GPS Accuracy

Those that purchased and used GPS units prior to May of 2000 might have been disappointed in the accuracy available. The military included random timing error in the GPS signals called Selective Availability (SA). This intentional error prevented all commercial GPS units without a method of correction, from obtaining accuracy any better than 30-100 meters. Even the cheapest handhelds are capable of accuracy within 10 - 15 meters or so without the intentional timing error.

So what's significant about May 2, 2000?

That is when the error (SA) was turned OFF !

The good news is that the GPS units everyone purchased are now accurate to better than 10 meters 95% of the time. That's only about two boat lengths.

I don't know about you, but I can return to individual stumps, brush piles, and points in no time if I know I am 30 feet or less from it.


Now what about the "WAAS Capable" thing I see in the ads for GPS??

All error has not been removed from the gps signals. Only the "intentional" error by the military has been removed.

Other sources of error include the slight deflection of the satellite signals as they come through a portion (ionosphere) of the atmosphere. Local satellite signal reflections (multi-path) occur around trees, buildings, and other stuctural features. Your own body can even reduce signal strength and quality when using a handheld gps.

The good news is that the ionospheric error can be corrected for by WAAS or DGPS enabled receivers. If a gps unit receives the timing adjustments along with the regular satellite signals, it can then use that information to compute a more accurate position with less error.

The WAAS signals are in the same frequency range of the GPS satellite signals, which means your 12 channel receiver will use 1 of it's channels to receive the WAAS corrections. A DGPS receiver is a separate device that has to be purchased and hooked to your existing GPS.

WAAS is an error correction system designed for use by the aviation industry to reduce the atmospheric error potential in gps calculations. A series of land based stations calculate and provide timing corrections to counteract the atmospheric signal delays. These land based receivers relay their information to satellites in geo-synchronous orbit (that means they are in the same place all the time relative to the earth -- unlike the moon and unlike normal GPS satellites ).

Land based DGPS is basically the same idea and has been around for a long time. DGPS corrections have been available through coast guard beacons distributed near most major rivers and lakes where barge traffic is common. In our area, there are beacons in St. Louis, Memphis, and Louisville. DGPS is a low frequency radio signal that requires a separate receiver that then hooks to your gps to provide correction information.

From what I can tell, there is only 1 WAAS satellite that we can receive the corrections from. That's OK, since it only takes 1 for the information we need. The interesting thing is that the WAAS satellites are located directly above the equator and relatively low on the horizon.

The WAAS satellite location relative to Kentucky is at an azimuth of approximately 130 degrees (roughly southeast) and 35 degrees above the horizon. That means if you are fishing a reservoir with fairly tall ridges, the WAAS signal may be unavailable since it is blocked by hills. The signal has to be "line of sight" or you won't get it. Folks in the western US may not benefit much at all since it will be only 10-15 degrees above the horizon there. WAAS is still in the development stage and we will hopefully see additional satellites come online in the future. A satellite directly south of the US would increase the height above the horizon significantly.

We should be fortunate here in western kentucky, since we are in rather "rolling" terrain, the hills shouldn't be too much of a problem.

GPS accuracy with WAAS corrections is specified to be < 3 meters 99% of the time. From some of the tests I have seen on consumer grade receivers, the specifications are accurate.

You will literally be within "spitting distance" of a spot when < 3 meters away.

If you have noticed, commercial GPS manufacturers are really starting to promote "marine" receivers. This is partly because of the WAAS availability plus they are adding moving map capabilities with lake information. They will not likely push the WAAS thing as hard for gps units designed for land use since the WAAS signal is more likely to be obstructed by trees or a variety of other structures.

Just to summarize the accuracy you might expect to get with a common gps receiver.

GPS with no correction capability -- <10 meters 95% of the time
GPS with DGPS receiver -- 3 - 5 meters 95% of the time
GPS with WAAS corrections -- < 3 meters 99% of the time

FYI there are survey-grade receivers and equipment available that can get accuracy in the sub-centimeter range. That kind of stuff usually sells in the neighborhood of 40 - 50,000 dollars.

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