|Radar in the 21st Century|
Part 3: True Motion Display
By Ben Ellison — March 2002
With the addition of accurate SOG (Speed Over Ground) data, several high-end radars can do all the navigational math to create a True Motion display. This is startling. Now your radar image sits there north up or course up with your vessel moving across it, just like a DR walking across a paper chart, and the true motion of other vessels is likewise deciphered.
Strong data integration is also behind the valuable ARPA (Automatic Radar Plotting Aid) -style feature that has migrated down from commercial ships. Manufacturers use a variety of acronyms for the yacht-level implementation, although MARPA (Mini or Manual ARPA, depending on which company you talk to) is the dominant one. MARPA, ATA (Automatic Tracking Aid), and ARP (Automatic Radar Plotting) all perform pretty much the same duty.
The traditional drill was to use EBLs and VRMs to get bearings and ranges of possibly dangerous traffic every six minutes while holding a steady course and to then plot the targets on a "maneuvering sheet." With two plots you can determine how close the other vessel will get to you, termed the Closest Point of Approach (CPA), and when, the T(Time)CPA. With a little vector work on the sheet, you can calculate the other vessel’s true speed and course, and even a new course to steer to achieve a certain desired CPA.
A typical MARPA set can lock onto user-selected targets and soon show each one’s true or relative forward motion track and a data window with CPA, TCPA, Speed, and Course. Most implementations can track 10 targets at once, which is handy, since multiple moving-target situations are when you might really appreciate some aid. MARPA can also change icons and/or sound alarms when a target’s CPA is dangerous. Solid speed and heading data is essential to smooth MARPA operation, as are strong and discrete targets (so maneuvering sheets, or just good radar sense, still has a place in the wheelhouse).
Yet another benefit of integration is the happy marriage of radar with chartplotting. Many products these days will show an electronic chart side by side with radar, either on the same split display or on synchronized screens. Heading modes and ranges can often be matched and changed with a single control–or not, as the user desires. Like a waypoint, the radar cursor position (TLL) can be passed to the plotter for target identification.
The ultimate is radar overlay, when the target images are placed semitransparent over the same-scaled chart image. Overlay started in megayacht systems and then appeared as an accessory to PC charting programs. Si-Tex and Nobeltec co-developed the original 2-kW RadarPC. Last year Si-Tex added 4-kW domed and open-array versions, which they market with their Genesis multifunction devices; Nobeltec sells the same units under its own brand for use with its Visual Navigation Suite. Furuno started offering overlay with its NavNet series, and Raymarine added it with the Pathfinder Plus upgrade. I’ve seen Nobeltec and Raymarine versions in action and was impressed; targets that corresponded to charted objects were immediately obvious. Also obvious was the necessity of an excellent color display and–like MARPA, for synchronization–fast, dead-on heading and position data.
I apologize for dragging you through a fog of acronyms, but in truth there are more useful features (with related screen acronyms) that I’ve had to leave out. Spend a little quality time with your radar and manual this season. Some dark and stormy night, you’ll be glad you did.
Nobeltec Phone: (800) 946-2877. Fax: (503) 579-1304. www.nobeltec.com.
Raymarine Phone: (800) 539-5539 (603) 881-5200. Fax: (603) 864-4756. www.raymarine.com.
Simrad Phone: (954) 922-7700. Fax: (954) 922-0707. www.simrad.com.
Si-Tex Phone: (727) 576-5734 Fax: (727) 570-8646. www.si-tex.com.
This article originally appeared in the February 2003 issue of Power & Motoryacht magazine.