A satellite works by pairing a body that keeps it alive with equipment that does a job, then flying the whole thing around the Earth fast enough to keep falling without ever landing. The body is called the bus. The working equipment is the payload. Solar panels power both, small thrusters hold the satellite in place, and antennas on the ground send commands up and pull data back down. Strip away the complexity and every satellite, from a weather watcher to a broadband relay, follows that same pattern.
The Bus and the Payload
Every satellite divides into two parts, and telling them apart makes the rest of the machine easy to follow. The bus is the standardized body: it supplies power, keeps the temperature in range, runs the onboard computer, and points the satellite where it needs to aim. Think of the bus as the truck. It is much the same whether it hauls one cargo or another.
The payload is the cargo, the reason the satellite was launched. On a communications satellite the payload is the set of transponders that relay signals, the heart of the SATCOM link. On a weather satellite it is a camera and sensors. On a navigation satellite it is a precise clock and a transmitter. The same bus design can carry very different payloads, which is why manufacturers reuse a proven bus across many missions and swap the payload to fit each customer.
Power: Solar Panels and Batteries
A satellite makes its own electricity, almost always from the Sun. Solar panels, the wide wings folded against the body at launch and spread open in orbit, convert sunlight into the power that runs everything on board. A large communications satellite can draw as much electricity as several homes, so its panels stretch tens of meters across once deployed.
Sunlight is not constant, though. A satellite regularly passes into Earth’s shadow, where the panels make nothing. To bridge those dark stretches it carries rechargeable batteries, charged by the panels in sunlight and drained in shadow. The pairing of panel and battery keeps the payload running around the clock. Only probes sent far from the Sun, where sunlight grows too faint to be useful, swap solar power for a nuclear source instead.
Orbits: Why a Satellite Does Not Fall
A satellite stays up because it is moving sideways fast enough to keep missing the Earth as it falls. Gravity constantly pulls it toward the planet. Its forward speed constantly carries it past the horizon. When the two balance, the satellite falls around the curve of the Earth in a closed loop, which is what an orbit is. Higher orbits need less speed to stay up, and lower ones need more.
The altitude a satellite is placed at sets what it can do.
| Orbit | Altitude | Behavior | Common use |
|---|---|---|---|
| Low Earth orbit | ~160 to 2,000 km | Circles the planet in about 90 minutes | Imaging, broadband constellations |
| Medium Earth orbit | ~2,000 to 35,786 km | Slower passes, wide view | Navigation, regional links |
| Geostationary orbit | ~35,786 km | Appears fixed over one spot | Broadcast, weather, wide-area comms |
A geostationary satellite circles at the exact rate the Earth turns, so from the ground it seems to hang motionless. That is why a home dish can point at one spot in the sky and never move. A low-orbit satellite, by contrast, races overhead and is gone in minutes, which is why low-orbit networks need many satellites. Our LEO satellites explainer covers that tradeoff in detail.
Station-Keeping: Staying in the Right Spot
Orbits are not perfectly stable. The Sun and Moon tug at a satellite, the Earth is not a smooth sphere, and the faint upper atmosphere drags on low-flying craft. Left alone, a satellite slowly drifts off its assigned slot. To fight that drift it carries small thrusters and fires them in short, scheduled bursts, a routine called station-keeping. A geostationary satellite gets nudged to hold its exact longitude so it stays lined up with the dishes aimed at it.
Pointing is a separate job from position, and it runs constantly. A communications satellite has to keep its antennas aimed at the right slice of the Earth, and a camera satellite has to hold steady on its target, so each carries an attitude control system. Spinning wheels inside the satellite, called reaction wheels, speed up or slow down to turn the craft without spending fuel, while sensors that track the Sun, the stars, or the Earth’s horizon tell the computer which way it is facing. This is why a satellite can stare at one spot for years without drifting off aim.
Fuel for those thrusters is finite, and it often decides how long a satellite lives. When the propellant runs low, the operator uses the last of it to move the satellite out of the way, either dropping a low-orbit craft into the atmosphere to burn up or lifting a geostationary one into a higher graveyard orbit. The payload might still work, but without fuel the satellite cannot hold its position, so its useful life is over.
Ground Stations and the Uplink
A satellite is useless on its own. It needs a ground station, the antenna and control room on Earth that talks to it. The ground station sends commands up on the uplink, telling the satellite where to point and what to do, and receives the satellite’s data and health reports on the downlink. Operators watch each satellite’s power, temperature, and position from these centers and schedule the station-keeping burns from there.
The ground segment is also where a satellite’s work reaches users. A broadband satellite relays traffic between a user’s terminal and a gateway ground station wired into the internet. A weather satellite dumps its images to a receiving station that feeds forecasters. To follow the signal path from the antenna up to orbit and back, read how a full SATCOM link uses that same uplink and downlink to move data across the world.