Hello deo!
Arpa interface:
ARPA: Contact 1 (selected contact to track)
CPA: (closest point of approach)
TCPA: (time of the closest point of approach)
Range: (distance to target/contact in Nm or "c", Cable lenght = 185.2 m)
Bearing: (bearing -azimuth- of contact relative to your vessel)
Relative co: (contact´s course relative to your ship/observed from your ship)
Relative sp: (contact´s speed relative to your ship)
True co: (true/exact course of contact)
true sp: (true/exact speed of contact)
ARPA Log: function of arpa log is intended for manual input of contact position so that radar system can calculate solution of target(contact) speed, course, CPA etc...
Let´s say that you see on your radar PPI (Plan Position Indicator/Display) a contact which is not automatically detected by ARPA and you want to determine his speed, course and other parameters.
Select on ARPA contact to have "M" designation, lets say M1 (M = manual )
You scroll radar cursor with arrow buttons over contact and press "log" button.
Contact current position,range and bearing is logged.
After few minutes you press "log" button again and now system have necessary data to calculate target speed,course and other information. It is basic calculation of two selected points in space to calculate object´s distance traveled and course in given time.
Now your radar system have enough information to calculate all necessary parameters to display contact CPA, TCPA, course, speed etc.
ARPA Trial: it simulates your ship alternative course and speed.
With trial maneuver on radar display you can test and decide what shall be your next move/maneuver in relation to other contact. ARPA Log (manual target tracking) is very well suited for that because automatic target tracking constantly resets contact on original position after few sweeps of radar antenna. While arpa Log (contact M1,M2,...so on) do not resets and require your manual input for next calculation.