Often an agent is uncertain what state the world is in. Filtering (or monitoring) is the task of tracking and updating the belief state (the distribution) $B_t(X) = P_t(X_t | e_1, \dots, e_t)$ as new evidence is observed.

We start with $B_1(X)$ with some initial setting (typically uniform), and update as new evidence is observed/time passes.

Particle filters

Sometimes we have state spaces which are too large for exact inference (i.e. too large for the forward algorithm) or just to hold in memory. For example, if the state space $X$ is continuous.

Instead, we can use particle filtering, which provides an approximate solution.

With particle filters, possible states are represented as particles (vectors); the density of these vectors in state space represents the posterior probability of being in a certain state (that is, higher density means the true state is more likely in that region), and the set of all these vectors represents the belief state.

Another way of putting this: Particles are essentially samples of possible states. Each particle can be thought of as a hypothesis that we are in the state it represents. The more particles there are for a state mean the more likely we are in that state.

So to start, these particles may be very diffuse, spread out across the space somewhat uniformly. As more data (measurements/observations) is collected, the particles are resampled and placed according to these observations, and they start to concentrate in more likely regions.

More formally, our representation of $P(X)$ is now a list of $N$ particles (generally $N << X$, and we don't need to store $X$ in memory anymore, just the particles).

$P(x)$ is approximated by the number of particles with value $x$ (i.e. the more particles that have value $x$, the more likely state $x$ is).

Particles have weights, and they all start with a weight of 1.

As time passes, we "move" each particle by sampling its next position from the transition model:

$$ x' = \text{sample}(P(X'|x)) $$

As we gain evidence, we fix the evidence and downweight samples based on the evidence:

$$ \begin{aligned} w(x) &= P(e|x) \\ B(X) \varpropto P(e|x)B'(X) \end{aligned} $$

These particle weights reflect how likely the evidence is from that particle's state. A result of this is that the probabilities don't sum to one anymore.

This is similar to likelihood weighting.

Rather than tracking the weighted samples, we resample.

That is, we sample $N$ times from the weighted sample distribution (i.e. we draw with replacement). This is essentially renormalizing the distribution and has the effect of "moving" low-weight (unlikely) samples to where high-weight samples are (i.e. to likely states), so they become more "useful".

The particle filter algorithm:

# s is a set of particles with importance weights
# u is a control vector
# z is a measurement vector
def particle_filter(s, u, z):
  # a new particle set
  s_new = []

  eta = 0

  n = len(s)
  for i in range(n):
    # sample a particle (with replacement)
    # based on the importance weights
    p = sample(s)

    # sample a possible successor state (i.e. a new particle)
    # according to the state transition probability
    # and the sampled particle
    # p' ~ p(p'|u, p)
    p_new = sample_next_state(u, p)

    # use the measurement probability as the importance weight
    # p(z|p')
    w_new = measurement_prob(z, p_new)

    # save to new particle set
    s_new.append((p_new, w_new))

  # normalize the importance weights
  # so they act as a probability distribution
  eta = sum(w for p, w in s_new)
  for i in range(n):
    s_new[i][1] /= eta

Particle filters do not scale to high-dimensional spaces because the number of particles you need to fill a high-dimensional space grows exponentially with the dimensionality. Though there are some particle filter methods that can handle this better.

But they work well for many applications. They are easy to implement, computationally efficient, and can deal well with complex posterior distributions.

DBN particle filters

There are also DBN particle filters in which each particle represents a full assignment to the world (i.e. a full assignment of all variables in the Bayes' net). Then at each time step, we sample a successor for each particle.

When we observe evidence, we weight each entire sample by the likelihood of the evidence conditioned on the sample.

Then we resample - select prior samples in proportion to their likelihood.

Basically, a DBN particle filter is a particle filter where each particle represents multiple assigned variables rather than just one.

