THHOR-BOLT REXUS 37 is a technology demonstration of LiFi (Light Fidelity) communication for intra-satellite applications. The mission flies three sub-experiments on the REXUS sounding rocket to characterise optical communication under realistic flight conditions and to validate the technology for upcoming projects, including the THHOR-HANS satellite mission.
Satellites contain many internal components that need to communicate with each other. These components are typically connected via wiring harnesses, which can account for 7 to 10 percent of the total satellite weight. The complex harness also has an adverse effect on the flexibility of the Assembly, Integration, and Test (AIT) processes, causing significant time and cost overheads.
LiFi offers an alternative: optical wireless communication for intra-satellite data transfer. Compared to traditional radio-frequency communication, LiFi provides higher bandwidth potential, eliminates electromagnetic interference between subsystems, and reduces susceptibility to eavesdropping due to signal confinement within structural walls.
Existing LiFi research has primarily been conducted in laboratory conditions. THHOR-BOLT extends this work by testing LiFi under realistic flight conditions including launch vibration, atmospheric variation, vacuum exposure, rapidly changing thermal conditions, and microgravity.
The mission has two main goals:
Technical Demonstration. Provide a technical demonstration for LiFi technology in a space context. Measured precision, signal transfer, and power transfer of the tested modules will be compared to commonly used wired data transfer.
Survivability Validation. Test the survivability of LiFi modules under launch and space conditions. Learned lessons and design decisions will be incorporated into upcoming projects like the THHOR-HANS satellite mission.
THHOR-BOLT consists of three independent sub-experiments, each housed in its own light-tight enclosure to prevent cross-experiment interference.
EXP1 characterises how atmospheric conditions and flight dynamics affect optical link quality across the spectrum.
An LED array containing RGB, white, infrared, and ultraviolet LEDs emits defined light patterns through an enclosed tube. On the opposite side, an AS7265X 18-channel spectrometer measures the received light. The system cycles through different LED states, brightness levels, and spectrometer integration times to characterise the optical channel under varying conditions.
Simultaneously, atmospheric conditions (pressure, temperature) are recorded to correlate environmental changes with spectral measurements.
Software role: Drive LED drivers via I2C, control spectrometer integration cycles, coordinate measurement timing across multiple LED states, and correlate spectral data with environmental sensors.
EXP2 measures the bit error rate (BER) of an optical link disturbed by free-floating particles.
Two LiFi transceivers face each other inside a sealed box. The transmitter sends a known PRBS-7 bit pattern via modulated light, the receiver decodes the signal, and the bit error rate is computed by comparing received against expected bits.
During microgravity, silicone particles inside the box float into the optical path between transmitter and receiver. The particles simulate loose or damaged components that may float through optical communication paths in real satellites. Two cameras record particle movement for post-flight correlation with measured signal degradation.
Software role: Coordinate LiFi transmitter and receiver via two separate UARTs, manage PRBS-7 bit pattern generation, perform per-tick BER computation, sweep through different LiFi data rates, and log full bit streams for post-flight analysis.
EXP3 demonstrates wireless power transfer combined with LiFi communication for distributed sensors.
Two magnetometer stacks operate in parallel: a Wired Stack receives power and data via cable, while a Wireless Stack receives power optically through high-power LEDs onto a solar cell with a supercapacitor as energy storage, and communicates exclusively via LiFi. Both stacks measure the magnetic field simultaneously.
The science goal is to compare measurements from both stacks. Magnetometers are particularly susceptible to electromagnetic interference from power supply currents, so we expect the wireless stack to provide more accurate readings free from cable-induced electromagnetic interference.
Software role: Coordinate the duty-cycled LED charging timeline, manage two LiFi transceivers with different latencies, synchronise sample streams from wired and wireless stacks, drive the laser/LED driver via PWM, and perform cable-LiFi cross-validation.
| Number | Objective |
|---|---|
| Obj. 1 | Develop three sub-experiments to test intra-satellite LiFi communications in an environment close to a satellite's life-cycle |
| Obj. 2 | Test the survivability of the module under launch and space conditions |
| Obj. 3 | Measure the perceived deviation of the RGB colour spectrum under changing atmospheric conditions and rocket launch conditions |
| Obj. 4 | Measure environmental conditions for later data analysis and data correlation |
| Obj. 5 | Measure error when a LiFi signal is interfered with by floating particles |
| Obj. 6 | Test feasibility of an electrically isolated sensor under space conditions |
| Obj. 7 | Gather data on and validate the usage of LiFi technology in space conditions |
Four STM32 flight computers -- one BTC master plus three experiment controllers -- coordinated over CAN and downlinked over RS-422 through the REXUS Service Module.
For the diagram, class hierarchy, communication interfaces, and the rest of the software architecture, see architecture.md.
THHOR-BOLT is software-heavy by aerospace standards. The flight software:
- Coordinates four flight computers with deterministic timing requirements
- Manages six LiFi transceiver co-processors with experiment-specific protocols
- Maintains scientific data integrity across mid-flight reboots
- Distinguishes valid measurements from sensor failures explicitly per packet
- Provides survivability guarantees for the local microSD storage
These properties make the mission a strong demonstration of how student-developed aerospace software can meet professional verification standards.
- Architecture -- Flight software architecture for this mission
- Architecture Decision Records -- Engineering decisions
- Interface Control Documents -- Interface specifications