Abstract
We present an overview of the Mars Laser Communications Demonstration (MLCD,) a joint project between NASA’s Goddard Space Flight Center (GSFC), the Jet Propulsion Laboratory, California Institute of Technology (JPL), and the Massachusetts Institute of Technology Lincoln Laboratory (MIT/LL). MLCD’s goal is to demonstrate the first high-rate, free-space laser communications link from deep space back to Earth. The lasercom flight terminal will be flown on the Mars Telecommunications Orbiter, to be launched by NASA in 2009, and will demonstrate a technology which has the potential of vastly improving NASA’s ability to communicate throughout the solar system.

Introduction
In the near future the National Aeronautics and Space Administration anticipates a significant increase in demand for long-haul communications services from deep space to Earth. Distances will range from 0.1 to 40 Astronomical Units (AU) with data rate requirements in the 1’s to 1000’s of Mbits/second. The near-term demand is for high-bandwidth deep space communications services for NASA’s Science Mission Directorate, which wishes to deploy more capable instruments onboard spacecraft. The longer-term demand is driven by NASA’s Exploration Systems Mission Directorate and the Science Mission Directorate, both of which envision missions with extreme communications challenges; examples include very high data rates supporting sub-surface exploration of outer planets, bi-directional high definition television transmission to and from the moon, and eventually supporting astronauts at Mars.
It has long been known that free-space laser communications has the potential of delivering extremely high data rates, or requiring very small terminals, or both, to space-based systems. Near-Earth lasercom systems have already been demonstrated (GeoLITE and GOLD in the U.S. and SILEX in Europe), with a number of operational follow-ons planned. Related technology also has the potential to revolutionize deep space communications. To realize this potential, NASA has undertaken the Mars Laser Communications Demonstration, whose goal is to be the first demonstration of high-rate optical communications from deep space back to Earth.
Under the management of NASA’s Goddard Space Flight Center, MIT Lincoln Laboratory and the Jet Propulsion Laboratory are working together to design a system that will deliver between 1 and 30 Mbps (depending on the orbital positions) from Martian orbit back to receiving terminals at the Earth’s surface. MIT/LL will develop the Mars Lasercom Terminal (MLT,) which is to fly on NASA’s 2009 Mars Telecommunications Orbiter (MTO). JPL will develop a ground terminal that will be based on the 5-meter Hale Telescope at the Palomar Observatory in California. JPL will also develop an uplink laser beacon, based at their Optical Communications Telescope Laboratory (OCTL) in Wrightwood, California. In addition, MIT/LL will develop a prototype of a scalable ground receiver telescope array technology. Finally, JPL will develop the control facility for the entire ground network.
Critical technologies for the mission include efficient doped-fiber laser amplifiers that can achieve high peak-to-average power performance; near-theoretically-efficient modulation, coding, channel interleaving, and decoding; highly accurate pointing and stabilization systems for pointing the narrow beam back to Earth; ability to point large ground-based telescopes to within a very few degrees of the solar disk; highly-efficient rejection of background sky brightness at the Earth terminal; and near-noiseless, high detection-efficiency photon-counting detection.
It is hoped that demonstration of all these technologies and reliable high-rate delivery of data back from Mars will pave the way toward operational use of Lasercom systems in future deep-space missions.

