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.
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
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 desired satellite
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