The on-board data handling (OBDH) subsystem of a

spacecraft A spacecraft is a vehicle that is designed spaceflight, to fly and operate in outer space. Spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth observation, Weather s ...

is the

subsystem A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and is exp ...

which carries and stores data between the various electronics units and the

ground segment A ground segment consists of all the ground-based elements of a spaceflight, space system used by operators and support personnel, as opposed to the Satellite space segment, space segment and user segment. The ground segment enables management of ...

, 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 A computer is a machine that can be programmed to automatically carry out sequences of arithmetic or logical operations ('' computation''). Modern digital electronic computers can perform generic sets of operations known as ''programs'', ...

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

telecommand A telecommand or telecontrol is a command sent to control a remote system or systems not directly connected (e.g. via wires) to the place from which the telecommand is sent. The word is derived from ''tele'' = remote (Greek), and ''command'' = to ...

s (TCs) from the TT&C * Forwarding of telecommands for execution by the target

Avionics Avionics (a portmanteau of ''aviation'' and ''electronics'') are the Electronics, electronic systems used on aircraft. Avionic systems include communications, Air navigation, navigation, the display and management of multiple systems, and the ...

* 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 MIL-STD-1553 is a military standard published by the United States Department of Defense that defines the mechanical, electrical, and functional characteristics of a serial data bus. It was originally designed as an avionic data bus for use w ...

* 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 Telemetry is the in situ collection of measurements or other data at remote points and their automatic transmission to receiving equipment (telecommunication) for monitoring. The word is derived from the Greek roots ''tele'', 'far off', an ...

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 Pulse-code modulation (PCM) is a method used to Digital signal (signal processing), digitally represent analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio application ...

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 A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated switc ...

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 A Sun-synchronous orbit (SSO), also called a heliosynchronous orbit, is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time. More technically, it is ...

orbits, take a precisely repeating track over the Earth. Observations which are taken from the same position may be compared using

interferometry Interferometry is a technique which uses the ''interference (wave propagation), interference'' of Superposition principle, superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important inves ...

, if they are in close enough register. The precise position required is sensed using

GPS The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide geol ...

. 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 Redundancy or redundant may refer to: Language * Redundancy (linguistics), information that is expressed more than once Engineering and computer science * Data redundancy, database systems which have a field that is repeated in two or more table ...

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.

References

{{reflist

External links

*https://ecss.nl/standard/ecss-e-st-50-04c-space-data-links-telecommand-protocols-synchronization-and-channel-coding/ Avionics