Air Navigation Services (ANS)

Aeronautical Telecommunication Network (ATN)
ATN is a global inter-network that provides digital communications necessary to the automated systems that include; air traffic service communication (ATSC), aeronautical operational control (AOC), aeronautical administrative communication (AAC), and aeronautical passenger communication (APC).
The ATN is composed of a network infrastructure and applications, and provides the global communication for ground-ground(G/G) and air-ground (A/G) services. The major components of ATN network are routers (intermediate Systems) including G/G routers and A/G routers, and communication sub-networks including air-ground and ground-ground sub-networks. The ATN applications includes, among others

• Context Management (CM)
• Controller-pilot data link communication (CPDLC)
• Air traffic service message handling services (AMHS)

The applications are hosted by the End Systems (ES). The Aeronautical Fixed Telecommunication (AFTN) and the Message Switching Systems are being incrementally replaced by a regional ATN ground network and AMHS.

 AMHS technology is the enabler for graphical depiction of aeronautical data through the automation system, thereby enhancing the performance and efficiency of the air traffic services. This will bring significant improvement in service delivery and air safety to the travelling public.

AMHS replace the legacy of AFTN connectivity which could cater to only small textual messages wherein binary attachements containing aeronautical maps, weather charts, digital NOTAM etc can be exchanged.

ATS Message Handling System (AMHS)
The ATS Message Handling System (AMHS), which has been defined in the ICAO Aeronautical Telecommunication Network (ATN) standards, is intended to be a replacement for the current Aeronautical Fixed Telecommunications Network (AFTN). AFTN is a store-forward messaging service for conveyance of text messages using character-oriented procedures.AFTN messages are forwarded on a hop-by-hope basis using pre-configured routes that are the most expeditious to affect delivery to the addressee. AFTN has diversion routing lists agreed to by the Administrations operating the communication centres where the AFTN switches reside. These lists are statically configured and used to immediately reroute traffic in the event of a circuit outage in a fully automatic communication center and to manually reroute traffic within 10 minutes in a non-fully automatic communication center.
Under AFTN procedures the sending station will hold messages transmitted, and in the event that continuity of message traffic is not maintained, they are re-transmitted. Continuity of message traffic is supervised using sequence numbers applied to all traffic over a particular channel. The AFTN system is depicted in the figure below:

Figure: AFTN system

OSI X.400 Message Handling System:

The ATS Message Handling System is based on the OSI X.400 Message Handling System. It is important to note that the X.400 Message Handling System is not a physical system with actual physical entities but rather is an architecural model for specifying functions or services provided by logical entities and the protocols between these logical entities and the protocols between these entities. The X.400 Architecture and Protocols can be depicted in the figure below:

The outer infrastructure is the message handling system (MHS), which is a generic messaging system from the perspective of MHS users. The inner infrastructure is the Message Transfer System (MTS), which provides the store and forward capability of the MHS. The MTS consists of a network of Message Transfer Agents (MTA).
 MTAs are message routers, they functions as routers at the message level to forward messages across the MHS. MTAs forward messages using the recipient Address in the message.MTAs communicate with one another using the P1 protocol, which defines a message envelope and contains routing and control information and thus determine how message exchange occurs among MTAs.
User Agent (UA)
MTS users within the MHS are either User Agents (UA) or Message Stores (MS). A UA functions to permit the user to send and receive mail. It is essentially a unit of software that interacts with the MTS on behalf of a user. UAs communicate with one another using the P2 protocol. The P2 protocol defines the content and format of an interpersonal message.
Message Stores (MS)
A MS is a unit that stores messages on behalf of a UA.When a MTA has a message to deliver to a UA it delivers it to the MS instead of the UA. The UA can then retrive the message from the MS at its convenience. A UA communicates with a MS using the P7 protocol. The P7 protocol defines procedures for message submittal, message retrieval, and message administration.
MTS users communicate with MTAs using the P3 protocol which defines procedures for message delivery and as in the case for the P7 protocol defines procedures for message submittal and message administration.
Access Unit (AU):
The AU is simply a gateway to another communication system. In the original X.400 specifications, AUs were expected to provide an interface to pre-X.400 technologies such as telex, and teletex. ICAO defines two AUs.

