science

On-board data handling

On-board data handling is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand On-board data handling rather than just read about it. In short: The on-board data handling (OBDH) subsystem of a spacecraft is the subsystem which carries and stores data between the various electronics units and the ground segment, via the telemetry, tracking and command (TT&C) subsystem. In the earlier decades of the space industry, the OBDH function was usually considered a part of the TT&C, particularly before computers became common on board.

Key takeaways

On-board data handling belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect On-board data handling to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of On-board data handling from memory before moving on to harder problems.

Reference excerpt

The on-board data handling (OBDH) subsystem of a spacecraft is the subsystem which carries and stores data between the various electronics units and the ground segment, via the telemetry, tracking and command (TT&C) subsystem. In the earlier decades of the space industry, the OBDH function was usually considered a part of the TT&C, particularly before computers became common on board. In recent years, the OBDH function has expanded, so much that it is generally considered a separate subsystem to the TT&C, which is these days concerned solely with the RF link between the ground and the spacecraft. Functions commonly performed by the OBDH are:

Reception, error correction and decoding of telecommands (TCs) from the TT&C Forwarding of telecommands for execution by the target Avionics Storage of telecommands until a defined time ('time tagged' TCs) Storage of telecommands until a defined position ('position tagged' TCs) Measurement of discrete values such as voltages, temperatures, binary statuses etc. Collection of measurements made by other units and subsystems via one or more data busses, such as MIL-STD-1553 Real-time buffering of the measurements in a data pool Provision of a processing capability to achieve the aims of the mission, often using the data collected Collation and encoding of pre-defined telemetry frames Storage of telemetry frames in a mass memory Downlinking of telemetry to the ground, via the TT&C Management and distribution of time signals

Telecommand reception The OBDH receives the TCs as a synchronous PCM data stream from the TT&C

Telecommand execution The desired effect of the telecommand may be just to change a value in the on-board software, or to open/close a latching relay to reconfigure or power a unit, or maybe to fire a thruster or main engine. Whichever effect is desired, the OBDH subsystem will facilitate this either by sending an electric pulse from the OBC, or by passing the command through a data bus to the unit which will eventually execute the TC. Some TCs are part of a large block of commands, used to upload updated software or data tables to fine tune the operation of the spacecraft, or to deal with anomalies.

Time-tagged telecommands It is often required to delay a command's execution until a certain time. This is often because the spacecraft is not in view of the ground station, but may also be for reasons of precision. The OBC will store the TC until the required time in a queue, and then execute it.

Position-tagged telecommands Similar to time-tagged commands are commands that are stored for execution until the spacecraft is at a specified position. These are most useful for Earth observation satellites, which need to start an observation over a specified point of the Earth's surface. The spacecraft, often in Sun-synchronous orbits, take a precisely repeating track over the Earth. Observations which are taken from the same position may be compared using interferometry, if they are in close enough register. The precise position required is sensed using GPS. Once a position tagged command has been executed, it may be flagged for deletion or left to execute again when the spacecraft is once again over the same point.

Processing function The modern OBDH always uses an on-board computer (OBC) that is reliable, usually with redundant processors. The processing power is made available to other applications which support the spacecraft bus, such as attitude control algorithms, thermal control, failure detection isolation and recovery. If the mission itself requires only a small amount of computing power (such as a small scientific satellite) then the payload may also be controlled by the software running on the OBC, to save launch mass and the considerable expense of a dedicated payload computer.

Worked examples

Example 1 — a first encounter with On-board data handling

Start with the simplest possible case. Write down what On-board data handling claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to On-board data handling before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about On-board data handling ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of On-board data handling

In research: On-board data handling appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses On-board data handling in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: On-board data handling is common in secondary-school and first-year university syllabi. It links to neighbouring topics Avionics, so understanding it makes those chapters shorter.
In everyday life: Look for On-board data handling outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study On-board data handling in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what On-board data handling means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain On-board data handling out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is On-board data handling in simple terms?

Why does On-board data handling matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study On-board data handling?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on On-board data handling.