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Benefits of Using Combined Marine Radar and Automatic Identification System (AIS)

Benefits and Shortfalls of AIS for Recreational Boaters

Boaters tend to regard AIS as a source of absolutely correct and valid information that they can rely on to provide them with data regarding approaching and overtaking vessels. But the fact is that AIS data can be erroneous and it can lack critical details about vessels that may pose a hazard to them.

Lets give at least two specific examples about AIS data that may be surprising. The first example is AIS data from a tug with a tow. The AIS reported position is at the location of the tug and does not depict the fact that a tow is following several hundred feet behind and that the length of the towed barge is not presented on the MFD. Thus if a boater were in bad weather or cruising at night with a tug and tow crossing perpendicular to the path of the cruising boat. If that boater follows a path that is immediately aft of the tug and does not see the trailing barge the mistake could be fatal when the cruiser hits the huge tow hawser just underwater.

The second example is when AIS data for direction, speed or location are erroneous. This author has witnessed several vessels that were traveling in directions nearly 180 degrees from the true course / heading of the vessel. The error arises when the AIS transponder uses a Heading or Course data source that has not been calibrated or validated at the time of the AIS transponder installation. This can occur because you cannot see your own AIS ICON on your own chart plotter or MFD. It requires a separate boat that has AIS receive capability to verify that the reported heading /course over ground is correct. This author has witnessed even County Sheriff Patrol boats transmitting wildly inaccurate heading or course data as shown in the image below.

Erroneous AIS Data Showing the vessel ahead approaching our Port Bow – In fact it was headed away from us at 15 – 20 Kts. Note that the AIS Target would have gone aground had it been following the plotted direction! Magenta color spots are radar returns from Raymarine Cyclone PRO Radar.

In the first example the AIS broadcast is valid and is not misleading in terms of the tug itself. But your MFD / chart plotter will not show an extended target that properly indicates the full nature of the threat. In the case of large ferries and other ships, the chart plotter or MFD usually reports a special AIS vessel icon that implies a ship or other large vessel is present. But there is no display of a hybrid vessel that consists of two vessels – one powered and the other towed. The only way to know that you are crossing paths with both a tug and tow is to either physically see the vessels or to observe them via radar.

In the second case one must be lucky to visually observe that the transmitted heading / course over ground data vector (usually a line with an arrow that is 3-6 minutes long at the vessel reported speed) does not fall anywhere along the actual periodic GNSS position updates that are reported by the vessel transponder. This is very hard to detect happening when there is a busy waterway involved. A skipper assumes the plotted direction vector data is correct and plans his course accordingly. The only way to detect the AIS course data is in error is to observe the vessel on radar and being using “trails” or MARPA / ARPA target vector data that is independently measured by the radar itself.

It is an interesting thing to observe that if an ownship course or heading data is in error then radar observations of other vessels will also show incorrect course data of EVERY other vessel that is being reported by the radar. Now one would hope that an error this large would be easily detected and corrected by the vessel owner.

Using only AIS Receive

Vessels equipped with no radar and AIS receive only are certainly ahead of the game when compared with a vessel with neither capability. AIS receive equipped vessels have the benefit of seeing other threats around them and certainly this dramatically improves their chances of safe travel at night or in poor visibility conditions.

These vessels can receive emergency man over board or other vessel emergencies in order to render assistance or to steer clear of a critical situation. They can also observe special Aids to Navigation (ATN), buoys or markers that physically are not present as anchored buoys or day marks but exist only as a mark that is present only on an AIS equipped vessel.

Thus the vessel equipped with AIS receive only has gained several advantages but it is not broadcasting its own presence to other vessels. Therefore this vessel must assume other vessels may not be observing them visually or on radar and must choose to navigation to avoid collision without interaction with other vessels or by contacting other vessels on VHF radio on 16 or other Vessel Traffic System channel to avoid a hazardous crossing.

The great value of AIS is in poor visibility conditions. So the AIS receive only vessel is burdened with making sure they have been observed when crossing with other vessels. Certainly they are far better off than a vessel with no AIS and no Radar but they do have some handicap.

