Frequently Asked Questions
The most-asked questions about satellite navigation and gps.
What exactly is satellite navigation and how does it differ from 'GPS'?
Satellite navigation is the broader technology where a receiver on Earth calculates its position by timing signals from orbiting satellites. GPS (Global Positioning System) is the specific U.S. military-built constellation that made the concept famous, but the term is often used colloquially to mean any satellite-based positioning service.
Who are the key figures behind GPS?
The system traces back to U.S. Navy and Air Force engineers in the 1960s–70s, with Ivan W. Geitlin and the broader DoD program office often cited as pivotal. On the civilian side, the 1983 Korean Air Lines Flight 007 incident is widely credited with pushing President Reagan to open GPS to non-military users.
How many satellites does the GPS constellation actually use?
The U.S. operates 31 active GPS satellites in medium Earth orbit, arranged in six orbital planes. A minimum of four satellites must be visible to a receiver for a full 3-D position fix, which is why the network is designed with redundancy well above that number.
What are the other global satellite navigation systems besides GPS?
Russia runs GLONASS, the European Union operates Galileo, and China fields BeiDou. All four systems are mutually compatible in modern receivers, so a phone or car navigator can draw on satellites from multiple constellations simultaneously to improve accuracy and availability.
How does a receiver actually figure out where I am?
The receiver measures the travel time of a coded signal from each satellite, converting that into a distance (a 'pseudorange'). By intersecting the spheres defined by at least four such distances, the receiver solves for its latitude, longitude, altitude, and clock offset in a process called trilateration.
How accurate is a typical consumer GPS fix?
Under open-sky conditions, a standard civilian receiver locks to roughly 3–10 meters of accuracy. With augmentation services like SBAS (WAAS, EGNOS, GAGAN) or dual-frequency L1/L5 tracking, that can tighten to the one-meter range or better.
Why does GPS sometimes fail or drift indoors and in urban canyons?
Satellite signals are extremely weak by the time they reach the ground, so building materials, dense foliage, and tall structures block or reflect them. Multipath—where a signal bounces off a surface before hitting the antenna—creates false distance readings that throw the calculated position off by tens or even hundreds of meters.
What is GPS jamming and spoofing, and why do they matter?
Jamming floods the receiver with noise on the GPS frequency, drowning out the genuine satellite signal, while spoofing broadcasts a convincingly fake signal to make the receiver compute a wrong position. Both have been documented in the Black Sea, near airports, and at military exercises, raising concerns for aviation, shipping, and emergency services.
When did GPS actually go live, and what were the milestones?
The first GPS satellite (NavStar 1) launched in 1978, the constellation reached full operational capability in 1995, and the civilian L1 signal was opened to the public in 1983 after selective availability was lifted. The modern L5 civilian band was added in the 2010s with the GPS III satellite series.
Where should a newcomer start to really understand satellite navigation?
A good entry point is learning the basic trilateration geometry and the role of atomic clocks on each satellite, since those two ideas underpin everything else. From there, exploring the ICD (Interface Control Document) for GPS or the Galileo Open Service Signal In Space Profile gives a concrete, technical look at what the receiver is actually decoding.