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post #1 of 1 Old 05-21-2000 Thread Starter
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GPS Gets More Accuracy

Wow! Talk about a bolt out of the blue. Without any warning or fanfare, the White House on the morning of May1 said that the President would have an announcement on GPS and Selective Availability (SA). Sure enough, at a 1400 EDT press conference they announced that effective immediately SA would be turned off. The next morning we all read a small blurb in the back pages of our newspapers that GPS accuracy would increase from that of a football field to one of a tennis court. Hikers, boaters and the makers of the newer automobile road navigation systems all welcomed the news.How did this come about and why so suddenly? I think that most of us remember the time back in early 1997 when the President wanted to turn SA off. The Joint Chiefs of Staff met with the President and afterwards announced that SA would remain on for the next five years. At the end of that time, they would reconsider the advisability of turning SA off once again. The Department of Defense (DoD) wanted to keep the increased accuracy out of the hands of terrorists and other enemies of the US. It sounded good on paper, but we all knew that anyone with a portable Differential GPS station could overcome SA easily enough.

Anyway, after that announcement back in '97, I and most other boaters decided to either bite the bullet and go get that DGPS unit, or to continue on with our old GPS receiver. I didn't mind it too much when I was on the edge of an electronic chart and the charting program changed back and forth between charts as the SA jumped around. My old 90 MHz computer struggled to keep up with the chart changes and was so slow on the redraws that most of the time I had a blank screen. Meanwhile the USCG continued on with their construction of DGPS stations and we continued to watch our boat's position on the electronic charts jump around while hoping for a change in 2002. So this is how things remained until May 1 when, with a stroke of the pen, our old GPS units became 10 times more accurate overnight.

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.

Differential Radio Beacons are land-based signals that corrected the effects of Selective Availability when it was turned on.
In case you missed the DoD press release on March 30, the Interagency Global Positioning System Executive Board (IGEB) announced a decision to place expanded civil capabilities on future GPS satellites. A new civil signal will be added to the current military GPS L2 signal. This additional signal will provide significant improvements in GPS positioning and navigation services to worldwide users. The IGEB was established to implement the President's GPS policy, and is jointly chaired by Jacques Gansler, the Under Secretary of Defense for Acquisition and Technology, and Mortimer Downey, the Deputy Secretary of Transportation.

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.

Heading into a difficult situation is not the time to start practicing traditional navigation techniques.
Part of the reason behind this sudden change is that the Federal Aviation Authority (FAA) was about to certify GPS for civilian aircraft navigation and instrument approaches to all airports—SA was interfering with this plan. The FAA is spending over $1 billion to implement the Wide Area Augmentation System (WAAS) and the GPS/WAAS precision instrument approach procedures for civilian and military aircraft. Eliminating SA would improve the GPS accuracy for everyone, provide more safety to the airline industry, and save money the FAA needed to counteract SA.Whether we agree or disagree with this decision, the fact remains that SA is off and an ordinary GPS unit is for all practical purposes just as accurate as a DGPS. Your GPS will have the same repeatability for saved positions as your Loran.

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.

Pre-planning your cruise on the computer allows uploading accurate waypoints into the GPS.
Occasionally one of the GPS satellites will malfunction and transmit an erroneous signal. Not all GPS units have the capability to detect this and warn you. If this goes undetected, your GPS position could be in error by a considerable amount, depending on the number of satellites in view. This condition will last until another satellite comes into view or the bad one goes over the horizon. It can take the USAF up to eight hours to correct a satellite with such errors. But the DGPS system will detect the bad signal and reject it. Therefore, not only is the DGPS more accurate, but it is always more reliable. So in case you were debating the upgrade to DGPS this may help your decision making process.

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.

 Always take care when you measure the latitude and longitude of your waypoints to avoid errors. When your waypoint is near a degree line on the chart, it is very easy to make an error in either measuring or recording the numerical values. For example, if a waypoint is a few miles east of the 65 degree West line of longitude, it is very easy to record it as 65 degrees and 57 minutes West as 65 degrees degrees and 03 minutes West instead of the correct answer of 64 degrees and 57 minutes West. When you enter a wrong set of coordinates into the GPS, it has no way of knowing the error. If you manually insert your navigation waypoints into the GPS, you should have another person cross check them to insure their accuracy. A degree error, or skipping a waypoint entry, can ruin your whole day.
 When you reach a waypoint, always turn to your precomputed compass heading first and then crosscheck the GPS heading before using it for steering. If the GPS is connected to the autopilot and is steering, always ensure that the GPS heading is within a few degrees of your computed heading after the turn. Any major difference needs to be rectified before proceeding. By the way, I don't recommend using GPS steering for navigation. You are better off using basic DR combined with coastal navigation procedures, comparing your fixes to the GPS position. Plot both your fix and the GPS position on the chart and label them. I restart my DR from my fix rather than from the GPS position.
 If you follow the GPS steering between waypoints, you will be using course and speed over the ground (COG and SOG), as the GPS will automatically compensate for leeway, set, and drift. This also holds true for the electronic charting and computerized navigation programs. Steering with the GPS will allow you to follow almost exactly the pre-planned route on the electronic chart. This is O.K. for short cruises when you just want to enjoy the day, or for single-handed voyages when you have other pressing duties. However, it doesn't allow you to practice your navigation.
 If your route or plans change, never steer directly from your present position to another waypoint or a newly entered waypoint, until the new route is plotted on a chart and the new data is cross checked. There may be dangerous waters between here and there.
 Make sure that your GPS is set to the same geodesy as that of the chart you are using, whether it's a paper or computerized chart. An NAD 83 chart works well with a WGS 84 GPS setting, while an NAD 27 chart will not. If you don't know what I'm talking about, you need to read up on this in The American Practical Navigator by Bowditch, or read my earlier articles on navigation.
 Remember that your GPS computes a great circle route while you will measure a rhumb line course on a Mercator chart. This is not a problem for short distances, but might cause a bit of confusion for distances greater than 200 NMs.
 If SA is on, do not save your present position as a waypoint in the GPS and then attempt to return to it at a later time using GPS steering. This position could be off by up to 200 meters (650 feet or one-tenth of a nautical mile). This is from 100 meters for the initial saved position plus another 100 meters for your current position, and could cause you to cross a hazardous area. Whether SA is on or off, always plot the saved position on the chart and draw a course line on the chart from your present position to it to check for dangers before engaging GPS steering.
 Almost all the current computerized and electronic charting programs have the capability of uploading waypoints from the computer to the GPS, and this feature will help prevent some of the errors previously mentioned. Use the route planning feature to debug your voyage, and then upload the waypoints to the GPS. Be sure you select the correct routing in the GPS, before following the steering.

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.


Jim Sexton is offline  
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