DMR or TETRA? Choosing a UHF Radio System for a Yacht

When someone reports that the yacht's radios are unreliable, the first question is rarely whether the vessel chose DMR or TETRA. I want to know where the call failed, which doors were closed, whether the repeater was alive, what else was transmitting and whether both radios were using the same programming.

That is the practical background to this comparison. DMR and TETRA organise radio traffic differently, but neither one can overcome a poor antenna layout, a dead repeater supply or a steel bulkhead in the wrong place.

For a single yacht, DMR Tier II is usually the sensible starting point. It can provide separate departmental talkgroups, two communication paths through each repeater carrier and a direct radio-to-radio fallback without requiring a large central system. TETRA starts to earn its extra infrastructure when the requirement includes central dispatch, controlled roaming, several busy sites, call recording or guaranteed priority during congestion.

The right choice is therefore not the technology with the longest feature list. It is the system the crew can use under pressure and the technical department can still support several years after handover.

DMR and TETRA are radio standards rather than frequency bands. This article focuses on their use in internal UHF yacht systems. The actual frequencies, permitted power and equipment variants remain subject to the vessel's licence and the relevant spectrum authority.

• How conventional DMR, trunked DMR and TETRA handle a crew call.
• Which yacht requirements genuinely justify a trunked radio system.
• Why coverage, power and fallback design matter more than brochure range.
• What to prove during commissioning before the radios enter operational use.
• Where to start when calls become intermittent, one-way or unavailable.

A conventional DMR Tier II repeater uses a 12.5 kHz radio channel divided into two alternating time slots. Those two slots provide two independent communication paths. Bridge and Deck might share one slot while Engineering and Interior use the other, depending on how the system is programmed.

The talkgroup name on the display does not create extra capacity. It tells the radio which group of users should receive the call. If six talkgroups are mapped across two slots, only two repeater calls can still take place at once.

DMR Tier III changes that arrangement by adding trunking. Radios register with the system and available traffic resources are assigned when a user presses PTT. It suits a larger, busier network, but introduces controllers, network services and a more involved recovery process.

TETRA is trunked by design in its normal Trunked Mode Operation, or TMO. A 25 kHz carrier is divided into four time slots. Radios register with the Switching and Management Infrastructure, which manages talkgroups, call setup, mobility, priority and the use of available traffic channels.

The important difference appears when the network is busy. A properly configured TETRA system can give an emergency or command call priority and pre-empt lower-priority traffic. That is more controlled than a conventional repeater where a user may simply hear a busy indication.

Both standards also provide off-network operation. DMR radios can use direct mode, while TETRA uses Direct Mode Operation, or DMO. Direct mode is a useful fallback only when the crew know how to select it and the tested range covers the locations that matter.

It is easy for a project specification to use words such as mission-critical, resilient and secure without defining what they mean. Those words do not by themselves justify TETRA.

TETRA becomes a serious candidate when the owner wants one managed communications environment across the yacht, tenders, a shore residence, marina security or a support base. It is also relevant when a dispatcher must see users, assign groups, record calls or ensure that an emergency transmission takes capacity from a lower-priority call.

DMR Tier III can meet part of that brief, so a trunked requirement should be evaluated rather than awarded by reputation. For one yacht with moderate radio traffic, several DMR Tier II repeater channels may deliver the required capacity with fewer dependencies and a recovery path the onboard team can own.

Before selecting either platform, record the real operating demand. How many calls overlap during mooring? Does the interior team need uninterrupted capacity while Deck and Bridge are working? Will tender operations extend beyond the ship's antenna coverage? Is central recording required, or merely offered? How many calls must remain possible after one repeater or base station fails?

At comparable UHF frequencies, DMR and TETRA face the same vessel. Steel and aluminium divisions absorb and reflect radio energy. Fire doors, watertight doors, shell doors and tender-garage doors change the path as they move. Machinery, converters and lighting equipment can raise the noise floor. A feeder fault or poorly terminated connector can remove the margin that made a previous survey look acceptable.

This is why I would not accept a radio system based on a walk-through with open doors and quiet machinery. Commissioning should be done with the yacht in a realistic state. Test from the actual watchkeeping and working positions rather than standing in the middle of each deck. Include the lower machinery spaces, bow, beach club, tender garage, external muster points and tenders alongside.

Do not rely on signal bars alone. Record successful call setup, speech intelligibility and any repeated or failed calls. Where the test equipment allows it, capture signal level and error-rate evidence. Digital audio can remain clean near the edge of coverage and then fail abruptly, so a radio that sounds acceptable once may still have very little margin.

