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The Importance of Heading

 

“To get the most out of any radar you need a good heading sensor, especially if you are using the more sophisticated functions such as chart overlays and VelocityTrack™,” says Navico® product expert Craig McMillan. “Garbage in/garbage out, as the saying goes.

 

“A really good compass on your NMEA 2000® is essential to get accurate bearings on everything that’s important from land features to underwater hazards, buoys and potential traffic conflicts.”

 

And as Simrad® Product Director Laurie Bates explains, it’s not just the radar that will benefit.

 

“From the autopilot to true wind speed and direction read outs and many other functions, all will operate far better with an accurate heading.”

 

Why is the compass so important?

 

“When we are proposing a system for a vessel one of the absolute keys is to make sure it has a good heading source, a good compass,” says Laurie. “Yet when people consider a radar and/or system upgrade, the compass is unlikely to be at the top if their list. They will often be looking at upgrading in stages rather than as a complete refit, perhaps spanning over a few seasons with the focus on say a new display, a new radar or a head unit.

 

“This is completely understandable, not everyone can or wants to do everything all at once. But the chances are that the compass, which is usually located out of sight under a bunk at the back of the boat, will be overlooked. Yet this one unit plays a huge part in the accuracy and general performance of many other functions. The compass might be out of sight and out of mind, but it’s playing a big part in your instrumentation system. Even so, it is surprisingly common to find that the compass in an otherwise new system aboard a customer’s boat is actually fifteen years old and will not be up to scratch for the job.

 

“So, unless you can be certain that bearings are accurate it will be hard to be 100% confident in identifying a specific target on your radar which clearly negates the main reason for having one. And while your radar is still going to be able to paint, without a decent heading sensor that is both accurate and can understand the motion onboard your boat and make allowances for it in real time, you are not going to be able to use your Velocity Tracking feature or rely on MARPA tracking. Again, this takes away another of the key reasons for having a radar.”

 

Like so many areas of modern electronics, in recent years marine instrumentation technology has advanced significantly and has led to the development of highly accurate compasses that can assess and compensate for a boat’s motion to a degree of accuracy that simply wasn’t possible, or affordable for most leisure craft before. So what are the broad options and key considerations? Craig explains.

FLUXGATE v Precision™ -9

 

“The development of modern heading sensors started with fluxgate compasses which have been around for a while,” he says. “Then, rate sensors were added which are very good at monitoring the boat’s motion. 

 

“Today our Precision-9 is a nine axis rate sensor that detects pitch, roll and yaw each in three different axes so it can map motion and provide rate of turn information extremely accurately”

“It’s a solid state compass that has proved to be very successful and dependable over the years and has benefitted from some big technical advances with faster updates and better processing.

 

“Fluxgate compasses need to be mounted somewhere along the centre line, low down in the hull near the waterline in the boat. They are very accurate, but it is very important that that they are installed clear of any other electrical or magnetic items such as big speakers or other sources of electrical or magnetic interference.

 

“One of the big advantages of the Precision-9 sensor is that is has an auto calibration routine. So whereas we used to have to do circles to calibrate, the Precision-9 will calibrate automatically as you move your boat around the harbour where it will also learn the anomalies of your boat.”

GPS COMPASSES

The other key group of heading sensors are satellite compasses that also have pitch and roll sensors in them which makes them good on moving platforms as well. Unlike fluxgate compasses, these are mounted on top of the superstructure where they have a good view of the satellites.

“GPS compasses are very, very accurate and work by measuring the phase difference on the signals from the GPS constellation,” continues Craig. “We have three in the Simrad® range, the HS™ 60, HS75 and the HS80.”

 

HS60

Accuracy of 2 degrees with a 10Hz output.

 

HS75

Accuracy of 0.75 degrees with a 10Hz output.

 

HS80

Top-of-the-range with an accuracy of 0.5 degree and a 20Hz output.

“With this kind of performance it is clear that GPS compasses are very advanced technical heading sensors and extremely accurate with it.

 

“In addition, GPS compasses don’t need to be calibrated because they use the GPS constellation to provide a heading so they’re not using magnetic flux to calculate it. This means that you don’t need to do a compass swing which makes them really easy to use and ensures that they are always accurate.”

 

So, while your compass may not be the first item you think of when you are considering upgrading your radar system, ensuring that the core heading data is as accurate as possible is one of the key cornerstones to ensuring an accurate, reliable and hence dependable system.

 

“Without an accurate and reliable heading you’re not going to be able to use your Doppler tracking feature or rely on MARPA tracking,” continues Laurie. “If your bearing is out by say 7-8 degrees you’re also not going to be 100% confident that a piece of paint on your radar at say, a bearing 045 is actually the channel marker that you’re looking for. It’s difficult then to maintain confidence in the correlation between what the radar paints and real world objects.

 

“The bottom line is that a poor compass will hamstring your system.”

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Laurie Bates

BE MSc CENG MIET

Laurie’s involvement in electronics has spanned his entire career. After studying electronic and electrical engineering at Auckland University he spent 20 years in the NZ Navy and gained an MSc in Explosive and Ordnance Engineering at Cranfield University (UK). With a wealth of specialist radar knowledge and having worked in the defence industry he joined Navico where he is now Product Director at Simrad Yachting.

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Craig McMillan

PRODUCT EXPERT

After a career in avionics in the New Zealand Airforce Craig was a distributor for Simrad® and B&G® products for 20 years before joining Navico as Product Expert, a role that he has held for the last six years.