||Ahmed H Mohamed
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The advent of satellites and the advances thereafter of satellite navigation from the early 1960s till now changed the face of our life on Earth; Google maps and GPS car navigation systems are just testimonial of the indispensability of its importance. And yet, men still look to the sky for help. In the past, it was stellar navigation that guided men in their journeys and voyages around the Globe. Radio navigation aids appeared in the late 1930s and matured in the 1940s. Early studies exploring the idea of navigation by satellites were carried out by the US Air and Naval Forces in the early 1960s. The Navy Navigation Satellite System (NNSS or Transit) based on measuring the Doppler frequency of the received radio signal was the first all-weather continuous global navigation satellite system to emerge. The DoD NAVSTAR Global Positioning System program was initiated almost a decade later, through the merger of US Naval and Air Force projects. The first experimental GPS satellite was launched in 1977, while the first GPS receivers were introduced during the 1980s; they were big, cumbersome and expensive, and used a single channel receiver which consecutively locked on to each visible satellite and performed the pseudo-range measurement. By the 1990s, the receivers were multi-channel, more compact and cheaper; thanks to the emergence of the microprocessor. This was the period when GPS started to migrate from aircraft and ships - large and expensive platforms - to become a ubiquitous navigation system in military and civilian applications.