Bradley G. Boone

Multiloop control of a nanopositioning mechanism for ultraprecision beam steering

Beam steering accuracy is critical to the successful operation of optical communications systems, especially those which take place over extreme length scales, such as for an interstellar spacecraft. In this paper, a novel beam steering mechanism and several control system approaches for ultra-precision beam steering are discussed. The beam steering mechanism is a nanopositioning device which utilizes a parallel cantilever configuration and a piezoelectric actuator to obtain extremely high positioning accuracy with minimal parasitic errors. A robust motion controller is presented for this mechanism which is designed to compensate for modeling uncertainty. This controller is intended for use with feedback from the nanopositioner’s built-in capacitance probe. Due to the need to track the trajectory of the steered beam, two additional control approaches are presented which combine the robust motion controller with additional feedback for the actual beam displacement. These multi-loop control approaches provide a level of robustness to thermal effects and vibrations which could not be obtained from a single sensor and feedback loop. Simulation results are provided for each of the control designs.

Optical communications development for spacecraft applications: recent progress at JHU/APL

Free-space optical communication systems for deep space as well as near terrestrial space environments are now under development for deployment aboard spacecraft within the next few years. Ever-increasing requirements for high data-rate communications are driving significant investments by NASA and DoD in critical technology readiness for spaceflight. One of the key NASA requirements is science data retrieval at rates much higher than heretofore possible with RF systems, for missions as far out as interstellar space and as close as geosynchronous Earth orbit (GEO). Recent efforts at Johns Hopkins University Applied Physics Laboratory (JHU/APL) are summarized that are focused on these requirements and challenges. We are developing a spacecraft optical communications terminal architecture initially using commercial off-the-shelf components while accelerating the development of state-of-the-art replacement components, which minimize mass and prime power while maintaining or improving performance. Recent technology development efforts will be summarized that include pulse position (PPM) modulator/demodulator chip development, compact optical beamsteering technology, including micro-electromechanical systems (MEMS), an ultra-lightweight deployable dual-band antenna concept, and a low-mass low-power optical downlink terminal design intended for deployment on a realistic interstellar explorer (RISE) mission

Comparison of macro-tip/tilt and mesoscale position beam-steering transducers for free-space optical communications using a quadrant photodiode sensor

The National Aeronautics and Space Administration (NASA) plans to develop optical communication terminals for future spacecraft, especially in support of high data rate science missions and manned exploration of Mars. Future, very long-range missions, such as the Realistic Interstellar Explorer (RISE)1, will need optical downlink communications to enable even very low data rates. For all of these applications, very fine pointing and tracking is also required, with accuracies on the order of ± 1 μrad or less and peak-to-peak ranges of ± 10 mrad or more. For these applications, it will also be necessary to implement very compact, lightweight and low-power precision beam-steering technologies. Although current commercial-off-the-shelf devices, such as macro-scale piezo-driven tip/tilt actuators exist, which approach mission requirements, they are too large, heavy, and power consuming for projected spacecraft mass and power budgets. The Johns Hopkins University Applied Physics Laboratory (JHU/APL) has adopted a different approach to beam-steering in collaboration with the National Institute of Standards and Technology (NIST). We are testing and planning to eventually package a highly accurate large dynamic range meso-scale position transducer under development at NIST. In this paper we will describe a generic package design of an optical communications terminal incorporating the NIST prototype beam-steerer. We will also show test results comparing the performance of the NIST prototype meso-scale position beam-steerer to a commercial macro-tip/tilt actuator using a quad-cell tracking sensor.