In the image below one can see radar returns painted in magenta (purple) for the nearby islands and a nearby AIS icon for a vessel passing well to starboard aft of the Own Ship Icon. The image shows that the actual position of the AIS target is in front of the current location. This reflects the delay in transmitting position for high speed vessels. Just to the right and below of the AIS target is another Radar target on a second vessel that is not carrying AIS. While this radar target for the second vessel shows it poses no problems for the Own Ship vessel it does point out that AIS does not report everything on the water.

By only carrying AIS receive and not using Radar a vessel still cannot detect other recreational vessels that are not equipped with an AIS transponder. This suggests that a vessel without radar and only AIS receive will be at risk in poor visibility conditions and should avoid being underway in such conditions. Certainly this is not too great a limitation in fine summer boating weather on salt water or small inland lakes where pontoon boats do not carry AIS transponders!

Low Cost AIS Protection

AIS is a great asset for safety and general situational awareness on board most recreational vessels. Given that there are now VHF communications radios that provide built in GPS receiver, AIS receive and even an AIS target display and target list even small boats can easily afford to have AIS monitoring capability even if they do not have a Chart Plotter or MFD installed. For vessels that do have a chart plotter or MFD installed these same VHF radios with built in AIS receive can provide AIS data to the Chart plotter or MFD via NMEA 2000 or NMEA 0183 High Speed data.

AIS receive capability is a tremendous help to displacement or semi-displacement vessels like sailboats and trawlers that travel at relatively slow speeds and therefore have limited ability to make meaningful course changes in the event of a close encounter with a high speed vessel like a ferry (330+ ft and 17+Kts in Pacific NW of USA) or standard shipping that can be 1000ft and traveling close to 20+kts.

If a small slow vessel does not carry Radar, the least expensive alternative to provide the most cost effective collision avoidance is a modest VHF Communications radio with AIS receive and display built in. These systems typically sell for under $500 and offer emergency distress calling when registered with an MMSI number in addition to providing AIS information. Examples are the Standard Horizon GX2400 and GX6000 that both retail for under $500 (The GPS Store prices May, 2022).

In congested waterways like the beautiful San Juan Islands of Washington State and The Gulf Islands of Canadian British Columbia AIS is a huge help in detecting the many ferries, ships and tugs that ply these waters. Being able to know that there is a high speed ferry approaching from behind an island is a great asset indeed! Being caught in summer time fog in these channels not only poses a navigation hazard but is a real threat when commercial ferries traveling close to 20Kts can be expected at any time.

Adding Marine Radar to Compliment AIS at Sea

In the image below we highlight a busy water way in the San Juan Islands of Washington State in the US. In this image the tow tug Island Mist AIS Icon can be seen with a radar detection that follows the AIS Icon. This is a direct indication that there is a TOW barge present that does not have a corresponding AIS Icon. While it appears that the Radar return (red blob) is very close to the reported AIS Icon, note that the image scale shows 1 Nmi is about 1/2″ on the display! So the red radar image is actually a long way aft of the reported AIS Icon for Island Mist at this scale. Zooming on the chart plotter would readily show the actual separation distance.

You can also notice that the “Own Ship” Vessel Icon in the center of the image is being followed close behind by an AIS Icon from another trawler and intends to pass to Starboard. At this range the radar return for the very close vessel may have been suppressed but would appear when zoomed in to shorter range for tactical information.

Radar Limitations

In the image above we notice that there is a large AIS target (designated as shipping by the AIS Icon type) just to the right of Island Mist. There is no radar detection of this vessel because it is just behind the high point of Cypress Island. The island is blocking the direct illumination of the vessel by the radar. This shows that while AIS signals can leapfrog over most nearby land masses radar cannot “see around corners”.

If you look closely at this image you will note that there appears to be a number areas that are missing radar detections, such as the left side of Guemes Island and no detections of Sinclair Island. In all of these cases the issue is “radar shadowing”. If the radar does not have a direct line of sight to the object because of a large object in the foreground the radar cannot see that target.

