Add information about dynamic estimators.

docs/source/filters.rst

@@ -13,33 +13,43 @@ For example, importing the QUEST estimator is simply done with:
    >>> from ahrs.filters import QUEST
 
 Most estimators are built to be working with signals of low-cost strapdown
-navigation systems. Three types of sensors are mainly used to this purpose:
+navigation systems. Three types of sensors are mainly used for this purpose:
 
 - **Gyroscopes** measure the angular velocity.
 - **Accelerometers** measure the acceleration (rate of change of velocity).
-- **Magnetometers** measure the geomagnetic field.
+- **Magnetometers** measure the local magnetic field.
 
 Gyroscopes, provide good short-term reliability and resistance to vibration,
 accelerometers provide information that is reliable over time, and
-magnetometers provide heading information in addition to limited attitude
+magnetometers provide heading information in addition to the limited attitude
 information (pitch and roll).
 
 For an attitude estimation we encounter two common strategies:
 
 - **Instantaneous estimation** calculates the attitude using vectors in two
-  frames (a body frame and a known reference frame). It finds the attitude in a
-  single point in time, whitout necessarily considering the kinematics of the
+  frames (a body frame and a known reference frame). It finds the attitude at a
+  single point in time, without necessarily considering the kinematics of the
   objective. Ideally, this works with a system in a quasi-static state. Thus,
   this estimation is sometimes called **Static Attitude Determination**.
 - **Recursive estimation** not only uses vectorial observations, but also takes
   the system dynamics into account to capture and predict the behaviour of the
   system. Because the system kinematics are considered, these type of strategy
   is also called **Dynamic Attitude Determination**.
 
-The most accurate estimators are the dynamic ones, but the are, generally, more
-computationally demanding, against the much simpler and faster static
+The most accurate estimators are the dynamic ones, but they are, generally,
+more computationally demanding, against the much simpler and faster static
 estimators.
 
+Dynamic estimators mainly use angular motions to compute the attitude. These
+displacements are normally measured by gyroscopes in a
+`strapdown <https://en.wikipedia.org/wiki/Inertial_navigation_system#Strapdown_systems)>`_
+system, where the sensors are rigidly attached to the body's frame.
+
+The gyroscopes capture the angular velocity of such body, which is then
+integrated over time and added to the previous estimations to continuously
+obtain a new estimation after every sample. For them to work, however, an
+initial orientation has to be known, so that it can *grow* on top of it.
+
 The following algorithms are implemented in this package:
 
 =============  =========  =============  ============