Follwing a tip from my friend KarapuZ and his recent tweet, I started to monitor 16941.0 KHz to study the STANAG-4285 naval broadcasts from the Indian Navy [1]. They use the quite rare 2400bps/Long sub-mode and decoding produces a lot of errors just due to the high data rate and the huge QSB that sometimes affects the signals. By the way, I used the KiwiSDRs VU Hams located in Kottarakkara Kerala and colombo4s7vk located in Colombo Sri Lanka, the latter is a bit less recommendable.
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| Fig. 1 - one of the S4285 2400/L heard broadcasts |
For what I could see, daily broadcasts starting around 1100 or 1200 Z are transmitted on that frequency. Broadcasts consist of clear-text weather bulletins and 4FG messages to VWGZ (VWGZ is the collective callsign for any/all the Indian Navy ships): indeed, they typically use a four FIG (off-line) encryption system. Either the bulletins and 4FG messages, are sent using the async ITA2 8N1 framing (Figs. 2, 3).
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| Fig. 2- 8N1 bitstream after decoding |
 |
| Fig. 3 - off-line decoding using Harris RF-5710A modem |
VWGZ
VND 677/16
ECHO BRAVO ZULU
ALPHA KILO UNIFORM
OSCAR KILO NOVEMBER
PAPA ECHO HOTEL
ROMEO QUEBEC XRAY
INDIA INDIA HOTEL
LIMA CHARLIE PAPA
DELTA HOTEL KILO
-P- 160732
GR 158
BT
ZERO ZERO ZERO EIGHT ALFA TWO TWO FOUR EIGHT 9838
6469 5315 6155 6433 5098 8353 7507 5237 5375 4271
...
8394 6708 1257 6554 6238 5987 3600 6023 9076 1083
4574 3021 1116 0342 6063 4300 2248 0008ALFA
BT
GR 158
NNNN
AAAAFIN0M0N9O8P7Q6R5S4T3U2V1
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
**************************************************
A1B2C3D4E5F6G7H8I9J0
where:
VWGZ
collective callsign for any/all Indian Navy ships
VND 677/16
indicates the daily serial number of the message in that broadcast, i.e. message #677 of day 16 (indeed 16 november, date of my reception). Don't know what VND stands for.
ECHO BRAVO ZULU
ALPHA KILO UNIFORM
...
most likely the daily encrypted callsigns for specific ships
-P-
precedence indicator of the message
-R- Routine
-P- Priority
-O- Immediate (Operational Immediate)
-Z- Flash
160732
date time of origination, no time zone indicator (!)
GR 158
the number of 4FG in the message (158 in this case)
BT
separation (break), as the usual Morse Code abbreviation
The 4FG block is always preceeded by a 9 chars string, i.e.:
ONE NINE NINE TWO ALFA NINE SEVEN TWO FOUR
I noted that this string is used to "signal" the last two 4FG in the block, respectively the last and the second-to-last:
ONE NINE NINE TWO ALFA NINE SEVEN TWO FOUR 9072
2299 4827 3953 0701 6748 2577 4084 8109 5655 4999
...
5904 4854 4358 8628 9964 9687 9032 0282 4140 7567
5029 5582 1302 9724 1992ALFA
;
ZERO ZERO ZERO EIGHT ALFA TWO TWO FOUR EIGHT 9838
6469 5315 6155 6433 5098 8353 7507 5237 5375 4271
...
8394 6708 1257 6554 6238 5987 3600 6023 9076 1083
4574 3021 1116 0342 6063 4300 2248 0008ALFA
;
ZERO THREE NINE NINE ALFA ZERO SIX FIVE NINE 7346
0822 9678 3021 3357 0524 0160 9645 0013 4927 1959
...
5457 3192 3301 5013 5856 9799 0272 2857 8727 9046
1854 5256 7000 0659 0399ALFA
The 4FG blocks usually end with the separation char (BT) folowed by the repetition of the number of encrypted groups in the message (GR nnn), the usual RTTY end-of-message (NNNN) and the strings:
AAAAFIN0M0N9O8P7Q6R5S4T3U2V1
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
**************************************************
A1B2C3D4E5F6G7H8I9J0
at present I do not know their scope/meaning, maybe test chars, but it makes some sense if they are read respectively as couples [0;1][M;V]:
AAAA
FIN (=finish ?)
0M
0N
9O
8P
7Q
6R
5S
4T
3U
2V
1
and [A;J][1;0]:
A1
B2
C3
D4
E5
F6
G7
H8
I9
J0
ONE NINE NINE TWO ALFA NINE SEVEN TWO FOUR
I noted that this string is used to "signal" the last two 4FG in the block, respectively the last and the second-to-last:
ONE NINE NINE TWO ALFA NINE SEVEN TWO FOUR 9072
2299 4827 3953 0701 6748 2577 4084 8109 5655 4999
...
5904 4854 4358 8628 9964 9687 9032 0282 4140 7567
5029 5582 1302 9724 1992ALFA
;
ZERO ZERO ZERO EIGHT ALFA TWO TWO FOUR EIGHT 9838
6469 5315 6155 6433 5098 8353 7507 5237 5375 4271
...
