A satellite data unit (SDU) is an

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 ...

device installed in an aircraft that allows air/ground communication via a satellite network. It is an integral part of an aircraft's SATCOM (satellite communication) system. The device connects with a satellite via ordinary

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...

(RF) communication and the satellite then connects to a ground station or ''vice versa''. All satellite communication whether audio or data is processed by the SDU. The SDU communicates with an onboard MDDU (multi-purpose disk-drive unit) which maintains an updatable table of ground stations near the aircraft's position and the order of preference for selection of which ground station to use which thus guides the choice of satellite. Along with analysing data continuously sent from all ground stations (such as station status and the error rate of signals from each station) the SDU receives information on the aircraft's position and orientation from another onboard system (ADIRU, air data inertial reference unit) which it passes to the BSU (beam-steering unit) to direct the signal beam from the aircraft to the chosen satellite. With the advent of cellphones and the Internet a separate or integrated SDU can be used to offer telephone and Internet services to passengers.A330 TECHNICAL TRAINING MANUAL MECHANICAL & AVIONICS COURSE, pages 58 - 69, copyright Airbus S.A.S, published before 2014, but copyright date not given Logs of satellite communication have been used to inform

search and rescue Search and rescue (SAR) is the search for and provision of aid to people who are in distress or imminent danger. The general field of search and rescue includes many specialty sub-fields, typically determined by the type of terrain the search ...

agencies of locations of missing aircraft, like

Malaysia Airlines Flight 370 Malaysia Airlines Flight 370 (MH370/MAS370) was an international passenger flight operated by Malaysia Airlines that disappeared from radar on 8 March 2014, while flying from Kuala Lumpur International Airport in Malaysia to its planned de ...

whose position was unknown due to loss of

radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...

contact and other communications. Automated SATCOM transmissions suggested it flew about off its designated flight path having flown approximately south-southwest rather than the intended approximately north-northeast."Information provided to MH370 by AAIB: Information provided to MH370 investigation by UK Air Accidents Investigation Branch (AAIB)". Department of Civil Aviation Malaysia. Retrieved 6 May 2014.

References

Navigational equipment Aircraft instruments Avionics Communications satellites {{aviation-stub