added a basic modes and health chapter

satrs-book/src/actions.md

@@ -29,7 +29,7 @@ There are 3 requirements for the PUS 8 telecommand:
 2. Bytes 0 to 4 of application data must contain the target ID in `u32` big endian format.
 3. Bytes 4 to 8 of application data must contain the action ID in `u32` big endian format.
 4. The rest of the application data are assumed to be command specific additional parameters. They
-   will be added to an IPC store, the the corresponding store address will be sent as part of the
+   will be added to an IPC store and the corresponding store address will be sent as part of the
    `ActionRequest`.
 
 ## Sending back telemetry

satrs-book/src/modes-and-health.md

@@ -0,0 +1,102 @@
+# Modes
+
+Modes are an extremely useful concept for complex system in general. They also allow simplified
+system reasoning for both system operators and OBSW developers. They model the behaviour of a
+component and also provide observability of a system. A few examples of how to model
+different components of a space system with modes will be given.
+
+## Modelling a pyhsical devices with modes
+
+The following simple mode scheme with the following three mode
+
+- `OFF`
+- `ON`
+- `NORMAL`
+
+can be applied to a large number of simpler devices of a remote system, for example sensors.
+
+1. `OFF` means that a device is physically switched off, and the corresponding software component
+does not poll the device regularly.
+2. `ON` means that a device is pyhsically switched on, but the device is not polled perically.
+3. `NORMAL` means that a device is powered on and polled periodically.
+
+If a devices is `OFF`, the device handler will deny commands which include physical communication
+with the connected devices. In `NORMAL` mode, it will autonomously perform periodic polling
+of a connected physical device in addition to handling remote commands by the operator.
+Using these three basic modes, there are two important transitions which need to be taken care of
+for the majority of devices:
+
+1. `OFF` to `ON` or `NORMAL`: The device first needs to be powered on. After that, the
+   device initial startup configuration must be performed.
+2. `NORMAL` or `ON` to `OFF`: Any important shutdown configuration or handling must be performed
+   before powering off the device.
+
+## Modelling a controller with modes
+
+Controller components are not modelling physical devices, but a mode scheme is still the best
+way to model most of these components.
+
+For example, a hypothetical attitude controller might have the following modes:
+
+- `SAFE`
+- `TARGET IDLE`
+- `TARGET POINTING GROUND`
+- `TARGET POINTING NADIR`
+
+We can also introduce the concept of submodes: The `SAFE` mode can for example have a
+`DEFAULT` submode and a `DETUMBLE` submode.
+
+## Achieving system observability with modes
+
+If a system component has a mode in some shape or form, this mode should be observable. This means
+that the operator can also retrieve the mode for a particular component. This is especially
+important if these components can change their mode autonomously.
+
+If a component is able to change its mode autonomously, this is also something which is relevant
+information for the operator or for other software components. This means that a component
+should also be able to announce its mode.
+
+This concept becomes especially important when applying the mode concept on the whole
+system level. This will also be explained in detail in a dedicated chapter, but the basic idea
+is to model the whole system as a tree where each node has a mode. A new capability is added now:
+A component can announce its mode recursively. This means that the component will announce its
+own mode first before announcing the mode of all its children. Using a scheme like this, the mode
+of the whole system can be retrieved using only one command. The same concept can also be used
+for commanding the whole system, which will be explained in more detail in the dedicated systems
+modelling chapter.
+
+In summary, a component which has modes has to expose the following 4 capabilities:
+
+1. Set a mode
+2. Read the mode
+3. Announce the mode
+4. Announce the mode recursively
+
+## Using ECSS PUS to perform mode commanding
+
+# Health
+
+Health is an important concept for systems and components which might fail.
+Oftentimes, the health is tied to the mode of a system component in some shape or form, and
+determines whether a system component is usable. Health is also an extremely useful concept
+to simplify the Fault Detection, Isolation and Recovery (FDIR) concept of a system.
+
+The following health states are based on the ones used inside the FSFW and are enough to model most
+use-cases:
+
+- `HEALTHY`
+- `FAULTY`
+- `NEEDS RECOVERY`
+- `EXTERNAL CONTROL`
+
+1. `HEALTHY` means that a component is working nominally, and can perform its task without any issues.
+2. `FAULTY` means that a component does not work properly. This might also impact other system
+components, so the passivation and isolation of that component is desirable for FDIR purposes.
+3. `NEEDS RECOVERY` is used to attempt a recovery of a component. For example, a simple sensor
+could be power-cycled if there were multiple communication issues in the last time.
+4. `EXTERNAL CONTROL` is used to isolate an individual component from the rest of the system. For
+   example, on operator might be interested in testing a component in isolation, and the interference
+   of the system is not desired. In that case, the `EXTERNAL CONTROL` health state might be used
+   to prevent mode commands from the system while allowing external mode commands.
+
+