Discovery
| Parameter | Value |
|---|---|
| Program | NAVSTAR GPS, US Department of Defense |
| Program start | 1973 (merger of Navy TIMATION and Air Force 621B projects) |
| First satellite | Navstar 1 (Block I), February 22, 1978 — preceded by the US Navy NTS-2 demonstrator, June 23, 1977 |
| Full operational capability (FOC) | July 17, 1995 |
| Nominal constellation | 24 satellites (6 orbital planes × 4 satellites) |
| Orbital altitude | 20,200 km (MEO orbit, semi-synchronous) |
| Orbital period | 11 h 58 min (2 orbits/sidereal day) |
| Civilian frequencies | L1: 1,575.42 MHz · L2: 1,227.60 MHz · L5: 1,176.45 MHz |
| Onboard clock | Cesium (precision ~10⁻¹³) and rubidium (10⁻¹²) |
Technical Explanation
1. Signal and pseudo-range measurement — Each satellite broadcasts a timestamped signal containing a unique pseudo-random noise (PRN) code. The receiver locally generates a replica of the same code and measures the time offset between the received signal and the replica. This offset, multiplied by the speed of light (c = 299,792,458 m/s), gives the satellite-receiver pseudo-range. With a temporal resolution of 1 ns, the theoretical precision is ~30 cm.
2. Trilateration with 4 satellites — Three satellites are sufficient to solve the 3 position unknowns (x, y, z). A fourth satellite is needed to correct the receiver's clock bias (quartz clock ≈ 10⁻⁶, far less precise than the onboard atomic clocks). The system thus solves 4 equations with 4 unknowns every second.
3. Relativistic corrections — Special relativity slows the satellite clocks by −7.2 μs/day (orbital velocity of 3.87 km/s). General relativity speeds them up by +45.8 μs/day (weaker gravitational field at 20,200 km). The net effect is +38.6 μs/day. Without correction, the positioning error would grow by 11.4 km per day. The onboard clocks are deliberately offset by −4.464733 × 10⁻¹⁰ s/s before launch to compensate.
4. Control segment — The ground network includes a master control station (Schriever Space Force Base, Colorado), 11 command antennas, and 16 monitoring stations that continuously track the satellites, calculate precise ephemerides, and upload clock corrections every 2 hours.
Why It Worked
GPS solves a problem that seemed intractable: nanosecond-level time synchronization between clocks separated by 20,200 km, moving at 3.87 km/s, in a different gravitational field. The solution rests on two pillars: onboard atomic clocks (cesium and rubidium) with stability exceeding 10⁻¹³, and the operational application of Einstein's general relativity — the first civilian technology requiring Einstein's physics to work correctly.
Bill Clinton's decision in May 2000 to remove the intentional degradation of the civilian signal (Selective Availability, ±100 m) multiplied civilian precision by 10 and triggered the explosion of commercial applications (car navigation, smartphones, precision agriculture).
Causal Chain
Transit (first navigation satellite, US Navy, 1960) → miniaturized atomic clocks (1960s) → NAVSTAR program initiated by DoD (1973) → US Navy NTS-2 demonstrator with cesium clock (1977) → first Block I satellite, Navstar 1 (February 22, 1978) → Selective Availability removed (May 1, 2000) → smartphone integration (iPhone 3G, 2008) → GPS III with L1C signal interoperable with Galileo (2018) → centimeter-level RTK positioning in real time (2020s)
Anecdote
On September 1, 1983, Korean Air Lines Flight 007 is shot down by the USSR after straying into Soviet airspace, killing all 269 passengers. President Reagan then announces that GPS, until then exclusively military, will be made freely available to civilians upon completion, to prevent such tragedies from recurring. This political decision transformed a military tool into a global public good.
Sources
References verified during the August 2026 fact-checking audit: these are the pages
against which this bulletin's claims were checked.