Understanding the picture

Anyone who has been to sea in poor visibility will appreciate the benefit of a system that can display where other vessels are in your vicinity. In the last decade the popularity of AIS (Automatic Identification System) in the leisure sector has grown considerably. AIS is cheaper than Radar and its installation on small craft maybe much easier, Radar also takes some skill to operate effectively. For these reasons, those who are not familiar will AIS and Radar often think it is an either/or option. Others don’t see the need for AIS when radar offers a truer picture of what is in front of the boat. The fact is, both are incredibly useful tools, so is one better than the other?

AIS, what are its benefits/disadvantages?

The advantage of an AIS transceiver is the amount of information that is broadcast to vessels that can receive AIS transmissions, including vessel name, its MMSI number, position, course, and speed. This can all be overlaid on a chart plotter to provide a picture of AIS targets around you. A benefit of sharing this level of vessel detail is that you can call specific vessels that are nearby and they are more likely to respond to your call.

However, a major downside is that not everyone has AIS and unless you are transmitting no one will “see” you. Not all ships are required to fit AIS and not everyone turns it on – maybe they forgot, the equipment is faulty or there could be security reasons why it’s not turned on. AIS uses VHF marine band frequencies and is basically line-of-sight VHF radio. Typically, transmission ranges are dictated by antenna height and maybe no more than 5-10 miles. AIS cannot “see” around solid objects such as harbour walls or headlands.

Radar, what are its benefits/disadvantages?

Radar stands for ‘Radio Detection And Ranging’. Radar sends out radio waves in pulses which reflects off targets and bounce back, whether or not it is light or dark or in poor visibility; the time taken for the signal to return allows its processor to calculate the angle, bearing, range and velocity of an object. Big targets reflect the pulse better than small wooden or GRP ones.

Unlike AIS, Radar does not rely on data input by an operator which may lead to incorrect information being displayed and is able to detect other objects such as windfarms, the coastline and rain squalls. The chances are if you see an object on Radar it will be a solid object, however, unwanted echoes may cause “clutter” and can cause confusion on the display screen and therefore an operator needs to be competent to use Radar effectively. Also, Radar will not give you the name of a ship or its MMSI number.

So, which is better? Well neither, both provide valuable information in different ways particularly when overlaid on a chart plotter. AIS information compliments Radar, which continues to be the primary aid for collision avoidance in the shipping sector and neither replace the need for keeping a proper look-out at all times

To find out more about how to keep yourself and everyone on-board safe whilst on the water, visit the RYA Safe Boating Hub.