thus be efficiently and accurately predicted for perturbed structures. Our sensitivities match those obtained using the accurate and time intensive central finite differences (CFD). The adjoint sensitivities are used in the solution of inverse problems through a number of approaches exploiting surrogate model optimization and direct optimization. Our technique enables efficient identification of object position, dimensions and material properties.