Module 10 — GPS, antennas, batteries and reserve power

While the ship is at sea there must be enough power to run the radio installations and to charge the batteries that form the reserve. There must also be a reserve source, independent of propulsion and the main electrical system, able to keep distress, urgency and safety communications going if the main and emergency sources fail.

The reserve must be able to feed the VHF and the equipment required for the sea area (MF, MF/HF or the ship earth station) simultaneously, together with their mandatory loads such as the position input and the emergency light. It does not need to feed two independent MF and HF installations at once unless the rule demands it.

Sizing rule. Battery capacity for the reserve is worked out from half the transmitter current, plus the full receive current and the additional loads, multiplied by the required hours (1 or 6), with a margin for temperature and age.

Usable capacity depends on the rated Ah, temperature, age, state of charge, discharge current and chemistry. A battery that shows the correct voltage off load can collapse under transmit load.

Battery types. Flooded lead-acid batteries need electrolyte checks and topping up with distilled water; a hydrometer reads their specific gravity, roughly 1.28 when fully charged and about 1.12 when discharged. Sealed gel and AGM batteries are not topped up or measured with a hydrometer: check their voltage on load, temperature and the charger instead.

Two-way equipment that includes position automatically in a distress alert must receive it from an internal or external navigation receiver. If that receiver fails, SOLAS requires manual updating of position and time at intervals not exceeding 4 hours while the ship is underway.

Working formulas: λ (m) ≈ 300 / f (MHz); quarter-wave L ≈ 75 / f (MHz); half-wave L ≈ 150 / f (MHz). Real antennas need corrections for end effects, structure, diameter, radome, earth plane and matching networks.

An MF/HF ship antenna has to work on many frequencies and cannot be resonant on all of them. The ATU (automatic tuning unit) transforms the impedance the transmitter sees so it can deliver power efficiently and safely.

VSWR (voltage standing wave ratio) measures mismatch. 1:1 is ideal; higher values mean more energy reflected back towards the transmitter. Modern transmitters reduce power to protect the final stage when they detect a severe mismatch, so "the set switches on" does not mean "the antenna works".

VSWR = (1 + |Γ|) / (1 − |Γ|), where Γ is the reflection coefficient; the reflected power fraction is |Γ|².

The decibel expresses a ratio on a log scale. For power: dB = 10 log₁₀(P₂/P₁); for voltage on the same impedance: dB = 20 log₁₀(V₂/V₁). Quick rules: +3 dB ≈ double power, +10 dB = ×10, −3 dB ≈ half.

In a link, gains and losses multiply physically but add in dB: transmitter power + antenna gain − cable loss − path loss + receive antenna gain. That is why link budgets are written in dB.

SNR (signal-to-noise ratio) often matters more than absolute signal: an S5 signal over S0 noise can be more readable than S9 over S8. Filters, antenna, band choice and killing on-board noise improve SNR without more transmit power.

Sensitivity is the ability to receive weak signals at a given quality; selectivity is rejecting adjacent signals; dynamic range is coping with very strong and very weak signals together. On a bridge full of electronics, intermodulation and blocking can matter as much as the weak signal.

1. 1Quarter-wave for 6215 kHz. 75 / 6.215 ≈ 12.1 m.
2. 2Endurance. A 24 V, 150 Ah reserve bank feeds an average radio load of 300 W. Energy ≈ 24 × 150 = 3,600 Wh; 3,600 / 300 = 12 h in theory. With about 70 % usable capacity in the cold, plan on roughly 8 h: comfortably above a 6-hour requirement.
3. 3Position age. The DSC position is 3 h old and the ship makes 14 knots: the alert could be up to 42 NM out.
4. 4Reflected power. A VSWR of 2:1 gives |Γ| = (2 − 1) / (2 + 1) ≈ 0.33, so about 11 % of the power is reflected.

STCW A-IV/2 asks every GOC holder to know the radio-frequency and electrical hazards of the station.

The GOC is not a repair technician, but must find simple faults and keep a station working.

Write every fault and repair in the radio record, and report what cannot be fixed on board.

References: SOLAS IV as amended by MSC.496(105)