Doppler Radar—Furuno NXT

[Sharphooks]

Not sure if any of you guys have tried out the new Doppler technology in recreational marine radar. I thought it would be a good addition to the boat for a safety factor —-boat positional awareness in pea-soup fog and for running at night—-and figured I’d post about some of its attributes

I just installed the Furuno DRS4D-NXT on my boat and had a chance to fire it up and put it through its paces yesterday. Pretty amazing target return when you switch it to “Target Analyzer”—-this setting is essentially the Doppler setting. The radar has built-in ARPA so if you’re near a shipping lane and get an ARPA target, you get a screen icon that shows the vessel, the direction it’s heading, the speed its running at and what the “CPA” will be (closest point of approach = potential collision)

I unintentionally disabled the AIS on my VHF radio when I switched chartplotters on my boat (the new chart plotter is strictly NMEA2K and the radio outputs in NMEA0183 so I lost my AIS returns on the chartplotter screen

But having ARPA and Doppler capabilities more then makes up for losing AIS

Here’s the first screen-shot I saw when I fired up the radar in the lagoon where I moor my boat: the red target just to starboard of my heading sensor line is a mud flat just starting to show in my lagoon as the tide recedes...






The second shot shows a duck (the blue target) that was swimming 10 feet off my port bow: I was impressed how the beam could pick up something so close to the boat




This next shot is in crappy focus—-there was some wave action and lots of boats around so I just grabbed a quick pic—-I was in a fleet of sailboats—-but they all had their spinnakers up and fully inflated which meant the wind was directly behind them and they were coming towards me——thus, the red color of all those targets which is how the Doppler marks approaching targets. The lone green target was a salmon troller going away from me—-thus, the color green

You can also see what the ARPA lollipops look like, showing course and speed



So the back-story of what made me bust this Doppler move was a little adventure I had crossing QCS last August—-impenetrable pea-soup fog....what else is new? I had my eyes glued to the radar screen (conventional radar with a magnetron) . I was going maybe 10 knots, minding my own business when a large cruiser showed up out of nowhere and steams right next to me on a similar course and similar speed—-a big red target showing on the screen off my port beam.

I have a full chart on my other chartplotter screen— I don’t like overlay—-too much information going on. I’m threading through a fairly narrow spot and I see it clearly on the chart—-rock on one side, reef on the other. It’s an “obvious” route if you’re crossing from Caution to Hardy and the cruiser next to me had the same idea I did. Then suddenly the two red targets ahead of me (rocks? Boats?) had become three red targets. I knew one, maaaybe two, of the targets were rocks (depending on the tide and how much they were exposed...) The third target had come out of nowhere—-but was it coming or going? Had it been ahead of me all this time but my gain hadn’t been cranked high enough to see it??

So all this sounds like small beer sitting in a brightly lit room at a table but when you’re solo in the middle of white-out conditions in QFC and you’ve lost all spatial and positional awarenes, it pretty quickly becomes sweaty palm stuff.

But you’ve made your move, you stick to the route because now there are known rocks straight ahead and you can’t afford to recalibrate at this stage....

So my first thought was after I’d threaded the rock/boat target needle was....wow, wouldn’t it be nice to have Doppler technology right about now?

So now I’m loaded for bear for my next trip to North Coast...but funny thing....while fiddling with the buttons of my new toy, I almost ran into a 300 foot long ferry...

So I’d better shut up about all this fancy new technology I just installed on my boat and going forward, promised myself I’ll focus on paying attention with my eyes and ears and use the Doppler as Plan B....

 

[Mike_R]

What do you mean by an ARPA target?

Thanks for sharing

 

[Sir Reel]

You can get an Interface Converter NMEA200-NMEA0183 for your AIS

https://www.internationalmarineservice.com/Furuno-IF-NMEA2K2-Converter-p/fur-if-nmea2k2.htm

Part number is FUR-IF-NMEA2K2

 

[Sharphooks]

Hey Sir Reel—thanks for that—-I tried the NM2K2 converter with my NMEA0183 heading sensor and I finally gave up due to inconsistent conversions...Furuno agreed it’s not an exact science. I think I’ll step up for a NM2K2 output VHF radio—several manufacturers just came out with upgraded models recently

 

[Sharphooks]

What do you mean by an ARPA target?

Thanks for sharing

ARPA is an acronym for automatic radar plotting aid. I recall reading that one of the EU radar manufacturers came up with the technology following the collision between the Andrea Doria and the Stockholm. The radar automatically will track a target ( an approaching vessel ) and using the change in distance between your vessel’s signal emitting magnetron and the approaching vessel, will calculate speed and heading and most importantly, both the CPA....(closest point of contact ) and the TCPA....(time of closest point of contact)——in radar-speak, ARPA creates a “predictive vector” and places it on your radar screen in real time

I clipped the following narrative off the Cruiser Forum and give a guy who’s screen name is Tanglewood full credit for a way more elegant and concise description of both ARPA and MARPA then I could even begin to give:

QUOTE


A radar has either ARPA or MARPA, but not both. It stands for "Automatic Radar Plotting Aid", and "Mini Automated Radar Plotting Aid". The main difference is that ARPA has the option to automatically acquire targets, where MARPA requires that you manually select any target that you want to acquire.

