Lasers have been considered for space communications since their realization in 1960.Specific advancements were needed in component performance and system engineering particularly for space qualified hardware. Advances in system architecture, data
formatting and component technology over the past three decades have made laser communications in space not only viable but also an attractive approach into inter satellite link applications.
Information transfer is driving the requirements to higher data rates, laser cross ?link technology explosions, global development activity, increased hardware, and design maturity. Most important in space laser communications has been the development of a reliable, high power, single mode laser diode as a directly modulable laser source. This technology advance offers the space laser communication system designer the flexibility to design very lightweight, high bandwidth, low-cost communication payloads for satellites whose launch costs are a very strong function of launch weigh. This feature substantially reduces blockage of fields of view of most desirable areas on satellites. The smaller antennas with diameter typically less than 30 centimeters create less momentum disturbance to any sensitive satellite sensors. Fewer on board consumables are required over the long lifetime because there are fewer disturbances to the satellite compared with heavier and larger RF systems. The narrow beam divergence affords interference free and secure operation.
Until recently, the United States government was funding the development of an operational space laser cross-link system employing solid-state laser technology. The NASA is developing technology and studying the applicability of space laser communication to NASA's tracking and data relay network both as cross-link and for user relay links. NASA's Jet Propulsion Laboratory is studying the development of large space and ground-base receiving stations and payload designs for optical data transfer from interplanetary spacecraft. Space laser communication is beginning to be accepted as a viable and reliable means of transferring data between satellites. Presently, ongoing hardware development efforts include ESA's Space satellite Link Experiment (SILEX) and the Japanese's Laser Communication Experiment (LCE). The United States development programs ended with the termination of both the production of the laser cross-link subsystem and the FEWS satellite program.
The frequency used for satellite communication should be selected from bands that are most favorable in terms of power efficiencies, minimal propagation of distortion, and reduced noise and interference effects. Terrestrial systems tend to favor these same bands. So, concern for interference effect between the satellite and terrestrial systems must be made.
? Satellite use from space must be regulated and shared on a worldwide basis. For this reason, frequencies to be used by the satellite are established by a world body known as the International Telecommunications Union (ITU) with broadcast regulations controlled by a subgroup known as World Administrative Radio Conference (WARC). An international consultative technical committee (CCIR) provides specific recommendations on satellite frequencies under consideration by WARC. The basic objective is to allocate particular frequency bands for different types of satellite services, and also to provide international regulations in the areas of maximum radiation?s level from space, co-ordination with terrestrial systems and the use of specific satellite locations in a given orbit. Within these allotments and regulations an individual country can make its own specific frequency selections based on intended uses and desiredsatellite services.
??????? Use of frequencies has been separated into military, non-military, and services has been designated as fixed point (between ground stations located at fixed points on earth), Broadcast (wide area coverage), and mobile (aircraft, ships, land vehicles). Inter satellite refers to satellite cross- links. Most of the early satellite was developed for UHF, C-band and X-band, which required the minimal conversion from existing microwave hardware. The foremost problem is the fact that the available bandwidth in these bands will be inadequate to meet present and future traffic demands. The advantage of using a carrier at higher frequencies is the ability to modulate more information on it.
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