{{Short description|US research satellites, 1965–1976}} [[File:Lincoln Laboratory LES-1 1965.jpg|thumb|LES-1]] [[File:LES 4.jpg|thumb|LES-4]] The '''Lincoln Experimental Satellite''' series was designed and built by [[MIT Lincoln Laboratory|Lincoln Laboratory]] at [[Massachusetts Institute of Technology]] between 1965 and 1976, under [[United States Air Force|USAF]] sponsorship, for testing devices and techniques for satellite communication.
==Development==
After the successful development and deployment of [[Project West Ford]], a passive communications system consisting of orbiting copper needles, [[MIT Lincoln Laboratory|MIT's Lincoln Laboratory]] turned to improving active-satellite space communications. In particular, Lincoln aimed to increase the transmission capability of communications satellites ("[[downlink]]"), which was necessarily constrained by their limited size. After receiving a charter in 1963 to build and demonstrate military space communications, Lincoln focused on a number of engineering solutions to the downlink problem including improved antennas, better stabilization of satellites in orbit (which would benefit both downlink and "uplink"—communications from the ground), high-efficiency systems of transmission modulation/de-modulation, and cutting-edge [[Error detection and correction|error-checking techniques]].<ref name=beyond>{{cite book|url=https://ntrs.nasa.gov/api/citations/19970026049/downloads/19970026049.pdf|editor=Andrew J. Butrica|title=Beyond the Ionosphere: Fifty Years of Satellite Communication|date=1997|publisher=NASA History Office|location=Washington D.C.|bibcode=1997bify.book.....B|access-date=18 February 2020}}</ref>{{rp|81–83}}
These experimental solutions were deployed in a series of nine spacecraft called Lincoln Experimental Satellites (LES). Concurrent with their development, Lincoln also developed the Lincoln Experimental Terminals (LET), ground stations that used interference-resistant signaling techniques that allowed use of communications satellites by up to hundreds of users at a time, mobile or stationary, without involving elaborate systems for synchronization and centralized control.<ref name=beyond/>{{rp|81–83}}
The 1st, 2nd, and 4th satellites in the LES series were designated "X-Band satellites," designed to conduct experiments in the [[X band]], the military's [[super high frequency]] (SHF) band (7 to 8 [[Hertz|GHz]])<ref name=compendium>{{cite book|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730022102.pdf|title=NASA COMPENDIUM OF SATELLITE COMMUNICATIONS PROGRAMS|date=1973|publisher=Goddard Spaceflight Center|location=Greenbelt, MD}}</ref>{{rp|160}} because solid-state equipment allowed for comparatively high output in this band, and also because the band had been previously used by West Ford.<ref name=beyond/>{{rp|83–84}}
==Overview== [[File:Titan-3A with LES-1 satellite 11-02-1965.jpg|thumb|Launch of the Titan IIIA rocket with satellite Lincoln Experimental Satellite 1.]] The series had satellites named LES-1 through LES-9. They suffered a number of launch problems - LES-1 and LES-2 were supposed to be delivered to the same 2,800 x 15,000 km orbit,<ref>{{cite web | url = http://www.astronautix.com/craft/les.htm | title = Astronautix.com | url-status = dead | archiveurl = https://web.archive.org/web/20130828112807/http://www.astronautix.com/craft/les.htm | archivedate = 2013-08-28 }}</ref> though a failure of a boost stage left LES-1 in a 2,800 km circular orbit.<ref name=":0">{{Cite web |title=LES 1, 2 |url=https://space.skyrocket.de/doc_sdat/les-1.htm |access-date=2024-09-06 |website=Gunter's Space Page |language=en}}</ref> LES-3 and LES-4 were intended to be delivered to [[geostationary orbit]], but a launch problem left them in their transfer orbit.<ref name=":1">{{Cite web |title=LES 4 |url=https://space.skyrocket.de/doc_sdat/les-4.htm |access-date=2024-09-06 |website=Gunter's Space Page |language=en}}</ref> All these satellites returned useful results despite the incorrect orbits. LES-5, -6, -8 and -9 ended up successfully in [[geostationary orbit]]; the project that would have been LES-7 ran out of funding and was cancelled.<ref name = "NASA">{{cite web | url = http://www.hq.nasa.gov/pao/History/SP-4217/ch8.htm | title = Thirty Years of Space Communications Research and Development at Lincoln Laboratory}}</ref><ref name=":2">{{Cite web |title=LES 7 |url=https://space.skyrocket.de/doc_sdat/les-7.htm |access-date=2024-09-06 |website=Gunter's Space Page |language=en}}</ref> [LES-9 was in an elliptical orbit in the mid-1980s and was used by environmental buoys to communicate data out of the very high northern latitudes. (Geostationary satellites are not visible at those latitudes as they are below the horizon.)]
