ANEP-87 digital underwater signalling standard (JANUS)

What it is

ANEP-87 is the NATO Allied Naval Engineering Publication that defines JANUS, a digital underwater acoustic signalling standard built so that submarines, surface ships, buoys, sensor networks and autonomous underwater vehicles (AUVs) from different manufacturers can discover each other and exchange short digital messages underwater. Before JANUS, each manufacturer's underwater modem used its own proprietary coding scheme, so NATO and allied navies had no way to communicate digitally underwater between assets built by different suppliers, even where each already carried some digital underwater capability of its own. JANUS fixes that at the physical layer: a single acoustic waveform, a 64-bit "Baseline JANUS Packet" structure, an error-correcting code and five defined frequency bands, so that any compliant modem can at least detect and decode a JANUS transmission regardless of who built it.

The baseline packet and physical layer

JANUS uses Frequency-Hopped Binary Frequency Shift Keying (FH-BFSK), a waveform chosen for robustness in the underwater acoustic channel and ease of implementation on existing hardware. Five frequency bands are defined, each with its own centre frequency, bandwidth and chip duration; bands A, B and C are intended predominantly for military use, while bands D and E serve both military and civilian applications, and band A is not expected to become obsolete before 2030. Every transmission starts from a fixed 64-bit packet: a version number, a mobility flag, a schedule flag, transmit/receive and forwarding flags, an 8-bit class-user identifier, a 6-bit application type, a 34-bit application data block and an 8-bit CRC. The packet is protected by a rate-1/2 convolutional encoder, based on the same coding used in the commercial IS-95 CDMA cellular standard, and then interleaved. An optional "Cargo Data" extension can follow the baseline packet for larger payloads, with the channel time for it reserved in advance or requested at a fixed repeat interval, for up to nearly eleven minutes at a time.

Coexisting with the existing underwater telephone

A large part of the standard's medium access control is built around one constraint: not disrupting NATO's existing analogue underwater telephone and distress traffic, which is standardised separately. JANUS uses an energy-detection carrier-sensing scheme with exponential backoff to avoid collisions, and adds a mandatory guard time before transmitting so that pauses between spoken words on an analogue underwater telephone are never mistaken for a free channel. Where the JANUS band overlaps the underwater telephone band, a JANUS node must not transmit immediately after detecting the channel was busy. The document leaves one detail deliberately open: the specific algorithm a modem uses to estimate background acoustic noise is not fixed, and an implementation is judged only by whether its false-alarm and missed-detection behaviour stays broadly in line with other implementations.

The application layer: user classes and message types

On top of the baseline packet, the standard allocates 256 numbered "User Classes", some assigned to specific message types, most left open for future assignment or reserved for individual nations. The message types actually specified cover emergency position, status and combined reports for a vessel in distress; an automated remote-query mechanism for retrieving sensor data; underwater AIS-style contact reports covering up to eight vessels' type, position, depth, speed and heading at once; oceanographic and meteorological data requests aimed at offshore installations; text chat with optional encryption (including AES-GCM) and delivery acknowledgement; two different node-discovery "first contact" handshakes for a device joining a network; a "keep silent" emission-control message for acoustic-sensitive operations; and a symmetrically pre-shared-key messaging scheme for pre-canned, codebook-style communication. Older equipment built to the previous packet-version number keeps working under separate legacy formulas the standard still carries, so this edition's changes do not break fielded systems built to the prior version.

Who has to act, and when

The standard does not speak in terms of "Supplier" and "Acquirer" the way NATO's quality-assurance publications do. It speaks in terms of the equipment and the people who build and operate it: manufacturers, who the document expects to bring existing systems into compliance mostly through firmware updates rather than a hardware redesign; and operational commanders, who choose which frequency band to use for a given mission. What brings JANUS into force is not the publication date but ratification: nations record their agreement to use ANEP-87 through STANAG 4748, and that ratification does not mean uniform or immediate rollout. Recorded reservations already show navies moving at different speeds - one nation's newest submarines and frigates carry JANUS from their in-service date, while its existing fleet is not expected to gain the capability for several more years, tied to its own industrial testing timeline rather than a single NATO-wide deadline.

What it does not fix

ANEP-87 leaves several things to the implementer rather than specifying them: the algorithm for estimating background acoustic noise, the addressing scheme used by the basic text-chat application, and the specific encryption method behind several of its "user-defined" encryption options. It also names no certification scheme, no accredited certification body and no notified body: nothing in the document supports the idea that a product, or a company, can be "JANUS certified."

Standards it references

• STANAG 1475Background