Challenges of Deep Space Lasercom
Link distances for deep-space missions can vary from tens of millions of kilometers to billions of kilometers. For the telecommunications engineer, this translates into distance-squared losses of between 60 and 100 dB IN ADDITION TO the losses of a geosynchronous-orbit-to-ground link. For the Mars link, the 80 dB extra loss means that, if we were to take a capable, near-Earth 10-Gbps Lasercom system and send one end to Mars, we would have power enough to support only 100 bps! To achieve even 1 Mbps from the maximum Martian distance, this means we need to create 40 dB more capability.
Following the lead of the RF telecommunications engineers, the first improvement is to employ as large a collector as is feasible. For Lasercom, this means a large, astronomical-type telescope. Although this does provide us with increased collecting capability, we find that options for detection in such a system are limited by atmospheric-turbulence-induced spreading of the signal at the telescope’s focal plane. (We will not consider adaptive optics systems, here, although it is known that such an approach could help mitigate this effect.) To capture most of the received light in this non-diffraction-limited spot, we are forced to detect all signals from a region of the sky large enough that there is appreciable collected background light during daytime operation. (A quick look at planetary orbits shows that such daytime conditions occur for much of the year.) The most straightforward detector for such a turbulence-limited system is one that provides so-called direct detection, and the most efficient such detector counts individual photons with high detection efficiency and low noise.
To get the highest communications performance, we must reject as much of the collected background light as is feasible using optical filtering before the detection. We must also employ modulation techniques that are the most robust against the residual light, and that are achievable at high powers and wall-plug efficiency at the transmitter. Furthermore, we must match all these techniques with modern error-correction codes and data interleavers in order to squeeze out the absolute best efficiency.
Orbital mechanics shows us that the terminals at each end of this link must look near the sun in order not to have an operational downtime of more than a few weeks during the planetary orbits. Scatter and thermally-induced telescope deformations must therefore be minimized, and telescope damage must be avoided.
* This work was sponsored by the National Aeronautics and Space Administration under Air Force Contract F19628-00-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Air Force.
Finally, there is the problem of stabilizing and pointing the narrow beam from the Mars terminal. A 30-cm telescope operating at a wavelength near 1 micron means the diffraction-limited beam is only about 3.5-4 microradians wide. Even if there were a bright reference beam coming up from the Earth, planetary velocities show us that we would need to point the returning beam to a spot that is up to 400 microradians away – a so-called point-ahead of well over 100 beamwidths – with an accuracy of a fraction of a beamwidth. Such pointing accuracy strains the capabilities of optical and measurement systems. Furthermore, to deliver such a reference beam to Mars, the uplink transmitter system needs to send many hundreds of Watts of power up through the turbulent atmosphere and then on a 400 million kilometer trip. With the atmospheric distortions at the beginning of this trip, it is known that performance will be tens of dBs away from the diffraction limit.
The MLCD program is facing up to these challenges. A very brief overview of the approaches being taken is given in the remainder of this short report.

The Mars Lasercom Terminal
The MLT is based on a 30-cm telescope that is carried on passive vibration-isolation struts. These both soften the vibrations during launch and reduce the in-space micro-vibrations that are present on the spacecraft during operation. Further beam stabilization is performed with a three-phase approach. The highest frequency stabilization is achieved by employing a high-speed tracking loop based on a fast steering mirror with measurements from a quad detector that monitors a pseudo-star. This signal is created by an on-board, inertially-stabilized reference called a MIRU (Magnetohydrodynamic (MHD) Inertial Reference Unit.) Mid-frequency stabilization is performed by monitoring the bright image of the Earth on a focal plane array (FPA) and using the tracking error signal to re-center the MIRU. Finally, a beacon signal, uplinked from an Earth terminal, is also monitored on the FPA for absolute pointing measurements accurate down to a small fraction of the beamwidth.
To point the outgoing beam, a tiny fraction of its light is monitored on the FPA as well. Calculated point-ahead angles between the beacon spot and this transmitter signal are then achieved by moving a separate Point-Ahead Mechanism.
The transmitter, at 1.064 microns, is constructed of a grating-stabilized fiber laser, an external (telecom-class) waveguide modulator, and a Ytterbium-doped fiber amplifier that emits five Watts of average power. The modulation format known to be most efficient (when bandwidth is not too much of a constraint) is pulse-position modulation (PPM) with a high alphabet size. The MLCD system uses 64-PPM as its baseline, and so the transmitter must emit pulses that have peak power of 320 Watts. To achieve the wide range of data rates required as the orbits and daylight conditions vary over the mission, a combination of pulsewidth variation and symbol repetition will be employed.