AIS can receive signals that are behind many (not all) other land masses because it does not depend on illuminating the target with a transmitted signal. But it can receive a signal that is either refracted (bent) or bounced from other land masses to the AIS receiver. These bounced or bent signals may flicker or come and go as the own ship changes position but they are at least observable part of the time.

Another significant problem for radar is angular resolution of multiple closely spaced targets. When it comes to radar systems, size does matter. A small radome radar that encloses a hidden rotating antenna that is barely 18″ across will produce a very wide beamwidth that can be as much as 6.5 degrees wide. Consider that at 3000ft (1/2 Nmi) a 6.5 degree radar beam will be 331ft wide! Clearly a couple of 100ft long boats could be within 100ft of each other and so far as the small radar is concerned there is just ONE vessel present because it cannot measure less than 331ft.

A 6ft long open array on the other hand will have a 1.2 degree beamwidth and can resolve 61ft of separation between targets. It will clearly show the two 100ft vessels placed 100ft apart and report them as both being present.

Finally the most challenging conditions for a radar system are very heavy seas or very heavy rain conditions. In these conditions waves themselves generate radar reflections that directly compete with the reflections from small vessels. Attempting to attenuate “sea clutter” or reflections from wave tops can also eliminate detection of small vessels in the same wave structure.

Rain can pose a similar problem when it is very heavy (1 – 2″ inches per hour – a very heavy storm by any measure) by generating large areas of “volume detections” – meaning that rain can fill the entire 22Degrees of vertical extent and say 1.2 degrees of azimuth extent with rain that is readily detected. This competing “rain clutter” can be attenuated but it will also cause small targets in the same area as the rain to be eliminated as well. Heavy rain (>1″ an hour) will also directly attenuate the radar signals themselves on both the transmit and receive paths.

While attenuation of typical X Band Marine Radar signals is virtually hill in heavy fog conditions, heavy rain poses a threat of total loss of visibility and attenuation of radar detections as well. When combined with heavy seas and wind, heavy rain conditions pose the most threatening conditions on the water so far as collision avoidance is concerned. Commercial shipping carry C Band radars that employ very large antennas of 6 -12 feet so that under heavy rain and wind conditions they can still maintain radar visibility. C Band radar incurs much lower attenuation of radar signals and can penetrate rain with less rain reflectivity. But C Band radars operate at 3 GHz and will require antennas that are on the order of 2- 3 times larger than a standard X band antenna to achieve the same angular resolution. Carrying a 12f – 18ft open array antenna on recreational vessels is totally impractical and is reserved only for large ships for that reason.

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The Next Marine Radar Technology

Back in 2007/8 I was employed by Honeywell Aerospace in Redmond Washington and was working on solid state pulse compression weather radars for aviation. The newly developed RDR 4000 (and now the RDR 7000) had just hit the market. It was the first replacement of the original simple pulse modulated solid state radars (replacing Magnetron tube transmitters with transistor based transmitters) with a pulse compression waveform (FM Chirp on top of a pulse waveform). This technology pre-dated the first of the commercially available solid state marine radars by several years. In 2008 we installed the Transmit Receive module of the RDR 4000 in the base unit of a 6 foot open array antenna and installed it on a 43Ft North Pacific Yachts Trawler. We took that radar out on the waters near Anacortes, WA (USA) and collected images of marine targets and in various wind conditions. This was the first time a solid state pulse compression radar had been demonstrated as a Recreational or Commercial Marine Radar.

Notice of those test results and some images were presented publicly in Seattle at a Maritime Navigation conference and published on the Panbo Marine Electronics blog – That was the beginning of what is now accepted as common place Marine Radar technology for both Recreational and Commercial applications. Major manufacturers Raymarine, Garmin, Furuno and Navico all sell very good Solid State Pulse Compression radar systems. At the time of this writing Garmin had just announced its new 250W peak Solid State Marine radar with performance rivaling 4KW and 12Kw Magnetron based radars of old.

