Jonathan R. Bruzzi

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.

MESSENGER mission: first electronically steered antenna for deep space communications

The MESSENGER mission to orbit the planet Mercury poses significant design challenges for its deep space communication system. These challenges include a wide pointing range, tight packaging, and a high temperature environment. To meet these challenges, the spacecraft incorporates the first steerable phased array antenna flown for deep space communications. The invention of a method for achieving circular polarization in a high-temperature (+300/spl deg/C) environment has doubled the science return of the mission relative to its initially proposed implementation. Cross-strapping between the phased array antennas and solid state power amplifiers (SSPAs) enables both amplifiers to be turned on when sufficient power is available, enhancing the scientific return at the planet. A science return of 25 Gbits/year is achieved using only one SSPA in Mercury orbit. The science return increases to 100 Gbits/year if both SSPAs are used in the orbital phase.

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.

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.

Implementation of an X-band phased-array subsystem in a deep space mission

The MESSENGER spacecraft, the first mission to the planet Mercury since 1975, will achieve Mercury orbit in 2011. The spacecraft uses two opposite-facing mission-enabling X-band (8.4 GHz) phased-array antennas to achieve high-rate downlink communications. The spacecraft orientation is constrained such that a preferred direction faces the Sun; rotation about the Sun-line is allowable. The main beam of each antenna is steerable in one dimension. These two degrees of freedom allow the main beam of the phased array to be pointed in any direction about the spacecraft. A novel system-level design requires many different subsystems of the spacecraft to interact together to achieve accurate beam-pointing, and thus, high-rate downlink data from Mercury to Earth

Spacecraft-level testing and verification of an X-band phased array

The MESSENGER spacecraft uses an X-band (8.4-GHz) phased array for high-rate downlink communications to meet mission data requirements yet still survive the extreme environment at the planet Mercury. To survive the solar intensity at the planet, the MESSENGER spacecraft uses a sunshade that must remain Sun-pointed; this restricts pointing of the spacecraft. The use of two phased-array antennas alleviates the need for a gimbaled high-gain dish. The RF signal is routed through on-board solid-state power amplifiers that control the phases of the signals fed to the phased arrays, thereby pointing without the need for any moving parts while maintaining a Sun-pointed attitude. Each phased array is composed of eight slotted waveguide sticks. This paper describes a method for a real-time, fast verification of the steering of the phased array during any phase of spacecraft-level testing (including thermal-vacuum) without the need to free radiate, which is specifically critical to a spacecraft during integration and test. This newly developed and implemented approach does not require near-field probing, in-line couplers, or extra flight mates and de-mates. Once the antennas are integrated onto the spacecraft, schedule constraints force the need for very quick verification methods. The technique described herein quickly samples the phase of the signal at each array element and, in conjunction with subsystem-level measurements, mathematically calculates the radiated antenna pattern. The phases within each array element are measured using innovative loop couplers that may simply be removed once testing is complete. These phases are combined using specifically designed software to calculate the far-field radiated pattern to verify pointing.

Telemetry recovery and uplink commanding of a spacecraft prior to three-axis attitude stabilization

After separation from the launch vehicle, a spacecrafts guidance and control system typically orients the spacecraft autonomously into a three-axis stabilized attitude for non-spinners. If an anomaly occurs, or if the spacecraft fails to orient itself appropriately, the Mission Operations team will want to observe spacecraft telemetry or may even be required to command the spacecraft before attitude stabilization. Antenna coverage in these cases is critical, since the command and control antenna may be rotating away from the ground station line-of-sight as the spacecraft tumbles. However, activating opposite-pointing hemispherical low-gain antennas on the spacecraft to obtain more complete angular coverage comes at the cost of presenting an interferometric signal to the ground station (or spacecraft receiver, depending on the radio-frequency subsystem configuration) which fluctuates as a function of the relative antenna positions and tumble rate of the spacecraft. Recent programs developed by The Johns Hopkins University Applied Physics Laboratory, specifically MESSENGER and STEREO, have investigated the capability of the deep space network and universal space network receivers to recover telemetry from a tumbling spacecraft at a low orbital altitude. Also investigated was the ability of a tumbling spacecraft utilizing a small deep-space transponder to register valid uplink commands, even in the presence of a second, closely-spaced uplink frequency, as expected for the two STEREO spacecraft

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.

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.