Development, test, and evaluation of MEMS micromirrors for free-space optical communications

MEMX Corporation in collaboration with Johns Hopkins University Applied Physics Laboratory (JHU/APL) has developed micro-mirror technology applicable to free-space multi-access optical communications terminals. Based on their previously developed micro-electro-mechanical systems (MEMS) optical switches, these new units are being evaluated for applications on spacecraft. These devices must operate within very accurate digitally-controlled pointing and tracking subsystems, which are an essential adjunct to the long-haul optical communication channels that would be operated potentially from geosynchronous earth orbit (GEO) to ground. For such spacecraft applications high-powered laser diodes are likely be the required transmitter. Coupled with their potential operation in a vacuum or at partial atmospheric pressures, MEMS mirror shape stability and fabrication tolerances are of key concern to a system designer. To this end we have measured the performance of preliminary micro-mirror units in terms of angular jitter, focal spot stability, and open and closed-loop response versus laser transmitter power in both ambient air and at low partial pressures. We will describe the fabrication process as well as the experimental test configurations and results in the context of optical beamsteering. We will also discuss the applicability and scalability of this technology to multi-access terminals.

Infrared communications for small spacecraft: from a wireless bus to cluster concepts

Nanosatellites operating singly or in clusters are anticipated for future space science missions. To implement this new communications paradigm, we are approaching cluster communications by first developing an infrared (IR) intra- craft wireless bus capability, following initially the MIL- STD-1553B protocol. Benefits of an IR wireless bus are low mass, size, power, and cost, simplicity of implementation, ease of use, minimum EMI, and efficient and reliable data transfer. Our goals are to maximize the reliable link margin in order to afford greater flexibility in receiver placement, which will ease technology insertion. We have developed a concept demonstration using a high-speed visible-band silicon PIN photodiode and a high-efficiency visible LED operating at a data rate up to 4 Mb/sec. In designing an internal IR wireless bus, we have characterized various candidate materials, emitters, and geometries, assuming a single reflection. Thus, we have measured the bidirectional reflectance distribution function (BRDF) for five different materials characteristic of typical spacecraft structures, which range from nearly Lambertian to highly specular. We have fit our data to empirical BRDF functions and modeled the detected irradiance anywhere in the plane of incidence for a divergent emitter. We have also determined the angular limits on the link geometry to remain within the required bit error rate by determining the received signal-to-noise ratio for minimum values of irradiance received at the detector.

Development of a laser transceiver system for deep-space optical communications

The National Aeronautics and Space Administration (NASA) continues to plan and anticipate the development of high data rate communications for future deep space missions. The Johns Hopkins University Applied Physics Laboratory is responding to this challenge by developing a breadboard laser transceiver package using commercial off-the-shelf components. We plan to demonstrate a breadboard transceiver unit, integrated with a fine pointing and tracking capability by the end of FY-03. A potential mission application is to ultimately demonstrate a live video link from Mars. Our near-term demonstration goals are to achieve a modest 5 Mbps data rate over an equivalent range of 2 AU. To achieve this we are modeling and testing the components for a hybrid analog/digital receiver in conjunctino with semiconductor laser diodes and silicon PIN and avalanche photodiodes. Our efforts leading up to hardware implementation and test have consisted of a trade-of between coherent and direct detection receiver architectures, and a link analysis for deep space applications, which established the laser power requirements for supporting a real-time video link from Mars as well as other missions, where the encoded bit error rate is from 10-6 to 10-9. Current efforts include the development of a direct-detection 4-ary pulse position modulation scheme using a FPGA-based modulator/demodulator as well as a separate quadrant photodiode receiver for tracking. We plan to integrate this transceiver with lightweight diffractive optical elements for beam-forming. The design and initial testing of the transceiver components will be discussed.

