AIS Reception

[dawntreader]

I happened to be offshore again and noticed my AIS showed contacts from 200 miles away (see attached). I'm using my Raymarine 54E VHF which has an AIS transponder T'd into it. How do these long-range contacts get onto my AIS given the (relatively) low power plus all the attenuation I've added?

[Aquila]

Don't know!! But weather condition will improve reception and some of the vessels way off maybe Class A AIS, ie cargo vessels giving much stronger ground waves!  Have you checked the type of vessel beyond 50-60 miles. Be interesting to know what they are.  At least it's working and you are on your way home!!

[Salty]

Atmospheric conditions can make a huge difference to the propagation of radio waves. Years ago when passing off West Africa on our way south towards the Cape but having not yet crossed the equator, we were able to pick up VHF radio transmissions from Cape Town which was more than a thousand miles away, indeed it became quite a regular occurrence.

[dawntreader]

OK, thanks. My understanding is that AIS is point-to-point so, as has been said,it must have been atmospherics combined with strong output. I was unable to receive ship's information just location but suspect it was one with VHF transmitter on tall mast and high powered. That being said, some large tankers close by showed up at the last minute. Anyone else had fun crossing the 5-lane TSS off Cape St Vincent in frisky conditions? 

[Aquila]

Thanks for feedback, amazing ground waves and right weather appears to give you some useful AIS data.  Good luck with home passage, Re TSS...try and dodge the big ones!!  PS Are you carrying the right passport to get back into UK as today the drawbridge went up on Europe.

[Salty]

OK, thanks. My understanding is that AIS is point-to-point so, as has been said,it must have been atmospherics combined with strong output. I was unable to receive ship's information just location but suspect it was one with VHF transmitter on tall mast and high powered. That being said, some large tankers close by showed up at the last minute. Anyone else had fun crossing the 5-lane TSS off Cape St Vincent in frisky conditions? 

There are several articles describing unusual propagation of VHF radio waves, one of which is at the following site    http://dxfm.com/content/propagation.htm

One of the points the article raises is that while VHF transmissions tend to be considered as line of site, it's actually a bit more than that, and considerably more if atmospheric conditions happen to be just right. In one article it reported transmissions exceeding 3700 kilometres. The extremely long ranges I experienced had been with standard equipment as provided on an ordinary merchant vessel where the radio systems provided onboard were standard off the shelf units, and not high powered. Indeed increased power would not have made any significant difference to the ranges achieved where this was due solely to the atmospheric conditions. As for the height of the ship's antenna, yes they would normally be placed as high up as possible in order to achieve maximum range.
I can't comment on the tankers that showed up at the last moment, perhaps something was shielding the transmissions.