Laser transponders are currently being developed for satellite laser ranging, but they can also be deployed on the lunar surface as an alternative to retroreflectors . Transponders are devices that both send and receive predictable signals and can be used for ranging and time transfer. They have an approximately r2 link advantage over direct ranging loss of 1/r4, as the signal is propagating in only one direction before being received. With the development and inclusion of laser communications for spaceflight missions, it is logical to include an optical transponder that uses the same opto-mechanical infrastructure with minimal impact on the mission resources. These instruments could be used to support the relativistic and lunar science in addition to providing communications support to astronauts and/or other scientific instruments. These lunar instruments would also provide a pathfinder for applications on Mars and other planetary bodies where the use of passive retroreflectors is not practical.
Asynchronous Laser Transponders (ALT) are relatively simple devises that have good potential for space applications. The ground and remote stations of ALT fire their lasers independently (as opposed to an echo transponder, which works by sending back a timing signal with a fixed delay from the receipt of the base-station signal). This allows the use of free-running lasers on the space-based terminal to operate at their most efficient repetition rates. It is also simpler and potentially more reliable than other types of laser transponders. However, it does require a clock on both ends.
Several interplanetary laser transponder experiments were successfully performed from the NASA Goddard Space Flight Center satellite laser ranging facility. The first (two-way, 24 × 106 km link) utilized the Mercury Laser Altimeter on the Messenger spacecraft  and the second (one-way, 80 × 106 km link) utilized the Mars Orbiting Laser Altimeter on the Mars Orbiter spacecraft . More recently, two-way ranging was successfully performed using the Lunar Obiter Laser Altimeter (LOLA) on the Lunar Reconnaissance Orbiter (LRO) orbiting the Moon . These experiments have proven the concept of being able to point both transceivers, detect the photons, and retrieve useful parameters at low-link margins over interplanetary distances.
An ALT, shown in Figure 9, is currently under development at NASA Goddard Space Flight Center that uses technology derived from the Next Generation Satellite Laser Ranging (NGSLR) system . Other efforts at Goddard and other institutions are currently underway that combine laser ranging and laser communications. The robust link budget combined with the potential compatibility with NGSLR would open up to the possibility of using a large number of existing ground stations, which would not only increase the scientific potential, but also significantly reduce the ground station and operational costs.
Living Rev. Relativity 13, (2010), 7
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