Conceptual design and algorithm evaluation for a very accurate imaging star tracker for deep- space optical communications

The National Aeronautics and Space Administration is planning high data rate optical communications for future deep space missions. The Johns Hopkins University Applied Physics Laboratory (JHU/APL) is responding by developing concepts for implementing optical communications terminals that are more compact and lightweight than heretofore. An essential requirement for these long-range optical links is a high-precision pointing and tracking system. Focal plane array (FPA)-based star trackers that enable open-loop pointing and tracking are necessary. Spacecraft attitude instabilities, emphemeris errors, tracking sensor noise, clock errors, and mechanical misalignments are among the error sources that must be minimized and compensated for. To achieve this JHU/APL has developed an imaging star tracker concept using redundant multi-aperture FPAs symmetrically disposed about the laser downlink. Centroid estimation and pattern matching techniques account for aberration and motion errors. Robustness, sensitivity to detection thresholds, field-of-view sizing, number of stars per frame, missed detections, false alarms, and position biases, as well as stellar catalog size and star selection, will be described. Finally the conceptual design of a frame-to-frame integration method and sensor fusion algorithm (such as a Kalman filter) will be considered. The goal is to achieve a system pointing and tracking error significantly less than 1 μrad.

Optical simulator and testbed for spacecraft star tracker development

The Johns Hopkins University Applied Physics Laboratory (JHU/APL) is currently developing a prototype star scanner design incorporating a variation on the V-slit design concept, called the N-slit, which is intended for deployment on future NASA spacecraft missions, such as the Radiation Belt Storm Probe (RBSP). In order to effectively test and evaluate alternative designs, including optics, sensors, and tracking algorithms, we have developed a laboratory testbed that simulates celestial objects, including stars down to a specified magnitude. We do this by creating a light-hermetic dome-shaped projection environment using light emitting diodes of specified brightness coupled to the dome exterior via fiber-optic patch cords, which can be adjusted by current bias and selected for color, if necessary, to simulate stars over a particular range of magnitudes required for the desired system accuracy. We also simulate the spacecraft platform spin dynamics using a two-axis servo-actuated mount for the star tracker test unit within the dome. This same actuator or a similar assembly can then be transitioned to actual field tests for sensor down-select and full functionality demonstrations prior to follow-on spacecraft-qualified design. We will describe the design, construction, calibration, and operation of this simulator and preliminary results of star scanner sensor evaluation using a photomultiplier-based N-slit sensor.

Optical beam spreading in the presence of both atmospheric turbulence and quartic aberration

Optical beam spread and beam quality factor in the presence of both quartic phase aberrations and atmospheric turbulence is numerically analyzed. We obtain analytical expressions for both the mean-square beam radius and the beam quality factor using the moment method, and we compare these expressions to the results from Monte Carlo simulations, which allows us to mutually validate the theory and the Monte Carlo simulation codes. We also discuss the reason for the discrepancy between the classical approach for calculating the ensemble-averaged mean-square beam radius in a turbulent atmosphere that is described by Andrews and Phillips and by Fante versus using the moment method.

Free-space high data rate communications technologies for near terrestrial space

Recent progress at the Applied Physics Laboratory in high data rate communications technology development is described in this paper. System issues for developing and implementing high data rate downlinks from geosynchronous earth orbit to the ground, either for CONUS or in-theater users is considered. Technology is described that supports a viable dual-band multi-channel system concept. Modeling and simulation of micro-electro-mechanical systems (MEMS) beamsteering mirrors has been accomplished to evaluate the potential for this technology to support multi-channel optical links with pointing accuracies approaching 10 microradians. These models were validated experimentally down to levels in which Brownian motion was detected and characterized for single mirror devices only 500 microns across. This multi-channel beamsteering technology can be designed to address environmental compromises to free-space optical links, which derive from turbulence, clouds, as well as spacecraft vibration. Another technology concept is being pursued that is designed to mitigate the adverse effects of weather. It consists of a dual-band (RF/optical) antenna that is optimally designed in both bands simultaneously (e.g., Ku-band and near infrared). This technology would enable optical communications hardware to be seamlessly integrated with existing RF communications hardware on spacecraft platforms, while saving on mass and power, and improving overall system performance. These technology initiatives have been pursued principally because of potential sponsor interest in upgrading existing systems to accommodate quick data recovery and decision support, particularly for the warfighter in future conflicts where the exchange of large data sets such as high resolution imagery would have significant tactical benefits.