Add doc for tracker plugin interface

Author: novakfi8

docs/45-plugin-interface/02-trackers.md

@@ -0,0 +1,70 @@
+---
+title: Tracker plugins
+pagination_label: API for the tracker plugins
+description: API for the tracker plugins
+---
+
+# The Tracker plugin interface
+
+
+The Tracker plugin receives the high-level references from a Mission \& navigation software and provides the UAV state reference to the [Feedback controller](https://ctu-mrs.github.io/docs/features/controllers/) that ensure the UAV follows the reference.
+In general, the high-level referene received from the Mission \& navigation software can be
+
+* a single position and heading reference,
+* a trajectory, consisting of several position and heading references separated by a fixed time constant.
+ 
+Diagram below shows the data architecture of the MRS system where the block containing the Tracker plugin is marked in red.
+
+The tracker plugin is compiled as [ROS plugins](http://wiki.ros.org/pluginlib) with the [interface](https://github.com/ctu-mrs/mrs_uav_managers/blob/master/include/mrs_uav_managers/tracker.h) defined by the [control manager](https://github.com/ctu-mrs/mrs_uav_managers).
+A tracker plugin from any ROS package can be loaded dynamically by the [control manager](https://github.com/ctu-mrs/mrs_uav_managers) without it being present during [control manager](https://github.com/ctu-mrs/mrs_uav_managers)'s compile time.
+Loaded trackers can be switched by the [control manager](https://github.com/ctu-mrs/mrs_uav_managers) in mid-flight, which allows safe testing of new trackers and adds flexibility the [MRS UAV system](https://github.com/ctu-mrs/mrs_uav_system).
+![](./fig/diagram_of_system_architecture.jpg)
+
+## Tracker plugin - outputs 
+The tracker plugin can output the combination following references:
+
+  * Acceleration 
+  * Jerk 
+  * Snap 
+  * Attitude rate
+  * Heading 
+  * Heading rate
+  * Heading acceleration
+  * Heading jerk
+  * Orientation 
+  * Throttle 
+  * Full state prediction 
+  
+In the tracker plugin source code, you can decide which references should be considered by the [Feedback controller](https://ctu-mrs.github.io/docs/features/controllers/).
+If you develop your own [controller plugin](https://ctu-mrs.github.io/docs/plugin-interface/controllers/), ensure that your controller supports the required tracker references.
+
+## Example tracker plugin 
+
+An example of a cutom tracker plugin can be found at [this link](https://github.com/ctu-mrs/mrs_core_examples/tree/master/cpp/tracker_plugin).
+It is highly reccomended to base your tracker plugin on this example.
+In the example, there is also a [tmux folder](https://github.com/ctu-mrs/mrs_core_examples/tree/master/cpp/tracker_plugin/tmux) containing a script that starts the simulation of a UAV in the [MRS multirotor simulator](https://github.com/ctu-mrs/mrs_multirotor_simulator), where the UAV uses the [example tracker plugin](https://github.com/ctu-mrs/mrs_core_examples/tree/master/cpp/tracker_plugin).
+You have to install [MRS UAV system](https://github.com/ctu-mrs/mrs_uav_system) first to run the example.
+To load the tracker plugin into the [MRS UAV system](https://github.com/ctu-mrs/mrs_uav_system), you need to provide a custom config file to the [MRS UAV Core](https://github.com/ctu-mrs/mrs_uav_core) containing the parameters for the tracker plugin.
+The custom config file should contain the following lines (taken from [custom config file of example tracker plugin](https://github.com/ctu-mrs/mrs_core_examples/blob/master/cpp/tracker_plugin/tmux/config/custom_config.yaml)):
+```yaml 
+mrs_uav_managers:
+
+  control_manager:
+
+    ExampleTracker:
+      address: "example_tracker_plugin/ExampleTracker"
+      namespace: "example_tracker"
+      human_switchable: true
+
+    # list of names of dynamically loaded trackers
+    trackers : [
+      "ExampleTracker",
+    ]
+```
+
+### Parameter description for tracker plugin
+
+```address: "example_tracker_plugin/ExampleTracker``` : Specifies the address of the plugin to be loaded into the interface as a tracker.  
+```namespace: "example_tracker"``` : Specifies the namespace of the tracker as per the package description in the plugin.  
+```human_switchable: true``` : Specifies whether the tracker can be switched by the user during flight.  
+