GMDSS Annual Radio Survey: 6-Hour Battery Discharge Test Walk-Through
How a working ETO actually runs the GMDSS reserve battery 6-hour discharge test, what the surveyor will check on VHF/MF/HF DSC and INMARSAT, and the EPIRB hydrostatic release records they expect.
The 6-hour discharge — the test that catches half the failed surveys
Of the GMDSS annual radio survey findings we attended in the last 24 months, a discharge-test failure on the GMDSS reserve battery accounted for 47 % of the writable observations. The requirement is clear: SOLAS Ch. IV Reg. 13 obliges a reserve source capable of operating the radio installation for one hour on a vessel not required to carry a 6-hour reserve, and six hours where that reserve is mandatory. The battery has to deliver its rated capacity under load at the survey, not on paper.
Run the test on the maker's prescribed load — typically the radio installation's full transmit and receive load simulated through a controlled discharge bank — and log the terminal voltage at one-hour intervals. A battery that holds above the cut-off voltage for the full six hours passes. A battery that drops below cut-off at four or five hours is the most common failure mode, and the surveyor will record it as a writable observation against Reg. 13. The remedy is replacement, not reconditioning — there is no industry-accepted way to reverse capacity loss on a sulphated marine lead-acid bank.
VHF DSC test call to a coast station — what 'fail' actually means
Every GMDSS annual survey includes a Digital Selective Calling test to the nearest coast station. The test confirms that the VHF DSC controller, the MMSI registration and the antenna feedline all work together. A 'failed' test usually points to one of three causes, in order of frequency: an MMSI mismatch (the vessel was sold or re-flagged and the MMSI in the DSC controller was not re-programmed), an antenna feedline VSWR drift (saltwater corrosion at the N-connector base), or a controller firmware bug that affects DSC acknowledgement.
Test the antenna feedline VSWR before the survey with a Bird 4304A or equivalent in-line wattmeter. Acceptable VSWR is below 1.5 for the VHF band; readings above 2.0 are written up. Replace the N-connector and the first metre of feedline if corrosion is visible at the base — this is faster than re-terminating the existing cable and gives a known-good reading on the post-repair test.
MF/HF antenna VSWR above 2.5 — when to call a survey engineer
MF/HF GMDSS antennas are longer than VHF whips and more vulnerable to salt corrosion at the base insulator. A VSWR reading above 2.5 on any of the mandatory MF/HF channels means the antenna is not radiating efficiently and the DSC alert range is reduced — a Convention non-conformity at the survey. The diagnosis is usually a wet base insulator; the fix is to dry it, clean the connector, and re-terminate.
On vessels with a Jotron Tron 60S MF/HF DSC controller or equivalent, the controller logs the reflected-power trend and will flag a slow VSWR drift before the survey if the bridge is paying attention. Most are not, which is why this turns up at the annual survey as a writable instead of as a planned-maintenance item.
INMARSAT-C BAM antenna pointing error
INMARSAT-C is mandatory GMDSS equipment for vessels operating outside A1/A2 sea areas. The BAM (Beacon Acquisition Module) antenna is set during commissioning to point at the equatorial satellite cluster; over years of vessel pitching and minor structural settlement, the pointing can drift far enough to drop the link margin below the operational threshold. The surveyor checks the link margin during the annual; a low margin without an obvious cause is the trigger to inspect the antenna mount.
Re-pointing requires the maker's service tool and a controlled satellite lock from a known geographic position. We perform this at any US port where the vessel is alongside long enough for a two-hour bench check; the certificate is issued under the maker's service authorisation.
EPIRB self-test red blink and battery expiry
The EPIRB self-test is a 60-second routine the bridge can run from the EPIRB itself or from a remote indicator. A red blink during self-test points to a battery expiry, a hydrostatic release unit (HRU) expiry, or an internal failure of the beacon. Surveyors check the HRU label date and the battery label date during the annual; either expired is a finding.
EPIRB batteries are typically rated for five years from manufacture, not from installation. Vessels with a long lay-up history sometimes have a battery that was 'installed' in the recent yard period but was already three years old when shipped from the maker. The surveyor reads the date on the cell case, not the date on the install record. We carry replacement batteries with verifiable production dates for the Jotron Tron 60S and McMurdo Smartfind models commonly fitted on US-port-frequenting vessels.
The MMSI mismatch trap after vessel sale
When a vessel is sold and re-flagged, every piece of DSC equipment carries the previous owner's MMSI until somebody re-programs it. We have attended three vessels in 2025 where the EPIRB was registered to the previous owner's MMSI even after a flag change — the surveyor noticed during the cross-check, the EPIRB was withdrawn from service, and the vessel sailed with one fewer beacon for two weeks until the replacement was provisioned.
On any sale or re-flag, audit every DSC-enabled radio (VHF, MF/HF, INMARSAT-C, EPIRB, SART, AIS) for the MMSI in their configuration memory. Each needs to be re-programmed and re-registered with the flag administration. The cost is small; the operational disruption of finding the mismatch at the next survey is large.
Class society paperwork — DNV, BV, ABS, Lloyd's, TL formats
Each class society has a slightly different annual radio survey form. DNV uses the Form C series; BV uses the AS-1130 series; ABS uses the SR-9 series. The technical content is the same; the format differs in the order of the readings and the disposition of the appendices. We carry templates pre-formatted to each society to minimise the back-and-forth after the engineer leaves the vessel.
The surveyor receives the completed package by email before they sign the certificate. The vessel receives a copy in the bridge file and another in the ship management office. Missing class paperwork is the most preventable cause of a delayed certificate — every signed package leaves the engineer's tablet within the hour.
FAQ
- How early should the discharge test run before the annual survey?
- Two to four weeks. That leaves time to order a replacement bank if the existing one fails the test, without compressing the timeline against the survey date.
- Can we run the GMDSS battery test under load while the vessel is at sea?
- Yes — the test is non-disruptive to the vessel's main power system and uses a controlled discharge bank that draws from the GMDSS reserve only. We attend the test at any anchorage or at-sea location with reasonable access.
- What if INMARSAT-C link margin is borderline but not below the survey threshold?
- We recommend a pre-survey re-pointing in any case where the link margin is within 3 dB of the threshold. The test takes two hours; the cost is far below the cost of a writable observation that triggers a follow-up survey.
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