The tiny amount of DC power available to the early satellites such as Explorer and Vanguard placed a limitation on the level of RF (radio frequency) power that could be radiated as a signal carrier; thus a satellite's signal was little more than a faint whisper from the cosmos. The solution was to build extremely large receiving dishes, or earth stations, which would act as a giant ear , collecting and amplifying the extremely low-power signals received from spacecraft.

In effect then, it was the mass and power constraints on spacecraft technology that led NASA to establish a network of tracking and communications stations around the world known as the Deep Space Network (DSN). Although deep space is generally defined as space beyond the vicinity of the Earth Moon system (the radius of the Moon's orbit being approximately 380,000 km), the DSN is also used for spacecraft in lunar orbit, highly elliptical Earth orbits, and for close-Earth fly-bys of interplanetary spacecraft using the planet's gravity to boost their velocity in a so-called gravitational slingshot manoeuvre ^[31].

The DSN evolved from the tracking and data recovery experience of the Jet Propulsion Laboratory (JPL) gained in work for the US Army in the 1950s. Specifically, it had developed Microlock, a phase-locked loop tracking system developed from early guidance research for the Corporal missile programme that could lock onto very low power signals.

For the Explorer 1 mission, JPL expanded its initial network of two Microlock tracking stations in California and Florida in...