Unit 5 navtex

Introduction

NAVTEX is an international automated direct-printing service, broadcast on 518 kHz, for the promulgation of navigational and meteorological warnings and urgent information to ships. It has been developed to provide a low cost, simple means of receiving marine safety information onboard ships at sea and in coastal waters.

NAVTEX fulfils an integral role in the GMDDS and is also a component of the World-Wide Navigational Warning Service (WWNWS).

1. Principal features of navtex

The service uses a single frequency (518 kHz), with transmissions in English from nominated stations within each NAVAREA being arranged on a time-sharing basis to eliminate mutual interference. All necessary information is contained in each transmission. The power of each transmitter is so regulated as to avoid the possibility of interference between transmitters.

A dedicated NAVTEX receiver which has the ability to select messages to be printed, according to (a) a technical code (B₁, B₂, B₃, B₄), which appears in the preamble of each message and (b) whether or not the particular message has already been printed. Certain essential classes of safety information such as navigational and meteorological warnings and search and rescue information are non-rejectable to ensure that ships using NAVTEX always receive the most vital information.

NAVTEX coordinators exercise control of messages transmitted by each station according to the information contained in each message and the geographical coverage required. Thus a user may choose to accept messages either from the single transmitter, which serves the se area around his position, or from a number of transmitters as appropriate.

2. Message priorities

Three message priorities are used to dictate the timing of the first broadcast of a new warning in the NAVTEX service. In descending order of urgency they are:

VITAL – for immediate broadcast, subject to avoiding interference to ongoing transmissions,

IMPORTANT – for broadcast at the next available period when the frequency is unused

ROUTINE – for broadcast at the next scheduled transmission period

Note: Both VITAL and IMPORTANT warnings will normally need to be repeated, if still valid, at the next scheduled transmission period.

3. The standard format of navtex messages

Phasing signals >10 sec

ZCZC (Start of message group) B₁, B₂, B₃, B₄

(B₁ – transmission identity,

B₂ – Subject indicator,

B₃, B₄ – consecutive number)

(Time of Origin) – Optional

Series Identity + Consecutive Number

Message Text

NNNN – end of message

Idle signals aa…a >2 sec

End of emission

The Transmitter Identification Character (B₁)

The Transmitter Identification Character (B₁) is a single unique letter that is allocated to each transmitter. It is used to identify the broadcasts, which are to be accepted by the receiver, and those, which are to be rejected.

In order to avoid erroneous reception of transmissions from two stations having the same B₁ character, it is necessary to ensure that such stations have a large geographical separation. This is achieved by allocating B₁ characters by alphabetical sequence through each NAVAREA of the World-Wide Navigational Warning Service.

NAVTEX transmissions have a designed range of about 400 nautical miles. The minimum distance between two transmitters with the same B₁ identifier is usually sufficient to ensure that a receiver cannot be within range of both at the same time.

Subject Indicator Character (B₂)

Information is grouped by subject on the NAVTEX broadcast, and each subject group is allocated a subject indicator character B₂. The subject indicator character is used by the receiver to identify different classes of messages as listed below. The indicator is also used to reject messages concerning certain optional subjects which are not required by the ship. For example, OMEGA messages might be rejected in a ship which is not fitted with an OMEGA receiver. Receivers also use the B₂ character to identify messages which, because of their importance, may not be rejected. The following subject indicator characters are in use:

A – Navigational Warnings (cannot be rejected by the receiver)

B - Meteorological Warnings (cannot be rejected by the receiver)

C- Ice Reports

D – Search and Rescue Information (cannot be rejected by the receiver)

E – Meteorological Forecasts

F – Pilot Service Messages

G – DECCA Messages

H – LORAN Messages

I – OMEGA Messages

J – SATNAV Messages

K - Other Electronic Navaid Messages (messages concerning radio navigation services)

L - Navigational Warnings-additional to letter A (should not be rejected by the receiver

(continuation of B₂ subject group “A”))

V – Special services – trial allocation

W – Special services – trial allocation

X – Special services – trial allocation

Y – Special services – trial allocation

Z – No messages on hand

Message Numbering (B₃, B₄)

Each message within a subject group is allocated a serial number B₃, B₄ between 01 and 99. This number will not necessarily relate to series numbering in other radio navigational warning systems. On reaching 99 numbering will re-commence at 01 but avoiding the use the message numbers still in force.

A shortage of numbers will, where possible, be alleviated by allocation of messages to other, relevant subject groups, and B₂=L may be used for additional navigational warnings, to receive the overflow from B₂=A when necessary.

2. Answer the following questions

1. What is the NAVTEX?

2. What are the principal features of NAVTEX?

3. What priorities exit for NAVTEX messages?

4. What is the standard format of NAVTEX messages?

5. How is the transmitter identified?

6. In what way is the subject indicated?

7. How are the messages numbered?

3. Translate the following NAVTEX message bearing in mind various indicators

LA 61

201355 UTC OCT 04

LIMNOS RADIO NAVWARN 193/04

TUG BOAT ALEXANDROS / SV5120

WITH TOWING LINE LENGTH 300 METRES

IN PSN 40 24 N – 022 47E ON 201430UTC

FROM THESSALONIKI PORT TO PIRAGDIKIA

POINT OF CHALKIDIKI PENINSULA

ETA 211105 UTC

COURSE VARIOUS SPEED ABT 5.5 KNOTS

WIDE BERTH REQUESTED

NNNN

UNIT 6

DISTRESS PROCEDURES