fkf {FKF}

Fast Kalman filter

Description

This function allows for fast and flexible Kalman filtering. Both, the measurement and transition equation may be multivariate and parameters are allowed to be time-varying. In addition “NA”-values in the observations are supported. fkf wraps the C-function FKF which fully relies on linear algebra subroutines contained in BLAS and LAPACK.

Usage

fkf(a0, P0, dt, ct, Tt, Zt, HHt, GGt, yt)

Arguments

a0

A vector giving the initial value/estimation of the state variable.

P0

A matrix giving the variance of a0.

dt

A matrix giving the intercept of the transition equation (see Details).

ct

A matrix giving the intercept of the measurement equation (see Details).

Tt

An array giving the factor of the transition equation (see Details).

Zt

An array giving the factor of the measurement equation (see Details).

HHt

An array giving the variance of the innovations of the transition equation (see Details).

GGt

An array giving the variance of the disturbances of the measurement equation (see Details).

yt

A matrix containing the observations. “NA”-values are allowed (see Details).

Details

State space form

The following notation is closest to the one of Koopman et al. The state space model is represented by the transition equation and the measurement equation. Let m be the dimension of the state variable, d be the dimension of the observations, and n the number of observations. The transition equation and the measurement equation are given by

\alpha_{t + 1} = d_t + T_t \cdot \alpha_t + H_t \cdot \eta_t

y_t = c_t + Z_t \cdot \alpha_t + G_t \cdot \epsilon_t,

where \eta_t and \epsilon_t are iid N(0, I_m) and iid N(0, I_d), respectively, and \alpha_t denotes the state variable. The parameters admit the following dimensions:

\alpha_{t} \in R^{m}	d_{t} \in R^m	\eta_{t} \in R^m
T_{t} \in R^{m \times m}	H_{t} \in R^{m \times m}
y_{t} \in R^d	c_t \in R^d	\epsilon_{t} \in R^d
Z_{t} \in R^{d \times m}	G_{t} \in R^{d \times d}

Note that fkf takes as input HHt and GGt which corresponds to H_t H_t^\prime and G_t G_t^\prime.

Iteration:

The filter iterations are implemented using the expected values

a_{t} = E[\alpha_t | y_1,\ldots,y_{t-1}]

a_{t|t} = E[\alpha_t | y_1,\ldots,y_{t}]

and variances

P_{t} = Var[\alpha_t | y_1,\ldots,y_{t-1}]

P_{t|t} = Var[\alpha_t | y_1,\ldots,y_{t}]

of the state \alpha_{t} in the following way (for the case of no NA's):

Initialisation: Set t=1 with a_{t} = a0 and P_{t}=P0

Updating equations:

v_t = y_t - c_t - Z_t a_t

F_t = Z_t P_t Z_t^{\prime} + G_t G_t^\prime

K_t = P_t Z_t^{\prime} F_{t}^{-1}

a_{t|t} = a_t + K_t v_t

P_{t|t} = P_t - P_t Z_t^\prime K_t^\prime

Prediction equations:

a_{t+1} = d_{t} + T_{t} a_{t|t}

P_{t+1} = T_{t} P_{t|t} T_{t}^{\prime} + H_t H_t^\prime

Next iteration: Set t=t+1 and goto “Updating equations”.

NA-values:

NA-values in the observation matrix yt are supported. If particular observations yt[,i] contain NAs, the NA-values are removed and the measurement equation is adjusted accordingly. When the full vector yt[,i] is missing the Kalman filter reduces to a prediction step.

Parameters:

The parameters can either be constant or deterministic time-varying. Assume the number of observations is n (i.e. y = (y_t)_{t = 1, \ldots, n}, y_t = (y_{t1}, \ldots, y_{td})). Then, the parameters admit the following classes and dimensions:

dt	either a m \times n (time-varying) or a m \times 1 (constant) matrix.
Tt	either a m \times m \times n or a m \times m \times 1 array.
HHt	either a m \times m \times n or a m \times m \times 1 array.
ct	either a d \times n or a d \times 1 matrix.
Zt	either a d \times m \times n or a d \times m \times 1 array.
GGt	either a d \times d \times n or a d \times d \times 1 array.
yt	a d \times n matrix.

BLAS and LAPACK routines used:

The R function fkf basically wraps the C-function FKF, which entirely relies on linear algebra subroutines provided by BLAS and LAPACK. The following functions are used:

BLAS:	dcopy, dgemm, daxpy.
LAPACK:	dpotri, dpotrf.