An example of the 40 Watt Honeywell Marine Radar images compared with Magnetron systems in 2008: Honeywell data on Left in the combined image taken in Guemes Channel just North of Fidalgo Island in Washington State.

Now that solid state pulse compression systems have been perfected and are being introduced in ever high power and more capable systems by competitors, where is the next form of Marine Radar Technology going to come from?

I propose that the next generation of Marine Radar will be a low cost Phased Array Radar with electronic beam steering in AZ only and multiple beams generated in the EL dimension. Traditionally Marine Radars have a fixed elevation beamwidth of 22 degrees that is designed to keep the sea surface in view as a function of boat motion. It is also useful to provide long range detection of rain squalls to provide some level of protection from thunderstorms and microbursts that can create exceptionally dangerous conditions in relatively small areas that are to be avoided. This 22 degrees of elevation could be broken into multiple contiguous beams to provide detailed elevation information. I further propose that while a single panel phased array system would normally be limited to about 90 to 100 degree field of regard in AZ, this array could be rotated 360 degrees as current traditional systems are done. While this 360 degree mechanical rotation of an AZ scanning radar may seem redundant note that the AZ scan is exceptionally fast electronic steering that permits revisiting of tracked targets during a single scan and for multiple AZ scans of the same area during a single mechanical rotation. The potential for vastly improved target detection probabilities is obvious.

So where will this low cost phased array radar technology come from? I propose that it can be found both in at least two of my patents (9,897,695 and 10,775,498) and others in my name and assigned to Honeywell. A current radar being flight tested and ground tested at Honeywell is known as the IntuVue RDR 84K. A brief video of the radar can be found here:

US Patent 9.897,695

US Patent 10,775,498

Where previous Phased Array Radar technology has been exceptionally expensive ( think millions of dollars) the small solid state systems that are possible today at K band (24GHz) and X Band (8 – 10 GHz) using digital beam steering rather than microwave phase shifters is on the order of thousands of dollars and rivals the cost of the most recent Garmin 250W mechanically scanned marine radars.

The potential of electronic beam steering in AZ when combined with multiple elevation beams would be unrivaled for marine radar applications. Consider an 8″ wide array (RDR 84K is 8″ x 4″) that can produce an 8 degree beamwidth in AZ and EL planes with the potential to subresolve that down to 0.8 degrees or better. A small system like this would be of tremendous value to the Washington State Ferry System that daily has to thread the needle among small pleasure boats that do not carry AIS transponders and that cannot be seen by standard radar systems at short (<1Nmi) when mounted some 50 – 75ft above the waterline. One or two small phased array systems similar to the RDR 84K could be mounted near the car deck level and provide exceptional images at ranges of 1Nmi with no mechanical scanning. The radars could also supplement visual systems for approaches to terminals in poor visibility conditions.

Similarly small recreational vessels like sailboats that cannot use anything larger than a 24″ radome base radar on the mast would see tremendous benefit in the imaging capabilities of a mechanically scanned phased array radar.

The technically savvy person may also immediately wonder about the applicability of the new Automotive Collision avoidance radars. But these systems operate at 80GHz and are intended to provide no more than about 300 meters of detection range. Hardly enough to provide sufficient warning about an approaching threat that is 1Nmi or more away and traveling at 20 Kts without AIS transmissions. Automotive radars are highly specialized to their application and would require considerable modification to provide any value in a marine environment.

There are other possible adaptations of the patented technology that is presented here that can be considered for a very low cost multiple beam mechanically scanned array with imaging capabilities but lacking multiple revisit features during a single mechanical rotation. None the less this could afford excellent radar performance for the tens of thousands of small fishing boats in the range below 30ft that currently blast along at 30 kts come fog or not without any collision avoidance capability beyond a chart plotter with AIS. Given the large number of non-AIS targets that can be present in littoral regions all over the world, it would seem this could be a large untapped market for Radar technology that currently is not served.

