Convergence hits the nav station

From Ocean Navigator #128
March/April 2003

Any navigator who has sweated across a busy shipping lane at night or in the fog has longed for one of the holy grails of navigation – an accurate chart overlaid with a combination of positioning information and detailed radar information. Until recently, this instrument integration was only a dream, but several companies have now introduced equipment and software that accomplish this difficult task and offer additional features and capabilities.

Electronic-chart and marine-electronics companies now offer the capability to view a composite image that shows a chart with a radar image overlay. At left is an example from Raymarine.

Image Credit: Courtesy Raymarine

There are three basic approaches to achieving radar/chart-plotter/GPS overlay and integration. Furuno and Raymarine have created networked instrument systems that link separate GPS/chart-plotter and radar instruments using proprietary technology. In addition to linking its own display units, Raymarine also offers RayTech software that allows a standard PC to be networked into the system. In addition to networking chart plotters and radars, both Furuno and Raymarine allow for the addition of data inputs from other instruments, including depth sounders, fish finders, wind and weather instruments, speed instruments, and aerial photo displays.

Other companies, including Global Navigation Software, MaxSea, Nobeltec and Transas Nautic, have created Windows-based software that allows an ordinary PC to be used as the integrating display for inputs from independent radar, GPS and other navigational devices. The PC-based systems can support data inputs from most manufacturers’ radar equipment. Some software companies have also begun to offer packages complete with radar arrays.

A third solution was introduced recently by Northstar – a single, standalone display that integrates radar, GPS/plotter and a fish finder/sounder into one package. According to Northstar, this integrated approach makes its system more user friendly.

Which way is up?

In addition to traditional radar images, with the vessel being at the center of the bullseye, integrated displays offer the same viewing options that are available on full-featured standalone radar units. Generally, one can choose between a north-up, heading-up and course-up display. These all do what they say – put the boat at the center of things and the selected option at the top of the screen. In order to use this option, some type of heading sensor is needed, either a flux-gate compass, gyrocompass or even one of the new GPS-based compasses that use multiple GPS antennas to determine vessel heading.

Electronic chart accuracy

Computers may have revolutionized navigation, but they haven’t made charts any more accurate. Why? A GPS-enabled chart plotter, or ECDIS (electronic chart display and information system) uses chart data from the same old paper charts that it replaced (You do still have them aboard somewhere, right?). Further, some of that data may have been distorted a bit during the conversion to digital format.

What does this mean in the real world? Let’s do some math. When we plot a round of bearings, we typically consider a triangle with 2-millimeter sides to be pretty accurate. That’s a pretty small “cocked hat.” But at a chart scale of 1:20,000, the triangle’s sides are 40 meters long. Now consider survey accuracy standards. Historically, surveys were generally conducted so as to have an error of 0.75 mm at the scale of the chart. On our 1:20,000-scale chart, that’s 15 meters. A plus-or-minus 40-meter fix on a chart accurate to 15 meters is fine.

But what happens when we add GPS? With Selective Availability off, accuracies of less than 10 meters are common. With DGPS added, three to five meters is a good bet, and with WAAS added, better than three-meter accuracy can be achieved over many areas. So now we’re plotting fixes accurate to a couple of meters on a chart accurate only to 15 meters. In other words, we now use a navigation system about an order of magnitude better than the surveyors who collected the data.

Worse, over half of the soundings portrayed on current editions of National Ocean Service charts were collected before 1940. Surveyors in those days used less-accurate positioning methods than are available today, and they knew that navigators could not hope to match their accuracy during routine navigation. Their sounding lines were less accurately run, soundings along those lines were less closely spaced, and no data was collected between lines because side-scan sonar had not yet been invented. Further, regardless of how accurate those surveys were, significant changes in bottom contours, submerged dangers and shorelines have occurred in the last 60+ years.

