Module 10 — GPS, antennas, batteries and reserve power

Many real-world GMDSS failures come not from the radio itself but from the support layer: a stale position, a damaged antenna or a flat battery.

GPS / DGPS connected to the equipment

All DSC sets and the Inmarsat C terminal need a fresh position. The operator must check that the GPS is outputting NMEA and that the set displays current time and position. In case of failure, enter the position manually.

Antennas

Each equipment needs its own antenna: VHF whip, MF/HF wire or whip with automatic tuner, Inmarsat C quadrifilar or helical patch, 518 kHz NAVTEX whip, EPIRB integrated antenna.

Power

Pre-watch checklist

Position current, DSC watch active, main power OK, batteries on charge, antennas visually intact, NAVTEX operating, Inmarsat C logged in.

Carriage and two independent means (6.1.2)

Part 6 reminds that every SOLAS ship must be capable of transmitting ship-to-shore distress alerts by at least two separate and independent means, each using a different radiocommunication service. That principle drives redundancy of antennas, power and electronic chains: if a single common failure (one antenna, one UPS) takes down both means, the requirement fails.

Three maintenance methods (6.4)

SOLAS IV/15.6 and 15.7 recognize three methods to ensure availability: (a) duplication of equipment; (b) shore-based maintenance; (c) at-sea electronic maintenance capability. In A1/A2 one is enough; in A3/A4 a combination of at least two is required. COMSAR/Circ.32/Rev.2 details the implementation.

Reserve source of energy (SOLAS IV/13)

Every installation must include a reserve source of energy capable of feeding VHF-DSC, MF/HF-DSC or RMSS and essential radio lighting for at least 1 hour (ship with emergency generator) or 6 hours (ship without). The reserve is load-tested weekly and logged.

GNSS and antennas

GNSS (GPS, GLONASS, Galileo, BeiDou depending on fit) feeds automatic position into DSC, Inmarsat C and EPIRB. A damaged cable, degraded splitter or wet connector can cause alerts to go out without position. VHF, MF/HF and satellite antennas must be visually inspected periodically and after severe weather.

Three test cadences the manual fixes

Batteries: chemistries, care and pitfalls

Reserve batteries are typically deep-cycle lead-acid: wet/flooded (need hydrogen venting and topped-up electrolyte), sealed gel (maintenance-free, intolerant of overcharge) or AGM (absorbed glass mat, very low self-discharge, very temperature-sensitive). Lithium shows up in some portable gear and primary beacons: higher density and lower weight, but needs electronic protection and does not charge like lead-acid. NiCd shows a memory effect (repeated partial charging cuts usable capacity). In lead-acid, sulfation from prolonged discharge destroys plates; it is prevented by a correct float charge and detected by measuring specific gravity with a hydrometer or open-circuit voltage. A periodic equalisation charge (where the manufacturer allows it) rebalances cells.

Antennas, cables and fuses

A typical installation combines: a 1 m vertical VHF whip with checkable SWR; an MF/HF fibreglass whip or, on larger ships, a backstay antenna with a copper-mesh counterpoise for RF return; omnidirectional satellite antennas (Inmarsat C, Iridium) that need sky visibility and a radar-blanking sector around radome-mounted radars. Every antenna has coax and connectors that must be inspected: moisture in a connector degrades SWR, a crushed cable adds loss, an unblanked radar sector can inject RF into the satellite antenna. Fuses must be of the rated value; swapping one for a larger one 'because it kept blowing' hides a real fault. The radio log records faults, replacements and tests as part of required maintenance.

Reserve-capacity sizing (SOLAS IV/13)

The minimum capacity (Ah) is computed as the sum of current draw for every mandatory GMDSS load fed during a typical 1-hour duty cycle (transmit + receive): Ah_req = Σ(I_load · t_load) · k_safety, with k_safety ≈ 1.2-1.25 to compensate for ageing and real discharge curve (a lead-acid battery delivers only ~80% of rated capacity in a 1-h discharge). Rough example: VHF-DSC 6 A TX (~10% of time) + 0.5 A RX (90%) ≈ 1 Ah; MF/HF-DSC 25 A TX (10%) + 1 A RX (90%) ≈ 3.4 Ah; Inmarsat C 0.5 A steady = 0.5 Ah; emergency lighting 2 A = 2 Ah; total ~7 Ah × 1.25 ≈ 9 Ah for 1 h. For 6 h (ship without emergency generator) the capacity scales to about 54 Ah. The weekly on-load test must keep terminal voltage above the critical equipment threshold (usually 20.4 V on a 24 V bank) throughout the test.

GNSS antennas: installation and maintenance

The DSC GNSS antenna sits in a clear spot with ≥360° sky visibility, at least 1 m above or below radar radomes to avoid saturation, and outside the main beam of high-power MF/HF antennas. Low-loss coax (RG-213, LMR-400 or equivalent) is routed with bending radii >10 cm; N or TNC connectors sealed with heat-shrink and electrical silicone; connector moisture is the most frequent fault and shows as poor SNR or intermittent fix. LNA power: typically +5 V DC injected through the coax by the receiver; measuring voltage at the antenna connector is a basic check when the fix drops out. Recommendation: a dual-GNSS (GPS + GLONASS + Galileo) receiver for polar or high-latitude operations where single-constellation geometry can degrade precision.

Pre-sailing GMDSS station checklist