Laser Communications for Angus Bay

Artemis will have a great need for high-bandwidth, very reliable communications links back to Earth. Initial plans call for beaming back large amounts of video imagery (most probably HDTV); tourists will want full audio/video and data communications back home; and scientific customers - most probably astronomical observatories located near and serviced by Angus Bay but run by scientists on Earth - will demand tremendous bandwidth for sending back data. In addition, the base itself will require a great deal of bandwidth for telemetry, control, and routine communications, and employee communications. Finally, if a high-bandwidth Luna-Earth communications system is in place, it could itself be a major source of income by providing communications services to non-Artemis customers, including other lunar bases and stations and even spacecraft in near-lunar space.

Until faster than light tachyon communication becomes a reality, electromagnetic (EM) transmission will be the preferred method for communication through space. Ever since Marconi invented the radio, communications engineers have reached for higher and higher frequencies, first long-wave radio, then short-wave, then microwave; the shorter the wavelength the more information that can be carried, the smaller the antenna required, and the sharper the transmitted beam can be.

Recently, significant developments have occurred in moving up one more notch in the spectrum, to the use of light as a carrier for space-based communications. Light has been used as a carrier for decades in ground-based applications, most notably the globe-spanning fiber-optic communications network upon which the telecommunications companies depend. However, NASA and the Ballistic Missile Defense Organization (BMDO) have been investigating the possibilities of laser-carried communications between Earth-based stations, aircraft, and satellites.

Laser-based communications offer several advantages over lower frequencies for point-to-point communications.

1. The transmitting and receiving stations are smaller and lighter for a given range.
2. Less overall power is required for a given distance and data rate.
3. Higher data rates may be achieved for a given distance and power output.
4. High security and resistance to interference.

In 1998, the BMDO will be launching the STRV-2 Satellite. The Space Technology Research Vehicle-2 is designed to test a number of new candidate technologies for space-based use, including an innovative all-composite structrure. One of the most promising experiments is a communications system designed by AstroTerra Corporation, of San Diego, California, a long-time manufacturer of laser communications systems for terrestrial uses. The system will be capable of transmitting 1.2 Gigibits/second over a distance of 1800 kilometers.

The STRV-2 lasercom system fits into a package less than 10 inches cubed, weighing 31.6 pounds (14.3 kg) including the transceiver telescopes and gimble, the electronics module, and a deployment mechanism. It utilizes a single 5.4"-diameter Schmidt-Cassegrain telescope as a receiver (satellite and ground stations have identical configurations); eight solid-state communications lasers with integral telescopes (125 mW each at 810 nm wavelength, 80 microradian divergence); and two acquisition/tracking lasers (100 mW each at 852 nm, 500/1500 microradian divergence). Four of the communications lasers form one 600 Mbits/second channel, transmitting with right-hand circular polarization and the other four form the other channel, transmitting with left-hand circular polarization. In the receiving telescope, cesium atomic line filters separate the communications signal from the tracking beacon, sending each in two seperate directions. The communications signal is then separated into the two channels by polarizers and passed to two avalanch photodiodes for conversion into an electrical digital signal, which is then decoded. The acquisition/tracking signal is focused onto a CCD camera sensor which tracks the spot and drives the gimbal system. The transmitted data is modulated onto the lasers by power switching (on-off keying) the laser diodes.

The basic AstroTerra system can be applied to an Earth-Luna communications system with some modification. Indeed, it is recommended that AstroTerra be approached to construct such a system. It would be necessary to increase the total laser power put out to 1 Watt, and increase the size of the receiving telescope to around 0.5 meters diameter. The pointing mechanism would need to be of extreme accuracy, however the behavior of the Earth-Moon system is extremely predictable, which makes the acquisition and tracking process much simpler.

Following is an initial set of calculations estimating the required characteristics for an Earth-Moon lasercom system. The received power required by the photodetectors is taken from a published estimate of the received power required by the STRV-2 system of 48 nW (Schuster, J, Hakakha, H, and Korevaar, E, "Optomechanical design of STRV-2 lasercom transceiver using novel azimuth/slant gimbal," SPIE Vol. 2699, January 1996, pp 227-239).

