# Why CPA, TCPA do not changes in STW or SOG?

CPA (Closest Point of Approach) and TCPA (Time to Closest Point of Approach) are nautical terms used in navigation and collision avoidance to assess the risk of vessels coming too close to each other. These parameters are typically used in conjunction with AIS (Automatic Identification System) and radar systems.

They are important for safety at sea, but they do not change with variations in STW (Speed Through Water) or SOG (Speed Over Ground) because CPA and TCPA are calculated based on the relative positions and courses of two vessels.

Here’s why they remain constant regardless of changes in speed:

**Relative Positions and Courses:**CPA and TCPA are determined by analyzing the current positions of two vessels in relation to each other and their respective courses (the direction in which they are heading). These calculations are based on vector algebra and take into account the relative motion of the vessels.**Speed Doesn’t Affect Geometry:**Changes in the speed of one or both vessels will alter their positions and courses over time. However, the geometric relationship between the vessels (i.e., how they are positioned relative to each other) remains the same as long as their courses remain unchanged. CPA and TCPA are calculated based on this geometric relationship and are independent of the vessels’ speeds.**Predicting Future Positions:**CPA and TCPA are used to predict the closest point at which two vessels will approach each other and the time it will take to reach that point. These predictions are made assuming that the vessels continue on their current courses and speeds. If the vessels change their courses or speeds, the CPA and TCPA values will need to be recalculated to account for these changes.

In summary, CPA and TCPA are valuable tools for assessing collision risk because they provide information about the closest point of approach and the time to that point based on the vessels’ current positions and courses.

These values remain constant until there are changes in the vessels’ courses or speeds, at which point they should be recalculated to ensure accurate collision avoidance measures are taken.