• AFTN/AMHS Gateway
• CDIN/AMHS Gateway

Almost all AUs provide gateway service to TCP/IP mail service in the form of an X.400 to SMTP gateway.

Basic ANS chart:

Some standard abbreviations used in aviation communication:

AGC: Automatic Gain Control
ATC: Air Traffic Controller
BIT:Built-in test
CAS:channel associated signalling
CCE: Control centre equipment
E1-RIC: E1- radio interconnect
E-BIT: external bit signal
HPA: High Power Amplifer
IBSU: In-band signalling unit
IDF: Intermediate distribution frame
LRU: Line replacement unit
MARC: Multi-access remote control
PCU: Protocol conversion unit
PTT: Press to transmit
RCMS: Remote control and monitoring system
RSSI: Radio signal strength indication
VCCS:Voice control and communication switch
VFP: Virtual front panel
VOGAD: Voice-operated gain adjusting device

T6T50 W VHF multimode transmitter:
It is intended for use in fixed ground environments such as airports and en-route centres. The transmitter operates voice and ICAO defined data modes at frequencies 118-136.975MHz for the standard model and 112-155.975MHz for the extended frequency model. Dependent on the software loaded into the radio there are two operating modes:

1. AM-Voice : This software provides voice via 4-wire E & M, E1 or VOIP. It provides SNMP (Simple Network Management Protocol) via Ethernet, and MARC via RS232, RS422, E1 and Ethernet. For VOIP operation it requires VOIP Configurator Application (VCA) software.
2. VHF Data Link (VDL) mode 2:VHF Data Link (VDL) Mode 2 radios support Controller Pilot Data Link Communications – sending information between aircraft and ground stations. It is latest upgrade to the VHF data link communications . VDL Mode 2, in its broadest sense, is the term that is used to describe a suite of air-ground protocols that increase the data rate of the air-ground link to 31,500 bits per second. VDL Mode 2 allows transitioning from an infrastructure that relies on character-oriented ACARS protocols for end-to-end delivery of messages to one that uses bit-oriented Aeronautical Telecommunications Network (ATN) protocols using the same VHF ground and aircraft radios.

The transmiiter may be connected to suitable control equipment using a variety of analog and digital methods. These include:

• 4-wire audio and signalling using analog lines
• An E1 digital link
• Ethernet links.

The transmitter is a single frequency synthesised radio that can operate with 25KHz and 8.33KHz channel spacing. The radio recognizes frequencies entered in ICAO format and automatically adjusts to the correct channel spacing. For multichannel operation up to 100 preset frequency channels can be stored in the radio for immediate recall; any combination of 8.33KHz and 25KHz channel spacing can be stored.

Transmitter RF characteristics:

• Output Impedance: 50 Ohms
• RF power output: The RF carrier output is adjustable in 1W steps from 5 W to 50 W.Output power is automatically controlled under the following conditions:
• Frequency range
• Low supply voltage
• High VSWR
• High RF PA temperature
• Rise Time
• Duty Cycle
• channel spacing
• offset carrier
• Harmonic outputs
• spurious outputs: The spurious outputs are less than -46dBm for modulation depths upto 90%, measured at greater than 500KHz from carrier in the frequency range 9KHz to 4GHz. There are no coherent spurious outputs above the spectral mask at less than 500KHz.
• Inter-modulation

Transmitter modulation characteristics:

• Modulation Depth
• Hum and Noise
• Frequency Response
• Distortion
• Residual FM
• VOGAD (AM-Voice only): TheVOGAD has an operational range of 30dB with the threshold level set at 10dB below the average speech line level setting. Within the VOGAD range the modulation depth remains at the set level +/- 10%.

