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Sept 2010 Electronics Column

Finding Your Place

An upgrade for GPS and a real-world test for AIS


The first of the line: a Block 2F satellite being prepared for launch at Boeing.

The first of a new generation of GPS satellites known as “Block 2F” was launched from Cape Canaveral at the end of May and should be “set healthy” as you read this. It joins an existing constellation of 32 satellites and will take over from a “Block 2A” satellite that was launched in 1996.

But there’s much more to this new $121 million satellite than just repairs and renewals. Improved technology has given the Block 2F satellites more accurate atomic clocks and a new frequency for civilian users, which will change the way the signals are transmitted. All together these improvements mean greater accuracy and better resistance to accidental interference or deliberate jamming.


New Block 2F satellites are expected to last 12 years, five years longer than those they replace.

Moreover, these new satellites also use a new reprogrammable processor so that they can be upgraded in orbit and their planned life expectancy has been extended to 12 years, compared to the seven years anticipated for their predecessors.

Colonel Dave Maddon, commander of the U.S. Air Force’s GPS wing, says, “The launch not only moves us a step closer to delivering sustained, reliable new-and-improved GPS capabilities, it also shows our commitment to maintaining GPS as the gold standard for positioning, navigation and timing information. The GPS constellation is healthy, stable and robust with 30 operational satellites orbiting the Earth delivering improved and enhanced GPS capabilities to the warfighter, our allies, and civil users.”

But despite Colonel Maddon’s bullish words, there’s no escaping the fact that this first Block 2F satellite is already four years later than originally planned, nor that the satellite that it is replacing is not being retired—despite being more than six years past its use-by date. Instead, the Block 2F satellite is being moved to a different position in the constellation where it will serve as a back-up for satellites that are even older and more frail.

Test-driving AIS

AIS—the Automatic Identification System—has been a fact of life for commercial ships for seven years now. And yet, among recreational boaters—at least on the U.S. side of the Atlantic—the technology has been slow to catch on. In my July column, I explained that AIS is an automatic communication system in which every ship equipped with it broadcasts its identity, position, course, speed and rate of turn several times a minute, in a short burst of digital data that can be picked up and decoded by anyone with an AIS receiver. I also pointed out that although some receivers have built-in displays, many others pass on the information to a chartplotter or radar display, which then displays it in the selected format.

Finally, I explained that there is also a cut-down version of the technology called AIS B that has been developed specifically for small craft, though it took the Federal Communications Commission quite a while to get around to approving it for use on U.S. vessels. Maybe that’s why AIS was accepted more quickly in Europe than in the United States.


AIS ship icons in the English Channel, home to some of the world’s busiest shipping lanes.

Indeed, for many European boaters AIS has become a fact of life. So I was interested in the experience of a European friend, Stephen Price, who used the latest-generation AIS equipment to help him cross the English Channel. To make his experience even more interesting, his course took him through some of the busiest shipping lanes in the world.

The trip to Cherbourg was the first useful test of the new system, (he wrote) so as soon as we started to encounter ships in the westbound lane I set about comparing the system’s calculations with my own best guess from visual assessment and a radar plot. Then, by moving the cursor over the ‘ship’ on the screen, I brought up all the AIS information, including its CPA (closest point of approach).

As each ship passed, I checked its actual CPA by radar and found that the AIS readings were more accurate than my best guess.

But when I got into the eastbound shipping lane two incidents convinced me of the value of the AIS. The first involved a ship that would previously have had me seriously worried. But the AIS told me that its CPA would be about a mile, so I stood on [maintained course]. True enough, as confirmed by radar, the ship passed a mile off. What was interesting is that the ship had altered course—only by a couple of degrees—about ten minutes before he passed. Had he seen me? Without AIS I would not even have realized that he had altered.

The second incident was a ship I could not even see at first but whose predicted CPA was only 200 yards from me. After about 45 minutes I could see that it was a small coastal tanker. He should have given way to me as the previous ship had done, but it was clear that he wasn’t intending to. Maybe he was happy with 200 yards clearance, but I wanted more, so I turned 30 degrees to starboard and watched the CPA grow to 500 yards.

Without AIS, the first of these encounters would have had me unnecessarily taking action. In the second, I had to take action... but heck! I did have an hour’s notice!

That was the good bit about AIS.

Heading back to Chichester, England, yesterday, I could see the Normandie [a big car ferry] heading up past Bembridge. I switched on my AIS just to see how fast she was going and how soon she would pass in front of us. It was just curiosity really, but if my AIS was to be believed, I was about to be in collision with any number of vessels from just about every angle!

In my opinion, the large number of leisure boats that are fitting AIS transponders will devalue the worth of AIS as a tool to safe navigation. How long will it be before tugs, ferries and other commercial traffic get fed up with a constant barrage of AIS targets?

So there you have a real first-person evaluation—AIS is good in open water, but a potential nuisance inshore. Personally, I don’t entirely agree with Stephen about the nuisance factor. But there’s a simple solution: follow Stephen’s example by using AIS for what it’s good at—like his open water experiences—and switch it off in situations where AIS doesn’t help.

This article originally appeared in the September 2010 issue of Power & Motoryacht magazine.