MEROD FSK 100Bd/600 (Indian BSF)

FSK 100Bd/600 signal (thanks to KarapuZ)

A few days ago my friend KarapuZ posted an interesting recording of a 100Bd/600 FSK signal which turned out to be a message structured according to MEROD-IND (Message Entry Read Out Device, Indian variant) message-handling principles, probably the MEROD MA4450 device [1]. MEROD  is the commercial name for the Racal manufactured equipment that uses the RACAL-ARQ mode.

Fig. 1 - main FSK parameters

MEROD-IND provided a simplified framework for organizing message elements such as originator, routing, and date-time group, supporting clarity and orderly handling under operational conditions.
While MEROD-IND is primarily associated with voice procedure mnemonics, its application in this context functioned as a content-structuring aid, with the actual transmission conducted as HF data. From a doctrinal perspective, such a transmission aligns functionally with ACP-125/ACP-131 data communication concepts, though it would normally be described using formal ACP message formats rather than the MEROD-IND designation.
The demodulated bitstream is shown in Figure 2.

Fig. 2 - demodulated bitstream

It's interesting to note that the first two 64-bit sequences are identical except for one bit, perhaps a reception error. This suggests a deliberately repeated sequence:
6D C5 F7 E0 B2 89 88 17
6D C5 F7 E0 F2 89 88 17
A confirmation comes from reading the characteristics of the MEROD MA4450 device [1], literally " [...] preamble of 64 bits used for detection and synchronization. Two sequences can be recognised on receive: one for secure messages and the other for clear messages. Sufficient redundancy is incorporated to achieve 98% probability of recognition of either sequence".

The message sent is in clear text and can be decoded by BEE, so the above 64-bit sequence refers to unencrypted messages.

OK TKS NR
DE NB SIG SRL NO 210004
RRR ZEQ APA
RRR DTO 211600
FROM FTR HQ BSF NB
TO CT WING N DELHI/SDGHE/W)/HZB/IND/TKP/ALL FTR
GR 20 BT
RESTD NO C/7101
FOR COMN. FOR AND FROM ADSO/CCT OPR.
HF COMMERCIAL REPORT UPTO 211600 HRS. FOR INFO PSE // BT
THI
CFN NO C/7101
SD B KR

It's clearly a plain-language military signal, just formatted in classic signal style, administrative/communications report. Indian origin (IND, Delhi, BSF strongly point to this). Indeed, the Border Security Force (BSF) is a central armed police force in India, under the Ministry of Home Affairs. It is responsible for guarding India's borders with Pakistan and Bangladesh. 

Line-by-line decode:

Header / routing
OK TKS NR
DE NB SIG SRL NO 210004
OK TKS – acknowledgment / thanks (common signal shorthand)
NR – Number
DE – “From”
NB SIG – NB Signals unit
SRL NO 210004 – Serial number of the message

Acknowledgments
RRR ZEQ APA
RRR DTO 211600
RRR – “Received, received, received” (strong confirmation)
ZEQ / APA – internal routing or reference codes
DTO 211600 – Date-Time Of message (21st at 1600 hrs)

Sender / recipient
FROM FTR HQ BSF NB
TO CT WING N DELHI/SDGHE/W)/HZB/IND/TKP/ALL FTR
FROM:
Frontier Headquarters, BSF NB
(BSF = Border Security Force)
TO:
Central Wing, New Delhi
SDGHE (signals directorate / HQ element)
HZB / IND / TKP (likely unit or regional designators)
ALL FTR – all frontiers
This is a broadcast-style admin signal.

Body
GR 20 BT
RESTD NO C/7101
GR 20 – Group count (20 word groups)
BT – Break (end of header, start of message)
RESTD – Restricted
NO C/7101 – Reference number

Actual message content
FOR COMN. FOR AND FROM ADSO/CCT OPR.
HF COMMERCIAL REPORT UPTO 211600 HRS.
FOR INFO PSE // BT
Plain English:
For information regarding communications for and from ADSO / CCT operations.
HF commercial report up to 211600 hours.
For information please.

