Laser Communication Between a UAV and the Ground
Successful implementation of a modulating retro-reflector link requires the integration of the device onto a platform as well as the ability to close a link while in flight. As a first step in developing an operational modulating retro-reflector communications link, NRL conducted two field tests to date with follow-on tests planned (Ref. 2). The aim of these tests was to demonstrate a short-range link to a platform that carried no active pointing system and indeed had relatively low platform stability. The tests were conducted in the fall of 1999 and in the winter of 2000 at the NRL Chesapeake Bay Detachment facility in Maryland.
A 0.5 cm diameter InGaAs transmissive MQW modulating retro-reflector was mounted on a small rotary-wing unmanned airborne vehicle . The modulating retro-reflector was placed on the tail of the UAV pointing down. Also mounted on the UAV were a camera, microprocessor, frame grabber and electrical drive circuitry for the modulator. The microprocessor could be programmed to send a pseudo-random bit stream at different bit rates ranging from 400 Kbps to 2 Mbps to the modulating retro-reflector. The UAV was flown at an altitude of about 35 meters and a range of 35 to 65 meters from the transmit/receive laser. The conditions for the test were somewhat adverse with a light rain, fog and low visibility. The second test was conducted with snow cover and in icy outdoor conditions. Nonetheless, due to the short range and the infrared wavelength of the laser no atmospheric effects were observed. Figure 1 shows the UAV, which is about 1-1/2 meter long.
The modulator used in the field tests was a monolithic 75 period InGaAs/AlGaAs MQW with an exciton resonance at 981 nm. The modulator was affixed to a mount centered above a corner-cube retro-reflector. Wire bonds to the p and n contact layers on the modulator were used to bias the modulator. The modulator/retro-reflector assembly, shown in Figure 2, was also ringed by infrared LEDs that were used to provide a beacon for acquisition and tracking of the UAV in the first test. The six element array was populated with 880 nm LEDs. Light was emitted from this array at a half angle of 15 degrees and served as a beacon for the tracking camera.
In addition to the modulator, the UAV carried video and driving electronics. The challenge in the design of the payload electronics was to design a lightweight but flexible payload that could meet the unique requirements of our demonstration. The payload had to survive vibration, oil mist, shock, fog, mist, and icy conditions.
Future tests will demonstrate higher data rates and video transmission from the UAV. Several modifications to the payload and Tx/Rx designs are being made to effect these goals. First, the payload will be replaced by an array of modulating retroreflectors. A lightweight holder (10 grams) with space-qualified heritage was designed and fabricated at NRL. This mass includes the retro-reflector, holder, modulator, cover plate, and wiring leads. A photo of the new mount is shown in Figure 3. The holders will be populated and arranged into a lightweight array that will replace the ring of LEDs used in the initial tests.
This modification to the payload will enable self-tracking from the UAV and will increase the FOV to 60 degrees. The larger FOV will further relax onboard pointing and tracking requirements. It should be noted that tracking and acquisition functions will be consolidated onto a single laptop in future tests but were separated initially to develop and test control algorithms that had to function in parallel in real-time.