An Introduction to Satellite Platforms
A satellite platform, often called a "bus," is the main body and structural component of a spacecraft that holds the payload and all the scientific instruments. The bus provides the essential "housekeeping" functions needed for the satellite to operate, such as power, propulsion, thermal control, telemetry, and attitude control. The payload, in contrast, is the equipment that performs the actual mission, such as a camera, antenna, or sensor. Platforms can be standardized and adapted for various missions, or custom-built for highly specific tasks.
Types of Satellite Platforms
Satellites are broadly categorized by their mass and size, which often dictates their capabilities and launch requirements. The U.S. industry utilizes a full spectrum of these platforms.
- Large Satellites (>1,000 kg): These are conventional, school bus-sized spacecraft typically launched into GEO for telecommunications (e.g., ViaSat-3) or into specialized orbits for flagship scientific missions (e.g., James Webb Space Telescope). They have long design lives, high power generation, and capacity for multiple complex payloads.
- Small Satellites (10-500 kg): This category, including "smallsats" and "minisatellites," represents a major shift in the industry. Lower development costs and the ability to launch multiple satellites at once have made them popular for LEO constellations and technology demonstration missions.
- CubeSats (1-10 kg): A standardized class of nanosatellites based on a 10x10x10 cm unit (1U). CubeSats have democratized access to space for universities and smaller companies. They are often deployed from the ISS or ride-share on larger rocket launches. While limited in capability, their low cost allows for novel mission concepts and distributed sensing networks.
Hosted Payloads: A Model of Efficiency
The concept of a hosted payload involves placing a secondary payload from one organization onto a commercial or government satellite owned by another. This model offers significant benefits by sharing the cost of the satellite bus and the launch, which are the two most expensive components of a space mission. For the payload owner, it provides a much faster and more affordable route to space than building a dedicated satellite. For the host, it generates additional revenue from unused capacity on their spacecraft.
Examples of Hosted Payloads
A prominent example is the Geostationary Lightning Mapper (GLM), a NOAA instrument that flies on the GOES-R series of weather satellites. While the primary mission of the satellite is weather imagery, the hosted GLM provides critical data on lightning activity. Similarly, the U.S. Space Force has utilized hosted payloads on commercial satellites to test new communication technologies and enhance space domain awareness without the expense of a dedicated military launch.
Relay Systems and Data Transmission
Getting data from a satellite back to Earth is a critical function of space infrastructure. For satellites in LEO, which are often out of sight of a ground station, communication relays are essential. These relays are themselves satellites, typically in a higher orbit, that can see both the LEO satellite and a ground station simultaneously.
The Role of TDRSS
The Tracking and Data Relay Satellite System (TDRSS) is a U.S. network of communications satellites in GEO. It provides a vital link for NASA's missions, including the ISS, the Hubble Space Telescope, and numerous Earth science satellites. By using TDRSS, these LEO assets can maintain near-continuous, high-bandwidth communication with mission controllers on the ground, a feat that would otherwise require a vast and expensive global network of ground stations.
Integration into Broader Infrastructure
Satellite platforms are not standalone assets; they are nodes in a complex, integrated system. A GPS satellite, for instance, is part of a constellation that must be perfectly synchronized. Its data is received by devices on Earth, which may then transmit their location via a commercial LEO communication satellite. This data might then be relayed through a GEO communication satellite back to a data center. This layered, interoperable architecture, combining government and commercial systems, is what gives modern space infrastructure its power and resilience. The design of any new satellite platform must consider how it will integrate with these existing ground and space-based networks to fulfill its mission effectively.