LES-3 was a very small (16 kg) satellite containing a radio transmitter,<ref>{{Cite web |title=LES 3 |url=https://space.skyrocket.de/doc_sdat/les-3.htm |access-date=2024-09-06 |website=Gunter's Space Page |language=en}}</ref> intended to measure the extent of multi-path interference due to reflection of 300 MHz radio waves off sufficiently flat parts of the Earth.
Amongst the technologies tested on LES-1 through LES-4 were solid-state [[X-band]] radio equipment, low-power logic circuits, electronic despinning (using optics to determine the location of the Earth and Sun relative to a spinning satellite at any moment, and then transmitting via whichever of several antennae were best positioned with respect to the Earth), and [[magnetic torquers]].<ref>{{cite journal | title=The Lincoln Experimental Satellite Program (LES-1,2,3,4): A Progress Report | journal=Proceedings of Communications Satellite Systems Conference May 2–4, 1966 | publisher=American Institute of Aeronautics and Astronautics}}</ref>
LES-8 and LES-9 were satellites of around 450 kg mass, launched on March 14, 1976.<ref name=":3">{{Cite web |title=LES 8, 9 |url=https://space.skyrocket.de/doc_sdat/les-8.htm |access-date=2024-09-06 |website=Gunter's Space Page |language=en}}</ref> They were originally planned to be equipped with [[Pulsed plasma thruster|pulsed plasma engines]] but actually launched with gas thrusters; unusually for communication satellites, they are powered by [[MHW-RTG]]s rather than by solar panels.<ref>{{cite web | url = http://www.aero.org/publications/martin/martin-8a.html | title = Aerospace Corporation article | access-date = 2007-12-07 | archive-url = https://web.archive.org/web/20120207125123/http://www.aero.org/publications/martin/martin-8a.html | archive-date = 2012-02-07 | url-status = dead }}</ref> There was a cross-link between them in the 36–38 GHz part of the [[K band (NATO)|K band]], with UHF up- and down-links; and the cross-link technology demonstrated was the basis for current [[Inter-satellite service|inter-satellite communications]] used by numerous satellites.<ref>{{Cite journal |last=Chan |first=Vincent |date=1988 |title=Intersatellite Optical Heterodyne Communication Systems |url=https://archive.ll.mit.edu/publications/journal/pdf/vol01_no2/1.2.4.intersatellitecomm.pdf |journal=The Lincoln Laboratory Journal |volume=1 |issue=2 |pages=169–186 |archive-url=https://web.archive.org/web/20240419025904/https://archive.ll.mit.edu/publications/journal/pdf/vol01_no2/1.2.4.intersatellitecomm.pdf |archive-date=19 April 2024 |access-date=4 September 2024}}</ref> The original intention was to run the cross-link at a frequency in the 55–65 GHz range, which is absorbed by water, so that it would be impossible for Earth-based receivers to pick up scattered signals, but technology at the time was inadequate. They operated in geostationary orbit until 1992 and now are drifting slowly.
[[Lincoln Laboratory]]'s next satellite-communication project after LES was the construction of [[FLTSATCOM|FLTSAT]] EHF Packages.
