Collision avoidance technology has changed dramatically over the last 20 years. Radar has become more affordable, more intuitive to use and easier to install. We can now track approaching vessels at the touch of a button while the hand bearing compass gathers dust in a locker.

But are we using this new technology to its full potential? I suspect not. CatZero, a Challenge 72 I co-skipper, has an all-new suite of instruments including B&G’s Halo 24 radar. I used a cross-Channel delivery from Cherbourg to the Solent to familiarise myself with the technology and reassess standard collision avoidance strategies.

How radar works

With radar, more than any other equipment, it’s important to understand how it works, especially what it can do and what it can’t. Radar works by emitting a powerful radio wave and timing any echoes to calculate the distance to targets. As such, distances can be calculated precisely. Angles are harder to differentiate: the beam width of a traditional radar is 3-4°, which makes bearings less accurate. The beam width of modern radars is still only around 4°.

In addition to the issue of beam width, radars transmit some energy outside the main beam and the pulse splays out into ‘side lobes’. This can make targets seem wider than they really are, especially large ones which return a signal from the side lobes as well as the main beam.

A traditional radar relies on a magnetron to send out a short pulse of very high energy at a set frequency. Magnetrons are heavy, slow to warm up and power hungry. Modern radars don’t have magnetrons and work quite differently. Known as solid state or broadband radar, they transmit a range of frequencies called a chirp – like sliding up through the ranges on a trombone, instead of giving out a single note blast.

Rather than timing how long it takes for a signal to be returned, broadband radars compare the frequency of the echo to the frequency currently being transmitted. This gives better definition, allowing the radar to see the returning signal in more detail and reducing background noise.

The latest generation of radars such as the Halo 24, or Raymarine’s Quantum 2, take this a stage further: they transmit a cluster of chirps, covering an even greater frequency range. The result of this new technology is a much clearer picture and better target differentiation.

Height equals radar range

Due to the curvature of the earth, radar range is limited by height. The radar horizon is 2.2 x √height in metres. Although CatZero is a 72ft boat, the radar is mounted on a 4m-high arch giving a horizon of only 4 miles to low-lying objects: 2.2 x √4 = 4.4m.

The range of target detection is increased by the height of the target, which allows us to see over the horizon. A ship with 25m freeboard would be detectable much further away than a semi-submerged container. Just 25m of height adds 11 miles to the detection range.

Most of the energy from a radar beam is concentrated within 12.5° either side of horizontal. Sailing vessels might often be operating at these angles of heel, so much of the available energy is heading for the sea or the sky, reducing the range (unless the antenna is gimballed). Leisure radars normally have a maximum range of 24 miles.

I’ve found CatZero’s radar has an upper limit of around 12 miles for detecting large vessels and 6 miles for smaller ones. It’s more than enough for most purposes, but considerably less than AIS, which regularly picks up vessels 20 miles away.

Setting up radar on a yacht

One of the challenges of traditional radar was set-up. Brilliance, gain, range and tune controls all required adjustment. Each time you changed the range, you needed to adjust the tune and gain settings too. Modern radars have automated most of this.

Preset modes such as harbour, coastal and offshore optimise the settings for each environment. In harbour mode the antenna spins faster, giving a higher refresh rate to the picture, which is important in close quarters scenarios. Offshore mode will assume a moderate sea state and rotates slower, which gives the radar energy a better chance of reaching targets further away.

Reducing clutter

The Halo 24 has a dual range function, transmitting for two ranges at the same time and overlaying them on the same screen. This is an impressive development that means close range performance isn’t compromised when scanning for targets further out. The Halo 24 worked well and allowed for quick range adjustments without the need for manual inputs.

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Rain and sea clutter tools can be automatic or manual. I’ve always been wary of these as they can remove small targets too, and I would want to play with it in more challenging conditions before relying on it.
The radar page can be displayed as north-up, course-up or head-up. Both north-up and course-up are stabilised, and on CatZero, we selected north-up, offshore mode and a 6-mile range for crossing the shipping lanes in the Channel.

