Gerry G. Ortiz

Fine pointing control for optical communications

Free-space optical communications requires precise, stable laser pointing to maintain optimal operating conditions. This paper describes the software and hardware implementation of the fine pointing control based on the Optical Communications Demonstrator architecture. The implementation is designed to facilitate system identification of the fast steering mirror mechanism. Models are derived from laboratory testing of two fine steering mirrors that are integrated into the fine tracking loop. Digital controllers are then designed to close the tracking loop using optical feedback. Results of the fine pointing control performance show an improvement of 20% in the jitter rejection bandwidth over previous experiments. A discussion of the computer delay and limited processing bandwidth in this particular implementation are included.

A high frame rate CCD camera with region-of-interest capability

This paper presents the design and preliminary results of a custom high-speed CCD camera utilizing a Texas Instruments TC237 CCD imager chip with sub-frame window read out. The camera interfaces to a C40 digital signal processor (DSP), which is used to issue commands and read images from the camera. The camera design consists of a two-card set including the CCD imager card and the focal plane array (FPA) interface card. The CCD imager card contains the level translator and buffer circuitry for the CCD strobe lines, the TC237 CCD imager chip and a pair of analog signal processor chips, each with a 10-bit analog-to-digital converter. The analog signal processor is a TLV987 with correlated double sampling (CDS) and serial programming capability to set amplifier gain, pixel bias level and background level illumination to name a few. The second card contains a pair of field programmable gate arrays (FPGA) used to interface the CCD imager card to the C40. The goal of this camera development is to provide a high-quality, high-speed camera as part of the tracking apparatus for a free-space optical communications terminal. Preliminary data suggests frame rates of 6 kHz for 8/spl times/8 subwindows in the current testbed with 7-bit pixel resolution. Refinements in camera and testbed operation target performance goals of 17 kHz for 8/spl times/8 sub-windows with 10-bit pixel resolution.

Exo-atmospheric telescopes for deep space optical communications

For deep space optical communications, optical telescopes located above the Earths atmosphere would have significant performance advantages over telescopes mounted on the Earths surface. Link outages due to cloud cover would be eliminated, atmospheric attenuation would be eliminated, and signal degradation due to stray light would be reduced. A study has been conducted to compare various exo-atmospheric platforms for the Earth end of the optical link. The three most promising platforms among many initially considered were selected for detailed study: satellites, free-flying airships and tethered airships. System configurations were compared that would have data rate capability comparable to a 6-m to 10-m diameter ground-mounted telescope, 100 percent line-of-sight coverage to a deep space spacecraft in the ecliptic, and at least 80 percent coverage in the event of failure of one Earth terminal. Based upon technical feasibility and readiness, life-cycle cost, performance and risk, a satellite platform is recommended. However, it is noted that airship technology may be advanced in the next decade or so to a level where airships should be reconsidered. Finally, this study provides a basis for a future study to compare systems using Earth-mounted and exo-atmospheric telescopes

Accelerometer-assisted tracking and pointing for deep space optical communications

NASA/JPL has been developing acquisition, tracking and pointing (ATP) technologies for deep space tracking and pointing of an optical communication beam using linear accelerometers to enhance pointing. Linear accelerometers provide excellent accuracy in sensing the vehicles acceleration with the advantage of small size, low power, low cost, and a broad range of well developed products. We present the concept of accelerometer-assisted tracking, error analysis, and progress made on its implementation.

Feasibility of infrared Earth tracking for deep-space optical communications

Infrared (IR) Earth thermal tracking is a viable option for optical communications to distant planet and outer-planetary missions. However, blurring due to finite receiver aperture size distorts IR Earth images in the presence of Earths nonuniform thermal emission and limits its applicability. We demonstrate a deconvolution algorithm that can overcome this limitation and reduce the error from blurring to a negligible level. The algorithm is applied successfully to Earth thermal images taken by the Mars Odyssey spacecraft. With the solution to this critical issue, IR Earth tracking is established as a viable means for distant planet and outer-planetary optical communications.

Inertial sensor assisted acquisition, tracking, and pointing for high data rate free space optical communications

We discuss use of inertial sensors to facilitate deep space optical communications. Implementation of this concept requires accurate and wide bandwidth inertial sensors. In this presentation, the principal concept and algorithm using linear accelerometers will be given along with the simulation and experimental results.

Acquisition, tracking, and pointing using Earth thermal images for deep space optical communications

The feasibility of using long wavelength Earth thermal (infrared) images for telescope tracking/pointing applications for both deep space free-space optical communications has been investigated and is reported here. The advantage of this technology rests on using full Earth images in this band, which yield more accurate estimates of geometric centroids than that of Earth images in the visible band. Another major advantage is that these images are nearly independent of Earth phase angle. The results of the study show that at a Mars range, with currently available sensors, a noise equivalent angle of 10 to 150 nanoradians and a bias error of better than 80 nanoradians can be obtained. This enables precise pointing of the optical communications beam for high data rate links.

Optical communication demonstration and high-rate link facility

Motivated by demands for faster, better, cheaper spacecraft, NASA is developing deep-space optical communication technology which promises reduced mass, volume, and power consumption compared to radio-frequency technology. While earth-orbiting optical receivers may eventually be employed, initial deep-space optical communication links are expected to utilize terrestrial telescope receivers. As the communication beam passes through the atmosphere, atmospheric turbulence causes the beam to scintillate, dramatically impacting its temporal and transverse nature. The statistics of these effects must be measured extensively if optical deep-space communication links are to be fully modeled and the design of deep-space communication links optimized. Sponsored by the Engineering Research and Technology Development program, the purposes of the Optical Communication Demonstration and High-Rate Link Facility are to demonstrate a Gbps-class optical downlink, gather extensive link statistics, and provide high-rate down...