digital slave, radar signals could be used to automatically maintain tracking. Spacecraft The Echo spacecraft (Echo 1, Echo 1A, and Echo 2) were large thin skinned spheres that were inflated in orbit after leaving the atmosphere. These balloon satellites were approximately in diameter with a thin skin made of Mylar (a trade name for stretched polyethylene terephthalate or BoPET), and were built by Gilmore Schjeldahl's G.T. Schjeldahl Company in Northfield, Minnesota. The satellites functioned as a reflector, not a transceiver; after being placed in low Earth orbit, signals could be sent from a ground station, reflected by its surface, and returned to Earth. As its shiny surface was also reflective in the range of visible light, Echo was easily visible to the unaided eye over most of the Earth. The spacecraft was nicknamed a "satelloon" by those involved in the project (a portmanteau combining satellite and balloon). It was used to redirect transcontinental and intercontinental telephone, radio, and television signals. During the latter portion of its life, it was used to evaluate the technical feasibility of satellite triangulation. Echo 1 Echo 1 was in diameter, had a non-rigid skin made of -thick Mylar, and had a total mass of , weighing at launch. During ground inflation tests, of air were needed to fill the balloon, but while in orbit, several pounds of gas were all that was required to fill the sphere. To address the problem of meteorite punctures and keep the sphere inflated, Echo 1 included a make-up gas system using two types of sublimating powders – of anthraquinone and of benzoic acid. It also had 107.9 MHz telemetry beacons, powered by five nickel-cadmium batteries that were charged by 70 solar cells mounted on the balloon. The spacecraft was useful to the calculation of atmospheric density and solar pressure, due to its large area-to-mass ratio. Echo 2 Echo 2 was a -diameter balloon satellite, the last launched by Project Echo. A revised inflation system was used for the balloon, to improve its smoothness and sphericity. Echo 2's skin was rigidizable, unlike that of Echo 1. Therefore, the balloon was capable of maintaining its shape without a constant internal pressure; a long-term supply of inflation gas was not needed, and it could easily survive strikes from micrometeoroids. The balloon was constructed from a -thick mylar film sandwiched between and bonded with two layers of -thick aluminum foil. It was inflated