Inter-Spacecraft Optical Communication and Navigation Using Multiple Quantum Well Modulating Retroreflectors
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J.A. Vasquez,1 G.C. Gilbreath,2 W.S. Rabinovich,3 and R. Mahon4
Introduction: Because of the benefits of autonomous spacecraft-to-spacecraft interrogation, communication, and navigation for civilian, commercial, and military space missions, there has been a significant amount of research and development on associated relative sensor systems over the past decade. These systems typically implement radio communication links, GPS sensing for long-range relative positioning, and combinations of visual and laser ranging for short-range proximity operations. In contrast, NRL has developed and tested a multifunctional device that uses solid-state multiple quantum well (MQW) modulating retroreflectors to provide inter-spacecraft laser interrogation, communication, and navigation. The modulating retroreflectors enable compact, low-power, and low-mass optical data transfer on the order of megabits per second, and relative navigation on the order of centimeters in three-axis position and arcminutes in two-axis orientation. Links over ranges of kilometers down to a few meters are possible.
Multiple Quantum Well Modulating Retroreflectors: An MQW modulating retroreflector (MRR) is an NRL-patented solid-state device that allows optical communication and ranging between two platforms.1 This device enables fast data rates at very low drive power and can be packaged in a lightweight, compact unit. Implementation requires that only one platform contain an onboard laser, telescope, and tracker; hence, the device is well suited to asymmetric problems in which one platform serves as the interrogator and pursuer and the other platform serves as the target. As depicted in Fig. 1, the pursuer illuminates the target platform carrying the modulating retroreflectors with a laser beam. The laser beam is modulated using an on-off keying mode and reflected back to the pursuer, with no need for precise laser pointing or tracking. Modulation is achieved by placing a moderate voltage (10-20 volts) across the MQW device in reverse bias. This causes an abrupt change in the optical transmissibility of the material, thus providing a controllable high-speed (up to 10 Mbps) solid-state shutter.
Target MRR Array: To achieve relative position and orientation knowledge of the target platform in addition to communication, an MRR array was designed with eight retroreflectors arranged in the configuration shown in Fig. 2. The center retroreflector and three outer retroreflectors lie in the same plane, and four inner retroreflectors are canted 20 degrees from this plane. Upon illumination of the entire array by the pursuer laser beam, the reflected signals from each MRR are uniquely modulated and reflected back to the pursuer. The intensity of each reflected signal is linearly proportional to the angle of the incoming laser beam relative to the MRR boresight direction, allowing discrimination between target translation and rotation.
Pursuer Spacecraft Laser Interrogator, Detector, and Signal Processing Logic: The pursuer spacecraft houses the interrogating gimbaled laser, an analog photodetector, an analog-to-digital converter, and the signal processing logic required to transmit the continuous-wave laser beam to the target and receive the modulated signals from the target. As depicted in Fig. 3, upon illumination, the aggregate photon return is captured by the analog photodetector and converted to a digital signal. Isolation of each individual MRR signal is then achieved through a set of matching filters tuned to the unique modulator code sequence associated with each MRR.
Target Tracking and Relative Navigation: Initial target detection is achieved by performing a series of rectangular searches in gimbal azimuth and elevation space until all retroreflectors are illuminated. Subsequent gimbal tracking is achieved by equalizing the signal intensities from the three outer retroreflectors, resulting in a laser beam that is continually centered on the MRR array. Relative navigation is achieved using the eight signal returns from the retroreflectors, the laser gimbal azimuth and elevation angles, and the range from the pursuer to the target. The range is determined by applying a pulse to the laser beam and measuring the round-trip flight time for the pulse to return to the pursuer.
Experimental Verification: The Naval Research Laboratory's Dual-Platform Motion Simulator (DMS)2 was used for validation and performance evaluation of the inter-spacecraft MRR sensor system. The DMS facility consists of a 6 degree-of-freedom pursuer platform and a 4 degree-of-freedom target platform, each driven autonomously and independently using a Pentium III personal computer. The pursuer platform was equipped with a gimbaled optical transmitter/receiver system comprised of a 100 mW-laser diode operating at a 980-nm wavelength, a 10 MHz Avalanche photodetector, a signal amplifier, an analog-to-digital converter, and eight digital matched filters. The target platform was equipped with the MRR array, consisting of eight 0.5-mm MQW modulating retroreflectors, each with a mass of 10 grams and a power draw of 75 mW. The devices were driven by a 15-volt modulator, sufficient to achieve a 3:1 optical on/off ratio. Figure 4 shows the pursuer and target platforms with the associated tracking hardware.
A target tracking maneuver was performed to demonstrate the capability of the MRR sensor suite for relative navigation. Using feedback from the MRR sensors, the pursuer platform was commanded to maintain a fixed relative position and orientation as the target rotated and translated along a 30-degree circular arc with a radius of 5 meters. For this simple test, we achieved a steady-state tracking capability of about 15 cm in position and 4.5 degrees in orientation and a static capability (no target motion) of 1 cm in position and 0.3 degrees in orientation. Of course, the tracking results are highly dependent on the controller feedback gains and can be reduced by increasing the bandwidth (selected to be 0.02 Hz for this test). More important from a sensor standpoint is the relative navigation knowledge capability, which has been demonstrated to be about 1 centimeter in position and 1 arcminute (0.017 degrees) in orientation.
Summary: We have developed and tested a multifunctional sensor technology for inter-spacecraft communication and navigation using solid-state modulating retroreflectors. This technology has the potential to simplify hardware requirements for asymmetric applications involving an agile pursuer spacecraft and a docile target spacecraft, such as autonomous rendezvous and capture of orbiting canisters containing fuel, electronic equipment, and supplies.
[Sponsored by NASA]