Understanding ACP and ARP Radar Signals

Our support team are frequently asked to explain ACP and ARP signals and how they are used, and in many cases why they do not appear to work correctly. The ACP and ARP signals are a standard way of representing the direction a rotating radar is pointing at any given time. They are used for conventional radars, which rotate around a central axis. The ARP signal (Azimuth Reset Pulse) is a pulse that appears once per revolution at a reference point. This reference point may correspond to North or, for a ship-based radar, the bow of the ship, or it could be any other fixed reference. At the time this pulse occurs the pointing direction of the radar is known. The ACP signal (Azimuth Change Pulse) is a pulse that occurs at each regular increment of angle after the reference point. There are a fixed number of ACP pulses between each ARP pulse. Commonly, this could be 360, 2048 or 4096.  If the number is 360 then it means there is an ACP pulse every 1 degree. So the angle of the radar in degrees is obtained by simply counting the number of ACP pulses after the ARP pulse. If there are 2048 ACP pulses per revolution, each pulse corresponds to 360/2048 degrees, so counting the number of pulses and multiplying by 360/2048 gives the angle in degrees. 

The radar equipment receiving the ACP and ARP signals has the task of interpreting the signals to correctly label each radar return with the azimuth that it was derived from.  To do this it needs to know the number of ACP signals in one revolution of the radar, ie the number of ACP pulses between consecutive ARP pulses – call this X.  It can then count incoming ACP pulses and calculate the azimuth in degrees, so that for the nth ACP pulse, the azimuth for the radar is n/X * 360 degrees. The value of X can be calculated automatically by the incoming processor by simply counting the number of ACP pulses observed between two consecutive ARP pulses.

So what can go wrong with ACP/ARP signal handling?

To begin with, what does the position represented by time of the ARP pulse represent? Is it the bow crossing of the ship, for example, or some other angle on the ship, which can happen if the radar has not been correctly aligned with the bow. It is necessary to have a reference point, but it is also necessary to know what that reference point is and, if necessary, apply a correction to adjust the azimuths to a more useful value. In a land-based radar, it is useful to have azimuth 0 representing North, but this is only true if the ARP pulse is aligned for that value. In general, the radar may not have been aligned like that so an offset is needed to adjust the reference value. This is simply an addition to the calculated ACP value...