IV Make up four types of questions to the text.

V Using figures describe the inverters.

 

Rotary inverter schematic

Static Inverter

(b)

UNIT 22AIR-GROUND COMMUNICATIONS

AIM OF THE UNIT:- to identifyair-ground communications

TASKS

1 Do your best to answer the brainstorming question. 2 Read the text for general understanding.

3 Make up questions to the text. 4 Find the sentences with the new words in the text. Give the Kazakh or Russian

equivalents of the words.

5 Write sentences with the new vocabulary. 6 Do the exercises for better remembering the topic.

7 Speak on the topic. Given schemes will help you to remember and understand

the topic.

8 Prepare a presentationor a project on the topic.

Brainstorming questions

 

 

1 What is ATC?

2 What isACARS?

3 What is AFTN?

4 What is CSMA?

 

All of these ATC applications use the ACARS air-ground link, which was neither designed for nor initially approved as an ATC communication medium. For that purpose, ICAO developed standards and recommended practices (SARPs) for the Aeronautical Telecommunication Network (ATN), which was designed for both air-ground and ground-ground communication. In the latter role, it is intended to replace the Aeronautical Fixed Telecommunication Network (AFTN), which has served the industry well as a teletype-based, message-switching network for many years. However, technology has overtaken AFTN, and the more modern packet-switched technology of the ATN is seen as more appropriate. The ATN SARPs development began in the early 1990s and, other than trial implementations, has yet to be fully implemented. In the past 15 years, the Internet, which is based on a different packet-switched technology, i.e., Transmission Control Protocol/Internet Protocol (TCP/IP), has had unprecedented success. It may be necessary for ICAO to make some accommodation to TCP/IP lest the cost of implementing the ATN be prohibitive.

As ACARS became essential to airline operations, the limitations of the initial VHF link became intolerable, first because of coverage limitations and then because of speed. The former was solved in two different ways. Long-range data link was implemented first using Inmarsat satellites; this was the basis for initial FANS implementations in the South Pacific. The oceanic coverage provided by satellites and data link was an improvement over HF voice services. All of the advantages of data link over voice communications were highlighted in the initial FANS trials and operational use. The advantages included: (a) consistent and rapid delivery of messages; (b) standardized message texts, which were understood by

all no matter what their native language; (c) automated delivery of position reports; and (d) integration of message content with flight management systems (FMSs). High frequency data link (HFDL) provided another long-range ACARS subnet work that covered the north Polar Regions, which are not reached by Inmarsat signals. VHF Digital Link (VDL) Mode 2 provided a higher-speed subnet work in continental airspace. These points will be elaborated in subsequent sections

The ACARS avionics architecture is centered on the management unit (MU) or communications management unit (CMU), which acts as an onboard router. All air-ground radios connect to the MU or CMU to send and receive messages. The CMU is connected to all of the various radios that communicate to the ground.

The MU or CMU acts as the ACARS router onboard the aircraft. All message blocks to or from the aircraft, over any of the air-ground subnet works, pass through the MU. Although the MU handles all ACARS message blocks, it does not perform a message-switching function because it does not recombine message blocks into a “message” prior to passing it along. It passes each message block in accordance with the “label” identifier, and it is up to the receiving end system to recombine message blocks into a complete message. The original OOOI messages were formatted and sent to the MU from an avionics unit that sensed various sensors placed around the aircraft and determined the associated changes of state. In the modern transport aircraft, many other avionics units send and receive routine ACARS messages.

The multifunction control and data unit (MCDU), along with the printer, is the primary ACARS interface to the flight crew. Other units, such as the FMS or the air traffic services unit (ATSU), will also interact with the crew for FANS messages. The vast majority of data link messages today are downlinks automatically generated by various systems on the airplane. The MU identifies each uplink message block and routes it to the appropriate device. Similarly, it takes each downlink, adds associated aircraft information such as the tail number, and sends it to one of the air-ground sub-networks. The latest avionics for each of the four sub- networks accepts an ACARS block as a data message over a data bus, typically ARINC 429. The subnet work avionics will then transform the message block into the signals needed to communicate with the ground radio. Each sub-network has its own protocols for link layer and physical aver exchange of a data block.

The original VHF subnetwork that was pioneered in 1978 uses the same 25 kHz VHF channels used by ATC and aeronautical operational communication (AOC) voice; the signal-in-space is sometimes called plain-old-ACARS (POA) for reasons that will become clearer when we discuss VDL Mode 2. The VHF subnetwork uses a form of frequency shift keying (FSK) called minimum shift keying (MSK) wherein the carrier is modulated with either a 1200 Hz or 2400Hz tone. Each signaling interval represents one bit of information, so the 2400 baud rate (i.e., rate of change of the signal) equals the bit rate of 2400 BPS. After initial synchronization, the receiver then can determine whether a given bit is a one or a zero. VHF ACARS uses the carrier-sensed multiple access (CSMA) protocol to reduce the effects of two transmitters sending a data block at the same or overlapping times. CSMA is nothing more than the automated version of voice radio protocols wherein the speaker first listens to the channel before initiating a call. Once a transmitter has begun sending a block, no other transmitter will “step on” that transmission.

The VHF ACARS sub network is an example of a connectionless link layer protocol in that the aircraft does not “log in” to each ground station along its route of flight. The aircraft does initiate a contact with the central message processor, and it does transmit administrative message as it changes sub networks.

EXERCISES FOR BETTER REMEMBERING THE TOPIC