Satellite constellation in the context of "Pass (spaceflight)"

A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation (GPS), broadcasting, scientific research, and Earth observation. Additional military uses are reconnaissance, early warning, signals intelligence and, potentially, weapon delivery. Other satellites include the final rocket stages that place satellites in orbit and formerly useful satellites that later become defunct.

Except for passive satellites, most satellites have an electricity generation system for equipment on board, such as solar panels or radioisotope thermoelectric generators (RTGs). Most satellites also have a method of communication to ground stations, called transponders. Many satellites use a standardized bus to save cost and work, the most popular of which are small CubeSats. Similar satellites can work together as groups, forming constellations. Because of the high launch cost to space, most satellites are designed to be as lightweight and robust as possible. Most communication satellites are radio relay stations in orbit and carry dozens of transponders, each with a bandwidth of tens of megahertz.

A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth. Communications satellites are used for television, telephone, radio, internet, and military applications. Some communications satellites are in geostationary orbit 22,236 miles (35,785 km) above the equator, so that the satellite appears stationary at the same point in the sky; therefore the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track the satellite. But most form satellite constellations in low Earth orbit, where antennas on the ground have to follow the position of the satellites and switch between satellites frequently.

The radio waves used for telecommunications links travel by line of sight and so are obstructed by the curve of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated geographical points. Communications satellites use a wide range of radio and microwave frequencies. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" certain organizations are allowed to use. This allocation of bands minimizes the risk of signal interference.

The A-train (from Afternoon Train) is a satellite constellation of four Earth observation satellites of varied nationality in Sun-synchronous orbit at an altitude that is slightly variable for each satellite.

The orbit, at an inclination of 98.14°, crosses the equator each day at around 1:30 pm solar time, giving the constellation its name (the "A" stands for "afternoon") and crosses the equator again on the night side of the Earth, at around 1:30 am.

Space Exploration Technologies Corp., commonly known as SpaceX, is a private American aerospace company and space transportation company headquartered at the Starbase development site in Starbase, Texas. Since its founding in 2002, the company has made numerous advances in rocket propulsion, reusable launch vehicles, human spaceflight and satellite constellation technology. As of 2025, SpaceX is the world's dominant space launch provider, its launch cadence eclipsing all others, including private competitors and national programs like the Chinese space program. SpaceX, NASA, and the United States Armed Forces work closely together by means of governmental contracts.

SpaceX was founded by Elon Musk in 2002 with a vision of decreasing the costs of space launches, paving the way to a self-sustaining colony on Mars. In 2008, Falcon 1 successfully launched into orbit after three failed launch attempts. The company then moved towards the development of the larger Falcon 9 rocket and the Dragon 1 capsule to satisfy NASA's COTS contracts for deliveries to the International Space Station. By 2012, SpaceX finished all COTS test flights and began delivering Commercial Resupply Services missions to the International Space Station. Also around that time, SpaceX started developing hardware to make the Falcon 9 first stage reusable. The company demonstrated the first successful first-stage landing in 2015 and re-launch of the first stage in 2017. Falcon Heavy, built from three Falcon 9 boosters, first flew in 2018 after a more than decade-long development process. As of May 2025, the company's Falcon 9 rockets have landed and flown again more than 450 times, reaching 1–3 launches a week.