Abstract:
The application of adaptive optics to the daytime deep space link is analyzed. Theoretical results show up to 8.4 dB gains for 64 PPM formats under daytime sky background conditions at 3 degrees sun angle.

Introduction
Future NASA deep space missions will fly high resolution, high data-rate-generating science instruments. Meeting the data rate needs of these missions will require higher communications bandwidths and higher carrier frequencies. In response, NASA is developing deep space optical communications technologies, with plans to demonstrate these technologies in the 2010 Mars Laser Communications Demonstration (MLCD) [1, 2]. Yet, future operational deep space optical communications ground station networks will need to implement strategies to mitigate the effects of cloud cover and atmospheric turbulence on the optical link.
Cloud cover mitigation strategies are currently being studied by JPL [3]. Recent initial results show 90% availability is achievable with a 3-station optical subnet whenever the probe has line-of-sight access to the stations. Clear air turbulence exacerbates the deleterious effects of background noise from daytime sky and sun-illuminated planets and results in degraded link performance [4]. Analysis shows that to maintain turbulence-induced link loss at the 1-dB level will require a multi-mode receiver with a field-of-view (fov) approximately 1.5 times the size of the seeing disk [5]. Yet for daytime links, the larger the fov the larger is the background noise at the receiver. Daytime seeing measurements at JPL’s Optical Communications Telescope Laboratory (OCTL) show that the receiver’s fov would need to accommodate 25 urad seeing at the 1064-nm wavelength. This paper assesses the performance improvement possible when adaptive optics (AO) is used in a daytime Mars-to-Earth optical communications link.

Sky Background and the Optical Link
Realizing the full potential of deep space optical communications will require daytime operation at small sun angles where sky background noise severely degrades the link. Maintaining link quality under these conditions will require combination of spectral and spatial filtering strategies, and higher order pulse position modulation (PPM) formats.

Figure 1: (a) Sky radiance at 3-degree Sun-Earth probe angle for 20 deg, elevation, sun’s zenith angle 70±3 deg, clear sky, and 2.2km altitude. (b) Sky background for Hale and OCTL telescopes as a function of FOV for SEP angle of 3 degrees.

Figure 1a shows the sky radiance at the 3 degrees operating sun angle required by MLCD. The sky background in a 1 Å optical bandwidth at the 1064-nm downlink wavelength computed from Figure 1a is plotted in Figure 1b as a function of fov for the OCTL and Hale-type telescope apertures. The data show background photon flux levels of 2X107 and 1X109 photons/second for the 1-m and 5-m apertures. Figure 2 shows the effect of the daytime sky on the Mars-to-Earth link with and without AO. Conditions are 25 urad daytime seeing, and what we consider as the best, nominal, and worst sky background. Other key inputs to the analysis are: 5W transmitted power, 30-cm transmitter aperture, 64 PPM, 5-m receiver aperture, and a photon counting communications detector. The plots show that at 0.01 uncoded bit error rate (BER), 8.4 dB gains can be realized when the AO system with d/r0 = 0.5 is used to reduce the receiver fov to 2.4-urad; d is the actuator spacing and r0 is the Fried coherence length [5].

Figure 2: Daytime BER vs. signal photons for 25- urad seeing. AO gains are 1.0, 4.0, 8.4 dB at 0.01 uncoded BER for photon counting detector.

We have built a laboratory AO testbed to validate our theoretical models. It includes a 635-nm “guidestar” laser a Shack-Hartman wavefront sensor consisting of a 10X10 lenslet array matched to a 40X40 segment of the EEV 39 CCD pixel array, a 1064-nm communications diode laser and APD communications detector, an off-axis parabola to collimate the beam to 7-cm for the 97-actuator deformable mirror, a tip/tilt steering mirror, a turbulator to generate wavefront aberrations, and an integrating sphere to simulate sky background noise. Preliminary laboratory tests at 100 Mbps and OOK modulation show 2 dB to 3 dB gains consistent with our predicted performance for APD detectors in nominal atmospheric turbulence conditions [6].

Conclusion
We have analyzed the benefit of adaptive optics to the deep space optical link. Results show up to 8.4 dB improvements with increasing gains at higher turbulence and noise background levels. Preliminary laboratory results show improvements of 2 dB to 3 dB, consistent with theoretical predictions. Future plans call for evaluating gains at higher order PPM formats, validating predicted AO gains for photon counting detectors, and performing field tests.

Acknowledgements
The research and development described in this paper was performed by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

References
[1] B. Edwards, et al. “Overview of the Mars Laser Communications Demonstration Project” Proceedings of AIAA Space Conference and Exposition, Long Beach, CA September 23-35 2003.
[2] K. Wilson, B. Parvin, S. Zingales, R. Fugate, P. Kervin, “Optical Ground Station Site Diversity for Deep Space Optical Communications: The Mars Telecom Orbiter Optical Link”, AMOS Technical Conference, September 2003, Maui HI.
[3] Robert Link, Mary Ellen Craddock, Randall J. Alliss, “Mitigating the Impact of Clouds on Optical Communications”, Proceedings of the IEEE Aerospace Conference Big Sky Montana, March 5-12 2005.
[4] K. Wilson M. Troy, M. Srinivasan, B. Platt, V. Vilnrotter, M. Wright, V. Garkanian, H. Hemmati, “Daytime Adaptive Optics For Deep Space Optical Communications” Conference proceedings International Space Conference of Pacific-basin Societies (ISCOPS), December 2003 Tokyo, Japan.
[5] Shinhak Lee, Keith E. Wilson, and Mitchell Troy, “Background Noise Mitigation in Deep Space Optical Communications using Adaptive Optics” to be published in Special Issue on Optical Communications: IPN PR, Vol.42-161, May 15, 2005.
[6] Malcolm W. Wright, Meera Srinivasan and Keith Wilson “Improved Optical communications Performance using Adaptive Optics with an Avalanche Photodiode Detector” to be published in Special Issue on Optical Communications: IPN PR, Vol.42-161, May 15, 2005.