ARPA Course Vector Length

[RickVicik]

Does the length of the course vector on an ARPA target indicate speed? I've seen different length vectors on the same screen (see photo). I couldn't find the answer in the TZT manual or by searching this forum but maybe I wasn't using the right keywords. My equipment is a DRS6A-NXT running v1.10 and a TZT19F running v3.5.
---rick

 

[Quitsa]

Yes, the length of the line is in proportion to the speed. Longer is faster. On your screen shot, the boat running away is going a lot faster than the one heading towards you. There are often slow-moving draggers operating in my area that might be towing a trawl at only 2 or 3 knots and have very short vectors. Also a stationary target like a buoy sometimes gets tracked and will show as an ARPA target with no visible vector line or a very short one. Was the large target with the short line a buoy?

The same is true for the vector lines on AIS targets. If there is an AIS equipped vessel that has been acquired by ARPA, you can see both vectors assuming you have the display set to show AIS and ARPA and you have an AIS receiver or transponder. Because the ARPA vector is recalculated constantly, sometimes the AIS vector will differ based on how frequently the AIS is transmitting if the target is changing course or speed.

Do you have target trails turned on with a green color? What is the heading source that the radar has available?

 

[RickVicik]

I was hoping the vector line length gave an indication of speed but couldn't find that stated explicitly. Thanks for confirming it.

Now I can look at the whole screen and focus on the targets that have the longest vectors that point in my general direction.

In my screenshot, the target with the short vector line was a slow moving boat, not a buoy. The screenshot was taken with true target trails turned on because I find them very helpful and always have them on. I set mine to green with multi-color shading and 15sec length. I find that makes the trail look like a boat wake and the length gives an indication of speed. My heading source is a SCX-20.

Once again, thanks for the info that vector length is proportional to speed.
---rick

 

[Quitsa]

I think the ARPA performance is one of the strongest features of the Furuno radars, especially compared to radar from other manufacturers. With an SCX-20, the ARPA is very accurate and provides a clear indication of the risk posed by other vessels. Its accuracy can be confirmed by comparing the ARPA target characteristics with the information received from a vessel with Class A AIS transponder, which transmits GPS derived course and speed every two seconds for a vessel moving over 14 kts. The ARPA calculated course and speed are very close to the AIS data.

I had the top of the line 25kW six foot open array radar from another manufacturer that should remain nameless for a couple of years. Its equivalent of ARPA required manual target acquisition and took a long time to generate course and speed vectors. Even worse, as the target moved, the vector would change dramatically as it was recalculated. I had to rely entirely on target trails to assess course and speed for boats without AIS. It was a great relief to go back to a Furuno radar, which I had had previously.