Command and Data Handling in Satellites
Madhukara Putty
Spacecraft today are intelligent machines, capable of making critical decisions without constant human input. Central to this intelligence is the Command and Data Handling (C&DH) subsystem. Think of it as the spacecraft’s brain and nervous system—processing commands, managing internal communication, and ensuring the vehicle’s smooth operation throughout its mission.
What Does the C&DH Subsystem Do?
The C&DH subsystem handles three major responsibilities:
Managing spacecraft operations through onboard software.
Processing incoming commands from the ground.
Facilitating internal communications between subsystems.
In earlier generations of satellites, many tasks were executed via analog circuitry. Today, the move to digital systems has made the term "Command and Data Handling" somewhat outdated—but the name has stuck around.
The Heart of C&DH: The On-Board Computer (OBC)
At the core of C&DH is the On-Board Computer (OBC). The OBC manages all operational aspects of the spacecraft and maintains close ties with the Electrical Power System (EPS).
For instance, if available power drops, the OBC may selectively shut down non-critical systems to preserve life-supporting subsystems. Communication between the OBC and EPS is typically over low-speed data links, given the modest amount of information exchanged.
The OBC is also responsible for:
Receiving commands sent from ground control.
Sending housekeeping data (health and status information) back to Earth through low-bandwidth transmitters.
Handling payload data — either directly or through high-speed links in larger spacecraft with substantial data generation.
By offloading heavy data handling to dedicated storage and transmitters, the OBC can remain focused on time-critical operations like maneuvering or system health monitoring.
How the OBC Communicates
The OBC connects to every subsystem—payload, communications, attitude control, power—primarily via low-speed data links. For spacecraft with substantial data needs, payload data is often managed separately through high-speed interfaces. The communication architecture of the OBC is shown in the figure above.
Types of Computing Hardware for the OBC
Depending on mission needs, the OBC can be built using:
1. Processor
High-speed general-purpose computing units.
Power consumption: typically 10W to 150W.
Capable of heavy computations and logical decision-making.
Space missions sometimes still use older, radiation-hardened processors like variants of the Intel 386 due to their proven reliability.
2. Microcontroller
Designed for embedded systems.
Much lower power consumption (often <1W).
Built-in peripherals like ADCs, PWM controllers, and bus interfaces.
Despite lower computing power compared to modern processors, they often outperform older space-qualified processors while consuming far less power.
There’s a growing trend toward microcontrollers in the satellite industry.
3. FPGA (Field Programmable Gate Array)
Reprogrammable integrated circuits.
Flexible architecture that can be customized after manufacturing.
Excellent for specialized tasks, offering speed and low power usage.
Often integrated with pre-designed modules, known as IP cores, such as the LEON processor developed by ESA specifically for space missions.
Selecting the Right OBC for a Mission
When choosing an OBC, engineers must consider:
Processing capability: Can it manage the payload and other systems efficiently?
Memory capacity: Adequate volatile (RAM) and non-volatile storage are crucial.
Interfaces: Must support the necessary communication standards like I2C, USB, etc.
Physical constraints: Must fit within the spacecraft’s mass and volume budget.
Not Just One Brain!
It’s important to remember that the spacecraft might have multiple computing units. Apart from the main OBC, subsystems like the payload, Attitude Determination and Control System (ADCS), and the power system often have dedicated processors for localized control.
TL;DR 🧠
The Command and Data Handling (C&DH) subsystem is the spacecraft's brain, managing operations, data flow, and internal communication. Centered around the On-Board Computer (OBC), it ensures mission success by balancing autonomy, data management, and subsystem coordination. Choosing the right OBC hardware—processor, microcontroller, or FPGA—is critical, depending on mission needs.
References:
Command and Data Handling. Lecture Notes - Spacecraft Technology (AE3534). Delft University of Technology, Delft, The Netherlands
Image credit: By NASA/Crew of STS-132 - https://images-assets.nasa.gov/image/s132e012208/s132e012208~orig.jpg(https://images.nasa.gov/details-s132e012208), Public Domain, https://commons.wikimedia.org/w/index.php?curid=10561008
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