Radar and AIS differences, and how they complement each other.

When I teach basic radar and AIS at Furuno Maritime Training, I often get the same question from my students: “why should I buy a radar? I already have my AIS where I can see all the other ships around me”.

My response to them will be “why don’t you use a radar as well”. Radar and AIS are the perfect couple, and they complement each other, like a married couple, which have been together for many years. 

In the following post, I will explain why, but first, I will give you some background knowledge about the AIS and Radar.

What is AIS?

What does AIS stand for:

AIS stands for Automatic Identification System.

How does AIS work?:

It's a bridge to bridge, bridge to the shore communication system. It uses two designated VHF channels to communicate. One might ask - “what does it communicate”?

It communicates four different types of information to other AIS receivers which are listed below.

• MMSI number

• Static data such as IMO number, call sign, length, beam, and type of ship.

• Dynamic data such as GPS position, speed, course over ground, heading and navigational status (underway using the engine or underway sailing).

• Voyage related such as, how many people are on board, where are you going and when are you going to be there.

Listed above, is some of the useful data you will receive about ships around you, if you have an AIS onboard. This means, that with the AIS onboard, you will no longer need to be calling out on the VHF, to know which ship is which.

What is Radar?

What does radar stand for:

Radar is an acronym derived from the words Radio Detection and Ranging. The questions is - how does it work?

How does radar work?

To make the understanding more figuratively, try to imagine somebody giving a short sharp shout through cupped hands, to focus the sound energy. The soundwave travels outwards, and some of it may strike, for example, on a cliff. Some of the energy which is intercepted will be reflected by the cliff. If the reflected energy returns in the direction of the caller and if the energy is of sufficient strength, then it will be heard as an audible echo.

If we transfer this example to the radar, the radar works the same way, but with radio waves instead of sound waves. The drawing below represents a simple radar installation.

So to transfer the analogy again, the cupped hands is the antenna, your voice is the transmitter, the signal amplification (receiver) is your ears, and the processor, display and interface is your brain. Then on the radar display, it shows what it has received back of reflected energy.

How do AIS and Radar work together?

This was a simple explanation of the AIS and Radar, and their functionality. But why are they great when working together? In the next section, I will discuss this, and hopefully, this will give you a newfound understanding on why Radar and AIS are the perfect couple. To begin with I will explain about Target Tracking.  

Radar and AIS - Target Tracking

Radar:

On a radar, you can have either manual or automatic tracking of targets. Manual tracking is where you have to plot the target manually again and again within a certain timeframe. Whereas automatic tracking like the ARPA (Automatic Radar Plotting Aid) does the plotting automatic continuously. 

AIS:

The AIS tracks targets in a different way than the Radar. The received data from the target is sent to you by the target, in the form of its position, speed over ground, course over ground and heading. Since there is no filtering done on the data received, it is immediately presented on your screen.

What is target tracking used for? 

The target tracking gives you a situational awareness about the traffic around you. With the target tracking, you can avoid collision with another ship. The data from the Radar and the AIS can only be useful if you understand the use of it.

True and Relative Vectors

Unfortunately, accidents caused by incorrect interpretation of vector information still occur. 

On the AIS and the Radar, you can use both relative and true vectors. 

A target that is travelling, in the same direction as your ship and with the same speed, has a relative speed on 0 knots and a constant range and bearing. but if a relative vector is pointing in the direction of own ship, then there is a risk of collision.

The picture shows how relative vectors look like. Here you can see two ships with a risk of collision. The one where there is a dot is the ship that travels with the same speed and direction as you.

True vectors can not be used for anti-collision matters, only to have a better situational awareness about the overall traffic situation as shown crudely in the picture below.

Understanding CPA and TCPA

When we are talking about anti-collision and relative vectors, then we are also talking about CPA and TCPA. Maybe you are thinking - what is that? 

CPA stands for Closest Point of Approach.

TCPA stands for Time to Closest Point of Approach.

To give an example, take a look at the above picture that shows that we have plotted a target in P1. Then let’s say that we 6 minutes later plot the same target again in P2. If the two relative positions, are connected with a line, then we will get the relative course of the target. you can also see where the CPA and TCPA is measured from and to. This calculation is what the ARPA radar and the AIS receiver does automatically.

So why is the Radar and AIS the perfect couple, and how do they complement each other? Listed below, I will give you some examples of such. 

Radar and AIS - the perfect couple

• They provide two independent ways of detecting targets.

• They give two independent estimates of the target range, bearing, course, and speed.

• Radar can detect targets that do not carry AIS.

• AIS transmissions are almost unaffected by sea clutter or heavy rain, allowing the detection of targets, in conditions where they are likely to be invisible to the Radar.

• The VHF frequencies used by the AIS are better at detecting around headlands and islands than radar is, allowing detection of targets that are in radar shadow.

• Radar is based on relative motion, aligned to ships heading and is, therefore, better suited for collision avoidance. It can continue to operate with no reliance on other internal or external navigational aids.

• AIS is more accurate than radar since it is continuously fed with data from reliable sensors.

• Radar can have very poor accuracy when tracking a target. AIS maintains its accuracy and gives a prompt indication of changes in heading, and will give the rate of turn if it is available from the target.

• The user normally has very good knowledge of his/her ship’s radar capabilities. They have no control over the effectiveness of the data supplied by other Ships AIS equipment, sometimes it can be of very poor quality.

•  In poor visibility, you can see which ship there is and if there is a need for special manoeuvres around it - maybe because it is restricted in some way, for example like a fishing vessel.

Conclusion - how to choose between Radar and AIS?

As a conclusion to this blog post, it should be clear to you, that I think that having both the Radar and AIS onboard, is the best solution as they complement each other very well.  Where one has a weakness than the other can supplement. So, get them both and use them to their fullest capacity, still with their weaknesses in mind.