A large yacht may need several repeaters feeding distributed antennas, discrete deck antennas or radiating cable. Check what supposedly redundant equipment still shares. Two repeaters in one rack may depend on the same switchboard, UPS, cabinet cooling, duplexer, feeder and antenna system. That is capacity duplication, not full resilience.

The 2021 engine-room fire on MPV Everest is directly relevant. The vessel used a UHF channel for its designated emergency communications. When power was isolated to the affected engine room and adjoining switchboard room, the UHF repeaters also lost supply. Crew did not fully understand that dependency, communications were disrupted and survival-craft VHF radios were used to restore contact.

The incident does not prove that one digital radio standard is safer than another. It shows why the technical boundary must include everything needed to complete a call: repeater or base station, controller, network switch, active antenna equipment, dispatch position, cabinet cooling and power supply.

During acceptance, remove those dependencies one at a time under a controlled test plan. Isolate the normal distribution board. Disable one repeater. Interrupt the supporting IP network. Remove the central service. Confirm what the crew sees, what calls remain possible and how they move to the fallback.

The fallback should then appear in the yacht's emergency procedures and drills. A direct channel hidden in every codeplug is of no practical value if nobody remembers its name during a real isolation.

Consider an 85-metre yacht carrying about 50 handhelds. Bridge, Deck, Engineering, Interior and Security all need routine groups. Mooring creates heavy Bridge and Deck traffic while Interior continues guest service and Engineering may need its own conversation.

Traffic observation shows that three simultaneous calls are occasionally needed, but central dispatch and multi-site roaming are not. A two-carrier DMR Tier II system can provide four time-slot paths. It is easier for the ETO to back up, keep spares for and recover than a trunked core.

That decision still has a weakness: losing one repeater removes half the capacity. The handover therefore documents which groups move to the remaining slots, who declares the degraded mode and which traffic takes priority.

Direct-mode tests also show that a handheld in the lower machinery space cannot reliably reach the forward muster area. The project does not pretend otherwise. The emergency plan retains independently powered survival-craft VHF radios for that failure state and the crew use them during drills.

The same yacht might justify TETRA if the owner later requires the vessel, tenders, residence and marina security team to operate as one controlled network with roaming, recording and pre-emptive emergency calls. The technology changes because the job has changed.

Calling a radio system encrypted is not enough. For DMR, identify the algorithm, key length, loading method, rotation process and action taken when a radio is lost. These details can vary across manufacturers and product families.

TETRA has a broader security architecture, including authentication, air-interface encryption and terminal disabling, with optional end-to-end encryption. Legacy installations still require review by algorithm and key-management practice. TCCA's published security disclosures identify reduced effective key lengths in some legacy algorithms and describe migration or end-to-end encryption as mitigations.

Interoperability also needs to be proved by function. A third-party radio may make a basic group call yet fail to support the required encryption, emergency button, roaming, remote disabling or dispatch feature. Keep one controlled programming baseline, restrict write access and record every codeplug or infrastructure change.

Capture evidence before rebooting the rack. Repeater alarms, controller logs, channel occupancy, power events and the last programming change can quickly separate an RF problem from a capacity or codeplug problem.

Planned maintenance should combine handheld checks with infrastructure checks.

• For handhelds, test battery capacity, antenna condition, microphone audio, emergency buttons and charging contacts.
• At the rack, inspect active alarms, supply condition, UPS autonomy and cabinet temperature.
• Measure transmitter output and reflected power, then investigate any change from the commissioning baseline.
• Check feeders, connectors and duplexers as an RF path rather than assuming the repeater is at fault.
• Prove that configuration backups can be recovered and that spare radios are charged, programmed and authorised to join.

The handover should let another technician understand the system without reverse-engineering it during a fault. At minimum, keep:

• the radio licence and authorised frequency plan;
• an inventory showing each radio's identity, firmware and programming version;
• the talkgroup, slot, priority and emergency-button matrix;
• the RF architecture and the power and network dependencies;
• controlled configuration backups and security-key responsibilities; and
• the lost-radio process, spare strategy, support contacts and change log.

Coverage results should identify the test location, door state, machinery state and failed calls. Store the direct-mode and isolation results beside the normal coverage survey.

That evidence says far more than a claimed range figure. It records what was designed, what was actually proved and what the crew should do when part of the system is unavailable.

Frequency authority, permitted power, product conformity, encryption controls and licensing vary with the vessel's flag, operating area and national administration. Confirm the authorised UHF arrangement before equipment is purchased or programmed.

An internal DMR or TETRA system is normally supplementary. It does not replace required GMDSS, marine VHF, survival-craft or distress equipment unless the competent authority has explicitly accepted that arrangement.