Figure 1: MLT Block Diagram

Abstract
We present The Mars Lasercom Terminal preliminary design that will demonstrate high-rate optical communications from Mars and provide NASA with the experience necessary to develop future operational deep-space optical communication systems.

Introduction
NASA’s plans for exploring the Solar System include the deployment of sophisticated instruments, such as hyperspectral imagers and synthetic aperture radars to Mars and beyond. Returning the large amounts of data that these types instruments are capable of collecting will place increasing demands on the present day Deep Space Network (DSN). Even with upgrades, it is not believed that the DSN will have the capacity to satisfy these demands. Optical communication has the potential to meet even the most demanding future data transmission requirements.
To demonstrate optical communication from interplanetary distances, NASA’s Goddard Space Flight Center (GSFC), MIT’s Lincoln Laboratory (MIT/LL) and the Jet Propulsion Laboratory (JPL) are jointly working on the Mars Laser Communication Demonstration (MLCD) project. MLCD will demonstrate optical communication from Mars to Earth as part of the Mars Telecommunications Orbiter (MTO) project to be launched in 2009 [1]. The MLCD project consists of the Mars Lasercom Terminal (MLT) and two pairs of Earth terminals. Each pair of Earth terminals provides two functions: receive the optical communication downlink signal from the MLT and transmit an optical beacon/uplink communication signal that provides a pointing reference for the MLT and has the capability to carry system commands. The overall architecture of the Mars-to-Earth optical communication link has been previously presented [2,3,4]; this paper will discuss the MLT.

Figure 2: MLT Configuration

The MLT
As with any deep space communication system, the primary function of the MLT is to transmit information over vast interplanetary distances back to Earth. The advantage of optical transmission is the very high signal gain available with relatively small transmit apertures and low-power transmitters. The MLT is designed to provide a minimum data rate of 1 Mbps at maximum distance and up to 30 Mbps at minimum distance when coupled with an Earth receive terminal aperture of approximately 20 m2 [3]. The MLT will employ a 30.5 cm transmit aperture and transmit laser operating at 5-W average power and a wavelength of 1.064 µm.

A. Pointing and Stabilization
The penalty for the high signal gain provided by the MLT design is a very narrow transmit beam, approximately 3.5 µrad, that must be pointed and stabilized with sub-µrad accuracy in the presence of spacecraft motions. The pointing and stabilization system operates in three frequency regimes; it provides DC (<0.1 HZ) pointing; it actively rejects disturbances up to mid-frequencies (<300 Hz) and it passively rejects disturbances at high frequencies (>300 Hz).
DC pointing is accomplished by tracking an Earth beacon on the Focal Plane Array (FPA). The narrow transmit beam and large relative transverse velocities between Mars and Earth require compensating for the point-ahead angle, which is as large as 400 µrad. This is accomplished by imaging a portion of the transmit beam on the FPA and offset pointing from the beacon using a Point Ahead Mechanism (PAM). When Earth and Mars are close, beacon tracking provides a bandwidth up to approximately 3 Hz. When Earth and Mars are far apart, the Earth itself is bright enough to provide a tracking bandwidth up to approximately 3 Hz.
Mid-frequency disturbances are compensated for by the use of a Magneto-hydrodynamic Inertial Reference Unit (MIRU), under development by Applied Technology Associates of Albuquerque, NM, and a quad-cell Fast Steering Mirror (FSM) tracking loop. The MIRU provides an inertially stabilized optical source; the quad-cell is used as a nulling sensor and, in conjunction with the FSM, rejects disturbances up to approximately 900 Hz.
High-frequency disturbances are rejected by passive vibration isolators arranged in a hexapod configuration. As a secondary benefit, the vibration isolators significantly reduce launch loads. The pointing and stabilization system and vibration isolator configuration are shown in Figures 1 and 2.