FKF is called through the .Call interface. Internally, FKF extracts the dimensions, allocates memory, and initializes the R-objects to be returned. FKF subsequently calls cfkf which performs the Kalman filtering.

The only critical part is to compute the inverse of F_t and the determinant of F_t. If the inverse can not be computed, the filter stops and returns the corresponding message in status (see Value). If the computation of the determinant fails, the filter will continue, but the log-likelihood (element logLik) will be “NA”.

The inverse is computed in two steps: First, the Cholesky factorization of F_t is calculated by dpotrf. Second, dpotri calculates the inverse based on the output of dpotrf.
The determinant of F_t is computed using again the Cholesky decomposition.

The first element of both at and Pt is filled with the function arguments a0 and P0, and the last, i.e. the (n + 1)-th, element of at and Pt contains the predictions for the next time step.

Value

An S3-object of class “fkf”, which is a list with the following elements:

att	A m \times n-matrix containing the filtered state variables, i.e. att[,t] = a_{t|t}.
at	A m \times (n + 1)-matrix containing the predicted state variables, i.e. at[,t] = a_t.
Ptt	A m \times m \times n-array containing the variance of att, i.e. Ptt[,,t] = P_{t|t}.
Pt	A m \times m \times (n + 1)-array containing the variances of at, i.e. Pt[,,t] = P_t.
vt	A d \times n-matrix of the prediction errors i.e. vt[,t] = v_t.
Ft	A d \times d \times n-array which contains the variances of vt, i.e. Ft[,,t] = F_t.
Kt	A m \times d \times n-array containing the “Kalman gain” i.e. Kt[,,t] = k_t.
logLik	The log-likelihood.
status	A vector which contains the status of LAPACK's dpotri and dpotrf. (0, 0) means successful exit.
sys.time	The time elapsed as an object of class “proc_time”.

References

Harvey, Andrew C. (1990). Forecasting, Structural Time Series Models and the Kalman Filter. Cambridge University Press.

Hamilton, James D. (1994). Time Series Analysis. Princeton University Press.

Koopman, S. J., Shephard, N., Doornik, J. A. (1999). Statistical algorithms for models in state space using SsfPack 2.2. Econometrics Journal, Royal Economic Society, vol. 2(1), pages 107-160.

See Also

plot to visualize and analyze fkf-objects, KalmanRun from the stats package, function dlmFilter from package dlm.

Examples

## <--------------------------------------------------------------------------->
## Example: Local level model for the Nile's annual flow.
## <--------------------------------------------------------------------------->
## Transition equation:
## alpha[t+1] = alpha[t] + eta[t], eta[t] ~ N(0, HHt)          
## Measurement equation:
## y[t] = alpha[t] + eps[t], eps[t] ~  N(0, GGt)

y <- Nile
y[c(3, 10)] <- NA  # NA values can be handled

## Set constant parameters:
dt <- ct <- matrix(0) 
Zt <- Tt <- matrix(1)
a0 <- y[1]            # Estimation of the first year flow 
P0 <- matrix(100)     # Variance of 'a0'

## Estimate parameters:
fit.fkf <- optim(c(HHt = var(y, na.rm = TRUE) * .5,
                   GGt = var(y, na.rm = TRUE) * .5),
                 fn = function(par, ...)
                 -fkf(HHt = matrix(par[1]), GGt = matrix(par[2]), ...)$logLik,
                 yt = rbind(y), a0 = a0, P0 = P0, dt = dt, ct = ct,
                 Zt = Zt, Tt = Tt)

## Filter Nile data with estimated parameters:
fkf.obj <- fkf(a0, P0, dt, ct, Tt, Zt, HHt = matrix(fit.fkf$par[1]),
               GGt = matrix(fit.fkf$par[2]), yt = rbind(y))

## Compare with the stats' structural time series implementation:
fit.stats <- StructTS(y, type = "level")

fit.fkf$par
fit.stats$coef

## Plot the flow data together with fitted local levels:
plot(y, main = "Nile flow")
lines(fitted(fit.stats), col = "green")
lines(ts(fkf.obj$att[1, ], start = start(y), frequency = frequency(y)), col = "blue")
legend("top", c("Nile flow data", "Local level (StructTS)", "Local level (fkf)"),
       col = c("black", "green", "blue"), lty = 1)