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2e. Using ARPA, MARPA, and Radar with Chart Navigator Pro

Created: 12/13/2006
Modified: 06/21/2007


Electronics Hookup and Configuration


Navigators rely on ARPA/MARPA radar devices to track ships and obstacles. Chart Navigator Pro can interface with these devices, and display targets in its chart windows.

What it is NOT: Radar Overlay
Be aware that, at this time, there is no way to overlay the actual radar image onto the nautical chart. This is a very powerful and complex feature reserved for the most high-end chart plotters (such as Maptech's i3.) What Chart Navigator Pro is capable of displaying, however, are targets identified through the ARPA/MARPA system of the radar.

ARPA & MARPA: What is it and what's the difference?
ARPA is an acronym for Automatic Radar Plotting Aid
MARPA is an acronym for Mini Automatic Radar Plotting Aid

The difference between the two is that with a MARPA system, the operator has to manually select radar signatures (or targets) for the radar to track and pass information about. With an ARPA radar, the target selection occurs automatically.

How is the information transferred from the Radar into Chart Navigator Pro?
By specification, ARPA/MARPA targets are passed via the NMEA protocol. Thus, to connect an ARPA/MARPA radar to Chart Navigator Pro, use the NMEA out from the radar and connect it to a serial (COM) port on the PC. If necessary, use a USB-to-Serial adaptor or NMEA multiplexer.

What is displayed?
When valid ARPA/MARPA targets are received via the NMEA protocol, they will be automatically displayed on the charts in Chart Navigator Pro. This information includes: the validity of the target, the risk (dangerous or harmless) of the target, and the target's CPA, TCPA, COG, Speed, and Range.

Targets are plotted on the chart display via standard symbols, as defined by the specification: a circle for a harmless target and a triangle for a dangerous one. Please note that the actual radar image itself is not displayed.

What NMEA sentences are used by ARPA/MARPA and Chart Navigator Pro?
$RATTM is the only sentence required. It contains all the necessary information to display ARPA/MARPA targets in Chart Navigator Pro.

$RATLL and $RATTD are also supported, but not needed to show targets. $RATLL is used to transmit a cursor location to Chart Navigator Pro. $RATTD is obsolete, but still used by certain radar units.

What is the difference between ARPA/MARPA and AIS?
ARPA/MARPA uses the radar signature transmitted from your vessel to locate other objects (vessels, etc.) AIS (Automatic Identification System) uses signals transmitted by commercial vessels to plot similar information in Chart Navigator Pro. Note that not all vessels carry AIS transmitters, so the "picture" of who is on the water can be incomplete. Regardless, the prudent mariner uses these both as tools to provide safe passage - but is no substitute for responsible navigation procedures.

Related Information
Connecting Multiple NMEA inputs to Chart Navigator Pro


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