USA 4

USA 4 is an active satellite operated by United States, launched on 1984-08-28 from Vandenberg SFB, California on the SDS 5 launch. With over 42 years in orbit, it has far exceeded many satellites’ design lifetimes. It orbits in Medium Earth Orbit (MEO) at altitudes between 2,720 km and 37,623 km with an inclination of 63.1°. It travels at approximately 13,951 km/h (3.88 km/s), completing one full orbit every 11 hours 58 minutes — that’s roughly 2.01 orbits per day. Its orbital eccentricity of 0.6575 gives it a noticeably elliptical path, with significant altitude variation between perigee and apogee. Orbital Radar tracks USA 4 in real time using the latest two-line element set (TLE) data, providing live position, altitude, speed and orbital path updated continuously.

USA 4 orbits at an average altitude of 20,172 km in Medium Earth Orbit, the region between LEO and GEO (2,000–35,786 km). MEO’s higher altitude gives each satellite a much larger ground footprint than LEO, meaning fewer spacecraft are needed for global coverage — but signal latency is higher and radiation from the Van Allen belts is a significant design constraint. Within ±50 km of USA 4’s average altitude, there are currently 105 active payloads and 13 tracked debris or rocket body fragments — notable neighbours include GPS BIIR-5 (PRN 22), GPS BIIR-8 (PRN 16), GPS BIIR-11 (PRN 19). With an inclination of 63.1°, USA 4 passes over latitudes between 63.1°N and 63.1°S, covering most populated land masses in both hemispheres. This mid-inclination band balances global coverage with efficient launch energy requirements. United States operates approximately 12,413 active satellites in total, of which 43 share a similar altitude band with USA 4.