On-Board Data Handling
Encyclopedia
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 (TTC) subsystem.
In the earlier decades of the space industry, the OBDH function was usually considered a part of the TTC, 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 TTC, which is these days concerned solely with the RF
link between the ground and the spacecraft.
Functions commonly performed by the OBDH are:
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.
, 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.
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 etc. 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.
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
is the subsystem which carries and stores data between the various electronics units and the ground segment, via the Telemetry, Tracking and Command (TTC) subsystem.
In the earlier decades of the space industry, the OBDH function was usually considered a part of the TTC, 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 TTC, which is these days concerned solely with the RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
link between the ground and the spacecraft.
Functions commonly performed by the OBDH are:
- Reception, error correction and decoding of telecommandTelecommandA telecommand is a command sent to control a remote system or systems not directly connected to the place from which the telecommand is sent. The word is derived from tele = remote , and command = to entrust/order...
s (TCs) from the TTC - Forwarding of telecommands for execution by the target AvionicsAvionicsAvionics are electronic systems used on aircraft, artificial satellites and spacecraft.Avionic systems include communications, navigation, the display and management of multiple systems and the hundreds of systems that are fitted to aircraft to meet individual roles...
- 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-1553MIL-STD-1553MIL-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 for use with military avionics, but has also become commonly used in spacecraft on-board...
. - 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 telemetryTelemetryTelemetry is a technology that allows measurements to be made at a distance, usually via radio wave transmission and reception of the information. The word is derived from Greek roots: tele = remote, and metron = measure...
frames - Storage of telemetry frames in a mass memory
- Downlinking of telemetry to the ground, via the TTC.
- Management and distribution of time signals
Telecommand reception
The OBDH receives the TCs as a synchronous PCM data stream from the TTCTelecommand 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 relayRelay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
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 which are stored for execution until the spacecraft is at a specified position. These are most useful for earth observation satellites, which need 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 interferometryInterferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...
, 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) which is reliable, usually with redundantRedundancy (engineering)
In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....
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 etc. 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.