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Ab8tmct-Projections of the growth of earth-sehg systems for the htter half of the 1980's show I data tmnanision requirement of 300 Mbit/s md &ow. Mission umstdnta md objectives led to the condusion that the most efficient technique to ntum the data h m 8 ensingsatellitetoagroundstrtionisthroughageosynrbronousdrtareky satellite. Of the two finlcs that are involved ( -sateIlite to relay satellite and relay satellite to grourrd), a laser system is most attractive for the sprce-tosprce link. A five-ym program was conducted which has &own the bmic technial feia'bitity of wmptishing thig link with a C02 laser system operaat a wavelength near 10 pm. The system wes a rnercury-cadmbm-telluride infraed mixer cooled to approxk m8tely 100 K by 8 ndi8tive cooler. The h r loal osciil8tor md couphgmoduhted I.ser tnnanittez use conductive& coded beryllium oxide construction. The optical 811-8 wed for t & r e -ceptionhasa~ofwminrlly92dBwitha3~bermwidthof80rru. T r r c~j i t t e r b l e s s t h . a t e n r r m d~P c q p h i t i o n o c c u r s m l~t h n n Manuscript received April 22, 1976; revised July 6, 1976. The submission of this paper was encouraged after review of an advanced proposal. are with the Space and Communication Group, Hughes Aircraft Company, El Segundo, CA 90245. Hammer, Inc., Melville, NY 11746. B. J . Peyton and T. Flattau are with AIL, a division of Cutler-PROCEEDINGS OF THE IEEE, FEBRUARY 1977 e LANDSAT-1 e TIROS-N e ITOS 0.001 O.O1 1960 i 1970 1980 Fa. 1. Space data transmission requirements. tions satellite. All intersatellite links are assumed to use laser communication systems, while all satellite-ground links use RF systems. The advantages provided by laser systems are compactness, freedom from frequency allbcation limits (which, for example, limit a &-bandlink to about 300 Mbit/s), and freedom from interference due to terrestrial RFI.Although the discussion has concentrated on a particular requirement which leads to the desirability of intersatellite links, such links have obvious utility in other applications. In domestic satellite systems, an intersatellite link would permit two satellites to serve essentially as a single system. By eliminating the transfer of data between satellites through a ground station, frequency allocations for ground-satellite links are conserved [ 181 . Similar considerations apply to international communication satellite systems [ 191 , where interregional telephony without intersatellite connections can require as many a s ' three hops-with an attendant excessive one-way propagation time.

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