Asymmetric Lasercom for Small Unmanned Aerial Systems

Introduction: As reliance on small unmanned aerial systems (UASs) expands, and sensors requiring higher-bandwidth downlinks are developed, the need for secure high-data-rate communications increases. Laser communication (lasercom), also known as free-space optical (FSO) communication, is inherently low-probability-of-interception and detection (LPI/ LPD). Lasercom has been previously demonstrated on manned aircraft; however, those systems require a high-precision pointing system and correspondingly high-accuracy navigation information hardware, both of which are prohibitively heavy for small UASs. The work described here opens up the use of lasercom to small UASs, which could not otherwise support lasercom terminals. NRL developed an alternative to conventional lasercom terminals and applied it to small UASs. The enabling technology in these new terminals is the modulating retro-reflector (MRR). NRL has actively developed MRRs since 1998, primarily for terrestrial and shipboard applications. An MRR architecture shifts most of the power, weight, and pointing requirements to one end of the link, allowing the other end to be extremely small, low-power, and require only rough pointing. Airborne Lasercom Transceiver Components: NRL's MRR transmitters and photoreceivers require only coarse pointing (±15°). An optical amplitude modulator is mounted in the path of a cornercube retro-reflector. The ground station's laser beam is retro-reflected back with data impressed on the beam. MRR transmitters and photoreceivers are installed in modified, low-cost, lightweight (65 g) camera gimbals to allow hemispherical coverage. Dakota Wingpod Lasercom System: The MRR transceiver gimbals, stabilized camera, and electronics are installed in a wingpod. Hardware providing GPS position, inertial sensing, and heading are included. An onboard processor maintains pointing to the laser ground station. An RF modem in the tail of one pod adds the capability to make hardware configuration changes while in flight for testing purposes. The final weights of the pods are 3.6 kg and 3.1 kg, including gimbaled lasercom transmitter and receiver, stabilized camera, video compressor/modem, navigation data sources, antennas, pod structure, and mounting hardware. The combined power draws and weights of the components required for communication are 6 W and 1 kg, and can readily be further reduced. The pods require only power and an optional GPS antenna feed from the Dakota. Figure 1 shows the Dakota with wingpods. Lasercom Ground Station: The lasercom ground station is based on the Dual Mode Optical Interrogator (DMOI) developed by NovaSol and NRL under the Office of Naval Research DMOI program. NRL extended the capabilities of the DMOI to allow it to track aircraft. …