Malaysia Airlines Flight 370

==== Technical background ====
[[File:Inmarsat-3 crop.jpg|thumb|A depiction of an [[Inmarsat#Satellites|Inmarsat-3 series]] satellite. Flight 370 was in contact with Inmarsat-3 F1 (also known as "IOR" for Indian Ocean Region).|alt=A depiction of a satellite in space.]]
Aeronautical satellite communication (SATCOM) systems are used to transmit messages sent from the aircraft cockpit, as well as automated data signals from onboard equipment, using the [[ACARS]] communications protocol. SATCOM may also be used for the transmission of [[Future Air Navigation System|FANS]] and [[Aeronautical Telecommunication Network|ATN]] messages, and for providing voice, fax and data links<ref name="classicaero">{{cite web|title=Classic Aero services and SwiftBroadband|url=http://www.inmarsat.com/service/aviation-safety/|work=[[Inmarsat]]|access-date=28 March 2014|archive-date=28 March 2014|archive-url=https://web.archive.org/web/20140328221911/http://www.inmarsat.com/service/aviation-safety/|url-status=live}}</ref> using other protocols.<ref name="Telegraph-delays"/><ref name=RGN>{{cite web|last=Kirby|first=Mary|title=SITA aids MH370 investigation; expert explains|url=http://www.runwaygirlnetwork.com/2014/03/14/sita-aids-mh370-investigation-expert-explains/|website=Runway Girl Network|date=14 March 2014|access-date=26 March 2014|archive-date=26 March 2014|archive-url=https://web.archive.org/web/20140326105450/http://www.runwaygirlnetwork.com/2014/03/14/sita-aids-mh370-investigation-expert-explains/|url-status=live}}</ref><ref name=ATM>{{cite web|last=Turner|first=Aimee|title=Malaysian MH370: SATCOMS 101 (Part One)|url=http://www.airtrafficmanagement.net/2014/03/mh370-satcoms-101/|website=AirTrafficManagement.net|date=16 March 2014|access-date=26 March 2014|archive-url=https://web.archive.org/web/20140322143315/http://www.airtrafficmanagement.net/2014/03/mh370-satcoms-101/|archive-date=22 March 2014|url-status=dead}}</ref> The aircraft uses a [[satellite data unit]] (SDU) to send and receive signals over the satellite communications network; this operates independently from the other onboard systems that communicate via SATCOM, mostly using the ACARS protocol. Signals from the SDU are transmitted to a communications satellite, which [[RF power amplifier|amplifies]] the signal and changes its [[Radio frequency|frequency]] before relaying it to a [[ground station]], where the signal is processed and, if applicable, routed to its intended destination (e.g. Malaysia Airlines' operations centre); signals are sent from the ground to the aircraft in reverse order.

When the SDU is first powered on, it attempts to connect with the Inmarsat network by transmitting a log-on request, which is acknowledged by the ground station.<ref name=ATSB />{{rp|17}}<ref name="ATM"/> This is partly to determine whether the SDU belongs to an active service subscriber, and also to identify which satellite should be used for transmitting messages to the SDU.<ref name="ATM"/> After connecting, if no further contact has been received from the data terminal (the SDU) for one hour,{{efn|The timing of the log-on interrogation message is determined by an inactivity timer, which was set to one hour at the time of Flight 370's disappearance (it was later reduced to 15&nbsp;minutes).<ref name=ATSB />{{rp|18}}}} the ground station transmits a "log-on interrogation" message, commonly referred to as a "ping";<ref name=ATSB />{{rp|18}} if the terminal is active, it will respond to the ping automatically. The entire process of interrogating the terminal is referred to as a "[[Handshake (computing)|handshake]]".<ref name="Inmarsat(26 March)" /><ref name="myDCA_20140325">{{cite web|title=Information provided to MH370 investigation by UK Air Accidents Investigation Branch (AAIB) |url=http://mh370.dca.gov.my/information-provided-to-mh370-by-aaib/ |date=25 March 2014 |author=Prime Minister's Department (Malaysia) |author-link=Prime Minister's Department (Malaysia) |publisher=[[Department of Civil Aviation Malaysia]] |access-date=6 May 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140406010301/http://mh370.dca.gov.my/information-provided-to-mh370-by-aaib/ |archive-date=6 April 2014}}</ref>