# Source code for stonesoup.updater.alphabeta

```
from functools import lru_cache
import numpy as np
from ..base import Property
from ..updater import Updater
from ..types.prediction import MeasurementPrediction
from ..types.update import Update
[docs]
class AlphaBetaUpdater(Updater):
r"""Conceptually, the :math:`\alpha-\beta` filter is similar to its Kalman cousins in that it
operates recursively over predict and update steps. It assumes that a state vector is
decomposable into quantities and the rates of change of those quantities. We refer to these as
position :math:`p` and velocity :math:`v` respectively, though they aren't confined to
locations in space. If the interval from :math:`t_{k-1} \rightarrow t_k` is :math:`\Delta T`,
and at :math:`k`, we can gain a (noisy) measurement of the position, :math:`p^z_k`.
The recursion proceeds as:
* Predict
.. math::
p_{k|k-1} &= p_{k-1} + \Delta T v_{k-1}
v_{k|k-1} &= v_{k-1}
* Update
.. math::
s_k &= p^z_k - p_{k|k-1} \: (\mathrm{innovation})
p_k &= p_{k|k-1} + \alpha s_k
v_k &= v_{k|k-1} + \frac{\beta}{\Delta T} s_k
The :math:`\alpha` and :math:`\beta` parameters which give the filter its name are small,
:math:`0 < \alpha < 1` and :math:`0 < \beta \leq 2`. Colloquially, the larger the values of the
parameters, the more influence the measurements have over the transition model; :math:`\beta`
is usually much smaller than :math:`\alpha`.
As the prediction is just the application of a constant velocity model, there is no
:math:`\alpha-\beta` predictor provided in Stone Soup. It is assumed that the predictions
passed to the hypothesis have been generated by a constant velocity model. Any application of a
control model is also assumed to have taken place during the prediction stage.
This class assumes the velocity is in units of the length per second. If different units are
required, scale the prior appropriately.
The measurement model used should be linear and a measurement model such that it provides a
'mapping' to :math:`p` via the :attr:`mapping` tuple and a binary measurement matrix which
returns :math:`p`. This isn't checked.
"""
alpha: float = Property(doc="The alpha parameter. Controls the weight given to the "
"measurements over the transition model.")
beta: float = Property(doc="The beta parameter. Controls the amount of variation allowed in "
"the velocity component.")
vmap: np.ndarray = Property(default=None, doc="Binary map of the velocity elements in the "
"state vector. If left default, the class will "
"assume that the velocity elements interleave "
"the position elements in the state vector.")
[docs]
@lru_cache()
def predict_measurement(self, prediction, measurement_model=None, **kwargs):
"""Return the predicted measurement
Parameters
----------
prediction : :class:`~.StatePrediction`
The state prediction
Returns
-------
: :class:`~.StateVector`
The predicted measurement
"""
# This necessary if predict_measurement called on its own
measurement_model = self._check_measurement_model(measurement_model)
pred_meas = measurement_model.matrix(**kwargs) @ prediction.state_vector
return MeasurementPrediction.from_state(prediction, pred_meas)
[docs]
def update(self, hypothesis, time_interval, **kwargs):
"""Calculate the inferred state following update
Parameters
----------
hypothesis : :class:`~.Hypothesis`
A hypothesis associates a measurement with a prediction
time_interval : :class:`~.timedelta`
The time interval over which the prediction has been made.
Returns
-------
: :class:`~.StateUpdate`
The updated state
"""
out_statevector = hypothesis.prediction.state_vector.copy()
# Check for the measurement_model in the measurement, if not present use the one in this
# updater
measurement_model = hypothesis.measurement.measurement_model
measurement_model = self._check_measurement_model(measurement_model)
# Check for a measurement prediction in the hypothesis
if hypothesis.measurement_prediction is None:
pred_meas = self.predict_measurement(hypothesis.prediction,
measurement_model=measurement_model)
else:
pred_meas = hypothesis.measurement_prediction
pmap = np.array(measurement_model.mapping)
if self.vmap is None:
vmap = pmap + 1
else:
vmap = self.vmap
innovation = hypothesis.measurement.state_vector - pred_meas.state_vector
out_statevector[pmap] = hypothesis.prediction.state_vector[pmap] + self.alpha * innovation
out_statevector[vmap] = hypothesis.prediction.state_vector[vmap] +\
(self.beta / time_interval.total_seconds()) * innovation
return Update.from_state(hypothesis.prediction, out_statevector,
timestamp=hypothesis.measurement.timestamp, hypothesis=hypothesis)
```