What is a CPA Alarm?
CPA means
Closest Point of Approach, you also have TCPA which is
Time to Closest Point of Approach.
You can set these 2 values in the radar. After plotting them on an ARPA (Automatic Range and Plotting Aid) radar, the radar keeps track of the vessel. If the vessel reaches the values of CPA and TCPA, e.g. the vessel will be closer than 1 nautical mile within 15 minutes, the radar will give an alarm to notify the officer on duty. Ofcourse the values of CPA and TCPA can be adjusted to the area that the vessel is sailing.
Back in the days when ARPA didn't excist, you had to plot the vessels manualy on a Plotting Sheet (http://gcaptain.com/maritime/tools/files/moboard-5090.jpg). You have to take the bearing and distance of another vessel on an interval (3 minutes, 6 minutes) and draw them in the plotting sheet. With a little bit of math and rocket science you can calculate the CPA and TCPA. You can also calculate how much degrees of course alteration you have to perform to avoid a collision