Standard radio-frequency communication links are bumping into their limits. RF technology is being pressed on one side by data overload from information-rich sources, such as synthetic-aperture radars and image sensors that provide multispectral resolution. From the other side, RF devices generally cannot expand in size and complexity to meet the demand for higher data rates because of economic limitations on size and weight and the political limitation of needing to share limited bandwidth capacity.
Fortunately, a technological solution to this problem has been arriving gradually over the last decade in the form of high-power, single-mode laser sources along with related technologies for detection and tracking. These technologies have enabled lasers to emerge as an alternative method for free-space communication and have raised exciting prospects for the future. The transition from RF to laser communication is likely to be slow, however, for a number of reasons that have just as much to do with the humans who use the technology as with the technology itself. These reasons become particularly evident when viewed from the perspective of NASA missions and space science.
RF systems entrenched
The motivation for transitioning to laser communication with a spacecraft is the ability to achieve a high data rate with a compact, lightweight system (see photo). At the same time, however, a robust, broad-beam RF beacon system may provide the best tracking device for a spacecraft during the immediate post-launch period. During this initial period, unpredictable atmospheric variables still play a larger role in determining vehicle trajectory than the calculable orbit mechanics that follow Kepler`s laws. So while a relatively narrow laser beam might provide an ideal tracking mechanism after the vehicle is in a stable orbit, a supplementary RF beacon is still essential for the launch phase.
Even after a spacecraft is in orbit, an RF beacon can provide the only tracking option during emergency situations, such as sun-locked modes in which the spacecraft faces away from the Earth for extended periods. Optical solutions are likely to be developed eventually for tracking both post-launch and sun-locked spacecraft, but historically these types of problems have been solved with radio-frequency solutions. So people who are used to solving such problems using RF will have to see proof that an alternative solution works before the alternative can be adopted.
A third reason for maintaining RF capability in space missions is that often one of the science objectives of a spacecraft is to make measurements using radio signals, or to perform radio science. So as laser communication comes on-line in the deep-space missions, there will be no sudden replacement of the large investment that currently exists in the radio-frequency infrastructure. Laser communication will simply add one more element to the mission planners` toolbox.
About 20 years ago, laser communication was just a gleam in the eye. A tremendous amount of development work over that time has brought the technology to a point now where it can and should be demonstrated in space so that people will start picking it up with a high degree of confidence. Aspects of the technology still need to be worked out, but meaningful work can be performed. The largest single impediment to moving forward at this point probably has less to do with technology and more to do with the emotional inertia associated with changing something that people are familiar with. It`s important to deal with that emotional inertia respectfully and gradually, because when people actually start using a brand new technique, they must also relearn important intuitive elements.
People have what might be described as a seventh sense that says, "This is what`s going on," when something doesn`t look right. So there`s a whole infrastructure that has to do with intuitive interpretations. People build up that infrastructure as they gain experience with the technology. Consequently, anything new that comes along presents exciting opportunities as well as huge threats and questions. With that in mind, our short-term focus at the Jet Propulsion Laboratory is on performing a number of demonstrations of the laser communication technology to familiarize people with the technology initially and ultimately lead to broad and confident acceptance.