White House officials claim that making the more accurate data available across the globe will not compromise national security. The Pentagon can implement SA over specific regions where there's trouble, or use GPS signal jammers that would not affect our military forces.
And that, my friends, is the rub. Pity the unfortunate cruiser who happens to be caught unaware in a troubled region. Lulled into a false sense of security, cruisers could quickly get into trouble when SA is turned back on or the GPS signal is jammed. This is why you must always back the GPS up with other navigation aids and Dead Reckoning (DR). You will never know ahead of time when they are going to take the GPS down or degrade the signal again.
Secretary of Defense William S. Cohen said, "This IGEB decision reinforces the continuing US commitment to provide the most capable, efficient and reliable satellite navigation system for use by all the world's nations well into the 21st century."
When this press release came out, we all missed the intent of the announcement and that the newly formed IGEB agency was set up to circumvent the desires of the Joint Chiefs of Staff, allowing the President, through another executive order, to turn SA off.
What do I mean by all practical purposes? I'm talking about being able to plot or use a position to three decimal places. On GPS, the first number displayed after the minutes of latitude and longitude are tenths of minutes and each tenth represents 600 feet. The second decimal place represents 60 feet for each hundredth, and the third decimal place represents six feet for each thousandth. On most coastal charts you can only use the first decimal place (tenths of a minute). On harbor charts you may be able to use the second decimal place, but nowhere can anyone use the third decimal place for normal operations. The GPS course will be computed to a tenth of a degree—but can you maintain the heading to within a degree without the help of an autopilot?
For years when SA was on, I criticized the GPS manufacturers for showing present position to three and four decimal places. At that time, the accuracy these decimal places represented was simply not available and gave the user a false sense of security. Now, with SA off, the normal GPS is accurate to two places and the DGPS is still accurate to three decimal places. I use these additional decimal places only to help determine when to round up the first decimal place. I will admit that there are specialized uses that require positions to four and five decimal places, but not for normal navigation purposes.
When I was in the Air Force we had a saying about planning air strikes versus the actual implementation. "Measure it with a micrometer, mark it with a grease pencil and cut it with an axe." The meaning here is that everything was computed down to a gnat's ass and then a No. 2 pencil was used to mark the target for a nuclear strike. The same thing applies to using the GPS on the sea.
So what's the bottom line on SA? The accuracy of non-differential GPS has improved about ten times. This means that fewer applications need differential correction. The commonly quoted "RMS" specification for GPS receivers is now in the less than 10 meter (33 feet) range.
For DGPS most of the effects of SA were minimized by differential correction and now you should see an improvement in the range of 10 to 20 percent, but certainly not the same ten times. The USCG claims an accuracy of 10 meters for DGPS' but in most cases it approaches five meters. Now it should become even more accurate.
There is one final thing you still need to remember. The number of available satellites is still the most important factor in determining your positional accuracy. Turning off SA does not reduce this requirement. If you want a 95% or better probability of your GPS position being within 10 meters (less than five meters for DGPS') you will need at least 5 satellites in view—more is better.Using your GPS for NavigationWith or without SA off, there are some important things you should always do when you use your GPS for navigation.
Using your GPS for Navigation
With or without SA off, there are some important things you should always do when you use your GPS for navigation.
Remember that one basic rule of navigation is to never blindly follow or trust any one electronic navigation instrument unless its accuracy is confirmed by other means and your DR position.