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AIS vs radar: Do you really need both?

AIS and chartplotters have come a long way in the last decade, but are they enough to replace radar? Jonty Pearce examines the latest technology

A yacht in boisterous conditions
Radar is more expensive than AIS, but you may wish you’d taken the plunge when the visibility closes in amid busy waters

All of us are keen to avoid collisions and to be aware of what other vessels might be in the vicinity.

We all try to maintain a good watch, but looking out on a drizzly night can deter tired crew putting their ‘heads above the parapet’.

Then of course there is restricted visibility.

Anyone who has sailed in fog, particularly in busy waters, will know how nerve-wracking it can be.

AIS MARPA

You can track a target with MARPA to see if it presents a collision risk

We would all welcome any extra help in spotting other vessels and to make sure we are seen.

This is where electronic aids can be of use. AIS, radar target enhancers and radar each give an added layer of observation to the ‘Mk1 eyeball’.

While radar was once the only real option for blind navigation and collision avoidance, there is now a range of technology to help yachts wanting to see and be seen, but each of the options has its own strengths and limitations.

With decisions to make for fitting out my own boat, Aurial, a Southerly 105 ketch, I set about exploring my navigation options.

Radar

Radar (Radio Detection And Ranging), which really came of age in the Second World War, is perhaps the most established of the three.

Put simply, it sends out a signal and listens for an echo in order to plot the range and bearing of anything solid, like land or other vessels.

By tracking the changes of range and bearing relative to your vessel, and applying Rule 19 when navigating in fog, it is a hugely useful tool for collision avoidance, but historically it needed some skill to operate effectively.

It also worked well as a means of blind pilotage, allowing a course to be steered relative to land otherwise unseen.

A commercial ship's bridge

Commercial shipping relies heavily on radar and AIS for collision avoidance. Making yourself as visible as possible on both systems is common sense

While GPS chartplotters have largely replaced this function, data provided by radar is not subject to datum shifts and gives a highly accurate picture of where your vessel is in relation to land.

It also has the benefit of revealing objects that are not charted or actively transmitting, including unmarked vessels, weather, and squalls.

More recently, radar technology has moved away from analogue magnetron systems to digital radar, which operates at a much lower power.

Manufacturers are constantly working to make their radars have lower power consumption, better image resolution and greater range.

The software that goes with them is also developing quickly, adding new functions and making it easier to interpret the key information from the radar display.

Without getting mired in proprietary names, the following are some of the key innovations.

Chart overlays and dual screen radar

If your radar is connected to your NMEA network and chartplotter it will be able to overlay the radar picture onto the electronic chart, helping you decipher the picture, as well as your AIS screen, so you can match up radar targets with AIS contacts.

The latest radars will also let you view your radar plot at two different scales simultaneously.

MARPA

MARPA (Mini Automatic Radar Plotting Aid) allows you to manually select radar targets which are then automatically tracked, giving you information such as closest point of approach (CPA) and time to CPA (TCPA).

Doppler

Much like the changing noise of a siren as it passes by, Doppler processing quickly detects if a target is moving towards you or going away and can then mark the target accordingly to identify risk of collision.

It doesn’t give you numerical data though.

Radar isn’t perfect, however.

The picture depends on the set being tuned properly to be as sensitive as possible, while editing out wave clutter and rain.

In doing so, it’s possible to lose sight of vessels with small radar signatures, such as yachts made of fibreglass.

The image also requires interpretation, and ‘radar assisted collisions’ have long been an issue.

Target enhancers

Radar target enhancers are much more simple pieces of equipment.

A powered antenna picks up when it is swept by the radar signal of another vessel, and sounds an audible alarm.

At the same time, it amplifies the signal returned to the source vessel, making a larger, more visible target appear on its radar screen.

The range at which you can pick up a signal depends on the height of the installation, but mounted on the mast, it can be more than 20 miles.

It does not, however, tell you where the source of the radar signal came from, or how far away it is, so you are then on alert for traditional watch-keeping skills.

AIS

In contrast to both of these systems, the Automatic Identification System, or AIS, works by vessels actively broadcasting their GPS positions, as well as a huge range of information such as vessel name, course, speed, appearance, and details about their cargo and route, over VHF radio frequencies and appearing on receiving vessels’ screens within range.

As with VHF radio, this is line of sight, and so limited to 20 miles or so.

This is a huge aid to collision avoidance, but also helps VTS services track and control shipping movements.

