AIS Range

[jstevens]

As other posters have written, comparing AIS reception ranges without knowing the specifics of the installations is probably a fool's errand. Of the installation parameters, I believe the receiver model, antenna height, possible RF interference, and condition of the cabling are probably the most important.
For example, I originally installed a NASA AIS engine in 2006 and mounted a Metz-type antenna on the mizzen masthead (about 35') using RG8X cables with one cable connection below the mast step. I got reasonable reception range on this setup, normally > 25nm. In 2008 a friend asked me to evaluate a SR161 receiver, so I cabled the SR161 in place of the NASA receiver using the existing antenna and cabling. Immediately I picked up several targets not seen with the NASA receiver. I switched the receivers several times and the SR161 consistently picked up targets not received by the NASA receiver. My friend got his first SR161 sale and the NASA receiver was sold on eBay.
My take away from this experience is that there can be significant performance differences between the units produced by different manufacturers. When I bought it, the SR161 product design was already several years old. I suspect if I were to replace the SR161 with a new manufacture model I would see another level of improvement in the reception range.

I have on occasion received targets from greater than 100nm, but that has been infrequent and short-lived. Mostly just a propagation anomaly, I suspect.

Over the intervening years, I've also noticed some reduction in the reception range even with the SR161. I suspect there has been some corrosion at the cable terminals and connectors. Cleaning those connections, or replacing the cabling and antenna will likely restore most of the original reception range.

One other thing to consider when evaluating reception range is the boat location. If the testing is being done at a berth in a large marina, the range is likely to be much less than would be experienced in open waters.

John

[colemj]

100+ miles is either tropospheric ducting (which would not be "routinely seen") or repetition of the signal along the way.

Normal reception of Class A AIS would not be at any greater range than VHF (maybe a tad bit further due to the digital nature).

If one is routinely talking to ships at 100+ miles, then never mind the above...

Mark

[FamilyVan]

Originally Posted by colemj

100+ miles is either tropospheric ducting (which would not be "routinely seen") or repetition of the signal along the way.

Normal reception of Class A AIS would not be at any greater range than VHF (maybe a tad bit further due to the digital nature).

If one is routinely talking to ships at 100+ miles, then never mind the above...

Mark

I thought 100 miles sounded odd too.

Sent from my SGH-I547C using Cruisers Sailing Forum mobile app

[ranger58sb]

Originally Posted by carstenb

I don't want to sound silly - but what range have to set your AIS signal to? Usually you can set this in your plotter. I've set mine at 20nm - but I could set it a 30 or more. You might want to check your settings.

Somebody in another thread mentioned range settings for AIS, but I can't find anything like that in our system. Near as I can tell, ours is what it is. (There are range settings for alerts, but not for reception... that I can find.)

We can at least see targets 20 nm out (the recent check I did in that other thread), but that was when we were at our marina, sort of landlocked, so it might be more out on the open water.

It's a Furuno FA-50, antenna approx. 18' off the water, no clue about coax type.

-Chris

[MYTraveler]

Originally Posted by colemj

100+ miles is either tropospheric ducting (which would not be "routinely seen") or repetition of the signal along the way.

Normal reception of Class A AIS would not be at any greater range than VHF (maybe a tad bit further due to the digital nature).

If one is routinely talking to ships at 100+ miles, then never mind the above...

Mark

I typically get those ranges when offshore 5nm or more. And I often have very good vhf comms at 80nm+ (using a 9db antenna). I know conventional wisdom is that VHF is only a little better than line of sight and that any range in excess of that must be due to an atmospheric anomaly. But I experience these ranges so commonly, especially with AIS, I do not believe that is the only explanation. I believe it begins with good equipment, an impeccable installation, and height. Anyway, here is a pic:

http://www.cruisersforum.com/forums/...1&d=1435680551

[colemj]

Originally Posted by ranger42c

Somebody in another thread mentioned range settings for AIS, but I can't find anything like that in our system.

There aren't range settings for the AIS itself - rather many AIS display units have range settings for how far targets will be displayed. For example, our AIS/VHF radio has a maximum display range of 15nm and our Furuno chart plotter 24nm. The AIS signals are receive further than those values, and are displayed on other devices that do not have limits.

Mark

[colemj]

Originally Posted by MYTraveler

I typically get those ranges when offshore 5nm or more. And I often have very good vhf comms at 80nm+ (using a 9db antenna). I know conventional wisdom is that VHF is only a little better than line of sight and that any range in excess of that must be due to an atmospheric anomaly. But I experience these ranges so commonly, especially with AIS, I do not believe that is the only explanation. I believe it begins with good equipment, an impeccable installation, and height. ]

We also occasionally see AIS targets from very long distances, however, we have never seen one at those distances when completely offshore and can correlated when we do receive long ranges to being within LOS of a repeater.