When you "acquire" a target with either ARPA or MARPA, the radar keeps track of the vessels movement relative to you, and from that it is able to calculate the vessels true course and speed, relative course and speed, CPA, and TCPA. You can do this manually by plotting the target's position over time, but it's a great job for a computer and is why the feature is called "automatic radar plotting aid"

Guard zones, or Target Alarms (different companies use different terminology) are adjustable ring segments around your boat, and an alarm is generated when a target enters the ring. On at least some Furuno radars, you can also alarm if a boat leaves the ring. Anything entering or leaving will alarm, be it a boat, channel marker, or land mass. And if you aren't tuned properly, waves and sea chop. Note that none of the targets that trip this alarm get tracked by ARPA, so you know the target is there and you can watch it on the screen, but you have no calculated course, speed, CPA, or TCPA.

Automatic target acquisition with ARPA uses an adjustable ring or a segment, and anything entering the zone gets acquired as an ARPA target. Once acquired, the targets will have speed/direction vectors just like an AIS target, and will have calculated CPA and TCPA, also like an AIS target. The difference between AIS and ARPA is that with AIS, the target has told you over the VHF what its Position, COG, SOG are. With ARPA, the radar observes and records the targets movement, and from that calculates its position, COG, SOG. In both cases, your chart plotter (or whatever) then calculates CPA and TCPA.

With ARPA and with AIS, alarms are based on CPA and TCPA, not just on the existence of the target. These alarms are independent of guard zone alarms, so for any given radar return, you could get alarms both when it enters your guard zone, and again if it has a CPA that is too close.

END QUOTE

 

[Mike_R]

Thanks I should have been more clear. Originally you stated something about an ARPA target near shipping lanes and I was wondering what you meant by that. Did you mean a target that was acquired by the ARPA on your radar?

I assume that your Radar gets input from your GPS for own ship heading and speed or do you need a fluxgate compass?

Personally I prefer to use chart overlay, if the position of land or other fixed objects doesn't line up between the chart overlay and the Radar image then you know something is wrong (likely the GPS).


I will have to read up on doppler RADAR. Thanks again for sharing.

 

[fishbadger]

I have been using Garmin's GMR Fantom 18 radar with an 7612xsv MFD for a couple years now, which has the doppler radar function and MARPA. I've enjoyed using the doppler radar while navigating in tight spots and in low visibility, it's really nice to know if something is coming at you or running away in an instant. It's been very reliable. I haven't used the MARPA function yet. While running in tight or low viz conditions I usually feel like I have my hands full and need to focus, so I haven't devoted bandwidth to any other new functionality (yet). Maybe next time I get a chance I'll do it,

fb

 

[Sharphooks]

Wow, impressive technology for a rec unit. This explains the guys blasting out of Sooke at 30kts in the pea soup fog. They must have these onboard!

Similar to mike R, I’m also wondering what this system needs for input data.When it produces a target vector, is it giving you relative course and speed, or true course and speed?

I agree that a chart/radar overlay is a great way to confirm you are where you think you should be. For strictly collision avoidance with moving targets I prefer a less cluttered radar. I think the art is finding a balance between the two or being able to switch the overlay on/off efficiently depending on your needs at the time.

Hi Graham—according to the quote I cited above, he mentions ARPA tracks both relative and true course and speed. I’m presuming if your boat and transmitting/receiving radar are tied to a dock, the ARPA target on your radar screen and the accompanying course and speed read outs on your radar screen would show true course and bearing, but if you’re out on the water, underway, then that same ARPA target would show relative course and speed.

I agree about having a less-cluttered screen when using radar overlay—-for me, there’s just too much information when I overlay —- radar targets get lost in buoy icons, tide and depth read outs, etc etc. That’s why I stepped up for two 14” MFD’s on my boat so I could give radar its dedicated piece of real estate....

Overlay is definitely cool for navigating but you have to be careful—-if you look at the top picture, the radar returns in the overlay are not tuned to the chart—-I had just installed the radar and plugged in the Ethernet cord and fired it up—first picture on the screen——-I had to finally use a 7% negative offset to match the radar return with the actual land configuration

Hey Mike R: yes, the radar automatically “acquires” the ARPA target and tracks its course and speed relative to your boat. Yes, a GPS signal is required to calculate own ship heading and speed so the radar micro-processor can in turn calculate the ARPA target’s course and speed relative to your boat. No, a fluxgate compass is not required for the radar to track the ARPA target—A heading sensor is only required to have the radar/chart overlay

Regarding the reason for land and other fixed objects not lining up properly in the radar image when using radar/chart overlay, as mentioned above, you address that problem by properly aligning (compensating) the radar via internal settings—-



Hey FB...that Garmin looks like a good unit but it appears not to have the ARPA function—-based on that guy’s description quoted above and what he says about the two functions, I’m curious why any given radar using Doppler technology cannot have both MARPA and ARPA. Any idea why?