The system was established internationally in the 1990s for commercial shipping (Class A), so it is still relatively new technology, but has been rapidly adopted by the yachting community.

Sailors initially had access to receive-only sets but transmit and receive sets – transceivers – are becoming commonplace in the leisure market (Class B).

SOTDMA and AIS SART

New AIS technology is about improving the way in which your data is transmitted, and the latest development for the leisure market is Self-Organised Time Division Multiple Access (SOTDMA).

Rather than waiting for a free moment to jump in and transmit your data, SOTDMA allows your AIS unit to pre-book a slot in which to can transmit, so you don’t get crowded out.

For vessels that are moving quickly, rapid transmissions are key to maintaining an accurate picture of traffic.

An additional benefit of AIS was the arrival of AIS SART (search and rescue transponder) beacons, which indicate the position of a man overboard to AIS-equipped vessels in range.

The limitations of AIS, as I discovered on my own boat, is that I remained invisible on other vessels’ AIS screens, as did any other vessels not transmitting on AIS.

Transmitting your own AIS position can warn receiving vessels of your existence and give them the option to avoid you, as well as facilitating them calling you using DSC radio.

AIS SART beacon

AIS SART beacons can pinpoint a man overboard to nearby vessels

This is all well and good for picking up commercial vessels that must carry AIS by law, but many smaller boats are either not equipped or prefer not to transmit their position.

The system is also reliant on the other vessels’ transmitted data being correct and up to date.

Nor can AIS show you the outline of land or navigation marks and is incapable of warning you of nearby squalls.

Many cruising yachts carry radar sets that are now ageing, and may speak a completely different language to modern instruments’ code of NMEA 0182 or NMEA 2000.

If you want to upgrade even a single part of your electronics, therefore, you risk making your radar set obsolete.

Faced with the choice, and the cost, do you do away with radar, keep your old one and add AIS, or update the lot?

I recently sailed across the Atlantic on a friend’s Beneteau 475 that had three alternative systems side by side: radar, AIS and, as a backup, an active radar target enhancer.

This proved an interesting test bed and made a good comparison to my own boat, Aurial, which is equipped with an AIS receiver only, to see precisely how much benefit radar adds.

What is clear is that a combination of AIS and radar gives you the best tools possible in every possible situation.

There are many who would argue that radar is still more useful for collision avoidance than AIS, and it certainly has a greater range of functions.

The final decision will depend primarily on your budget, but as with all things safety related, one close call in fog is often enough to persuade people to take the plunge for both AIS and radar.

Range

On the west-to-east Atlantic crossing from Antigua to the Azores we had the benefit of a Raymarine RD418 combined with an E120 MFD.

While scanning we could see targets well over the horizon at up to 24 miles and were able to track them accurately at a distance of 12 miles; we could also set guard zones to alert us to other vessels’ positions.

The radar was handy for spotting and tracking squalls; several times we used it to take avoiding action for blobs of nasty weather shown on the screen.

AIS and radar

AIS and radar can be just as useful for keeping watch crossing an ocean as in busy coastal waters

Our AIS should have been a boon, but a faulty aerial limited its range from what should have been 20 miles to less than two, but my AIS aboard Aurial allows me to identify and track craft well over the horizon when at sea as well as being useful to spot tanker and tug activity in the confines of Milford Haven.

We were also equipped with an Echomax Active-X radar target enhancer.

We found our unit had a range of at least 25 miles as it regularly picked up radar sweeps from large ships beyond our own radar range.

The option of activating AIS or radar tracking could then be reserved for occasions where more detail was warranted.

Mounting it lower down would, however, have given a more useful warning range when we would then be more able to assess the risk.

Power

Jonty Pearce

Jonty Pearce is a lifelong cruising yachtsman and retired GP. He keeps his Southerly 105 in Milford Haven

Few small cruising yachts enjoy the reserves of battery power required to support a continuously transmitting radar.

Our radar and MFD drew over 5.8A on transmit – 160Ah in 24 hours was just not practical given our battery bank and power generation capacity.

If you are going to use radar as a lookout, you need to be confident that its power draw, combined with the other instruments you are running, can be supported by your battery capacity, and that you have sufficient power generation available to recharge the batteries.

By comparison, AIS units sip power far more gently, though this will also depend on the interface with which you use them.

Frugal sailors could install a Radar Target Enhancer.

Their tiny current draw means that they can be left on day and night, though they are less useful for collision avoidance.

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