Since repeaters are often mounted high, LOS can be a long way. Definitely much more than 5nm.

If you are experiencing routine VHF comms at 80nm+ with reasonable antenna height, good equipment and a consumer radio, then you are exceeding the realm of physics or live in a permanent atmospheric ducting zone. Comms with coast guard doesn't count because they use repeaters high on hills to allow long distance comms.

Mark

[MYTraveler]

Originally Posted by colemj

Since repeaters are often mounted high, LOS can be a long way. Definitely much more than 5nm.

I mention 5nm only because when closer I don't achieve these ranges, presumably due to local interference. Much of my time is spent much further offshore and distant from population centers. For example, 100nm straight offshore, I will pickup inbound (eastbound) freighters another 100nm west of my position. So if that is a repeated signal, the repeater station, if on land, is itself receiving the signal from much further 100nm+. Also, since targets are often approaching from a range of directions, the separation between them is huge -- they couldn't all be on the repeater.

As far as VHF, I wish I could document my experience with a picture, but I can't see how to do that.

[DotDun]

Originally Posted by TxCoastSailor

As to why I might want a larger range, if I'm closing with a ship at a relative speed of 25 knots I will be on top of him in 24 minutes. That might sound like a lot of time but it really isn't particularly if there is only one person on watch and they need to wake a crew member to help tack or gybe.

Your crew must be hard to wake.

With 24 minutes TCPA, I would do nothing but watch the other vessel. Too many variables in play to make decisions whether to tack or gybe at that point.

[mikereed100]

Originally Posted by TxCoastSailor

My range is about 10nm even for large commercial vessels. Seems like I should be able to see them farther out.

I think you are right. How high is your antenna? Have you checked the connections at the radio and antenna? The connections between the coax and the PL259's?

We normally get 30-40 miles with our Matrix GX2150, 66' antenna height, RG8X cable. We often get over 100 miles and very occasionally get over 500 miles when the signals are bouncing around the atmosphere.

[carstenb]

Originally Posted by colemj

There aren't range settings for the AIS itself - rather many AIS display units have range settings for how far targets will be displayed. For example, our AIS/VHF radio has a maximum display range of 15nm and our Furuno chart plotter 24nm. The AIS signals are receive further than those values, and are displayed on other devices that do not have limits.

Mark

yes, I meant that whatever your display instrument has a range setting for when it will show the AIS traffic. My furuno is set for 20nm

[FamilyVan]

In navigator school (which was before AIS), we learned roughly, 8-10 miles should be your detection phase, 6-8 miles decision phase and 4-6 miles your action phase. That's for slow turning ships. Anything outside of 10 miles was considered to be nice to know but not immediately relevant to safe navigation.

My GX2200 has a maximum display range of 10 miles (as pointed it out- it will detect further, just not show the vessels on the graphical display. This seems to be in keeping with the above guidelines.

Sent from my SGH-I547C using Cruisers Sailing Forum mobile app

[TxCoastSailor]

Originally Posted by carstenb

I don't want to sound silly - but what range have to set your AIS signal to? Usually you can set this in your plotter. I've set mine at 20nm - but I could set it a 30 or more. You might want to check your settings.

Thanks, that occurred to me too. I will check it next time I'm on the boat.

TCS

[TxCoastSailor]

Originally Posted by mikereed100

I think you are right. How high is your antenna? Have you checked the connections at the radio and antenna? The connections between the coax and the PL259's?

We normally get 30-40 miles with our Matrix GX2150, 66' antenna height, RG8X cable. We often get over 100 miles and very occasionally get over 500 miles when the signals are bouncing around the atmosphere.

Mike,

Antenna is on the mast head about 58ft above the water. The antenna on a ship must be close to 100ft off the water. My distance to the horizon is about 9 mi and the ships distance to the horizon is about 12 miles so a large ship should be visible out to about 20 miles. My distance might come down a bit when heeled over. Not sure how folks are seeing targets at 100 miles or more. I thought VHF was strictly line of sight.

TCS

[MarkJ]

Originally Posted by TxCoastSailor

Mike,

Antenna is on the mast head about 58ft above the water. The antenna on a ship must be close to 100ft off the water.

TCS

100 feet? Thats a tiny ship. Panamx air draft is 190 feet so all the container ships will be up there somewhere.

My setup is good, and last trip i saw everyone at more than 40nms.
Here we see them at 50nms. And its not ducting!