8394 6708 1257 6554 6238 5987 3600 6023 9076 1083
4574 3021 1116 0342 6063 4300 2248 0008ALFA
;
ZERO THREE NINE NINE ALFA ZERO SIX FIVE NINE 7346
0822 9678 3021 3357 0524 0160 9645 0013 4927 1959
...
5457 3192 3301 5013 5856 9799 0272 2857 8727 9046
1854 5256 7000 0659 0399ALFA
The 4FG blocks usually end with the separation char (BT) folowed by the repetition of the number of encrypted groups in the message (GR nnn), the usual RTTY end-of-message (NNNN) and the strings:
AAAAFIN0M0N9O8P7Q6R5S4T3U2V1
&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&
**************************************************
A1B2C3D4E5F6G7H8I9J0
at present I do not know their scope/meaning, maybe test chars, but it makes some sense if they are read respectively as couples [0;1][M;V]:
AAAA
FIN (=finish ?)
0M
0N
9O
8P
7Q
6R
5S
4T
3U
2V
1
and [A;J][1;0]:
A1
B2
C3
D4
E5
F6
G7
H8
I9
J0
It's worth noting that in each transmission the most recent message is sent as first (a kind of LIFO). Moreover, some of the messages that were sent in the previous broadcast are re-inserted in the current one, i.e. the broadcast of 1304 Z contains the last message (#679) and the twos (#678 and #677) sent in the previous broadcast of 1255 Z
[2019-11-16 1152Z]
VND 677/16
VND 676/16
VND 675/16
VND 674/16
[2019-11-16 1255Z]
VND 678/16
VND 677/16
VND 676/16
[2019-11-16 1304Z]
VND 679/16
VND 678/16
VND 677/16
Probably this method is used to improve the reliability of the system but it is not clear to me how the number of messages to be repeated is determined (precedence? duration?).
Sometimes it's possible to see short messages as:
VWGZ
VND 675/16
ZFA
VTH DE GOLF YANKEE
-O-
LIMA ROMEO MIKE
FOXTROT KILO NOVEMBER
160801
ZBQ 0805
BT
NNNN
VTH is listed as the callsign of Indian Navy Mumbai.
Note the Z codes ZFA (Following message has been received) and ZBQ (Message was received at).
Weather bulletins report Weather, Surface Wind, Visibility, Sea State, Swell, and Warnings for specific areas and period of validity (12 hours). The bulletins header indicates the originator of the message just after the precedence indicator:
-P- 150320VWGZ
VND 675/16
ZFA
VTH DE GOLF YANKEE
-O-
LIMA ROMEO MIKE
FOXTROT KILO NOVEMBER
160801
ZBQ 0805
BT
NNNN
VTH is listed as the callsign of Indian Navy Mumbai.
Note the Z codes ZFA (Following message has been received) and ZBQ (Message was received at).
Weather bulletins report Weather, Surface Wind, Visibility, Sea State, Swell, and Warnings for specific areas and period of validity (12 hours). The bulletins header indicates the originator of the message just after the precedence indicator:
FROM FOCINC EAST
TO ALL CONCERNED
-R- 141004Z
FROM NAVAREA VIII CO-ORDINATOR
TO NAVAREA VIII
-P- 160900
FROM CINCAN
TO ALL CONCERNED
where:
FOCINC EAST: Flag Officer Commanding-in-Chief Eastern Naval Command. The Indian Navy operates three operational Commands, each headed by a Flag Officer Commanding-in-Chief (FOCINC): FOCINC East (Visakhapatnam HQ), FOCINC West (Mumbai HQ), FOCINC South (Kochi HQ).
NAVAREA VIII CO-ORDINATOR: the Chief Hydrographer to the Government of India.
CINCAN: Commander-in-Chief of the Andaman & Nicobar Command. The Andaman and Nicobar Command is the first and only Tri-service (army, navy, air force) theater command of the Indian Armed Forces.
Since some of the weather bulletins also report detailed "LOCAL WEATHER FORECAST FOR VISAKHAPATNAM", probably the broadcasts are transmitted from a COMCEN belonging to the Eastern Naval Command (ENC) HQ in Visakhapatnam [2]. In this respect it's noted that some logs in old WUN/UTNL newletters report "VTP Visakhapatnam" as Indian Navy station operating in CW and RTTY 50Bd/850, but not on 16941 KHz. Actually, I didn't find any "official" allocation for 16941.0 KHz but only a clue related to one of the frequecies that are used for HF communications in the Indian activities in Antarctica (IAP, Indian Antarctic Programme) [3].
Given the period of validity (12 hours, except for the forecast for Visakhapatnam which have 24 hours validity) it makes sense to expect similar broadcasts around 0000Z, likely on a lower HF band.
(to be continued)
kiwisdr.vuhams.net_2019-11-16T12_55_40Z_16941.00_usb.wav [1] https://en.wikipedia.org/wiki/Indian_Navy
[2] https://www.indiannavy.nic.in/node/1399#
[3] inpre07e.doc