==LES Missions== {| class="wikitable sortable" border="1" |+Lincoln Experimental Satellites<ref name="log">{{Cite web |last=McDowell |first=Jonathan |title=Launch Log |url=http://planet4589.org/space/log/launchlog.txt |access-date=December 30, 2018 |publisher=Jonathon's Space Report}}</ref><ref name="cat">{{Cite web |last=McDowell |first=Jonathan |title=Satellite Catalog |url=https://www.planet4589.org/space/log/satcat.txt |access-date=February 11, 2020 |publisher=Jonathon's Space Report}}</ref> |- ! Name ! COSPAR ID !Launch Vehicle ! Launch Date ! Reentry Date |- | [[LES-1]] | 1965-008C<ref name=":0" /> |[[Titan IIIA]]-3 | 11 Feb 1965 | |- | [[LES-2]] | 1965-034B<ref name=":0" /> |Titan IIIA-6 | 6 May 1965 | |- | [[LES-3]] | 1965-108D<ref>{{Cite web |title=LES 3 |url=https://space.skyrocket.de/doc_sdat/les-3.htm |access-date=2024-09-06 |website=Gunter's Space Page |language=en}}</ref> | rowspan="2" |[[Titan IIIC]]-8 | rowspan="2" | 21 Dec 1965 | 6 Apr 1968 |- | [[LES-4]] | 1965-108B<ref name=":1" /> | 19 Aug 1970 |- | LES-5 | 1967-066E<ref>{{Cite web |title=LES 5 |url=https://space.skyrocket.de/doc_sdat/les-5.htm |access-date=2024-09-06 |website=Gunter's Space Page |language=en}}</ref> |Titan IIIC-14 | 1 Jul 1967 | |- | LES-6 | 1968-081D<ref>{{Cite web |title=LES 6 |url=https://space.skyrocket.de/doc_sdat/les-6.htm |access-date=2024-09-06 |website=Gunter's Space Page |language=en}}</ref> |Titan IIIC-5 | 26 Sep 1968 | |- | LES 7 | colspan="4" | Cancelled<ref name=":2" /> |- | LES-8 | 1976-023A<ref name=":3" /> | rowspan="2" |Titan IIIC-30 | rowspan="2" | 15 Mar 1976 | |- | LES-9 | 1976-023B<ref name=":3" /> | |}
==Current status== {{see also|LES-1#Legacy and status}} [[Amateur radio]] enthusiasts were able to detect the radio signals emitted from LES-1 at the end of 2012 and the telemetry of LES-5 in 2020.<ref name="npr">{{cite news |last1=Palca |first1=Joe |last2=Neuman |first2=Scott |date=24 April 2020 |title=Long-Lost U.S. Military Satellite Found By Amateur Radio Operator |url=https://www.npr.org/2020/04/24/843493304/long-lost-u-s-military-satellite-found-by-amateur-radio-operator?t=1587809667239 |work=NPR |access-date=25 April 2020 }}</ref> LES-9 was decommissioned in 2020 after 44 years of service, whilst LES-8 was decommissioned in 2004.<ref>{{Cite web | title=Lincoln Laboratory decommissions Lincoln Experimental Satellite-9 {{!}} MIT Lincoln Laboratory | url=https://www.ll.mit.edu/news/lincoln-laboratory-decommissions-lincoln-experimental-satellite-9 | archive-url=https://web.archive.org/web/20200713134957/https://www.ll.mit.edu/news/lincoln-laboratory-decommissions-lincoln-experimental-satellite-9 | access-date=2025-09-07 | archive-date=2020-07-13}}</ref>
==References== {{reflist}}
* [http://www.hq.nasa.gov/pao/History/SP-4217/ch8.htm NASA ''SP-4217 Beyond the Ionosphere'' (derived from William W. Ward, Franklin W. Floyd (1989), ''Thirty Years of Space Communications Research and Development at Lincoln Laboratory'')] is a detailed description of satellite communications development at [[Lincoln Laboratory]], and was used as a reference for much of this article.
==See also== *[[Voyager Program]] (also used MHW-RTGs, which operated into the 21st century)
[[Category:Massachusetts Institute of Technology]] [[Category:Communications satellites]] [[Category:Nuclear-powered robots]] [[Category:Satellite series]] [[Category:Lincoln Experimental Satellite|*]]