Using Light Emitting Diodes for Intersatellite Links full report
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03-04-2010, 11:38 PM

We examine the utility of Light-Emitting Diodes (LEDs) for short-range intersatellite links (ISLs), and compare and contrast LEDs with existing laser technologies used for long-distance ISLs. A hypothetical low-end LED ISL link is described, and applications are suggested

Presented By:
Lloyd Wood
Surrey, England
Will Ivancic
NASA Glenn Research Center
Klaus-Peter Dörpelkus
Munich, Germany

Current thinking on intersatellite link communication is that either the links use radio (e.g. operationally in the existing Iridium constellation) or via laser (demonstrated in experimental LEO/GEO connections, e.g. the SILEX payload onboard the geostationary Artemis satellite.) We can classify intersatellite links (ISLs) for geostationary satellites according to the distance between the communicating satellites:

(a). Long-distance ISLs,
connecting distant geostationary satellites. Examples of this are the long connections outlined in Arthur C. Clarke's 1945 paper in wireless communications, making a triangle around the Earth , or the high-speed laser intersatellite links that were originally proposed for the satellite component of the US TSAT (Transformational Satellite Architecture) program . (Its proposed link rate of 40Gbps was the maximum rate then available on commercial terrestrial optical fibres.) Since these links between geostationary satellites traverse tens of thousands of km through free space, use of expensive highly directed lasers is expected. Such longdistance ISLs are actually not that useful for network communications, as their propagation delays add significantly to the end-to-end path delay that the geostationary satellite uplink and downlink previously dominated. This long delay degrades the performance of many network transfer protocols such as TCP, and is far less than ideal for interactive real-time applications. Still, the military desire to be able to communicate to anywhere, without relaying through bottleneck ground stations, drives their demand for long-distance ISLs. Laser links have also been proposed for connecting geostationary satellites with low-Earth-orbiting satellites, with aircraft or direct to the ground. Connecting to rapidlymoving LEO satellites poses engineering challenges for acquisition and tracking; connecting through the atmosphere adds additional signal loss and interference. 1

2 (b). Short-distance ISLs,
etween formation-flying satellites that are relatively near to each other (tens of km or less) and stationkeeping together. The satellites are close enough to be able to interact to create a 'virtual satellite' cluster that is made up out of all the communicating satellites, and is more than the sum of its parts. This clustering can also be applied to nearby stationkeeping geostationary satellites. Formation flying is the foundation of the DARPA F6 effort. Because the local links in b) are much shorter distance than the links discussed in a), use of lasers and the extremely accurate pointing they offer can be considered engineering overkill for this application. These shorter local links are commonly expected to be high-frequency wireless links. Reuse of terrestrial wireless technologies, adapted for space and for increased distance, has been proposed. However, these terrestrial technologies are constrained in frequency to the intersection of the sets of what works well in atmosphere, and what is permissible in allocated terrestrial regulatory bands. One disadvantage to using high-frequency radio is the possibility of electromagnetic interference from sidelobes affecting other payloads, complicating electromagnetic interference, testing and payload integration.

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