DIY Boating
Understanding radar
An old naval salt once told me that nothing beats a ‘Mark 1 eyeball’ for safely navigating your vessel. He was right, of course, and many accidents have been caused by people failing to simply keep a good lookout. However, there are times when visibility is restricted, and our eyes alone are not good enough.
At night, or when there is fog or heavy rain, technology can help to dramatically upgrade our vison and enable us to see things that would otherwise be hidden.
Radio Detection and Ranging, or radar, has been around since the Second World War and is often credited with helping to win the Battle of Britain. This technology, even in its primitive form, enabled British fighter planes to accurately intercept incoming German bombers at night.
Since those early days the technology has developed hugely, and compact digital radar systems are now readily available and priced to suit even moderate-sized trailer boats and yachts.
Large commercial radar units, and larger recreational ones, are characterised by a horizontal antenna (called an open array) which spins slowly around. This transmits a strong pulse of electromagnetic energy, which then bounces off objects in the distance. By knowing the exact angle of the rotating beam and measuring the length of time for the echo to come back, the software can build up a picture of the surrounding area. The strength of the returning signal gives an indication of the size of the object and builds up a fairly accurate picture of distant objects.
One of the issues with radar has always been that large rotating antenna; another the relatively high energy output of traditional systems. Since radar technology uses microwave radiation, just like microwave ovens, much has been said about the risks of exposure to that energy. However, while it is still not a good idea to stand in front of a large commercial radar unit, modern units only transmit incredibly short pulses of radiation and pose no threat to humans.
Those in the industry joke that you are more at risk of being hit in the head from a spinning open-array antenna than suffering ill effects from the energy coming from it. And most recreational units mitigate even that risk by using a smaller antenna inside a completely enclosed cylindrical housing called a radome.
So, given that you probably already have a compatible multifunction display (MFD) on your boat, as discussed last issue, what are the considerations when looking to add a radar? Prices start from just over $2000 (assuming you have an MFD), but there are a bewildering number of options.
Even if you stick with just one manufacturer you will notice numerous models, each with different acronyms, specifications and prices. At the time of writing we counted no less than 40 different models from the four major marine electronics suppliers, all aimed at the recreational section. To confuse matters further, different manufacturers often use their own proprietary name for the same feature. Let’s try and make some sense of this.
The first decision to make is around the base technology, and the difference largely depends on the range you require. Traditional ‘pulse’ radar uses a high-powered magnetron to generate a very short duration burst of energy. These units transmit at high power outputs, typically 4kw to 25kw in
One of the issues with radar has always been that large rotating antenna.
recreational models, and have a range close to 100 nautical miles. For maximum range, a pulsed unit is the best option.
New solid-state technology uses a different technique called FMCW (Frequency Modulated Continuous Wave). This ‘broadband’ radar sends a continuous signal wave (rather than a pulse) with an increasing frequency (hence called broadband) and is roughly similar to the CHIRP used in sonar. That wave travels out, retaining its frequency and bouncing back from any object.
At the same time the system continues to output a wave at an increasing frequency. Some clever electronics compares the difference between the returning frequency and that currently being transmitted and calculates the precise distance to the target. Since this happens continuously over different frequencies throughout its range, a broadband system provides far better target detection and differentiation than a pulse radar.
Broadband units also transmit at far lower energy levels, making them safer for smaller boats. Naturally they also draw considerably less power, making them suitable for smaller
runabouts or sailboats with limited power. The downside is they have limited range. Earlier models had very poor performance outside the three-mile range, but new models, notably the 3G and 4G models from Navico, now have an effective range of up to 36 nautical miles.
And then, just to confuse things further, there is another new technology called pulse compression radar, offered in different variants by Simrad, Raymarine, Furuno and Garmin. Basically a hybrid between traditional pulse and CHIRP technologies, this provides lower-emission signals than a standard pulse but which produces excellent range and target resolution. This technology is also superior at detecting small moving targets, like a flock of flying birds and fast-moving runabouts. As such, it is best suited to medium range, with options of 24 to 72 nautical miles maximum range.
So, having chosen the base technology you want, the next decision is whether to go radome or open array. The radome encloses all the moving parts inside a composite housing, leaving nothing to snag on rigging, fishing lines or sails.
Some manufacturers offer equivalent models in both radome and open array configurations, but generally open arrays are larger and therefore have
a higher output and greater range. Broadband units are generally radome, and pulse units may only be available as open arrays, but it is in that mid-range where the difficult decision comes. Note that the effective range is also affected by how high the antenna can be mounted above the water, with more height giving greater range.
Apart from the maximum range, the minimum distance is also an important aspect when deciding on a model, and units vary in this regard from as little as 10 metres to more than 100m. Boaties who have travelled out of Havelock in the Marlborough Sounds on a foggy morning will know how crucial this distance is, since the navigable channel is only 20 metres wide and it is a hairraising experience to hear another boat close by when you cannot see more than a few meters ahead of you. Virtually every boat operating out of that harbour has a radar installed.
The software supporting the radar signal is equally important, and of course this depends on what MFD or dedicated display you connect your antenna to. Most MFDS allow you to display the radar either in its own section of the screen or overlaid directly onto the marine chart, and some will integrate AIS signals to identify vessels which the radar detects. If your MFD does not support your desired radar unit, you can opt to add a dedicated display with specific features for your chosen model.
Another useful option is MARPA (Mini-automatic Radar Plotting Aid) Target Tracking, a collision-avoidance software feature supported by some models. This enables the display to identify and track other vessels, showing their speed and bearing, warning of closest point of approach and the time to the closest point of approach.
This can be combined with proximity alarms to sound a warning if a vessel is too close and heading for a collision. This feature would be important to vessels operating in busy shipping lanes, such as cruising boats and large commercial vessels.
A variation on this is a feature displaying an echo trail which shows where the radar target has been, so you can easily see the direction it is moving and work out what the risk is to yourself.
So, knowing all this, how do you decide what unit to buy? A few fundamental questions should reduce the options considerably, leaving the final decision down to personal preference and budget:
Large commercial radar units, and larger recreational ones, are characterised by a horizontal antenna (called an open array)
If you are integrating to an existing multi-function display, then naturally start with the same manufacturer and look at what models are compatible. However, avoid limiting yourself too early, as you may find that the feature you really want requires a new MFD, possibly even a different brand.
Next decide on the maximum range you require, based on your typical usage. For recreational fisherman, 24nm (so, one of the broadband models) is most likely adequate. For passage-making sailing boats a greater range may be vital. This is where the greatest cost trade-off comes – generally, the greater the range, the higher the cost.
Next, check how much power is available on your vessel. A high-power pulse radar will quickly suck the battery dry on a yacht that does not have an engine continuously charging it, while a broadband unit will draw a fraction of the amount of current.
Do you need close-range capability? If your boating region regularly experiences fog then this may be more important to you than the maximum range. Also, if you often go into busy anchorages at night, then the close-range target resolution capabilities may be the most important feature.
For game fisherman, the ability to identify a flock of birds flying far enough away to be out of sight would be an extremely valuable feature. In that case look for a unit offering ‘Bird mode’ or similar. BNZ