MIT Lincoln Laboratory demonstrates reliable air-to-ground laser communications

A seven-month rapid development project of the Advanced Lasercom Systems and Operations Group at MIT Lincoln Laboratory culminated in the successful demonstration of a multi-gigabit-per-second air-to-ground optical data link. Communication to a ground station was achieved over nearly horizontal links at aircraft ranges out to 60 kilometers, in spite of severe atmospheric channel disturbances. By implementing coding and interleaving on the link and by employing a receiver with spatial diversity, the research team attained error-free data transfers of 100-gigabyte imagery files. This achievement opens a new path for high-bandwidth connectivity between airborne sensor platforms and the end users of sensor data. 

Aircraft beam directorThe aircraft beam director used in the demonstration is a commercial product enhanced with an inertial measurement unit, an optical tracking system, and a fiber coupler for the transmit and receive paths.

The data link was conceived as a critical step toward transmitting high-resolution persistent surveillance imagery data in near real time. Without this link, airborne imagery data must be stored on board until landing or transmitted over radio-frequency links in a highly compressed form. 

Although a highly reliable fiber-optic-based communications infrastructure is widely deployed for terrestrial links, laser communications (lasercom) in free space are hindered by numerous impediments. Precise pointing of narrow optical beams is a challenge, especially when one terminal is moving rapidly relative to the other. When the optical beam passes through the air, turbulence from atmospheric thermal fluctuations can spread and distort the laser beam into a complex pattern that presents rapidly varying bright and dark spots to the receiver. These fluctuations (or "scintillation") can cause received power to vary by a factor of 10,000. In addition, when one of the terminals is on an aircraft, turbulence from the flow of air around aircraft structures can also steer or disrupt the optical beam. The power fades and beam distortion induced by the free-space channel must be addressed by the lasercom system design to ensure good beam tracking and quality of service of the data transfer.

Twin Otter aircraftThe Twin Otter aircraft was used as the laser communications terminal platform. The transmit and receive beams passed through the circular optical window in the left aft door.

For this demonstration, the flight tests consisted primarily of flights along arcs of constant distance from a ground station atop a Lincoln Laboratory building in Lexington, Massachusetts. The aircraft platform used in the demonstration was a DeHavilland Twin Otter, selected for its versatility, affordability, and availability. The transmit and receive optical beams passed through the aircraft's 66-centimeter-diameter fused-silica window in the aft fuselage.

The flight tests assessed the terminals' pointing, acquisition, tracking, and communications performance. Communication links were established during both day and night, in atmospheric conditions ranging from strongly turbulent in the early afternoon to benign in the early evening. Communications at ranges from 15 to 60 kilometers were evaluated. The communications testing included the transmission of communications test signals and 100-gigabyte files of stored imagery. This stored imagery data set was a stand-in for a future live stream of sensor data.

False-color infrared imageA false-color infrared image of the intensity of the beam from the aircraft, when viewed on a temporary reflective card at the ground terminal. Dark red represents a strong intensity; light greens and blues represent weaker signals. (The four circles are the receiver apertures, where holes were cut out of the reflective card.) At the particular instant illustrated, most of the summed energy arrives through the aperture at the lower left.

To assist the aircraft in pointing its downlink beam accurately toward the ensemble of four ground receivers, each ground receiver transmitted a weak (10 mW) laser toward the aircraft. This cooperative tracking scheme was initiated by the aircraft's pointing a temporarily widened beam toward the known position of the ground station. The widened beam allowed the ground station to locate the aircraft rapidly and accurately, enabling transition to fine tracking and communications. Other acquisition schemes are possible, but this one required no radio signaling.

The aircraft terminal included a laser transmitter steered through a commercial beam director with a 25-millimeter-aperture, along with an integral beacon receiver for tracking the optical beams from the ground. The aircraft's laser transmitter was an eye-safe laser, operating at 1550 nanometers, that was modulated by a test data stream. An onboard, custom-built data coder and interleaver embedded the "hooks" required in the data stream to allow the ground station to reconstruct bits missing because of severe short-duration channel fading.

The ground terminal consisted of a set of four optical receivers mounted in a rooftop dome, as well as communications, control, and telemetry equipment located in a nearby room. Each of the four receivers was fed by its own tiny, 12-milllimeter-diameter telescope that collected a sample of the light transmitted from the plane. The receivers were spaced so that each receiver would sample a different portion of the scintillating downlink beam. When the results were summed, deep fades caused by the atmosphere were reduced substantially.

Graph of received powerThe graph shows the received power at the ground receiver. Each ground receiver aperture (channel) receives the same average power, but with fast, deep fades occurring at different times. Because these fades are only weakly correlated, summing the four channels reduces the fading substantially. The received power is shown for each channel, along with the sum of four receiver signals.
 
Ground station receiver
In the ground station receiver front-end, the four receiver apertures in the black box sample weakly correlated portions of the downlink beam in order to reduce the impact of spatial intensity fluctuations.
The size and spacing of the apertures were chosen on the basis of simulations of the atmospheric scintillation. Outside the black box are an uplink beacon aperture to aid acquisition and tracking as well as an infrared camera for acquisition, diagnostics, and telemetry. For scale, the black box's face is 22 × 32 cm.

For deeper fades, for which the sum of the received signals fell below the communications threshold, coding and interleaving were employed to correct bursts of corrupted bits. Coding uses extra, overhead bits to detect and then correct an occasional corrupted bit. Interleaving is added to reorder the bits so that the bits corrupted by a single atmospheric fade are no longer adjacent to each other in time.

Lincoln Laboratory is planning future flight testing campaigns in which a persistent surveillance camera collects live imagery while the data are being continuously downlinked via the optical beam to a transportable ground station. From there, the data can be distributed through a ground network for processing and analysis. The Laboratory envisions additional enhancements that use a beam director external to the aircraft fuselage so that the transmitting laser can point over nearly 360 degrees of azimuth, allowing the aircraft to follow nearly arbitrary flight paths while still providing reliable links to one or more ground stations.

Posted June 2010

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