ALIGN Mission: Radio Intersatellite Link Feasibility Demonstration for Hybrid Free-Space Optical Communications

The ALIGN (Autonomous Laser-based Intersatellite Gigabit Network) mission comprises a pair of 6U CubeSats equipped with FOCUS (Free-space Optical Communications Unit for Space), an experimental payload demonstrating high-data-rate optical intersatellite links in Low Earth Orbit (LEO) and advanced hybrid optical/radio intersatellite communications. Each satellite implements a dual RF-link architecture: the Radio Ground-to-Satellite Link (RGSL) for telemetry, tracking, and command, and the Radio Intersatellite Link (RISL) for direct satellite-to-satellite exchange. Both employ identical COTS-based designs with simplified interfaces.

Optical intersatellite links require strict pointing, acquisition, and tracking (PAT) accuracy due to extremely narrow beam divergence, presenting significant operational challenges. To address this, RF intersatellite capability provides essential support for optical link establishment. RF signals operate with wider beamwidths and less severe alignment requirements, enabling coarse acquisition and initial ranging. Following Space Development Agency standards, the RF subsystem performs distance verification, relative localization, and coarse alignment before initiating the optical PAT sequence. The dual S-band architecture operates three links: full-duplex ground at 2.085/2.245 GHz and half-duplex intersatellite at 2.0305 GHz, providing 54.5 MHz minimum separation. Implementation within the 6U platform creates co-site interference challenges addressed through orthogonal antenna placement: ground link patch antenna (RHCP) on +Z nadir face and ISL patch antenna (linear polarization) on +X forward face, providing 15 cm separation and 90-degree angular isolation.

Initial hardware feasibility assessment used two satellite mockups in an anechoic chamber positioned 2 meters apart. Testing confirmed successful bidirectional RISL communication with all RF links operating simultaneously, with no observable cross-talk. Results establish functional feasibility and validate key design decisions. Comprehensive quantitative characterisation is ongoing prior to launch. ALIGN aims to demonstrate practical hybrid optical/RF architectures for CubeSat constellations requiring high-throughput data transfer and reliable telemetry.