shape and size, construction materials, temperature control and satellite tracking. Echo was designed as an experiment to demonstrate the potential of satellite communications, not to function as a global communications system. Echo was designed, approved and built with the following objectives: Observe and measure the effects of atmospheric drag Passively reflect ground based transmissions Demonstrate two-way communications Demonstrate commitment to the development of an American space program Provide precedent for the overflight of other nations by surveillance satellites All of these objectives were accomplished with Project Echo. Further experiments used the satellite to engage a two-way telephone conversation on 15 August 1960 and to relay a live television transmission in April 1962. Ground stations Two ground stations were used for testing Project Echo. The Goldstone facility located at Goldstone Dry Lake in the Mojave Desert, California and the Holmdel 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 dish antenna built for Project Echo by JPL. The signals were received at Holmdel by a aperture horn-reflector antenna. Horn antennas were known to have low-noise properties. A transmission frequency of 2390 megahertz was selected, as this was the planned frequency band for future satellite experiments. East-to-west transmissions were sent from Holmdel using a 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. 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 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