B. Transmitter
The MLT transmitter utilizes the Master-Oscillator- Power-Amplifier (MOPA) architecture commonly used for high-rate communications in the telecom industry [5]. Advantages of the MOPA architecture are that it supports a wide variety of transmit formats and that each element, the master laser, modulators and power amplifier, can be optimized for the application. Commercial-Off-The-Shelf (COTS) electro-optics components will be qualified for the space environment, and in some cases repackaged, for our application. Transmit formats and coding are critical in designing a highly efficient communication link. The MLT will utilize Pulse Position Modulation (PPM) in conjunction with a serially concatenated turbo-code to provide performance within approximately 1 dB of the theoretical channel capacity. The MLT is designed with transmit data rates from approximately 25 kbps up to 50 Mbps to allow closing the link under a very wide variety of conditions.

C. Telescope and Electronics
As shown in Figure 2, the MLT consists of separate optical and electro-optics modules. The optical module incorporates a diffraction limited low distortion 15X telescope with an approximately 1.5 mrad field of view. A telescope design with low distortion is critical due to the large point ahead angles. A solar window is included to reduce the thermal load on the optical module. The solar window is protected during launch by a one time open-able cover.
Any communication system from Mars must be capable of operating while pointing close to the Sun to minimize outages. The MLT will be capable of operating within 2º of the Sun, resulting in an annual outage of 23 days, similar to the existing limit for the DSN. This requires maintaining optical surfaces to cleanliness level 300 and designing them for low scatter. The optical module is designed to minimize mass, while utilizing proven technologies.
The optical module is thermally isolated from the host spacecraft and incorporates an active thermal control system that will maintain its temperature to 21ºC ±2ºC. Active temperature control is required to maintain the optical wavefront quality during flight operations.
The electro-optics module is designed to minimize mass and electrical power. As previously described, the electrooptic transmitter is designed to be modular, while the electronics are highly integrated to reduce mass and power. An industry standard compact Peripheral Component Interconnect (PCI) bus is used to distribute signals and electrical power within the analog and digital electronics chassis. This configuration saves mass by reducing the number of interconnect cables required.
The electronics have been carefully designed to apportion functionality between hardware and software. A highly capable space qualified single board computer, the SCS750A by Maxwell Technologies of San Diego, CA, will serve as the spacecraft interface via MIL-STD-1553B, PCI bus controller and command and telemetry processor. In addition, it will calculate centroids for the FPA and quad-cell, decode system commands from the uplink beacon and implement the digital control loops required for the MIRU, FSM and PAM.
The electro-optics module is thermally isolated from the host spacecraft. Its temperature is passively controlled to maintain average operating temperatures from 0ºC to 40ºC. The temperature must be controlled to this relatively narrow band for space flight, to allow the use of COTS electro-optic components.

Conclusion
The MLCD project is one of NASA’s first steps in establishing an optical communication network to support planetary exploration. We have presented a design for the MLT, a critical component of the MLCD project, that will support all project goals and provide NASA with the experience necessary to develop future deep space optical communications systems.

Acknowledgment
This work was sponsored by the National Aeronautics and Space Administration under Air Force Contract F19628-00-C- 0002. Opinions, interpretations, recommendations and conclusions are those of the author and are not necessarily endorsed by the United States Government.

References
[1] S.F. Franklin et al., “The 2009 Mars Telecom Orbiter Mission,” IEEE Aerospace Conference Proceedings, pp. 1-20, 2004.
[2] D. M. Boroson, R. S. Bondurant, J. J. Scozzafava, “Overview of High Rate Deep Space Laser Communications Options,” SPIE Proceedings, vol 5338 : Free-Space Laser Communications Technologies XVI, January 2004.
[3] B. L. Edwards et a, “Overview of the Mars Laser Communications Demonstration Project,” American Institute of Aeronautics and Astronautics Space 2003, September 2003.
[4] D. M. Boroson, A. Biswas, B. L. Edwards, “MLCT : Overview of NASA’s Mars Laser Communications Demonstration System,” SPIE Proceedings, vol 5338 : Free-Space Laser Communications Technologies XVI, January 2004.
[5] M. L. Stevens, D. M. Boroson, D. O. Caplan, “A Novel Variable-Rate Pulse-Position Modulation System With Near Quantum Limited Performance,” LEOS, November, 1999.