Kalman Filters

(note: the images below are all sourced from How a Kalman filter works, in pictures, Tim Babb)

Kalman filters can provide an estimate for the current state of a system and from that, provide an estimate about the next state of the system. They make the approximation that everything is Gaussian (i.e. transmissions and emissions).

We have some random variables about the current state (in the example below, they are position $p$ and velocity $v$); we can generalize this as a random variable vector $S$. We are uncertain about the current state but (we assume) they can be expressed as Gaussian distributions, parameterized by a mean value and a variance (which reflects the uncertainty).

Gaussian random variables
Gaussian random variables parameters

These random variables may be uncorrelated, as they are above (knowing the state of one tells us nothing about the other), or they may be correlated like below.

Correlated Gaussian random variables
Correlated Gaussian random variables

This correlation is described by a covariance matrix, $\mathbf{\Sigma}$, where the element $\sigma_{ij}$ describes the correlation between the $i$th and $j$th random variables. Covariance matrices are symmetric.

We say the current state is at time $t-1$, so the random variables describing it is notated $S_{t-1}$, and the next state (which we want to predict) is time $t$, so the random variables we predict then are notated $S_t$

The Kalman filter basically takes the random variable distributions for the current state and gives us new random variable distributions for the next state:

From current distributions to predicted distributions for the next state
From current distributions to predicted distributions for the next state

In essence it moves each possible point for the current state to a new predicted point.

We then have to come up with some function for making the prediction. In the example of position $p$ and velocity $v$, we can just use $p_t = p_{t-1} + \Delta t v_{t-1}$ to update the position and assume the velocity is kept constant, i.e. $v_t = v_{t-1}$.

We can represent these functions collectively as a matrix applied to the state vector:

$$ \begin{aligned} S_t &= \begin{bmatrix} 1 & \Delta t \\ 0 & 1 \end{bmatrix} S_{t-1} \\ &= F_t S_{t-1} \end{aligned} $$

We call this matrix $F_t$ our prediction matrix. With this, we can transform the means of each random variable (we notate the vector of these means at $\hat S_{t-1}$ since these means are our best estimates) to the predict means in the next state, $\hat S_{t}$.

We can similarly apply the prediction matrix to determine the covariance at time $t$, using the property $Cov(Ax) = A \Sigma A^T$, so that:

$$ \Sigma_t = F_t \Sigma_{t-1} F_t^T $$

It's possible we also want to model external influences on the system. In the position and velocity example, perhaps some acceleration is being applied. We can capture these external influences in a vector $u_t$, which is called the control vector.

For the position and velocity example, this control vector would just have acceleration $a$, i.e. $u_t = [a]$. We then need to update your prediction functions for each random variable in $S$ to incorporate it, i.e. $p_t = p_{t-1} + \Delta t v_{t-1} + \frac{1}{2} \Delta t^2 a$ and $v_t = v_{t-1} + a \Delta t$.

Again, we can pull out the coefficients for the control vector terms into a matrix. For this example, it would be:

$$ U_t = \begin{bmatrix} \frac{\Delta t^2}{2} \\ \Delta t \end{bmatrix} $$

This matrix is called the control matrix, which we'll notate as $U_t$.

We can then update our prediction function:

$$ S_t = F_t S_{t-1} + U_t u_t $$

These control terms capture external influences we are certain about, but we also want to model external influences we are uncertain about. To model this, instead of moving each point from the distributions of $S_{t-1}$ exactly to where the prediction function says it should go, we also describe these new predicted points as Gaussian distributions with covariance matrices $Q_t$.

Modeling uncertainty in the predicted points
Modeling uncertainty in the predicted points

We can incorporate the uncertainty modeled by $Q_t$ by including it when we update the predicted covariance at time $t$:

$$ \Sigma_t = F_t \Sigma_{t-1} F_t^T + Q_t $$

Now consider that we have sensors which measure the current state for us, though there is some measurement error (noise). We can model these sensors with the matrix $H_t$ (which would include measured values for each of our state random variables) and incorporate them:

$$ \begin{aligned} \mu_{\text{expected}} &= \mathbf{H}_t \mathbf{\hat{S}}_t \\ \mathbf{\Sigma}_{\text{expected}} &= \mathbf{H}_t \mathbf{P}_t \mathbf{H}_t^T \end{aligned} $$

This gives us the final equation for our predicted state values.