End / authentication
THI
CFN NO C/7101
SD B KR
THI – End of text (Indian signals usage)
CFN – Confirmation
SD – Signed
B KR – Initials of the duty signal officer

So what does it mean? In normal language: “Frontier HQ BSF is informing all formations and central HQ about the status of HF commercial communications up to 1600 hours on the 21st, for operational coordination. This is a restricted administrative signal.”

Below a quite complete abbreviation map about the Indian Signals (CAPF doctrine) (1)

PROCEDURAL SIGNALS (PROSIGs)
OK:
Meaning: Message understood / acknowledged
Type: Informal procedural
Usage: Common in Indian HF nets
TKS
Meaning: Thanks
Type: Informal procedural
RRR
Meaning: Received, Received, Received
Doctrine: ITU / military standard
Usage: Strong confirmation of receipt
BT
Meaning: Break
Doctrine: ITU prosign
Usage: Separates header from text / clauses
DE
Meaning: From
Doctrine: ITU prosign

MESSAGE IDENTIFIERS
NR
Meaning: Number
Usage: Message or signal number
SRL NO
Meaning: Serial Number
Doctrine: Indian Signals standard
GR
Meaning: Group count
Usage: Number of word groups in body

DATE / TIME
DTO
Meaning: Date Time Origin
Doctrine: Military standard (Indian & NATO usage)
Format: DDHHMM (local or Z as context dictates)
HRS
Meaning: Hours
Usage: Indian military style (vs “hrs” in NATO)

SECURITY / CLASSIFICATION
RESTD
Meaning: Restricted
Doctrine: Indian security classification
Levels (India):
Top Secret
Secret
Confidential
Restricted

ORGANIZATIONS / FORMATIONS
BSF
Meaning: Border Security Force
Type: CAPF under MHA
FTR
Meaning: Frontier
Usage: BSF regional command (equivalent to Army Corps-area)
HQ
Meaning: Headquarters
SIG
Meaning: Signals
Context: Signals unit / communications staff
NB
Meaning: Likely North Bengal
Usage: BSF Frontier designation

ADDRESS / ROUTING TERMS
TO
Meaning: Addressee
CT WING
Meaning: Central Wing
Context: BSF / CAPF HQ directorate
DELHI
Meaning: New Delhi (HQ location)
ALL FTR
Meaning: All Frontiers
Usage: Broadcast instruction

OPERATIONS / STAFF FUNCTIONS
COMN
Meaning: Communications
Doctrine: Indian spelling (COMN vs COMM)
OPR
Meaning: Operations
ADSO
Meaning: Assistant Director (Signals / Special Operations)
Context: CAPF / HQ staff appointment
CCT
Meaning: Command & Control Team
(sometimes “Counter-Communication Team” depending on unit)

REFERENCES / FILE NUMBERS
NO C/7101
Meaning: File / reference number
Doctrine: Indian HQ filing style
CFN
Meaning: Confirmation
Usage: Cross-reference to original file number

REPORTING / CONTENT
HF
Meaning: High Frequency
Usage: 3–30 MHz band
HF COMMERCIAL
Meaning: Civil / leased HF services
Context: Backup or contingency comms
REPORT UPTO
Meaning: Status report valid until stated time
FOR INFO PSE
Meaning: For information please
Usage: Indian service English

TERMINATION / AUTHENTICATION
THI
Meaning: This is the end of message
Doctrine: India-specific signal terminator
Equivalent: NATO “AR”
SD
Meaning: Signed
Usage: Precedes sender authentication
B KR
Meaning: Initials / callsign of duty signal officer
Doctrine: Indian Signals authentication practice

UNKNOWN / CONTEXTUAL CODES
ZEQ / APA
Meaning: Internal routing or acknowledgment codes
Usage: Network-specific, not globally standardized
Common in: Indian HF fixed networks
SDGHE / HZB / IND / TKP
Meaning: Likely:
Directorate codes
Geographic or functional routing
Network node identifiers
Status: Internal BSF / Signals net designators

https://disk.yandex.com/d/nx8YASlWvDZvIQ

[1] https://www.cryptomuseum.com/crypto/racal/ma4450/files/merod_ma4450.pdf

(1) CAPF = Central Armed Police Forces, under India’s Ministry of Home. How India’s military communication systems and Central Armed Police Forces are supposed to operate, communicate, and coordinate according to official principles.