Which statement best describes a GPS development requirement?

• A. It required an accurate model of Earth's shape, along with precise timekeeping and geometry.
• B. It relied on ancient navigation methods only.
• C. It used satellites exclusively.
• D. It required a manual map of sea routes.

Answer: (D)

The fundamental idea is that GPS position comes from converting precise timing signals from satellites into a location on Earth using a solid mathematical framework. Satellites carry atomic clocks and broadcast their time and orbital positions; your receiver measures how long those signals take to arrive, turning that into distances to multiple satellites. With at least four satellites, you use the geometry of intersecting spheres—trilateration—to solve for your three spatial coordinates plus the receiver’s clock bias. But to translate those coordinates into a real place on the globe, you need a reliable model of Earth’s shape, typically a reference ellipsoid, so coordinates map correctly to latitude and longitude. That combination of precise timekeeping and the Earth’s shape model is what GPS development relies on, enabling accurate positioning. The other statements don’t capture this mechanism: navigation from ancient methods isn’t the basis here, satellites aren’t used in isolation without a shared time reference and ephemeris data, and a manual sea-route map isn’t what enables computing current position.