SAFE-T SDR Design Micro-g (SEMAPHITES)

Accepted SAFE-T design for the 2024-2025 NASA Micro-g Challenge.

Github:
https://github.com/LiamUV/SAFE-T

IV. Hardware/Software Design

i. The SEMAPHITES team’s system is a portable, rugged, and autonomous RF signal detection platform designed to support post-landing recovery operations for the Orion spacecraft. The system is built around a Raspberry Pi 5, which serves as the central processing unit, and a PLUTO+ SDR that monitors two critical frequencies: 121.65 MHz for homing beacons and 406.025 MHz for SARSAT emergency signals. The SDR is connected to external antennas mounted on a commercial tripod and linked via coaxial cables to maintain signal integrity. The entire system is housed within a Pelican case, which contains the Raspberry Pi, SDR, a PCB with an LED and throw switch, and a rechargeable battery. A 7-inch touchscreen display is mounted on the inner lid to provide real-time data visualization and signal decoding feedback. Python3 software running on the Raspberry Pi performs signal demodulation, decoding, and analysis. It displays decoded beacon data, including the country code, hex ID, encoded location, and timestamp on the GUI. The system also controls an LED indicator for immediate visual signal detection feedback. Additionally, the software generates a KML file to log detection events with geographic coordinates for post-mission analysis using Google Earth. The design prioritizes simplicity, portability, and durability, enabling rapid deployment in remote or aquatic recovery environments. Its modular layout supports easy hardware replacement and future software upgrades, making it scalable for other SAR and space applications.

ii. The SEMAPHITES SAFE-T system incorporates commercially available, off-the-shelf components that are compact, rugged, and optimized for portability and splashproof field operation. The system is housed in a Pelican 1400 case, protecting internal electronics and enabling reliable poolside testing.

Key Hardware Components and Interfaces:

Antennas (2 Units):
Two antennas are used to detect RF signals: one tuned to 121.65 MHz for homing beacon signals and one to 406.025 MHz for SARSAT emergency signals. The antennas are mounted externally on a lightweight 60-inch tripod for optimal signal reception and elevation adjustment.

Coaxial Cables (2 Units):
Shielded coaxial cables connect each antenna to its corresponding RX port in the PLUTO+ SDR inside the Pelican case, preserving signal quality during transmission.

PLUTO+ SDR:
The Software Defined Radio dual channel, capable of intercepting both frequencies. The SDR converts analog RF signals from the antennas into digital signals and transmits them to the Raspberry Pi 5 via USB. It serves as the primary RF front-end of the system.

Raspberry Pi 5 (4 GB):
The Raspberry Pi is the central processing unit of the system. It runs Python3 software to decode incoming signals, drive the GUI, manage GPIO-based LED indicators, and generate historical KML files. It receives digital signal data via USB connections from the SDR and interfaces with external display and control components.

7-Inch LCD Touchscreen Display:
This screen is mounted inside the Pelican case lid and displays real-time signal data, including beacon Hex ID, country code, encoded location, and timestamp. It provides an intuitive GUI for user interaction.

PCB LED and Slide Switch:
A custom PCB includes a slide switch for system activation and an LED connected to a Raspberry Pi GPIO pin. The LED provides immediate visual feedback when a 121.65 MHz signal is detected.

Rechargeable Battery Pack:
A portable charger powers all components inside the Pelican case. It ensures the system can operate independently in field settings without external power sources.

Tripod:
The tripod provides a stable base for the antennas and is essential for aligning them to achieve maximum signal reception performance.

Supporting Components:
Wiring to connect components to the Pi
Resistors to regulate LED power
3D-printed Raspberry Pi case for secure internal mounting
Plastic brackets to mount antennas to the tripod

Component Interfacing Summary:
RF Path: Antennas → Coaxial Cables → SDR
Data Path: SDR → USB → Raspberry Pi
User Interface Path: Raspberry Pi → Display (HDMI/GPIO), LED (GPIO), Python3 GUI
Power Path: Battery → Raspberry Pi, SDR, Display
Control Path: Slide Switch → PCB → Raspberry Pi (to trigger Python3 code)
The modular architecture ensures easy replacement, testing, and expansion, while the robust interconnections between RF hardware, processing unit, and visual feedback systems allow for real-time operation with minimal setup time.