Now say we've come to the next state and we get in new sensor values. This allows us to observe the new state (with some noise/uncertainty) and combine it to our predicted state values to get a more accurate estimate of the new current state.

The readings we get for our state random variables (e.g. position and velocity) are represented by a vector $z_t$, and the uncertainty/noise (covariance) in these measurements is described by the covariance matrix $R_t$. Basically, these sensors are also described as Gaussian distributions, where the values the sensor gave us, $z_t$, is considered the vector of the means for each random variable.

Uncertainty in sensor readings
Uncertainty in sensor readings

We are left with two Gaussians - one describing the sensor readings and their uncertainty, and another describing the predicted values and their uncertainty. We can multiply the distributions to get their overlap, which describes the space of values likely for both distributions.

Overlap of the two Gaussians
Overlap of the two Gaussians

The resulting overlap is, yet again, also a Gaussian distribution with its own mean and covariance matrix.

We can compute this new mean and covariance from the two distributions that formed it.

First, consider the product of two 1D Gaussian distributions:

The product of two 1D Gaussians
The product of two 1D Gaussians

$$ \mathcal{N}(x, \mu_0, \sigma_0) \cdot \mathcal{N}(x, \mu_1, \sigma_1) \stackrel{?}{=} \mathcal{N}(x, \mu’, \sigma’) $$

As a reminder, the Gaussian distribution is formalized as:


We can solve for both $\mu’$ and $\sigma’^2$ to get:

$$ \begin{aligned} \mu’ &= \mu_0 + \frac{\sigma_0^2 (\mu_1 – \mu_0)} {\sigma_0^2 + \sigma_1^2} \\ \sigma’^2 &= \sigma_0^2 – \frac{\sigma_0^4} {\sigma_0^2 + \sigma_1^2} \end{aligned} $$

To make this more readable, we can factor out $k$, such that:

$$ \begin{aligned} k &= \frac{\sigma_0^2}{\sigma_0^2 + \sigma_1^2} \\ \mu’ &= \mu_0 + k(\mu_1 - \mu_0) \\ \sigma’^2 &= \sigma_0^2 - k \sigma_0^2 \end{aligned} $$

In dimensions higher than 1, we can re-write the above with matrices ($\mu$ are now vectors here):

$$ \begin{aligned} K &= \frac{\Sigma_0^2}{\Sigma_0^2 + \Sigma_1^2} \\ \mu’ &= \mu_0 + K(\mu_1 - \mu_0) \\ \Sigma’^2 &= \Sigma_0^2 - K \Sigma_0^2 \end{aligned} $$

This matrix $K$ is the Kalman gain.

So we have the two following distributions:

And using the above, we compute their overlap to get a new best estimate:

$$ \begin{aligned} H_t \hat S_t’ &= H_t \hat S_t + K(z_t - H_t \hat S_t) \\ H_t \Sigma’_t H_t^T &= H_t \Sigma_t H_t^T - K H_t \Sigma_t H_t^T \\ K &= \frac{H_t \Sigma_t H_t^T}{H_t \Sigma_t H_t^T + R_t} \end{aligned} $$

Simplifying a bit, we get:

$$ \begin{aligned} \hat S_t’ &= \hat S_t + K’(z_t - H_t \hat S_t) \\ \Sigma’_t &= \Sigma_t - K’ H_t \Sigma_t \\ K &= \frac{\Sigma_t H_t^T}{H_t \Sigma_t H_t^T + R_t} \end{aligned} $$

Which are the equations for the update step, which gives us the new best estimate $\hat \Sigma’$.

Kalman filters work for modeling linear systems; for nonlinear systems you instead need to use the extended Kalman filter.