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ECE5550: Applied Kalman Filtering
NONLINEAR KALMAN FILTERS
6.1: Extended Kalman filters
■ We return to the basic problem of estimating the present hidden state (vector) value of a dynamic system, using noisy measurements that are somehow related to that state (vector).
■ We now examine the nonlinear case, with system dynamics xk = fk−1(xk−1, uk−1,wk−1)
z k = hk (xk, uk,vk ),
where uk is a known (deterministic/measured) input signal,wk is a process-noise random input, and vk is a sensor-noise random input.
■ There are three basic nonlinear generalizations to KF
• Extended Kalman filter (EKF): Analytic linearization of the model at each point in time. Problematic, but still popular.
• Sigma-point (Unscented) Kalman filter (SPKF/UKF): Statistical/ empirical linearization of the model at each point in time. Much better than EKF, at same computational complexity.
• Particle filters : The most precise, but often thousands of times more computations required than either EKF/SPKF. Directly approximates the integrals required to compute f (xk | Zk ) using Monte-Carlo integration techniques.
■ In this chapter, we present the EKF and SPKF.
The Extended Kalman Filter (EKF)
■ The EKF makes two simplifying assumptions when adapting the general sequential inference equations to a nonlinear system:
• In computing state estimates, EKF assumes E[fn(x)] ≈ fn(E[x]);
• In computing covariance estimates, EKF uses Taylor series to
linearize the system equations around the present operating point.
■ Here, we will show how to apply these approximations and
assumptions to derive the EKF equations from the general six steps.
EKF step 1a: State estimate time update.
■ The state prediction step is approximated as
ˆ(x) = E[fk−1(xk−1 , uk−1,wk−1) | Zk−1]
≈ fk−1(ˆ(x)1 , uk−1 , ¯(w)k−1),
where ¯(w)k−1 = E[wk−1]. (Often,¯(w)k−1 = 0.)
■ That is, we approximate the expected value of the state by assuming
it is reasonable to propagateˆ(x)1 and¯(w)k−1 through the state eqn.
EKF step 1b: Error covariance time update.
■ The covariance prediction step is accomplished by first making an
approximation for˜(x) .
x˜k − = xk − ˆxk −
= fk−1(xk−1, uk−1, wk−1) − fk−1(xˆk + −1, uk−1, w¯ k−1).
■ The first term is expanded as a Taylor series around the prior
operating “point” which is the set of values {ˆ(x)1 , uk−1 , ¯(w)k−1}
■ The distinction between the total differential and the partial differential is not critical at this point, but will be when we look at parameter
estimation using extended Kalman filters.
EKF step 1c: Output estimate (where 
k = E[vk ]).
■ The system output is estimated to be
k = E[hk (xk, uk,vk ) | Zk — 1] ≈ hk (^(x)k— , uk, v-k ).
■ That is, it is assumed that propagating xˆk − and the mean sensor noise is the best approximation to estimating the output.
EKF step 2a: Estimator gain matrix.
■ The output prediction error may then be approximated
using again a Taylor-series expansion on the first term.
z k ≈ hk (^(x)k— , uk, v-k )
■ Note, much like we saw in Step 1b,
■ From this, we can compute such necessary quantities as
z˜,k ≈ˆCk − ˜x,kˆCk T +ˆDk v˜ˆDk T , − ˜x˜z,k
≈ E[(x˜k −(ˆCk x˜k −ˆDkv˜k) T ] = x − ˜,kˆCk T .
■ These terms may be combined to get the Kalman gain
Lk = − ˜ x, kCk T Cˆk x − ˜,kCˆTk+ Dˆk v˜ DˆTk−1.
EKF step 2b: State estimate measurement update.
■ The fifth step is to compute the a posteriori state estimate by updating the a priori estimate
^(x)k(十) = ^(x)k— 十 Lk (z k — k ).
EKF step 2c: Error covariance measurement update.
■ Finally, the updated covariance is computed as
+ ˜ x, k = − ˜ x, k − Lk z˜ , kLk T = (I − Lk ˆ Ck)x − ˜ , k.