Inmarsat-C Login Failure After Antenna Cable Replacement
While crossing the Pacific in Area A3, a 12,000 GT container ship loses its Inmarsat-C terminal. The display alternates between 'Login failed' and 'LES not available'; signal strength is 0 dB; the GPS position input is valid and updates. The terminal was working before a new L-band antenna cable was fitted in the last port. The installation sheet shows the cable as RG-8/U, 28 m long, with three additional connectors. The terminal's ocean-region setting is 'AOR-W'; the vessel is at 35°N 170°E. The master needs to send a routine email and receive SafetyNET MSI; the next port is four days away. The ship also has MF/HF DSC and radiotelephony. You have a spare Inmarsat-C antenna and a spectrum analyzer but no spare terminal. The antenna is mounted on the monkey island, 12 m from the Inmarsat F terminal.
1. What is the most likely cause, and what test sequence would you use to confirm it?
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There are two strong candidates, and the order matters. At 35°N 170°E the vessel is in the Pacific Ocean Region, but the terminal is set to AOR-W, so it may be looking at the wrong satellite. That alone can produce a login failure and zero signal. First set the ocean region to POR or to automatic and try a login. If that fails, investigate the new cable. RG-8/U is not a good choice for a 1.5 GHz Inmarsat-C antenna run: 28 m plus three additional connectors can introduce excessive loss even if the cable is intact. Use the spectrum analyzer at the antenna end to confirm whether the antenna is receiving a signal and whether DC power is reaching the low-noise amplifier. Check each connector for poor crimp, short, or water ingress. Then substitute the spare antenna and a short known-good cable at the terminal. If the terminal logs in on the short cable, the fault is in the installed cable or antenna; if it does not, suspect the terminal or its configuration. Do not start by replacing the terminal.
2. What operational and reporting actions are required while the terminal is unusable?
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The ship has lost its primary Inmarsat-C capability in Area A3, so the master must fall back to the MF/HF installation for distress alerting and watchkeeping, and must obtain MSI by another means. Use MF/HF DSC and radiotelephony as the distress and safety link, and use HF NBDP, radiofax, or another Inmarsat terminal for MSI if available. Record the failure time, symptoms, and actions in the radio log. Notify the coast station or RCC serving the area and the flag administration or its recognized authority as required by the ship's GMDSS failure-reporting procedure. The company should also be informed so that shore support can arrange repairs at the next port. Because the next port is four days away, the master should confirm that the MF/HF installation is fully operational and that the required watches can be maintained. A routine email outage is not a distress matter, but the loss of the Inmarsat-C SafetyNET receiver is a GMDSS equipment failure and must not be treated as an ordinary IT problem.
3. What permanent corrective action and documentation should follow?
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Replace the RG-8/U run with an approved low-loss L-band coaxial cable sized for the actual length and connector count. Minimize connectors, use correct crimp or solder practices, weatherproof every outdoor joint, and route the cable away from sharp edges and sources of heat. Confirm that the antenna has a clear view and that the terminal is set to the correct ocean region or automatic mode. After repair, perform a login test, a SafetyNET reception test, and a signal-strength check. Record the cable type, length, connector count, measured loss or signal level, and the date. Update the installation drawing and the maintenance schedule. If the original installation used RG-8/U, check whether other GMDSS antennas have the same problem. The documentation should let the next technician see why the cable was changed and what test result was accepted, rather than repeating the same diagnosis.