Acars Via Iridium

ACARS via Iridium Iridium is a large satellite constellation that consists of 66 low-earth orbiting (LEO), cross-linked satellites operating as a fully meshed network providing truly global coverage (including oceans, airways and polar regions). Iridium satellites are in a near-polar orbit at an altitude of 485 miles. The network is composed of 66 active satellites that fly in six orbital planes, each with 11 satellites per plane. Satellites circle the earth once every 100 minutes. ACARS is an Aircraft Communication Addressing and Reporting System otherwise known as Digital DataLink System.

ACARS was first developed in the 1970’s. It is a point-to-point service that either sends air-to-ground messages or receives ground-to-air messages from the cockpit. ACARS via Iridium now gives Flight Departments the capability to communicate with the aircraft in their fleet all over the globe. ACARS is now a viable option via Iridium. Until now ACARS was only available by 2 different mediums. ACARS via Very High Frequency (VHF) communication is only available through line of site.

ACARS via Inmarsat (Satcom) is only available over the populated continents and does not provide communications over polar routes. The cost of Iridium service is far less than that of Satcom and is available over oceans whereas VHF is not. This allows aircraft to access various services provided by ACARS Service Providers. Aircraft can now send a number of automatic reports (OOOI, engine oil pressure, position reports, fuel, and etc) using ACARS via Iridium. Pilots can request relevant information pertaining to flight safety and receive necessary in-flight updates.

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Some of the data services include: Text-based and Graphical Weather, Flight Planning and Clearances, Text Messaging, Position Reporting, Automatic Terminal Information Services (ATIS), Terminal Weather Information for Pilots (TWIP), and OOOI messages (Out of the gate, Off the ground, On the ground, and Into the gate). New flight safety services such as Controller Pilot Data Link Communications (CPDLC) and Future Air Navigation System messaging (FANS) are currently undergoing live tests to help improve flight times and routes for Air Transportation.

Typical ACARS messages are only 100 to 200 characters in length. Such messages are made up of a one-block transmission from (or to) the aircraft, constrained to be no more than 220 characters within the body of the message. Iridium’s Short Burst Data Service (SBD) has the network transport capability to transmit short data messages between field equipment and a centralized host computing system. ACARS blocks are 238 bytes maximum; segmenting and distributing them across multiple SBD messages is not necessary. Each ACARS block is encapsulated in a unique SBD message.

ACARS requires ARINC-429 Interface, compliant with ARINC 741 characteristics for satellite communication systems. Standardized nature of the ARINC 429 digital data bus allows Satcom Data Unit (SDU) to interface with wide variety of ACARS capable hardware. Air-to-ground messages are originated from the Control Display Unit (CDU), which is located in the cockpit of the aircraft. The pilot will initiate his request through the CDU, where the Data Management Unit (DMU) will pass the ACARS requested message onto the Iridium Satcom Data Unit (SDU).

The SDU encapsulates each ACARS block into an SBD message and sends the SBD message to the Iridium gateway. The SBD messages are received by the Iridium gateway, converted back into ACARS blocks, then passed on to the chosen DataLink Service Provider (DSP) through the service provider gateway. Conversely, ground-to-air messages from the data provider are passed through the service provider gateway as ACARS message blocks. Once received by the Iridium gateway, they are encapsulated as SBD messages and delivered to the aircraft.

Once the SDU has received an uplink SBD message, it removes SBD-specific information and passes the remaining data to the DMU as an ACARS message block via the ARINC 429 data bus. The DMU then interprets and displays the ACARS data via the CDU. ARINC (Aeronautical Radio, Incorporated) and SITA (Society of International Telecommunications Aeronautics) are the two DSPs available to the airlines. ACARS messaging allows the airlines to identify abnormal flight conditions and automatically send real-time messages to an airline, thereby enabling airlines to improve their aircraft performance.

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