SpaceWire-based Data Acquisition Network for the Solar Flare Sounding Rocket Experiment FOXSI-4 and FOXSI-5

We developed a SpaceWire-based data acquisition (DAQ) system for the FOXSI-4 and FOXSI-5 sounding rocket experiments, which aim to observe solar flares with high sensitivity and dynamic range using direct X-ray focusing optics. The FOXSI-4 mission, launched on April 17, 2024, achieved the first direct focusing observation of a GOES M1.6 class solar flare with imaging spectroscopy capabilities in the soft and hard X-ray energy ranges, using a suite of advanced detectors, including two CMOS sensors, four CdTe double-sided strip detectors (CdTe-DSDs), and a Quad-Timepix3 detector. To accommodate the high photon flux from a solar flare and these diverse detector types, a modular DAQ network architecture was implemented based on SpaceWire and the Remote Memory Access Protocol (RMAP). This modular architecture enabled fast, reliable, and scalable communication among various onboard components, including detectors, readout boards, onboard computers, and telemetry systems. In addition, by standardizing the communication interface and modularizing each detector unit and its associated electronics, the architecture also supported distributed development among collaborating institutions, simplifying integration and reducing overall complexity. To realize this architecture, we developed FPGA-based readout boards (SPMU-001 and SPMU-002) that support SpaceWire communication for high-speed data transfer and flexible instrument control. In addition, a real-time ground support system was developed to handle telemetry and command operations during flight, enabling live monitoring and adaptive configuration of onboard instruments in response to the properties of the observed solar flare. The same architecture is being adopted for the upcoming FOXSI-5 mission, scheduled for launch in 2026.

💡 Research Summary

The paper presents the design, implementation, and flight‑tested performance of a SpaceWire‑based data acquisition (DAQ) network developed for the FOXSI‑4 sounding‑rocket mission and slated for reuse in the upcoming FOXSI‑5 mission (scheduled for 2026). FOXSI‑4 marked the first direct‑focusing observation of a GOES M1.6 solar flare, employing a heterogeneous suite of detectors: two soft‑X‑ray CMOS sensors, four hard‑X‑ray CdTe double‑sided strip detectors (CdTe‑DSDs), and a Quad‑Timepix3 detector. The high photon flux associated with flare observations, together with the need to support multiple detector types, drove the development of a modular, high‑speed, and low‑latency DAQ architecture.

Core to the architecture is the use of SpaceWire, a standardized high‑rate serial bus (≈100 Mbps) widely adopted by ESA, NASA, JAXA, and Roscosmos, combined with the Remote Memory Access Protocol (RMAP). RMAP enables direct read/write access to remote node memory over SpaceWire, allowing the flight computer to fetch or store data without involving a CPU on the remote node. This combination provides a uniform interface across all payload subsystems, reduces integration effort, and supports future scalability.

Two custom FPGA boards were created to implement the protocol stack and provide detector‑specific I/O. The SPMU‑001 board (Spartan‑7 FPGA, 128 MB DDR2‑SDRAM) hosts a ten‑port SpaceWire router (via an auxiliary SPMU‑001‑SpW daughterboard), an AXI‑RMAP bridge, and a SpaceWire‑to‑Ethernet bridge. It supplies power and a Raspberry Pi 4 header, allowing the Pi to act as the host processor while the FPGA handles all SpaceWire traffic and memory mapping. SPMU‑001 is used for each CdTe‑DSD module; detector ASIC configuration registers are written via RMAP, and raw event data are buffered in on‑board SDRAM before being handed to the Pi for storage on an SD card and for Quick‑Look (QL) telemetry.

The SPMU‑002 board (Zynq UltraScale+ MPSoC) serves the two CMOS sensors. It provides high‑throughput data capture onto an onboard SSD, while a subset of the data is mirrored to SDRAM for real‑time QL telemetry. Like SPMU‑001, it presents a SpaceWire interface so that the Formatter can pull data using RMAP.

The Timepix3 detector uses a distinct approach: an AMD Kintex‑7 FPGA performs event packetization and writes data to a local PCAP file on a Raspberry Pi 3B+. Telemetry and command exchange with the rest of the payload are carried out over a simple UART link rather than SpaceWire, a decision made to minimise development risk because the Timepix3 readout system already existed and had not yet earned flight heritage.

Timing synchronization across all subsystems is achieved with a 1 Hz pulse‑per‑second (PPS) signal derived from the rocket’s GPS. During FOXSI‑4 only the CdTe‑DE correctly logged PPS timestamps; the issue was resolved for FOXSI‑5, where every board will record absolute UTC timestamps aligned to the 64 Hz SpaceWire timecode, enabling precise post‑flight alignment of multi‑instrument data.

The ground segment consists of a Formatter board that aggregates QL data from all detectors in pull mode, forwards it to a ground support equipment (GSE) computer via an Ethernet‑via‑Telemetry (EVTM) link limited to ~20 Mbps, and receives uplink commands over a 1.2 kbps UART channel. The GSE software provides live monitoring of light curves, spectra, and housekeeping, and allows operators to adjust detector bias, readout modes, and other parameters in real time, adapting to the evolving flare intensity.

Flight performance demonstrated that the system could sustain the high count rates expected from an M‑class flare without data loss, while maintaining deterministic latency for command execution. The modular design permitted distributed development: institutions in Japan, the United States, and elsewhere supplied detectors, readout electronics, and software independently, then integrated them through the common SpaceWire/RMAP interface.

The authors conclude that the architecture meets the stringent requirements of high‑flux solar flare observations, offers a clear path for scaling to additional detectors or higher data rates, and will be directly reused for FOXSI‑5. Future work includes expanding the SpaceWire router capacity, further optimizing RMAP transaction handling, and fully validating the PPS‑based absolute timing across all subsystems before the 2026 launch.