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Project Echo
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== Ground stations == [[File:Horn Antenna-in Holmdel, New Jersey - restoration1.jpg|thumb|upright=1.0|left|[[Holmdel Horn Antenna]], constructed for Project Echo, and later used to discover the [[cosmic microwave background radiation]].]] Two ground stations were used for testing Project Echo. The Goldstone facility located at [[Goldstone Lake|Goldstone Dry Lake]] in the Mojave Desert, California and the [[Crawford Hill]] facility located at Holmdel, New Jersey. Both sites used separate antennas for transmitting and receiving. West-to-east transmissions were sent from Goldstone by a {{cvt|26|m}} dish antenna built for Project Echo by JPL. The signals were received at Crawford Hill by a {{cvt|6|xx|6|m}} aperture horn-reflector antenna. [[Horn antenna]]s were known to have low-noise properties. A transmission frequency of 2390 [[Hertz|megahertz]] was selected, as this was the planned frequency band for future satellite experiments. East-to-west transmissions were sent from Crawford Hill using a {{cvt|18|m}} diameter parabolic antenna and received at Goldstone using the existing [[Pioneer program]] antenna. A transmission frequency of 960.05 megahertz was used for westbound communications because the JPL receiver was already tuned to this frequency from the Pioneer lunar program.<ref name="Butrica1997"/> Satellite acquisition and tracking were accomplished by three methods: optical, digital slave, and automatic radar. Optical tracking was the easiest method but could only be used at night when the [[Sun]] illuminated the satellite. Broad and narrow field telescopes with a [[Professional video camera|television camera]] were mounted to the structure of the antenna at each site. The camera images were displayed to a servo operator who would control the position of the antenna to track the satellite. When optical tracking could not be used, a computer system called digital slave could acquire and track Echo. Digital slave worked by receiving primary tracking data from the NASA [[Minitrack]] network of stations. The computer would then issue antenna-pointing commands to control the antenna. The third tracking method was a continuous-wave radar subsystem. Radar was not suitable for acquisition of the satellite, but once Echo was acquired by optical, or digital slave, radar signals could be used to automatically maintain tracking.<ref name="Butrica1997"/>
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