Relative motion

Radar sees things in relative motion: we’re stationary in the centre of the display and things move around us. This is the language of collision avoidance: a collision course is a steady bearing and range decreasing. To identify dangerous targets we don’t need to know another vessel’s speed or course, we just need to know whether their bearing is constant and range decreasing.

Radar is extremely good at presenting this information visually, without the need for any further interpretation.

Merchant ships use ARPA – automatic radar plotting aid – to track targets. ARPA locks onto an object and calculates the other vessel’s course, speed and CPA. Leisure radars often have a simplified version of ARPA called MARPA, where the M stands for mini.

The problem with using MARPA (and AIS for that matter) to determine whether a collision course exists, is that it relies on you sailing a constant course and speed. We’ve all seen the CPA jump from 1.5 miles to 0.5 miles as you surf down a wave. On older radar sets, target tracking can be done more simply using the VRM (variable range marker), and EBL (electronic bearing line). They’re the hand-bearing compass of the radar realm and track targets without the need for any computer or human processing. There’s very little interpretation required: a target sliding down the EBL towards you is a problem; one moving well off it isn’t.

There are now more target tracking functions to add to the toolbox which don’t rely on speed and heading inputs: relative trails and Doppler target identification. Doppler technology identifies which targets are moving towards or away from you by colour highlighting them, helping you to focus on the hazards.

Tracking and trails

This is particularly useful when there are a lot of targets on screen, such as busy inshore waterways or the shipping lanes.

Another function, relative trails, allows you to monitor the relative track of a target, which gives a good indication of the CPA. I was keen to see how these would work out in practice.

Back on CatZero, the helmsman was becoming agitated. “Rachael, are you actually gonna use that thing or just play with the settings?” asked our Irish crew, Kenny. “Because I can’t see the horizon any more.”

Right on cue fog had rolled in. Fog has an intimidating effect and the mood on board changed instantly as we started tracking targets in earnest.

A small target appeared on the beam at just under 2 miles range. It wasn’t on AIS but we had been aware of a yacht heading in the opposite direction to us on the starboard side. It hadn’t posed a threat, and sure enough on the VRM and EBL it tracked astern of us, slipping behind the blue line.

On the quarter, another target appeared, this time highlighted yellow by Halo’s Doppler velocity tracking system.

We set up a second VRM and EBL on the approaching target. It appeared to be overtaking, slipping inside the range marker but moving forwards off the bearing line and passing well clear, but it was helpful to have it identified as an approaching target by Doppler.

Next, we turned on the trails function in relative motion. This leaves a slug-like path behind the vessel which, when projected forwards, indicates an approximate CPA. With trails turned on, the screen can become quite cluttered.

You can choose the length of the trail. The shorter the trail the less clutter, but the less history you have on an approaching vessel. In congested waterways they might not work so well, but the passive approach is useful where the density of traffic isn’t too demanding.

A vessel ahead of us presented with a very short trail; it was slowly converging but travelling at a similar speed to us, so would cross our path at a safe distance ahead. A vessel going in the opposite direction which was diverging more rapidly had a longer trail. When combined with an AIS overlay indicating each vessel’s true course, we had a really comprehensive picture of what was going on.

Guard zones & blind spots

As we approached the west-bound shipping lane, things became busier. On the port quarter, Lilbo, an 83m cargo vessel, was passing astern of us. This gave a relatively small target compared to the Star Athena, a 200m bulk carrier heading in the opposite direction and colour-coded grey. However, DS Ocean, a 140m tanker approaching 20° off the starboard side, was invisible on radar. AIS gave a CPA of less than two cables so this was a major omission.

We reduced the sea or rain controls which might be masking a target, and increased the gain but there was still nothing.

“Could it be the mast or the rigging?” asked watch leader Steve. Altering course 10° towards the vessel was enough to bring it into view on screen and prove that something on board was creating a significant blind spot.

With the radar mounted at the stern of the boat, and the mainsheet eased it was possible that the boom was getting in the way.

It wasn’t long before another anomaly occurred. A large container ship, Atlantic Star, was tracking down the port side four miles off. It was showing on AIS and posed no threat, but another substantial target appeared in what should have been the radar blind spot. This one wasn’t on AIS, but from the size of the target on the display it should have been.

