Eyes of the Fleet

Eyes of the Fleet

Wednesday, April 04, 2007

Iran's seizing Brits? Just a GPS error?

According to this piece of bilge: Don’t Forget the Astrolabe - New York Times:
After all, Global Positioning System receivers aboard the inflatable British patrol boats and the merchant vessel they were investigating clearly showed that they were in Iraqi, not Iranian, territorial waters. Surely, once Iran was made aware of the mix-up the situation could be cleared up and the prisoners would be returned. After all, military-grade G.P.S. does not lie.

But nothing involving global positioning technology is quite so simple. First, Iranian officials produced map coordinates that placed the sailors in Iraqi waters, though not where the British government had located them. Then, when it was pointed out to the Iranians that this didn’t constitute an offense, they produced a second, more damning set of coordinates. Not that it mattered. Regardless of how dubious the maps were, Iran still holds the prisoners. Regardless of how accurate the G.P.S. reading was, it was not, in itself, a resolution.

G.P.S. has become a remarkable tool, solving problems for farmers, prison wardens, broadcasters and concerned parents, not to mention drivers, pilots and ship captains. But, as this incident serves to remind us, it is not a panacea: relying too heavily on it can be dangerous. G.P.S. — the ability to know exactly where you are — has the potential to provide a false sense of security...
An old sailor’s rule of thumb warns against depending on a sole means of navigation; just because you have G.P.S. does not mean you should leave your compass and sextant on dry land. Failure, whether due to an electrical problems or a drained battery or old fashioned human error, can be devastating. The real lesson here is that, by itself, accurate G.P.S. will not save you from rough seas — or an international diplomatic crisis.
Let's see - using an extremely accurate navigational system (with several backups) makes it your fault when hostile forces invade another country's territorial waters and captures innocent sailors?

Further, I'm a little fuzzy on how celestial navigation (the purpose of the sextant, after all) would be of much use during a broad daylight attack. Going to shoot some sunlines and plot them on your chart's DR course? Sorry, GPS is very much more accurate than that...

Do a little radar or visual bearing plotting? Take some soundings? All have their own problems.

And, further, Mr. Rothman, I seriously doubt there was anything wrong with the ship's compasses, whether gyroscopic or magnetic. Or, for that matter, with the GPS system on the ship or in the helicopter hovering above the sailors, or in the sailor's handheld GPS...

GPS accuracy?
When taken together, autonomous civilian GPS horizontal position fixes are typically accurate to about 15 meters (50 ft). These effects also reduce the more precise P(Y) code's accuracy.

Sources of errors Source Effect
Ionospheric effects
± 5 meter
Ephemeris errors ± 2.5 meter
Satellite clock errors ± 2 meter
Multipath distortion ± 1 meter
Tropospheric effects ± 0.5 meter
Numerical errors ± 1 meter or less

I spent a couple of years professionally navigating ships before GPS came online. I can assure you that if ever I was accurate to within the civilian GPS possible error of 50 feet (about 1/8 the length of the ship I was on) I was the best danged pre-GPS navigator ever. You know, unless I was tied to pier or something...in which case my navigation was perfect.

Get a grip. The Iranians were the bad actors in this little power play.

U.S. Navy Photo:
USS Alaska (CB-1)- Chief Quartermaster John P. Overholt taking a sun sighting with a sextant from the ship's navigating bridge. Taken circa 6 March 1945, during the Iwo Jima operation. Taking notes on the observations is Quartermaster Third Class Clark R. Bartholomew.

UPDATE: Reader "Worker" sends along a link about the glitches caused to GPS by solar flare ups and advises to "keep that sextant handy..."

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