While I am the inventor of record, these patents are the property of Honeywell. The patents are available in the public domain and are presented here as such. Vacanti Yacht Design LLC does not represent that it has any ability to field these systems in the proposed applications but as the inventor I find the potential for their use to be obvious and are news worthy in that regard. I suggest any interested party contact Honeywell directly using the contact information provided with the Video and Website for the RDR84K. VYD LLC has not been paid for this article in any way and the article is the creation of the inventor of record solely for the interest of patrons of my website. I am available for consultation on matters of radar design and development as Vacanti Consulting Services LLC.

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Vacanti “Active Fence” Keel Design

Well known Yacht Designer / Naval Architect Dave Gerr approached me some time back as he was writing a keel and rudder design methods article for a well known Yacht Design School. He had noticed the article I had published for Sailing World that described a keel design that was used on a winning J36 on the Newport – Bermuda race.

I provided some details of how the design was intended to work and why I believed that it was an improvement over many keel designs of that era. Dave included the description of the keel in course work he was writing and also published a short article that compared several keel design types. At the time the keel I used on the J36 was radical because it defied the then current concept that the keel / hull joint was the source of a great deal of drag. The conclusion by many designers at the time was to dramatically reduce the root chord length (keel to hull location) as much as physically possible in order to reduce this keel to hull drag.

My design, seen in the downloadable article listed below, flew in the face of that concept by using an even longer hull to keel joint but done using a method perfected on aircraft wing to fuselage joints going back 80 years (1940’s). The most typical design could be seen on the exceptionally fast P51 Mustang fighters of WWI and still seen on modern Piper low wing aircraft today. The concept is to use a highly swept back (~45 degrees) short span segment that sharply breaks into a far lower sweep back angle on the order of 10 degrees. The highly swept portion of the root section of the keel or wing causes span wise flow to be induced away from the fuselage / hull joint until the sharp leading edge change is encountered. The sharp break in the leading edge induces a local vortex to form that effectively seals the upper portion of the keel and enhances steady flow on the remainder of the keel / wing span. The very slight leading edge sweep angle of the remaining leading edge resists span wise flow and reduces tip vortex drag and reduces keel drag overall. Leading edge sweep or at least quarter chord sweep angle of near zero degrees is required for maximum keel lift and reduced keel drag.

Here is the article showing the various keel types that includes the Vacanti Keel concept.

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Keel and Rudder Design Methodology


I published a number of keel and rudder design articles over the course of about 3 decades in magazines ranging from SAIL, Sailing World, 48 North (Seattle Sailing Local Magazine) and most recently in Professional Boat Builder. I have provided a link below to the article that I last published in Professional Boat Builder because it is the most comprehensive article of all of those that I published. I have not seen any similar set of details provided by any other author to my knowledge. While there are excellent hydrodynamic articles by well known author C J Marchaj (Aero-Hydrodynamics of Sailing – Dodd-Mead) specific details of specific foil shapes and definition of the true value and function of the once popular Winged Keels are not provided in most books.

Our WINGS and FOIL programs are designed to allow the optimization of Keels and Rudders for low speed power ( Speed to Length Ratios under 2) and sail applications. True state of the art performance analysis of Keels and rudders would require the use of 3D Computational Fluid Dynamics software and a detailed knowledge of the modeling methods needed to use the this advanced analysis software. We have endeavored to offer programs and design methods (described in the download article) that are within the grasp and financial resources of the great majority of designers and hobby builders. We recommend the more advanced software in our companion blog article on “How to Make a Good Boat Faster”.

Please consider the following article as a technical introduction to Keel and Rudder design based on well known Aero-Hydrodynamic principles. Many keels and rudders have been designed with our software that have contributed to winning major offshore Sailing races, including the Newport- Bermuda race in which a Vacanti Keel modification to a J36 resulted in an overall victory over Farr and other well known boat designs.

