The payload is the part that does the satellite's job, such as a camera or radio; the bus is everything that keeps the payload alive and pointed.
Every satellite has two halves. The {{payload}} is the reason it was launched: a camera, a weather imager, a set of radio transponders, a navigation transmitter. The bus is the rest of the spacecraft. It carries the payload and supplies everything the payload needs.
Think of a delivery van. The cargo is the payload. The van — engine, fuel tank, battery, steering — is the bus.
Worked example: ISRO's small IMS-1 bus. The whole satellite is about 100 kg, and up to 30 kg of that can be payload. So roughly 70 percent of the satellite exists only to support the other 30 percent. Its solar arrays make 330 W, shared between the two.
The bus does the same jobs on almost every satellite:
ISRO's GSAT-19 shows all of these in one machine. Its two solar arrays give about 4,500 watts. Sun, Earth and star sensors plus gyroscopes tell it which way it faces, and momentum wheels, magnetic torquers and thrusters turn it. A liquid apogee motor raised its orbit.
ISRO builds a few standard buses and fits these different payloads on top (see ISRO's satellite buses).
Compare the categories in the list — weather, navigation, communication, Earth observation: each one is a different payload riding on a similar ISRO bus. Open the ISRO satellite tracker
Engineers judge a bus by how much useful payload it can carry for its own mass and power. A simple measure is the payload mass fraction:
payload fraction = payload mass ÷ total spacecraft mass
ISRO's IMS-1 microsatellite bus is a clean example. ISRO gives the whole satellite as about 100 kg with a payload capacity of 30 kg. So:
payload fraction = 30 ÷ 100 = 0.30, or 30 percent.
The other 70 kg is bus: structure, solar array, battery, radios, attitude sensors and the computer. IMS-1's solar arrays produce 330 W at a raw bus voltage of 30–42 V, and that power budget is shared between bus and payload.
A second constraint is power. The bus must feed the payload and run itself, in sunlight and in shadow. If a payload needs P_payload watts and the bus needs P_bus watts, the solar array must supply at least P_payload + P_bus plus the energy to recharge the battery after each eclipse, with margin for the cells ageing.
For big geostationary satellites the biggest single item is often not the payload at all but propellant (see launch mass vs dry mass). This is why designers count payload against dry mass when they compare buses.
ISRO's subsystem list for GSAT-19 maps cleanly onto the standard bus functions: power (solar arrays, Li-ion battery), attitude and orbit control (sensors, momentum wheels, magnetic torquers, thrusters), propulsion (liquid apogee motor and chemical thrusters) and telemetry (it even flew a Ku-band TTC transponder as a new technology).
The payload is the equipment that does the mission, such as a camera or transponder. The bus is the supporting platform — structure, power, propulsion, attitude control, thermal control and communications — that keeps the payload working.
Kalpana-1 carried a Very High Resolution Radiometer (VHRR) for weather imaging and a Data Relay Transponder (DRT).
The supporting jobs — power, pointing, propulsion, heat and communication — are similar on most satellites, so a proven bus can carry many different payloads, saving design time and risk.