LES-4

After the successful development and deployment of Project West Ford, a passive communications system consisting of orbiting copper needles, 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-checking techniques.^{[2]: 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.^{[2]: 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 SHF (super high frequency) band (7 to 8 GHz)^[3]: 9–1 because solid-state equipment allowed for comparatively high output in this band, and also because the band had been previously used by West Ford.^{[2]: 83–84}

LES-1, launched 11 February 1965, failed to depart from its original circular medium orbit when its onboard thruster failed to fire. The resultant tumbling and the improper orbit rendered the satellite useless for experimentation purposes.^{[2]: 83 [3]: 161} LES-2, launched 6 May 1965^[4] was used in a number of communications experiments and met all expected objectives.^[3]: 9–3 The satellite was automatically shut down by its internal clock in 1967.^[3]: 9–4

With medium orbit experiments complete, the experimenters then focused on conducting X-band experiments from geosynchronous orbit^[2]: 83

LES-4 was an evolution of the LES-1/2 design for a much higher orbit. It carried more solar cells^[2]: 83 to run a more powerful transmitter^{[3]: 9–12} and a greater number of Sun and Earth sensors.^[2]: 83 In addition, LES-4 was equipped with an electron spectrometer^[1] to measure radiation in orbit in the 130 KeV to 4 MeV range,^: 9–12 both for scientific purposes and to correlate the effect of radiation from the intense Van Allen Belts on spacecraft functions.^[2]: 83 The satellite was spin stabilized, designed to rotate perpendicular to its orbital plane.^{[3]: 9–12}