Occasionally some of a returning radar signal will bounce off part of your own vessel before being received by the antenna. This creates an indirect echo from a different angle, as well as the direct echo which has come straight from the target.

The telltale sign is that it occurs at the exact same range as the original target. We set up a VRM (bright blue) at 4.8 miles to see whether the two target ranges matched, and they did.

It seemed likely that the target 15° on the starboard bow was an indirect echo which had bounced off the boom, the very thing causing our blind spot in the first place.

It was a timely reminder that, despite the sophisticated technology, we should never believe everything we see on screen, at least not unless it is confirmed by an independent source.

AIS and true motion

AIS works in true motion, which is not the natural language of collision avoidance.

It receives the actual course and speed of another vessel and then calculates the CPA, but on sailing boats where speed and heading can fluctuate, the CPA can vary significantly, making it a crude tool for assessing risk of collision.

New software is addressing this. Most chart plotters now offer the ability to set up AIS alarms for targets with a CPA under a certain limit.

More recent editions of Raymarine Lighthouse show ‘Interception Zones’, shaded areas around another vessel which must be avoided in order to avert the risk of collision. This can make for a very cluttered screen, however. B&G recommend setting the chartplotter to display the true vectors of your own vessel and those of AIS targets. If they’re both set to the same length, such as 12 minutes, it’s possible to identify a collision course by how close the vectors are.

AIS is also useful for monitoring the course and speed of another vessel. A speed reduction in a vessel approaching could be easier to identify on AIS than it is visually, and assists understanding their intentions.
AIS doesn’t suffer from blind spots and provides more information, but not every vessel or floating object transmits on AIS and any calculation of a collision course is, in my mind, to be taken with a pinch of salt.

Also, AIS is not yet part of COLREGs and the UK Maritime and Coastguard Agency guidance says it should be used to enhance situational awareness rather than be relied upon as the primary tool for collision avoidance.

An essential everyday tool

The latest radar technology is impressive, and it’s quicker and easier to get useful information from it. Preset modes and automatic gain and tune controls save time and reduce the chance of user error. The picture on the Halo 24 was much clearer than that of a traditional radar and the Doppler target identification of approaching vessels feels like a real gamechanger.

Since they draw less power, they can be left on in the background, transforming them from an occasional tool to an everyday essential, and improving our familiarity with the equipment for when we really need it.
Despite the advances in target tracking, I still found the VRM and EBL one of the clearest, simplest ways to monitor dangerous targets without cluttering up the screen.

Advances in technology haven’t eliminated all the pitfalls of radar use though, and there’s a risk that the simplicity of using it masks the underlying complexities of radar, which still exist, so we need to understand how radar works in order to appreciate its limitations.

We also need to understand the limitations of each individual radar installation, such as CatZero’s boom height being similar to the height of the radar antenna. This is particularly true on large yachts where biminis, davits and spars may be significant obstructions. As ever, training and practice are important investments, alongside the equipment.

In conclusion

As the tools for collision avoidance become more sophisticated, it takes longer to master them. I was still discovering more functions of the Halo 24 as I researched this article.

Too much information can be just as problematic as too little, and to get the most from your AIS or radar you need to spend time setting them up properly. Being able to adjust settings on the fly is important too so that you can respond to changes in traffic density or conditions.

Whether you’re using AIS or radar, no system is entirely comprehensive. During the Channel crossing the radar had failed to show one target, and given us an extra target that wasn’t there, while a passing yacht didn’t show up on AIS but did appear on radar.

In good visibility you’ll often identify a dangerous target by eye and instinct before the electronics, but when you’re tired or visibility is reduced, AIS and radar offer valuable assistance. The crossing reminded me how a relaxed trip can quickly become fraught when the visibility reduces, and how unnerving fog can be.

For vessels making more ambitious voyages and in more challenging areas, radar is an indispensable tool.

Thanks to Richard Jennings of B&G and Richard Marsden of Raymarine for their technical input.

CatZero is a sail training charity based in Hull which works with disadvantaged people to create positive change in their lives. They also have a programme of adventure sailing and racing which anyone can join. See catzero.org

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