Method of characterizing a coated body by at least one coating parameter based on fitting to a physical model, wherein the coated body comprises a substrate coated by a polymeric coating such as a paint film, the polymeric coating having at least one layer, and wherein the method being carried out by a sensor system in a non-contact manner, the sensor system comprising an emitter system for emitting THz radiation, a detector system for detecting THz radiation, and a processing unit operationally coupled to the emitter system and the detector system, the method comprising: - Emitting, by the emitter system, a THz radiation signal towards the coated body such that the THz radiation interacts with the polymeric coating; - Detecting, by the detector system, a response signal being the detected THz radiation signal having interacted with the polymeric coating; - Determining model parameters of the physical model by optimizing the model parameters such that a predicted response signal of the physical model is fitted to the detected response signal, the model parameters including at least one thickness of the polymeric coating, the model parameters being indicative of optical properties of the polymeric coating describing the interaction of the THz radiation signal with the polymeric coating, the model parameters including a parameterization of the index of refraction of the at least one layer; - Determining, from the determined model parameters, the at least one coating parameter, wherein the at least one coating parameter includes the at least one thickness of the polymeric coating; - Fitting the predicted response signal of the physical model to the detected response signal by an iterative procedure comprising the steps: (a) calculating a predicted response signal as a simulated response signal based on the physical model using an initial guess for the model parameters (92, 93); (b) calculating an error function expressing a deviation between the predicted response signal and the detected response signal; (c) iterating steps (a) and (b), whereby the model parameters (92, 93) are varied until the error function satisfies a best-fit criterion, (d) obtaining the fitted parameters as the final parameters satisfying the best-fit criterion in step (c), and calculating at least one of the one or more coating parameters from the fitted parameters (92, 93), wherein step (a) includes calculating a simulated response signal in time domain; and step (b) includes calculating the error function such that the error function has a time-domain error function component. - Emitting, by the emitter system, a THz radiation signal towards the coated body such that the THz radiation interacts with the polymeric coating; - Detecting, by the detector system, a response signal being the detected THz radiation signal having interacted with the polymeric coating; - Determining model parameters of the physical model by optimizing the model parameters such that a predicted response signal of the physical model is fitted to the detected response signal, the model parameters including at least one thickness of the polymeric coating, the model parameters being indicative of optical properties of the polymeric coating describing the interaction of the THz radiation signal with the polymeric coating, the model parameters including a parameterization of the index of refraction of the at least one layer; - Determining, from the determined model parameters, the at least one coating parameter, wherein the at least one coating parameter includes the at least one thickness of the polymeric coating; - Fitting the predicted response signal of the physical model to the detected response signal by an iterative procedure comprising the steps: (a) calculating a predicted response signal as a simulated response signal based on the physical model using an initial guess for the model parameters (92, 93); (b) calculating an error function expressing a deviation between the predicted response signal and the detected response signal; (c) iterating steps (a) and (b), whereby the model parameters (92, 93) are varied until the error function satisfies a best-fit criterion, (d) obtaining the fitted parameters as the final parameters satisfying the best-fit criterion in step (c), and calculating at least one of the one or more coating parameters from the fitted parameters (92, 93), wherein step (a) includes calculating a simulated response signal in time domain; and step (b) includes calculating the error function such that the error function has a time-domain error function component. (a) calculating a predicted response signal as a simulated response signal based on the physical model using an initial guess for the model parameters (92, 93); (b) calculating an error function expressing a deviation between the predicted response signal and the detected response signal; (c) iterating steps (a) and (b), whereby the model parameters (92, 93) are varied until the error function satisfies a best-fit criterion, (d) obtaining the fitted parameters as the final parameters satisfying the best-fit criterion in step (c), and calculating at least one of the one or more coating parameters from the fitted parameters (92, 93), wherein step (a) includes calculating a simulated response signal in time domain; and step (b) includes calculating the error function such that the error function has a time-domain error function component.