CLOSEST POINT OF APPROACH

The spreadsheet cpa.xls computes the closest point of approach (CPA) of another vessel. This type of computation is useful for collision avoidance. All bearings and ranges (in nautical miles) are relative to your vessel’s heading. The calculation encoded into this spreadsheet works with a locally flat Earth’s surface (i.e. it is only valid for small distances) and assumes that both vessels in question move with constant speeds and tracks during the relevant time interval. The vessel is observed at two different ranges (cells A2 and C2) and relative bearings (cells B2 and D2) separated by time interval entered into cell E2 in the HH:MM:SS format. From this information the spreadsheet calculates the relative speed of the other vessel in knots (cell A5), range (cell B5) and relative bearing (cell C5) at the CPA, and the time interval between the second observation and the moment of the CPA (cell D5, in the HH:MM:SS format). If the range at CPA (cell A5) is close to zero, and if the “at” time (cell D5) is positive, the two vessels are headed for collision.