NAVTEX data format

NAVTEX messages are sent in SITOR-B (AMTOR) format. The CCIR 476 character set is used. This character set is arranged so that each 7 bit character contains 4 1's and 3 0's for error detection purposes. This restriction reduces the number of characters you can squeeze into 7 bits so two character sets are used. The receiver switches between them upon recipt of the control characters 'FIGS' which selects the numeric set, and 'LTRS' which selects the alphabetic set. Below are tables of the two character sets.

CCIR 476 Alphabetic character set

0

1

2

3

4

5

6

7

8

9

A

B

C

D

E

F

0

ALPHA

1

J

F

C

K

2

W

Y

P

Q

3

BETA

G

FIGS

M

X

V

4

A

S

I

U

5

D

R

E

N

LTRS

SP

6

Z

L

REP

H

CH32

LF

7

O

B

T

CR

CCIR 476 Numeric character set

	0	1	2	3	4	5	6	7	8	9	A	B	C	D	E	F
0																ALPHA
1								\				!		:	(
2								2				6		0	1
3				BETA		&	FIGS			.	/		=
4								-				BEL		8	7
5				$		4	3			,	LTRS		SP
6				+		)	REP			#		CH32		LF
7		9	?		5				CR

More protection from transmission errors comes from sending each character twice, delayed by three characters. Both characters must arrive at the receiver intact for the character to be displayed. As an example the following message:

...ABCDEFGHI...

Would be interleaved:

. . . A B C D E F G H I . . .
     . . . A B C D E F G H I . . .

And transmitted as:

...A.B.CADBECFDGEHFIG.H.I...

This encoding scheme dates from the 60s and its simplicity kept the cost of the transmitting and receiving equipment down. In practice it works well enough for the purpose. The low data rate of 100bps also helps keep errors to a tolerable level.

The interleaving requires that the receiver periodically resynchronises with the transmitter to ensure correct decoding. This is done by alternately sending the ALPHA and BETA characters a number of times. These 'phasing pairs' can be heard on the generated files as the regular bleeps at the start and end of the message.

Click here to generate NAVTEX test messages