One note for those not familiar with optical systems: For a laser transmitter, the magnification of the telescope serves to decrease the divergence of the beam, thus making it spread out less. Any laser beam has a characteristic divergence angle and a beam width at the laser's exit aperture. The magnification of the telescope will increase the beam's width by a factor equal to the magnification, and decrease its divergence by the same amount. For instance, a typical commercially available NdYAG laser might have a beam width of 1 mm and a divergence of 1 milliradian would increase its width to 1 meter wide, but would decrease its divergence to 1 microradian. However, for the receiver, the primary purpose of a large telescope is to increase area, and thus the power received by, the photodetectors which convert the light into electrical impulses.

As can be seen, the system requirements are quite reasonable. 1.0 Watt is well within the range of currently available laser diodes, and a half-meter telescope is large but not unreasonable for a fixed-site system. The part of the system with the greatest risk would be the pointing system. The laser beam would have a Gaussian distribution, and in order to maximize the power received it is necessary to keep the receiving station within +/- 30% of the center of the beam (approximately). Given a beam divergence out of the telescope of 3.33 microradians, it is therefore necessary to have a pointing system capable of point the telescope accurately to within +/- 1 microradian. This will be difficult, the STRV-2 lasercom system has a pointing error budget of 40 microradians, but can be achieved through the use of high- precision bearings and resolvers and by using high-accuracy micro-positioners, such as piezoelectric drivers. That the stations are located on solid ground and their positions and movements will be known to a very high degree of accuracy will help. Of course it may also prove necessary to simply increase the laser's divergence and up the transmitted power to compensate.

Given the lunar environment, a composite telescope of the Dobsonian design would be suitable, with its very lightweight mounting structure and shroud. Given that the Earth is practically stationary in the Moon's sky, after initial alignment less than +/- 5 deg of movement should be required to acquire and track the ground stations on Earth through the Earth's rotation and the Moon's libration. If possible, a sun-shade should be deployed over the unit (a simple mylar film on poles would suffice) to reduce unwanted light and heating.

The Terrestrial transceivers would be identical to the Lunar systems, except that an equatorial mount with approximately 200 deg of excursion. would need to be used for tracking the Moon in its orbit. At least three ground stations would be required to ensure continuous communications, with five being preferred for redundancy. The stations would need to be in high locations where the weather is usually dry and cloudless, regularly spaced around the Earth's circumference, not necessarily on the equator. (Go to John Walker's The Earth-Moon Viewer and experiment). Some possible locations are Australia, Peru or Denver, and Italy. Mountaintop locations nearby astronomical observatories would be ideal. For the most part, the stations could operate unattended, with need only for scheduled and emergency maintenance.

Since the ground stations would necessarily be at remote locations, reliable high-bandwidth communications links to the rest of the world are vital. Fortunately, there are several global communications systems either in existance or forthcoming which will make such links available at reasonable cost. Several low-Earth orbit satellite systems dedicated to commercial data transmission are coming on line (see the Byte Magazine article on The Orbiting Internet: Fiber in the Sky). In addition, Project Oxygen - being developed by CTR Group, a global consortium of telecommunications companies - is planning on having a world-spanning 1-Terrabit/second fiber-optic network in place by 2003.

The major laser safety concern regarding an Earth-Moon lasercom system is that of blinding anyone that accidently wandered through the beam. Given the geometry of the situation, this could practically only involve crew and passengers on aircraft and spacecraft flying through the transmission beam, which would be highly improbably. Nonetheless the probability is finite and must be accounted for. The pertinent regulations are contained in ANSI Z136.1, American National Standard for Safe Use of Lasers, published by the American National Standards Institute.

Assuming the use of 810nm and 850nm lasers as in the AstroTerra system, Table 5 from ANSI 136.1-1993 gives the Maximum Permissible Exposure (MPE) as 320 x 10^-6 W/cm² (810 nm is beyond visible range, so the aversion response time is not applicable). The proposed system meets this criteria assuming that the transmitting telescope has an exit aperture of at least 300 mm. It should be noted that 1 Watt system output does constitute a Class 4 laser system and appropriate safety precautions must be taken at the ground stations (both Terrestrial and Lunar). These numbers are appoximate, and a final safety analysis will have to be done at the time of final system design.

This paper proposes a laser communications system to provide a lightweight, high-bandwidth link between Angus Bay and Earth. The system can be constructed using currently available, albeit state-of-the-art, technology currently being developed for high-bandwidth satellite links. Using such a system would give Artemis the communications bandwidth required to easily support its own internal data, video, and audio requirements, high-resolution video streams for the entertainment industry, and high-capacity data channels for scientific and industrial customers.