A Study Guide to Radar: CPA, TCPA and ARPA
- Reece Smith-ward
- Aug 1
- 3 min read
Radar can look intimidating at first: a glowing display, a sweep, and a scatter of blips that seem to move in their own directions. This study guide introduces the core ideas at a gentle, conceptual level so you can build a mental model before you revise the detail. It also shows how the Maritime Master Radar app lets you practise these ideas on a simulated display.
Everything here is about learning and revision. The concepts below are explained to help you understand what a radar picture represents, not to prepare you to operate real equipment or make real navigation decisions.
What a radar picture shows
A marine radar sends out pulses and listens for the echoes that bounce back from objects around the vessel. On the display, those echoes appear as blips whose position reflects range and bearing. You can view the picture in relative or true motion, and orient it head-up, course-up, or north-up, each of which changes how movement is presented. Learning to read these choices is the first step in making sense of what you see.
Clutter, palettes and controls
A real picture is rarely clean. Sea and rain clutter, squalls, and receiver noise can obscure contacts, while gain and tuning affect how strongly echoes show. Chart overlays with day, dusk, and night palettes, land, and shadow sectors help set the scene. Understanding how these elements interact is a large part of interpreting a display accurately, rather than guessing.
CPA and TCPA explained
Two ideas sit at the heart of studying how vessels approach one another: CPA and TCPA. CPA, the closest point of approach, describes how near another vessel would come if both continued on their present course and speed. TCPA, the time to closest point of approach, estimates how long until that moment would arrive. Together they give a conceptual sense of which contacts matter most and how much time there is to think. As a learner, your goal is simply to understand what these values mean and how they change.
What ARPA is for
ARPA, the automatic radar plotting aid, is the technology that tracks targets and calculates figures such as CPA and TCPA. Conceptually, it acquires a contact, follows it over time, and presents its predicted behaviour so that the situation can be reasoned about. Studying ARPA is really about understanding what the numbers represent, where they come from, and why they must always be treated with judgement rather than blind trust.
Practising in the simulator
The Maritime Master Radar app is a radar simulator for learning and revision, where you practise reading and plotting on a simulated display. It is a study and practice tool, not a substitute for an approved radar or ARPA course. You can work with electronic bearing lines, variable range markers, parallel index lines, and true or relative trails, and set up circular or sector guard zones with anti-grounding alerts. For ARPA practice you can acquire and track targets, read CPA and TCPA, set danger thresholds, and run trial manoeuvres, all alongside a simulated AIS overlay. Optional sensor error models such as gyro drift and log errors let you study how imperfect inputs shift the picture.
Scenarios, packs and debriefs
Free classic scenarios give you a starting point, and procedurally generated practice watches of fifteen minutes to two hours keep each session fresh. Directed situations such as crossing rushes, fog close-quarters, late give-way vessels, and developing distress calls let you focus on particular challenges. Six guided training packs cover COLREGs crossings, restricted visibility, ARPA tracking, pilotage, traffic separation schemes, and yacht radar, with levels, goals, badges, and shareable completion certificates. Afterwards, a debrief gives a score out of one hundred, highlights closest points of approach, early action analysis and missed contacts, and offers a replayable timeline so you can review your reasoning.
Free features and Pro upgrades
The classic scenarios are free, and the app keeps things private with no accounts, analytics, or tracking, so your data stays on the device. Voiceover briefings help set up each exercise. Optional Pro and Pro Max subscriptions, each with a free trial, unlock unlimited procedural watches, all training packs, full debriefs, the scenario creator, import and export, debrief reports, radar profiles, and instructor tools. It is available on iPhone and iPad.
An important safety note
Everything is simulated for learning only and must never be used for real navigation or collision avoidance; it does not replace an approved radar or ARPA course; the app has no connection to real radar, AIS, or GPS; completion certificates record practice in the simulator and are not qualifications or certificates of competency.
