We frequently refer to AIS in our Blogs and are regularly asked what it is. Here’s a brief layman’s explanation of AIS and Radar.

AIS stands for “Automatic Identification System”. It is a digital VHF radio-based transponder system used to prevent collisions. It is similar in concept to the Collision Avoidance system used in Aircraft.

AIS is NOT Radar. We also have radar on board, but AIS provides enhanced information over Radar. Both have their uses and anyone who is doing the Great Loop or any significant amount of cruising, especially in the river system, should have both.

AIS uses GPS (Global Positioning System), VHF radio, and sophisticated digital processing to automatically communicate information between vessels. AIS allows you to see the name, size, position, distance, and speed of other vessels equipped with an AIS system. The range of AIS is similar to the VHF voice radios used on boats, so about 5 miles depending on obstructions. The great thing about AIS is that it is not Line-of-sight like Radar, you can see other boats around corners and behind objects like buildings, docks, trees, or hills.

AIS Screen

AIS View of our friends on Brand New Day coming into Pensacola, Florida. They are doing 8.5 knots, at a bearing of 182.0 magnetic, and are 1.5 nautical miles away. You can also see other boats in and around the marina.

The technology uses GPS Satellites to determine your boat’s position. It then transmits that information, along with your boat ID (called an MMSI Number), speed, heading, and length, over the VHF radio waves. Other vessels equipped with AIS receivers pick up that information and display it on a map showing your boat’s position relative to them. Larger commercial vessels (cargo ships, cruise ships, fishing trawlers) also transmit this information using satellite signals so that their owners, the Coast Guard, and harbor controllers can track their position anywhere in the world.

One of the great things about AIS, especially on the rivers where Radar is pretty much useless because of the twists and turns, is that frequently the Towboats will reach out to you by name and give you passing instructions or warn you about hazards coming up. In my humble opinion, any boat over 30 feet should be required to have AIS onboard.

MarineTraffic.com AIS Clutter
AIS Clutter in Fort Lauderdale

One of my (and many other boaters’) pet peeves is AIS Clutter. This is when boats in a marina leave their AIS turned on when they are not moving. This covers the chart information so that all you see are boat markers, and you can’t see the marina entrance or docks on your map.

In this image, every boat that is a little round dot is docked, so there is no need to have its AIS on. There are so many purple dots that you can’t see the waterway. Every AIS system has a “stealth mode” capability, where you can see other boats, but your boat does not transmit a signal. I think it should be a regulation that you turn off your AIS if you are on a fixed dock and not preparing to move.

Boats at anchor SHOULD leave their AIS on so that other boats can see them to avoid them.

AIS vs Radar

As mentioned, AIS provides more information and, in most cases, overlays on your charts so that you can see the information clearly and relative to your own boat. On pleasure boats, AIS has a range of between 1 and 3 miles depending on the terrain, and the height of your antenna and the antenna of the other boat. You only see boats that have AIS systems installed. In larger ports, some of the channel markers are equipped with AIS transmitters, and those are visible as well. AIS can see around corners.

Radar, on the other hand, is a line-of-sight device. It can’t see around obstructions like trees, buildings, or land masses. It works by transmitting a signal from your boat that bounces off objects around you and then reads the return signals to give a display of what is around you. With radar, you are looking at colored blobs. Those blobs can be land, other boats, channel markers, waves, birds, or rain. Unless you have practice with it and a chart to reference, it can be hard to tell one blob from the next!

Example of a radar screen
(click to enlarge)
Google Earth Fort Myers Beach Area
(click to enlarge)
The same approx view as above in Google Earth

The advantage of radar is that, typically, the range is longer than with AIS. If your radar is, say, 16 feet up on your mast, it can see an object at sea level 5 miles away. If that object sticks up, like a ship, bridge, or island, then the distance is improved. In the right conditions, radar can see 10-15 miles ahead and pick up weather even farther out. Also with radar, as long as the signal bounces off of it, you can see it. It doesn’t matter if the object has AIS, or radar. That said, most boats are fiberglass and fiberglass is a bad reflector of radar signals. Some small fishing boats, may not show up until you are very close. Many boats, us included, have Radar Reflectors on our masts to improve our visibility.

The disadvantage of radar is that it can be difficult to tell what it is that you are looking at. I frequently run my radar on clear days so that I can compare what I’m seeing out the window to what is showing up on the radar screen to learn what blog is what.

Modern boat electronics have high-bred modes that overlay charts, radar, and AIS data on a single screen to give you the big picture.

Enough science, now for something cool!

There are companies that gather AIS information from satellites and ground stations around the globe and display it online. You can see where ships are around the globe. It is the marine version of FlightAware. One of the most popular sites is called “MarineTraffic.com.” Click the link for the Live Map. It’s amazing to see how many ships are out there!

(click the for the Live Map)
MarineTraffic.com map of the area around Florida

On the live map, you can click on a boat icon, and see details about the boat. Boats outside of the range of a ground station require a subscription to see their information. We run a ground station from our home in Palm Coast and contribute to the community with AIS data from the area around us. You can see our station information here.

If you want to learn more about AIS or Radar there are great resources avaiable online. Just Google “Marine AIS” or “Marine Radar” and you’ll find 1,000s! One of our favorite resources for information about marine technology is “PANBO Marine Technology Hub.”

Kiss Some Frogs To Find Your Prince
Thanks for visiting! –Tom & Brenda

Write A Comment