Once last critical thing that must be understood in Keel and Rudder design is implementation accuracy. Any keel or rudder design can be rendered useless if the actual implementation or build of the design is not done with sufficient accuracy. A good example was a keel design I created for a Six Meter Sailing yacht named Steverino. The builder of the keel did a great job but at the last minute decided that the “look” of the keel was not agreeable to him. The builder then heavily modified the intended leading edge nose of the keel by grinding the off the forward 6 inches of the leading edge into a more visually appealing (to him) rounded shape. This modification destroyed the entire foil shape structure of the keel tip design and resulted in poor performance. Similarly if good CNC or other high accuracy mold methods are not used the expected performance of the keel or rudder may be destroyed.

Boat Stability and Keel Design

While most designers worry about athwartship stability normally provided by a keel, one client and user of our software reported that he was working on a boat that had the terrible tendency to bury its bow while running downwind. We looked carefully at the choice of foil shape used on the keel and determined that it had exceptionally poor stall characteristics that could result in very high drag. While most consider the keel to operate near zero angle of attack when running downwind, the boat is constantly being steered to keep the apparent wind at the required location relative to the spinnaker and mainsail. This particular boat required that all crew remain aft in the cockpit to keep the bow up and prevent submerging in waves.

After the keel was redesigned with an appropriate foil shape, specifically designed for the Reynolds number range of that boat, the boat downwind characteristics changed dramatically. Literally the boat was “tripping” over its own keel and was being forced nose down by the large moment arm of the keel drag located well below the waterline. Many have not accepted this hypothesis but none the less, accuracy of build and proper foil shape selection (use of FOIL and WINGS) resulted in a boat that won and was sailed safely off the wind.

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Marine Radar Technology

I have worked in radar technology for over 44 years in application to military missiles, radar altimeters, aviation weather radar and small phased array systems for drone collision avoidance, mapping radar Altimeters and brown out landing aids for helicopters.

I’ve watched as recreational and commercial marine radars transitioned from very high power systems ( up to 25,000 watts peak) based on nearly 80 year old magnetron transmitter technology to low power solid state radars that might transmit a mere fraction of a watt.

Solid state systems offer Doppler speed measurement to detect relative motion between the radar and other vessels. Older magnetron systems are not sufficiently stable in operating frequency to permit Doppler detection. Older magnetron tube systems transmit exceptionally short pulses at very high power. But a receiver designed to collect those very short pulses must have a very wide bandwidth that includes competing noise. Solid state systems transmit far longer pulses that are typically encoded with linear frequency modulation called “chirp”. The combination of a long pulse that allows a narrow bandwidth receiver with far less noise results in greater detection range per watt of transmit power. The encoded frequency chirp achieves fine range resolution. Low power solid state systems can also use very short pulses for very fine resolution short range detection to a fraction of a mile.

Therefore newer solid state systems from Raymarine, Navico, Garmin and Furuno offer excellent performance in open array scanner systems and fine performance in small dome packages. These systems have virtually standardized on Ethernet digital data and simple 12 to 24vdc power.

With the advent of low power transmitters radiating less than 0.1% of the power of some older radars, the danger of suffering physical damage to eyes due to microwave heating has all but been eliminated.

Only the Navico Broadband Radar employs FMCW modulation and radiates less than a watt of power. This system enjoys favor with small boats where use is confined to 5 Nmi or less. In places like the San Juan Islands of Washington and the Gulf Islands of Canadian British Columbia that is more than sufficient for collision avoidance.

FMCW or Frequency Modulation Continuous Wave is a very powerful radar scheme when properly implemented. I have several patents related to this technology that includes new products currently in production as radar Altimeters and Millimeterwave Phased Array Radar.

A PDF presentation on recreational marine Radar will be added to this blog in the near future. The file includes more on radar modulation technology, installation advice, complimentary functionality with AIS and much more.

Contact us for consulting on marine Radar selection, site selection for optimal performance and comments on radar technology in general related to marine applications.

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PROLINES First Design Project

Learn how to use PROLINES for fast and easy development of a boat, yacht, ship, SUP, Canoe or Kayak! PROLINES uses powerful NURB mathematics to instantly create a full 3D Hull shape from a few basic values for Length, Width (Beam) and depth (Draft) of the boat you want to design.