A final problem occurred during conversion of the paper-based data to digital format. Before the 1990s, U.S. chart compilation was a manual process. Projection lines, based on one of a number of datums, would be drawn by hand, and charted features were plotted according to these lines. During digital conversion, these projection lines had to be converted to a single common datum, and computerized averaging techniques were used to do this. You can imagine the process as if the chart were a sheet of rubber, which has to be stretched and pulled so that the projection lines all conform to the new datum. Different areas of the chart move different distances and in different directions. This introduces an additional source of error. Small, of course, but a little error goes a long way at ordinary chart scales.

So when your GPS says you’re in deep water while you struggle to kedge off a shoal, or when it plots your boat in the parking lot while you’re safely moored in your slip, it’s probably not the fault of the GPS. More than likely, it’s the data.

Richard Hubbard is a sailor, freelance writer and one of the editors of American Practical Navigator, published by the National Imagery and Mapping Agency.

Richard Hubbard

Some units also offer a true-motion display option. The true-motion display uses a stationary chart image and shows the boat’s position moving across the screen and/or the radar targets moving (if they aren’t stationary objects). This display option is particularly good at keeping track of other vessels that may be paralleling your course at a similar over-the-ground speed. Of course, you might want to get used to just one type of display method. Switching between true motion and relative motion can take some getting used to, and you might not want to start that type of juggling in the middle of a tricky situation, like finding yourself in heavy traffic at night.

On target

It quickly becomes apparent that integrated radar/chart plotters help solve one of the radar users most difficult problems – how to identify targets. Putting the radar image on the chart will help to quickly differentiate between buoys, fixed objects and moving vessels.

This can be particularly useful when the coastline offers few or poor radar targets. For example, radar may only pick up a portion of a low coastline, which appears on the radar screen as a broken-up image resembling some islands or separated rocks. Overlaying the radar image on a chart of the area can help fill in the missing information. Small buoys or wooden navigation aids can also provide poor radar returns, but an up-to-date chart (a key point here), viewed in conjunction with the radar image, will provide the locations of these objects.

In addition, the navigator has various software-generated tools to help identify things. An electronic range marker can quickly determine the distance to an object. Or guard zones can be set with alarms when radar targets appear within the zone. Courses to a radar target can be set and sent to an autopilot if one is integrated with the system.

One of the potentially most useful features of these systems is ARPA (automatic radar plotting aid) or MARPA (mini automatic radar plotting aid) technology. Generally, these systems can track up to 10 radar targets simultaneously, providing anti-collision alarms and warnings. ARPA capability means you don’t have to transfer radar target information to a paper plotting sheet in order to get information on another vessel’s closest point of approach (CPA) and time to closest point of approach (TCPA). Once you designate a target, the unit automatically calculates CPA and TCPA, and displays that data on the screen. While it’s easy to assume you know the relative collision danger posed by other vessels by simply looking at the radar screen, only by knowing a vessel’s CPA or TCPA can you be confident that you are either clear of collision risk or must maneuver your vessel to get clear.

In order for the software to be able to integrate and properly overlap target images from radar and chart plotters, these systems generally require highly accurate heading information from real-time course sensors that input both heading and speed.

ARPA, while available on standalone radar units, becomes even more useful when overlaid on the chart-plotter image. For example, if one is proceeding parallel to a shipping channel to avoid close encounters in the fog, ARPA and MARPA will indicate which vessels are sticking to the channel and which ones are cutting across and may pose collision threats. The ability to provide an assessment of collision risk based on the relative motion of your vessel and other targets designated by you makes ARPA or MARPA capability a valuable feature for a voyaging vessel.

Software and hardware

Furuno’s NavNet network system is a 10-Base-T Ethernet-based system of instruments that allow all displays to be operated as standalone units or as part of the network, sharing the information coming from the other instruments. Standard off-the-shelf Ethernet hubs can be used to link up to four instruments simultaneously. Each display can act as the master controller of the network, with each proprietary display unit using a similar control interface. The units also have NMEA 0183 ports to accept inputs from non-NavNet electronics.

