What Your RAIM Residuals Aren’t Telling You About Multipath in Confined Waters
You are coming into Rotterdam at half-tide. The channel narrows to 180 meter. Your ECDIS shows the ship exactly on the centerline, RAIM residual blinking green at 0.12 NM. everyth looks fine. But the helmsman says the bow is falling off to starboard. You check the rate-of-turn indicator: 3° per minute, steady. The RAIM residual haven't twitched. So what is going on? The snag is that RAIM — Receiver Autonomous Integrity Monitoring — was designed for en-route oceanic flight, not for squeezing a 50-meter beam between concrete walls. In confined water, multipath reflec off nearby structures can introduce pseudorange error of 10 to 50 meter without pushing residual above the alarm threshold. The residual are telling you the satellite are consistent with each other, not that the posi is correct.