For nonlinear estimation, the linear minimum mean square error (LMMSE) estimator using the measurement augmented by its nonlinear conversion can achieve better performance than using the original measurement. The main reason is that the original measurement cannot be fully utilized by the LMMSE estimator in a linear way. To effectively extract additional measurement information which can be further utilized by a linear estimator, a nonlinear approach named uncorrelated conversion (UC) is proposed. The uncorrelated conversions of the measurement are uncorrelated with the measurement itself. Two specific approaches to generating UCs are proposed based on a Gaussian assumption and a reference distribution, respectively. Then a UC based filter (UCF) is proposed based on LMMSE estimation using the measurement augmented by its uncorrelated conversions. To minimize the mean square error of the overall estimate, an optimized UCF (OUCF) with an analytical form is proposed by optimizing the reference distribution based UCs. In the UCF, measurement augmentation can be continued using the proposed nonlinear UC approach, and all augmenting terms are uncorrelated under certain conditions. Thus, the performance of the UCF and the OUCF may be continually improved.Simulation results demonstrate the effectiveness of the proposed estimator compared with some popular nonlinear estimators.
I. INTRODUCTIONNonlinear filters aim at estimating a quantity of a nonlinear problem from noisy measurements. Design, derivation, and application of nonlinear filters have received extensive attention over the past several decades, because nonlinear estimation has widespread applications in many fields, e.g., navigation, target tracking, and guidance systems. As an extension of the Kalman filter [17] for linear-Gaussian problems, the well-known extended Kalman filter (EKF) for nonlinear estimation is widely applied to many practical problems of a low nonlinearity. If the problem is highly nonlinear, however, EKF may diverge [13] [16].To solve this problem, some extensions of EKF have been proposed (e.g., the second-order EKF [4] [13] and iterated Kalman filter [8] [13]). These filters can be classified as function approximation methods [21]. Another class of nonlinear estimators based on approximations of the posterior state density [23] using the sequential Monte Carlo (SMC) method has also been proposed and applied (e.g., the particle filters [7] [9] [27] [18]). In this paper, we consider only the estimators which directly obtain the estimated quantity without obtaining its posterior distribution, since they are simpler and are adequate for many practical applications. Using deterministic sampling, the unscented transformation (UT) was proposed to calculate the first and second moments of a nonlinear function of a random variable [15] [16]. In UT, deterministic sigma points are generated and put through the nonlinear function. Then the moments are calculated using the obtained points [29] [25] [28]. Using UT to compute the moments in the linear m...