## SITUATION REPORT 3 (31/10/2022)
# How we can introduce a Rocket Model to a Pixhawk based board
## Introduction
This report aims to study the viability of using a *Pixhawk* board as the central controller of a model rocket. The system must be able to capture relevant data from sensors, send telemetry information to a ground station, store logs in persistent memory, and trigger parachute systems.
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## High level perspective of a Flight Stack
In order to understand the requirements of the model rocket system, I'll present an overview of a typical flight system for drones, since the architecture is very similar.
Fig. 1 - System Overview
In the image above, to translate the system to the context of a rocket, the only component that differs is the **Drone Hardware**. This component represents the elementary drone components, such as the battery, the motors, and the GPS. Thankfully, some of these elementary components are common to both drones and rockets.
### Drone Section
Now focusing in the Drone section:
Fig. 2 - Drone Overview
As we can see, above the Drone hardware is the *Flight Controller Hardware*. This is the responsability of the **Pixhawk Board**, which is the object of study. Focusing on the *Pixhawk*, the sensors and components from the layer below (GPS, telemtretry modules, etc.) hook into it, forming an essential **hardware hub**. This hub is then used by the *Software layer* to control the drones' basic components.
Regarding the top layer, the most common *Flight Controller Software* are:
1. *Ardupilot*
- Oldest software
- There are more examples using this software
- Operates under the GPL license, i.e any changes made to the source code are required to be pushed back to the master branch
2. *PX4*
- Created based on *Ardupilot*
- Feature-wise they seem to equivalent
- Official documentation available.
- Operates under the BSD license, i.e changes made to the source code don't need to be updated to the master branch
For the purpose of our system, any of the two softwares should work. Based on the official docs, I personally prefer the *PX4* organization.
### Simulation
Thankfully, it is possible to test the *Flight Controller Software* without acquiring the hardware, through *STL (Software In The Loop)*.
Fig. 3 - STL Simulation
### Ground Control Station
Now focusing in the Ground Control Station:
Fig. 4 - Ground Control Station
In terms of hardware, the ground station just needs a telemetry module, identical to the usb module represented in the figure, that will communicate with the antenna in the rocket, using the *MAVLINK* protocol.
In the software layer, resides the user interfaces. The softwares represented here are essentially GUIs that utilize the *MAVLINK* protocol. Namely, *QGroundControl*, *APM Planner* and *Mission Planner*. The first 2 are for linux, while the last one is exclusively for Windows.
Additionally, this layer also contains packages that can control vehicles, with the *MAVLINK* protocol through a coding language. In another words, these packages allow you to treat *Ardupilot* and *PX4* as an API to control the vehicle. Here we have represented the package **DroneKit**.
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## PX4 Deep Dive
As mentioned above, PX4 is one of the most popular *Flight Controller Software*. For that reason, we will explore it in more detail since its supported features are very similar to *Ardupilot* and the documentation is easier to follow.
### Supported Vehicles
Although PX4 supports a high variety of vehicles, model rockets are not part of that list. Nonetheless, I believe that is not an issue since this list is targeted for the usage of the software as a *Professional Autopilot*, that is, controlling the vehicle's trajectory and moving it as desired. In our use case, that feature isn't required so it should be fine as long as the features mentioned in the [Introduction](##Introduction) are supported.
### System Architecture
The *PX4* documentation provides a high level overview of a typical PX4 system based around a flight controller.
Fig. 5 - PX4 System Architecture
Note that this architecture is similar to what we wish to achieve, as it contains most of the sensors required for the model rocket. The missing pieces in the diagram are the [*parachute triggering*](https://docs.px4.io/main/en/peripherals/parachute.html) and [*persistent storage*](https://docs.px4.io/main/en/dev_log/logging.html), which are also supported and documented.
### Flight Controllers
There are many *Pixhawk* boards to choose from, such as the [Pixhawk 4](https://docs.px4.io/main/en/flight_controller/pixhawk4.html). The comparison and analysis of the most suitable board should be made in colaboration with the *Eletrical Engineering Department*.
### Supported Sensors
It is confirmed that the Pixhawk + PX4 combination supports the sensors and radio systems we are looking for. Anyhow, there's a lot of options to choose from and, once again, there needs to be contact with the *Eletrical Engineering Department* to reach a consensus.
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## Conclusion and Next Steps
After analyzing this information, *Pixhawk* seems to be a promising board to introduce in our model rocket. However, the current information isn't enough to reach a conclusion. To do so, we must convert this theoretical information into a practical example. Thankfully, the *Software in the Loop* solution mentioned in this report provides a way to simulate the *Pixhawk* and the rocket model. Thus, the next steps involve testing this *PX4* and *Ardupilot* software, in order to ensure they fit our needs and are capable of adapting to our scenario.
## References
\[1\] [Drone Programming Course](https://www.youtube.com/playlist?list=PLgiealSjeVyx3t4N9GroE29SbVwhYrOtL)
\[2\] [PX4 User Guide](https://docs.px4.io/main/en/)
\[3\] [Ardupilot](https://ardupilot.org/)
Published by João Mesquita