This page explains, in simple terms, how ships use technology to avoid collisions at sea, based on a research article about advances in marine navigation.

Imagine two ships moving. How do you know if they're going to get too close? Traditionally, sailors used radar to measure the distance and direction (bearing) to the other ship. They would take two measurements with a short time gap (like 6 minutes) and then use math (called “ratio plotting”) to calculate the closest they would get (the “closest point of approach,” or CPA).

Then came AIS (Automatic Identification System). Think of it as ships constantly “texting” each other their position, speed, and direction. This is much more precise than radar. Now, you could use the same “ratio plotting” math, but with much better data from AIS.

The old way, even with AIS, still had a problem: you had to *wait* for that second measurement. This takes up valuable time when you need to make quick decisions to avoid a collision.

The article describes a new trick: “virtual second epoch of observation.” Instead of waiting for a second measurement, they *predict* where the other ship will be in a few minutes using the AIS data (speed and direction). They create a *virtual* second measurement.

Basically, they use some trigonometry (fancy math with triangles) to calculate where the other ship *should* be in, say, 6 minutes, based on its current course and speed. This gives them their “virtual” second measurement.

This new method is much faster because you don't have to wait. However, because it's a prediction, it's not quite as accurate as waiting for a real second measurement. It's like predicting the weather – it's helpful, but not always perfect.

The article also figured out the best time to make this prediction. If you predict too soon (like 1 minute), the change in position is so small that it's hard to get a good CPA calculation. If you predict too far into the future (like 15 minutes), the prediction becomes less accurate. They found that around 6-12 minutes is usually the best, depending on how far apart the ships are.

So, this new method using AIS data to predict a “virtual” second measurement is faster, which is very important for avoiding collisions. It's not as accurate as the traditional method, but it's accurate *enough* in most cases and saves precious time. It's a trade-off between speed and accuracy.

This page describes the procedure of a new method for calculating the Closest Point of Approach (CPA) using AIS data, as detailed in the article “Determining of the Distance of the Nearest Approach of Vessels with Using Information from AIS Method of Relative Ship' Plotting with a Virtual Second Observation Epoch”.

Instead of waiting for a second measurement of distance and bearing to another ship, this method *predicts* those measurements using AIS data. This creates a “virtual” second observation, allowing for faster calculation of the CPA.

# Gather Initial Data: ## Obtain the current position, speed, and course (direction) of both the observer's ship and the target ship using AIS. This provides the first set of measurements: Distance 1 (D1) and Bearing 1 (B1). # Predict Future Position: ## Using the speed and course of both ships, calculate where the target ship will be after a short time interval (the “virtual second epoch”). This involves some trigonometry to account for the movement of both vessels. This calculation results in a predicted Distance 2 (D2) and Bearing 2 (B2). # Calculate CPA: ## With the two sets of measurements (D1, B1 and the predicted D2, B2), use the traditional “ratio plotting” technique (or the associated formulas) to determine the CPA.

* 'No Waiting:' The traditional method requires waiting for a set time (e.g., 6 minutes) to take a second measurement. This new method eliminates that waiting time. * 'Prediction:' The core of the new method is the prediction of the target ship's future position, which introduces a degree of potential error.

* 'Speed:' Provides a much faster estimation of CPA, allowing for quicker decision-making in collision avoidance.

* 'Reduced Accuracy:' Due to the prediction involved, the CPA calculation is not as accurate as the traditional method with two real measurements.

* The accuracy of the prediction depends on the time interval chosen for the “virtual second epoch.” Too short an interval results in minimal change between the measurements, making CPA calculation difficult. Too long an interval reduces the accuracy of the prediction. The article suggests that 6-12 minutes is generally optimal, depending on the distance between the ships.

In essence, this new method prioritizes speed over absolute accuracy, providing a timely warning of potential collisions, which is crucial in dynamic maritime situations.

Navigational Tools and Equipment Advances in Marine Navigation – Marine Navigation and Safety of Sea Transportation – Weintrit (ed.)