The meteoric rise from the first use of GPS in the 1990s to billions of users of multiple constellations 30 years later can be understood from several key design choices made by GPS early on. GNSS is a one-to-many broadcast service, with signal power below the noise floor and clever signal processing techniques to make use of them. The civilian signals have publicly available Interface Specifications (ISs) or Interface Control Documents (ICDs) that allow anyone to develop and market GNSS receivers.
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