Sunday, July 22, 2012

Radar overlay heading gyrations

This all started with hearing a rumor about a boat accident, that led to a lawsuit. This caused Raymarine to turn off their system's long existing capability of using GPS generated course over ground data as the radar overlay heading input. This was reinforced by Compass Marine's rant on Panbo that was similar to what I heard.

So I explored the issue, and was initially confused by what I was finding. There is the recently settled patent lawsuit that involved almost all of the big MFD manufacturers. It appeared that since there were aspects of the case that could have involved radar overlay, this could be the culprit. In the end this was not the case, and it was settled by all parties in American GNC's favor.

As for the rumored boat accident, I just couldn't find it. The one that was closest to the described accident was wrapped in somewhat of a mystery. Despite the fact that three souls were airlifted off the vessel, there is zero reference to it in the USCG records. There were lawsuits, but I could find no information regarding them. In the end I had spent way too many hours looking, and I picked up the phone, and called Raymarine.


Here is the crux of the issue. COG (course over ground) is calculated by the GPS. The last fix, you were there, on this fix you are here. From this your direction of travel is calculated. The problem occurs when your bow is not pointing in the direction you are actually traveling in.

Huh, how can that be? Wind and waves have an impact on all boats, to some degree. In the picture above, the boat could could have strong winds pushing at it on the port side. In order to stay on the course line, the boat has to turn up a bit into the wind to maintain the course line. It's sort of like a long term cross wind landing scenario for an aircraft.

I have often been on boats in this scenario with the autopilot crabbing the boat upwind a bit, and you can clearly see it on the display. That being said, the view of your radar is always looking off the bow. Now if the chart orientation is being determined by course over ground, the radar image is going to be shifted by the amount of difference between the direction the bow has, and the course over the ground direction. In the example above, you can see the radar image is painting the shore to the left of where it really is along with the marker.

So is this dangerous? I have mixed emotions about it, but if I didn't have a heading sensor, I would rather have the course over ground overlay information available, rather than none. But in truth, a better solution is to have a heading sensor which largely removes the errors in the overlay. For boats that move slowly, the COG error can be larger than boats that travel more quickly. The picture above was taken yesterday in Sarasota bay. The MDF is a newly installed Raymarine C90 wide with radar, and no heading sensor. We are traveling at 18.2 knots, and it works perfectly. It was a calm day, and at idle speeds it worked well also.

So what did Raymarine tell me? Yes, they had disabled the capability to use COG as a heading source based on some complaints from slower traveling boats (I think that translates to sail boats). Their quality assurance professionals had voiced some safety concerns about the issue, and multiple staff assured me it was not an accident or  lawsuit that lead to the decision, just possible safety concerns. Then shortly afterwards, the market place pushed back, and they are restoring the feature, and I think this is a good idea.

Here's the catch. Their software group is very busy, and I can believe that given all of the innovation I'm seeing in their products. The earliest possible software update will be in the fall, and at worst case, early spring. The E&C wide's and newer systems will regain the overlay capability. The classic C's, and E's at this point will not get the change back. So for the classic C's software version 4.29 is as high as you can go and keep COG heading capability, and software 5.52 is as high as you can go on the classic E series. In theory, you might be able migrate backwards if you had the software, but Raymarine doesn't recommend trying this, and I certainly have never done this.

In the new Raymarine software, it won't do COG radar overlay if you are moving at less than than 2 knots, and as you slowdown, and approach 2 knots, the system will warn you. There are ever increasing demands on overlay capabilities in today's high tech navigation systems. MARPA/ARPA, AIS, waypoints, weather, radar, and if it was my boat, I would prefer a good heading sensor, despite the difficulties sometimes in getting it well placed in some vessels.

Unlike Compass Marine, I haven't had a problem with a frying pan, but anchors, buckets full of chain, the air tank for a life raft, fire extinguishers, boomboxes, and many others have given me fits. I like the fact that as far as I know, only Raymarine provides a sticker that indicates fluxgate compass location. As a last note, Furuno, and Navico system always required a heading sensor, and Garmin still allows COG heading information to be used for overlay use.

I know this was a little mind numbing because Kate was rolling her eyes, and mumbling blah, blah, blah while she read it. I am working on a batch of lighter toned, yet erudite pieces. Kate has already said no to half of my thank you notes I have in a Jimmy Fallon take off. I'm gunning for that Pulitzer.

5 comments:

  1. My new Garmin Nuvi in the car has a self-contained heading sensor...from a cold start with the car motionless, it knows which direction the car is pointing, so it obviously is not relying on COG.

    If a $79 car GPS has this feature, why doesn't a marine GPS have it?

    I realize a remote heading sensor away from iron and magnets would be better, but the Nuvi approach works remarkably well.

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  2. I just updated my older c-80 to 5.5 and lost my over lay. Raymarine wants $200 for a heading sensor. Is there another sensor at a lower price that you would use? Also, do I need just the sensor or the sensor and teh control head? who is this installed. Tom in marysville Wa.

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  3. Starting with the NUVI, only the higher end units have a compass that is used for walking and hiking navigation. What most Garmin car units do is to remember the orientation, so the next time you turn it on, it puts the orientation you stopped with as your current orientation. The usage of compasses in cars are somewhat problematic because of the amount of steel in the car.

    The $200 dollar heading sensor is a stand alone fluxgate sensor, that requires additional hardware to convert the data to something the chartplotter can use. Raymarine at several points in the past had a standalone heading sensor that output heading data in Seatalk, if my memory was correct. Ray is planning to reintroduce this product at some point again in the near future. I think at this point you are stuck. There is a possible scenario that backward migration in software might be possible if the unit is old enough, but I would not recommend trying it. The consequences could be severe for both the unit and your pocketbook. The best bang for the buck for a heading sensor I have found to date is the KVH 1000 which can be purchased online from Cascade GPS about $562. you might also haunt Ebay and the ilk for a used or remanufactured unit. Sorry, but I know it's not good or inexpensive news.

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  4. What is the latest on this issue? Raymarine issued update v5.69 on the E Classic series and v5.16 for the C Classic series the time this was written (July 2012). I had the E120 with version 5.52, but since I'm actively cruising the Caribbean right now, I didn't even know about it.

    I read about this issue a few days ago while helping friends re-configure their E120 Classic network. They're getting magnetic heading info from their autopilot, but it's multiplexed together with speed/log/depth/wind from their Nexus 2 instruments at 4800b/s over NMEA-0183, so it's not true Fast Heading data at 10Hz.

    Since the new 5.69 software is supposed to address performance issues with AIS targets (they're also installing a new StdHoriz Matrix AIS VHF radio), I'd like them to upgrade, but that would require a NEMA multiplexer - and they're already in the middle of a "surprise" deck core replacement, so I'm loath to tell them they have to purchase more hardware. So I'm telling them to stay with v5.52 and the slower heading updates and see how the AIS data works.

    Boatts - Break Out Another Ten Thousand...

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  5. Jon, there is only one input port, and if you multiplex data to it, all data will come in at 4800 bps. I might try to just add the SH VHF/AIS first to the NMEA input with everything else to see if the Ray will take it. If it doesn't then add the multiplexer. I don't think in either case the upgrade would hurt them, and more likely will help.

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