The Palomar Receive Terminal (PRT)
MLCD is very lucky to be able to have extended access to the Hale Telescope at Palomar Observatory, which is run by Caltech Optical Observatories. The JPL team will both retrofit the telescope with required upgrades ([1]), and deliver to the facility a suite of receiver subsystems.
In order to allow the telescope to point not only into the daylight sky, but in fact as close as 3 degrees from the sun, JPL will design and install a grid of dielectrically-coated windows mounted in the dome’s aperture, along with a wind baffle. Achieving low in-band scatter while rejecting a high percentage of out-of-band sunlight is the driving engineering task.
The receiver consists of coupling optics, an optical filter of width 1 angstrom or less, a photon-counting detector, and a suite of electronics for spatial tracking, synchronization, demodulation, decoding, and demultiplexing. It is predicted that the PRT will be able to support links from the MLT from several hundred Kbps to 30 Mbps as the conditions change.
At the time of writing this report, several options are being evaluated for the photon-counting detector technology, including avalanche photodiodes and hybrid photo-multiplier tubes.
It is expected that MLCD will use the Hale Telescope during the 2-year mission about 2 weeks per month.

The OCTL Transmit Terminal (OTT)
To provide the uplink beacon signal, and to provide a very low rate (1-100 bps) commanding capability (in addition to an RF commanding capability through the MTO,) JPL is instrumenting their OCTL facility with a highly-capable uplink laser system. Employing multiple commercially-available fiber laser modules, each emitting well over 100 Watts, the OTT will couple between 500 and 1000 Watts of power through small subapertures for transmission to Mars. It is widely known that multiple, small, incoherent beams constitute an efficient technique for sending beams up through and out of the turbulent atmosphere.
These beams must be aligned and pointed to a small fraction of a beamwidth in order to maximize power delivery. This capability will be achieved by using pointing calibrations based on bright stars and the Martian disk. The uplink will be on-off modulated at 500 Hz so that the MLT can separate it efficiently from the bright Earth background. The on-off modulation is then used in special patterns that are used to carry framed and encoded uplink data.

The Telescope Array Receive Terminal
In addition to the Palomar Receive Terminal, MLCD will employ an Earth receiver based on the novel concept of an array of small telescopes ([2]) whose photon-counted outputs are synchronized and summed to create a virtual large telescope. Four (or more) 0.8-meter telescopes, each on its own gimbal and housed in its own inexpensive dome, comprise the Link Demonstration and Evaluation System (LDES) being developed by MIT/LL. In addition to demonstrating this scalable approach to receiver design, the LDES will provide a secondary receiver whenever the PRT is under clouds or not available. In addition, the concept of “handover’ between Earth terminals as the Earth rotates, presently standard for deep-space RF systems, can be demonstrated.

Mission Operations
An MLCD Mission Operations System will be constructed and staffed by JPL. Presently planned for residence at Palomar, the facility will include control and monitoring capability for all the Earth terminals, as well as command generation and telemetry monitoring capabilities for the MLT. Using a combination of pre-planned operations and nearer-to-real-time responses to Earth conditions, operators will coordinate link parameters and experiments such as trying to maximize data delivery in the face of changing atmospheric conditions and the long round trip time.

Conclusion
NASA GSFC, MIT Lincoln Laboratory, and the Jet Propulsion Laboratory are working together to create the Mars Laser Communications Demonstration. The goal of the program is to demonstrate the potential of high-rate, deep-space laser communications.

Acknowledgements
The designs and concepts sketched out in this overview have been developed by talented and dedicated teams at NASA GSFC, MIT/LL, and JPL.

References
[1] W. Tom Roberts, Hal L. Petrie, Andrew J. Pickles, Robert P. Thicksten, Chris Echols , “Feasibility of utilizing the 200-inch Hale Telescope as a deep-space optical receiver,” SPIE Free-Space Laser Communications IV, Vol 5550, 2004.
[2] Don M. Boroson, Roy S. Bondurant, Daniel V Murphy, “LDORA: A Novel Laser Communications Receiver Array Architecture”, SPIE LASE 2004, Free-Space Laser Communications Technologies XVI, Vol 5338.