The HABET Electrical System (HES) defines the avionics used on HABET flights. It determines which hardware is certified for current flights, provides guidelines for future hardware, and provides the framework for component interactions when needed. Currently, HABET is using HESv5 and is developing HESv6.

HESv6 contains the following projects/sub-system boards with new boards in bold.

• High Altitude Report (HAR)
• Backup Emergency Recovery Transmitter (BERT)
• The High Output Regulator (THOR)
• Advanced Telemetry and High-Altitude Environmental Networking Assistant (ATHENA)
• Hermod - Backplane interconnection system

All HES components must meet minimum requirements to be flown on HABET flights, and they must be tested to ensure they can withstand these conditions. The list below is the current hardware components certified for HESv5.

• High Altitude Reporter (HAR)
• Backup Emergency Recovery Transponder (BERT)
• Payload Support Computer (PSC) codenamed Athena
• Spot Tracker (Spot is being phased out).

In HESv5, all subsystems have their own battery pack and supply. This results in heavier weights due to the multiple batteries needed. HESv6 moves power to a central location that can be better managed. HAR and BERT will still have a backup battery, but its size will be drastically reduced to save weight and space.

The HES provides the spacecraft with key functions necessary for a successful mission. The HES as a whole provides the following key functions:

• Power
• Data Downlink/Uplink
• Spacecraft Tracking Data
• Interfaces to key sensors and components.

Generally speaking, Power and some sort of data management are usually the bare minimum needed for most payloads. However, many payloads either benefit from or require the additional items listed. Therefore, Athena was designed with all of the items listed. As of the date of this document, the following features are currently provided.

HABET uses a dedicated onboard computer for all payloads. A Raspberry Pi Zero 2 W is used as the main computer. This computer provides power, logs data, and provides commands when needed. The onboard computer has a dedicated communication link for sending commands and monitoring data via a 70 cm-band LoRa radio. A USB OTG board provides USB connectivity via serial emulation and power. Finally, this system has its own dedicated power system independent of the main avionics flown on the spacecraft.

HABET uses Raspbian Lite as the base operating system. This version does not include a GUI to provide as many resources as possible for managing the payloads. We use Python to help log data and act as a controller when needed. The payload computer and operating system are not available to those flying payloads unless a special arrangement has been made in advance.

Power to payloads is provided through four USB-A ports. This means that HABET can support up to four payloads with this configuration. The USB port provides 5 VDC. The maximum current the USB OTG board can supply to all USB devices is 2 A. This means we may be unable to fly all four payloads if the combined payload weight exceeds this power limit. If the power drawn from the USB OTG port exceeds 2 A, power will be cut off via a non-resettable fuse to protect the board. All payloads are encouraged to use the minimum power required.

A 6600 mAh battery powers all payloads and the onboard computer. In most cases, this should result in run times of three hours or more. The payload system is hooked to external power up to the launch point. The Raspberry Pi computer has a dedicated battery system to power it.

Payloads may elect to provide additional power, subject to the following restrictions.

• Only primary cells are allowed.
• The power system is counted with the total mass of the payload.
• The power system must be isolated from the USB power input if used.
• The power system must be fuse-protected with a properly rated fuse.
• The power system must have an on/off switch that can be accessed from the outside of the spacecraft.

The HABET team recommends using Energizer Lithium AA or AAA batteries. These provide better cold-temperature performance and higher energy density than alkaline batteries. Rechargeable batteries, such as Li-Ion or Li-Poly, are not allowed.

External power is provided to the spacecraft before the balloon is filled, so operations checks can be performed. This external power is disconnected just before launch to ensure the onboard battery is fully charged. The HABET has calculated that the internal battery will operate for three or more hours with a full 10 watts of power from payloads and with the onboard computer drawing maximum power. As these are worst-case scenarios, we should expect operational uptime of 4 or more hours in most cases.

The internal battery does communicate the State of Charge (SOC) to the Linux operating system. At 5% SOC, the onboard computer will begin a shutdown sequence to ensure all data is saved and reduce the risk of corruption in onboard memory.

The onboard computer will scan for all serial ports and, once found, will determine the baud rate. The Raspberry Pi supports baud rates up to 115,200 bps. HABET recommends a baud rate of 9600 bps unless a higher one is required.

All data will then be logged into the internal memory. A file will be created for each payload (USB port). The data format is up to the PI. HABET recommends using a comma-delimited value (CSV) format, but it is not required. A request can be made during a flight to see the last line of data logged to that file.

HABET records and timestamps GPS data from the flight. All GPS data can be made available to anyone flying a payload. HABET will help with decoding the data, but otherwise does not provide services to sync it with the data in the payload.