Every NavNet radar display includes an integral chart plotter using either Furuno CDC and Navionics, or C-Map NT charts. Models range from the 1833C with a 4-kW output, a 10.4-inch display and a 24-inch radome up to the 1953C with a 12-kW output, 10.4-inch display and either a 4- or 6-foot open-array antenna.

The chart plotter can be displayed full-screen, split-screen, overlaid on the radar, and/or on a separate chart-plotter display. Users can select from true-motion north-up, true-motion course-up, relative-motion north-up, and relative-motion course-up modes. The true-motion modes keep the coastlines stationary while your boat moves, and the relative-motion modes keep your boat at the center of the screen.

When connected to electronic heading sensors and the optional ARP11 radar auto plotter, these units provide ARPA tracking of up to 10 targets simultaneously.

Global Navigation Software Company provides an inexpensive Windows-based chart-plotting program that includes an ARPA interface and is compatible with various raster chart formats and C-Map NT vector charts. A standard PC video capture card is used to interface with the radar image. Despite its low price, many, if not most, of the features of other more costly programs are included. The company is planning a new software upgrade for release in June 2003.

Image Credit: Courtesy Global Navigation Software

Global Navigation Software’s NavPak has an automatic radar plotting aid (ARPA) feature that can track targets.

MaxSea Yachting is a Windows-based software package providing a radar overlay on a chart-plotter display with full ARPA functions, including the tracking of up to 300 targets. A pop-up menu lets the navigator choose various colors for the radar image, and slider-type controls adjust for sea state, clutter and gain. The company says its software is compatible with most radars using various hardware options for networking. Though previous versions of MaxSea software were compatible with Macintosh operating systems, this is no longer the case.

Image Credit: Courtesy MaxSea

The MaxSea Yachting package radar/electronic chart overlay has ARPA functions that can track up to 300 targets.

Nobeltec was one of the first companies to develop Windows-based software that integrated radar images with chart-plotter images when connected to the proper radar and GPS equipment. Nobeltec’s Visual Navigation Suite 6.5 special radar edition or Admiral software uses SoftChart, Passport or Passport Deluxe charts. SoftCharts are raster-type charts with enhanced colors, and Passport Deluxe are the latest proprietary vector charts derived from Transas Marine charting. The entire world can fit on a single Passport Deluxe CD (chart detail may not be equivalent to raster charts in some areas).

Image Credit: Courtesy Nobeltec

Nobeltec’s Visual Navigation Suite 6.5 special radar edition. This is split-screen mode with the chart and radar at right and the radar alone at left.

In addition, Nobeltec offers a range of radar/software packages ranging from a 2-kW unit with a 12.4-inch enclosed radome to a 12-kW unit with a 6.5-foot open array.

Northstar decided that rather than using standard PCs for display and computing power, or linking a variety of separate navigational devices, its 958 Integrated Navigation System is a radar, GPS/plotter and fish finder/sounder all-in-one instrument. Complementing the display-chart plotter is a variety of radars ranging from a 4-kW, 36-mile unit up to a 12-kW, 72-mile package. The company claims that this integrated approach makes its system particularly user friendly. A variety of split-screen display options can be used, as well as a radar overlay on the chart-plotter image.

Image Credit: Courtesy Northstar

A screen shot from the Northstar 958 showing the side-by-side chart and radar-image option. This unit can also display depth-sounder info and picture-in-picture video overlay.

Raymarine, long known for its standalone small-boat radars, offers its hsb2 networking, integrating its Pathfinder radar displays with Raychart plotters. In addition, sailing instruments, fish and depth finders, standard PCs, heading sensors, and autopilots can be linked together using SeaTalk or standard NMEA interfaces. Rather than using hubs, instruments are daisy-chained together.

Image Credit: Courtesy Raymarine

The Raymarine RL70C with radar overlay. This unit can use Raymarine’s hsb2 networking capability.

