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This paper proposes an observer-based method for adaptive nonlinear guidance considering line-of-sight (LOS) measurement. Previously, an adaptive nonlinear guidance law was proposed considering target maneuver and control loop dynamics. However, some feedback quantities such as higher-order LOS rate derivatives, Chwa and Choi, "Adaptive nonlinear guidance considering control loop dynamics", IEEE Transactions on Aerospace Electronic Systems, are not directly measurable in this guidance law, and therefore these need to be estimated for more practical application. Accordingly, assuming the unavailable information as bounded time-varying uncertainties, an integrated guidance and control model is reformulated in normal form with respect to available states including target uncertainties and control loop dynamics. Then, a nonlinear observer is designed based on this integrated guidance and control model using only measured LOS rate. Finally, by using the estimates of states and uncertainties, an observer-based adaptive guidance law is proposed to guarantee the desired interception performance against maneuvering target. The proposed approach can be effectively used against target maneuver and the limited performance of control loop. The stability analyses and simulations of the proposed observer and guidance law are included to demonstrate the practical application of our scheme.