ACP (Azimuth Change Pulse) and ARP (Azimuth Reset Pulse) signals serve as a standardized method for indicating the direction a rotating radar is pointing at any given moment, primarily in the context of conventional radars that revolve around a central axis.
The ARP signal, manifesting as a pulse once per revolution, serves as a reference point. This reference point can correspond to North or, in the case of a ship-based radar, the bow of the ship, or any other fixed reference point. The moment the ARP pulse occurs, the radar's pointing direction is known.
On the other hand, the ACP signal, occurring at regular angular increments after the reference point, plays a crucial role. The fixed number of ACP pulses between consecutive ARP pulses is a key factor, often set at values like 360, 2048, or 4096. For example, if the number is 360, it implies an ACP pulse occurs every 1 degree. Consequently, determining the radar's angle in degrees involves counting the ACP pulses after the ARP pulse. In cases where there are 2048 ACP pulses per revolution, each pulse corresponds to 360/2048 degrees. Thus, multiplying the count of pulses by 360/2048 provides the radar angle in degrees. Understanding this mechanism is essential for accurate radar positioning.
The radar system tasked with receiving ACP and ARP signals plays a crucial role in interpreting these signals accurately. Its primary objective is to assign the correct azimuth to each radar return, derived from the signals received. To achieve this, the system must determine the number of ACP signals in one radar revolution, denoted as X, or the count of ACP pulses between consecutive ARP pulses. The azimuth calculation for the nth ACP pulse is then given by n/X * 360 degrees. Automatic calculation of X involves counting the ACP pulses between two successive ARP pulses.
However, several challenges can arise in the handling of ACP/ARP signals. Firstly, understanding the position represented by the time of the ARP pulse is crucial. It might denote the bow crossing of a ship, but this assumes correct radar alignment. Establishing a reference point is necessary, and adjustments may be needed to correct azimuth values. For instance, in land-based radars, setting azimuth 0 to represent North is useful but requires alignment with the ARP pulse.
Practical issues common to signal transmission also impact ACP/ARP signal handling. Ensuring impedance matching between transmitter and receiver is vital to prevent transmission line effects like signal overshoot and bounce, potentially causing incorrect or variable ACP pulse counts per scan. Different radars may present signals in various electrical forms, such as RS422 differential, open collector, or single-ended signals referenced to ground.
The absence or incorrect processing of the ARP pulse can lead to misleading radar data. Even if the received and displayed data seems accurate, the absence of the ARP pulse can result in incorrect radar picture angles or gradual drift over time.