Relative Motion and True Motion on Radar
Radar is one of the classic subjects in navigation study, and one of the first things a learner meets is a simple but important question: how is the picture arranged? The screen shows a pattern of echoes, yet the way those echoes are presented can follow two quite different conventions. Understanding both is a cornerstone of radar theory, and it helps to explore them calmly on shore before they ever matter at sea.
Those two conventions are usually called relative motion and true motion. They describe the same world through different frames of reference, and each has its own character. This guide looks at what each one means in general terms, and why a student is encouraged to become comfortable moving between them. It is a study companion only, not a navigational authority.
What Relative Motion Shows
In a relative motion presentation, your own vessel sits fixed at the centre of the display. Everything else on the screen moves in relation to you. A target's trail therefore reflects how it is closing or opening relative to your own position, rather than the path it is genuinely tracing across the sea. This is the traditional way a radar picture is drawn, and it is often the first that a student meets.
The appeal of relative motion is that it keeps the question of approach front and centre. Because your vessel stays put, the way a contact drifts across the screen reflects the developing relationship between you and it. In broad terms, this is the frame in which ideas such as closest point of approach are studied. It is worth remembering, though, that a fixed object such as a mark will appear to move on this kind of display, which can puzzle a newcomer until the concept settles.
What True Motion Shows
In a true motion presentation, both your own vessel and other contacts move across the display according to their actual travel. Your own symbol is no longer pinned to the centre; instead it works its way across the screen, and so do the targets, each along its own genuine track. A truly fixed object stays where it is, which many learners find more intuitive at first glance.
True motion can make it easier to picture the real geometry of a situation, because what you see is closer to an overhead view of everyone's movement. The trade-off is that the display needs information about your own course and speed to build this picture, and the presentation resets as your symbol reaches the edge. Neither view is simply better than the other; they answer slightly different questions, and a good student appreciates why.
Why Students Learn Both
A confident radar student learns to read each presentation fluently and to switch between them. Relative motion keeps attention on how a contact is approaching, while true motion helps reveal what that contact is actually doing. Being able to move between the frames, and to understand why a target looks the way it does in each, is part of building a sound mental model of the radar picture. The aim of study is understanding, not memorising a single view.
None of this replaces the official rules or proper training. Real decisions about risk of collision are governed by the COLREGs, by a proper lookout using sight and hearing, and by approved instruction. A study guide like this one is only about what the concepts mean; it is not guidance on what to do in any particular encounter, and it is not an emergency procedure.
Practising With Maritime Master Radar
Maritime Master Radar is a radar simulator built for learning and revision, and it lets you explore both presentations for yourself. You can see how the same scene looks in relative and in true motion, follow procedurally generated watches, work through guided training packs, and review debriefs afterwards. It also includes ARPA practice and a simulated AIS overlay, so the wider radar picture can be studied together in one place.
Everything in the app is simulated. There is no connection to real radar, real AIS, or real GPS, and nothing on the screen represents an actual vessel. That makes it a patient environment for turning theory into familiarity, at your own pace and as often as you like. Maritime Master Radar is available on the Apple App Store for iPhone and iPad, free to download with optional Pro and Pro Max subscriptions.
For Study Only
This guide, and the Maritime Master Radar app, are study, learning, and revision aids only. They are not for real-world navigation or collision avoidance, and they are not a substitute for approved training or equipment. The simulator must never be used for real navigation, and it is not a replacement for an approved radar or ARPA course. Real navigation is governed by official charts, publications, and proper training. Nothing here should be treated as operational instruction or as a navigational authority.
