The chapter introduces a software library for addressing the propagation of systematic and random measurement uncertainty in the frequency domain for system identification purposes. The MATLAB-based library aims to overcome the prevailing neglect of systematic uncertainties, e.g., for large data sets, and offers the possibility to evaluate their significance in obtaining accurate and reliable system models. It uses a sophisticated sensor error model in combination with simple propagation laws, initially obtained by Monte Carlo simulations, to propagate the modelled errors into the frequency domain. The modelled errors include four of the most widespread errors for arbitrary sensor signals.