For an advanced tactical fighter or high performance missiles, the dynamic characteristics could be highly nonlinear so that the conventional linear control design techniques may not be applicable. Recently, various nonlinear control techniques such as feedback linearization have been applied to overcome this problem with some success. However, the prerequisite of these nonlinear methods is that states of the system are available for feedback, which requires nonlinear estimation of state variables not directly measured.
In this paper, recent nonlinear observer theories are applied to the state estimation of a nonlinear aircraft model. Three different nonlinear observer design techniques are considered : canonical form observer, nonlinear state observer and variable structure system observer. The basic theory on observer construction and error convergence for each technique is presented. The techniques are also applied to the estimation of the angle of attack for a flight vehicle which has nonlinearity in $C_m$.