A PC running RayTech Navigator software can act as the central navigation station, providing a display for both the radar and the chart plotter, or sending the charting information to the radar display. In addition, charting information can be sent to the PC from chart cards installed in the Pathfinder displays.

Raymarine offers mini-ARPA capability on some units.

Raymarine’s Pathfinder radars RL70C Plus (7-inch display) or RL80C Plus (10.4-inch display) can have built-in Raychart electronic charting and/or can be linked with Raychart systems to overlay radar images on C-Map NT+ charts. Scanner options include 2-kW and 4-kW radomes, and 4-kW and 10-kW open arrays, providing ranges up to 48 nm.

MARPA is standard on Raymarine Pathfinder displays and the targets can be overlaid on the charts. This feature requires input from a heading sensor or from a Raymarine autopilot sensor. The radars provide automatic adjustment of gain, sea clutter and tuning, while charts and radar images are automatically synchronized.

Transas Nautic is the pleasure-boating division of a company that specializes in providing professional-grade electronic navigation systems for commercial shipping and fishing interests. In addition to the usual chart-plotter features, Windows-based NaviGator PRO software will interface with many ARPA radars for automated plotting of radar targets overlaid on the charts.

Transas vector charts provide a layered digital image with worldwide coverage on a single CD. Like the Nobeltec Passport charts (derived from Transas vector charts), detail in some areas of the world might not match that found on raster images of standard paper charts. However, unlike raster charts, vector charts provide the ability to view layers selectively – for example, all soundings can be turned off to reduce clutter. Also, when you zoom out on a chart (the scale is decreased), the text on the charts remains the same size, allowing for easier reading. When charts are rotated, the text remains horizontal.

Other instruments can be interfaced with the system, including Navtex, and a split-screen mode can be used instead of a radar overlay. An unusual mode is 3-D chart imaging that generates a three-dimensional view of the active chart and your vessel’s location. n

John Kettlewell is a sailor and freelance writer who currently lives in New York state.

On target

It quickly becomes apparent that integrated radar/chart plotters help solve one of the radar users most difficult problems &mdash how to identify targets. Putting the radar image on the chart will help to quickly differentiate between buoys, fixed objects and moving vessels.

Image Credit: Courtesy Global Navigation Software

Global Navigation Software’s NavPak has an automatic radar plotting aid (ARPA) feature that can track targets.

Image Credit: Courtesy MaxSea

The MaxSea Yachting package radar/electronic chart overlay has ARPA functions that can track up to 300 targets.

Image Credit: Courtesy Nobeltec

Nobeltec’s Visual Navigation Suite 6.5 special radar edition. This is split-screen mode with the chart and radar at right and the radar alone at left.

Image Credit: Courtesy Northstar

A screen shot from the Northstar 958 showing the side-by-side chart and radar-image option. This unit can also display depth-sounder info and picture-in-picture video overlay.

Image Credit: Courtesy Raymarine

The Raymarine RL70C with radar overlay. This unit can use Raymarine’s hsb2 networking capability.

Raymarine offers mini-ARPA capability on some units.

Software and hardware

When connected to electronic heading sensors and the optional ARP11 radar auto plotter, these units provide ARPA tracking of up to 10 targets simultaneously.

In addition, Nobeltec offers a range of radar/software packages ranging from a 2-kW unit with a 12.4-inch enclosed radome to a 12-kW unit with a 6.5-foot open array.

Transas vector charts provide a layered digital image with worldwide coverage on a single CD. Like the Nobeltec Passport charts (derived from Transas vector charts), detail in some areas of the world might not match that found on raster images of standard paper charts. However, unlike raster charts, vector charts provide the ability to view layers selectively &mdash for example, all soundings can be turned off to reduce clutter. Also, when you zoom out on a chart (the scale is decreased), the text on the charts remains the same size, allowing for easier reading. When charts are rotated, the text remains horizontal.

John Kettlewell is a sailor and freelance writer who currently lives in New York state.

Convergence hits the nav station

By Ocean Navigator