Transmitter Control:

• Audio inputs: Audio can be  connected via the 600 ohm balanced line inputs.
• PTT: The transmitter can also be keyed via a direct PTT  input, via phanton keying superimposed on the audio lines and through PTT tone signalling over the audio lines.
• PTT Time out: The PTT time out period is adjustable from 2 to 510 seconds in 2 seconds of steps or it can be disabled.
• PCM Voice: Digitized voice can be  connected to the transmitter via the E1 or IP interfaces. 
• E1-64kbit/sec digitized 8-bit A-law encoded PCM voice can be connected to the transmitter via the T1/E1 connector. Audio uses TS1 and keying is achieved using the four associated CAS bits on TS16.
• VOIP- 64kbit/sec digitized 8-bit A-law encoded PCM voice can be connected to the transmitter via the IP connector using VOIP Ethernet protocols.

VDL Mode 2:

VDL mode 2 uses CSMA differentially encoded 8-phase shift keying (D8PSK) using a raised cosine filter with alpha = 0.6. Information is differentially encoded with 3 bits per symbol transmitted as changes in phase rather than absolute phase. The data stream is divided into groups of 3 consecutive data bits, least significant bit first. Zeros are padded to the end of transmissions if needed for the final channel symbol.

 VDL mode 2 parameters are identical to AM-Voice mode with the following exceptions.

• RF power output
• RF power Rise Time
• RF Power Decay Time
• Channel Spacing
• Harmonic Outputs 
• Spurious Outputs

VDL mode 2 modulation characteristics:

• Modulation rate:  The symbol rate is 10500 symbols/second, resulting in a nominal bit rate of 31500 bits/sec.
• RMS Phase Error: The RMS phase error is less than 3 degree. The error magnitude is less than 6 %.

Color code for standard RJ 45:

Color     Abbreviation
White    W
Red        R
Black     BK
Yellow   Y
Blue       B
Grey       G
Orange   O
Green     G
Brown    BR
Violet     V

Radio Communication Frequency used in Nepali Aviation
Kathmandu

• Ground(GND/SMC FC/CD)   121.9MHz/121.75MHz
• Tower(TWR)[Main/Standby]   118.1/118.5MHz
• Area Control (ACC)[Main/Standby]   126.5/124.7MHz
• Approach Control (APP)[Main/Standby]   120.6/125.1MHz
• Digital Air Traffic Information System(D-ATIS)   127MHz
• Emergency(EMG)   121.5MHz
• SSB EAST: 5805.5KHz /6607.0KHz
• SSB WEST:5858.0KHz

Outside Kathmandu 

Only Tower (TWR)

Bajhang (VNBG)   122.50MHz

Bajura(VNBR)   122.50MHz

Bhairahawa(VNBW)   122.50MHz

Bharatpur(VNBP)   122.30MHz

Bhojpur(VNBJ)   122.30MHz

Biratnagar(VNVT)   123.80MHz

Bhojpur(VNBJ)  122.30MHz

Chandragadi(VNCG)  122.50MHz

Dang(VNDG)  122.30MHz

Dhangadi(VNDH)  122.30MHz

Dolpa(VNDP)  122.50MHz

Janakpur(VNJP)  122.50MHz

Jomsom(VNJS)  122.50MHz

Jumla(VNJL)  122.50MHz

Kangeldanda(VNKL)  122.30MHz

Khanidanda(VNKD)  122.50MHz

Lamidanda(VNLD)  122.50MHz

Lukla(VNLK)  122.30MHz

Mahendranagar(VNMN)  122.30MHz

Manang(VNMA)  118.30MHz

Meghauli(VNMG)  122.50MHz

Nepalganj(VNNG)  118.30MHz

Palpa(VNPL)  122.50MHz

Pokhara(VNPK)  123.80MHz

Rajbiraj(VNRB)  118.30MHz

Ramechhap(VNRC)  122.50MHz

Rara(VNRR)  122.50MHz

Rukum(chaujahari)(VNCI)  122.50MHz

Rumjatar(VNRT) 122.30MHz

Salley(VNSL)  122.50MHz

Sanfebagar(VNSR)  122.50MHz

Simara(VNSI)  118.30MHz

Simikot(VNST)  122.50MHz

Surkhet(VNST) 122.50MHz

Thamkharka(VNTH)  122.30MHz

Tumlingtar(VNTR)  123.95MHz

Taplejung(VNTJ)  122.50MHz