[Silver Hilton]

Anyone know if the GPS "fuzzies" are still on, and whether or when we'll get high accuracy back?

 

[STGRule]

I believe they are still on. There are a couple of websites that have some kind of corrections available after a couple of days. I'm not sure how it works but I assume you need the time of day along with the day to correct your saved coordinates. Of course, that doesn't do you any good when you need to use those coordinates. :shrug:

 

[Pete]

What I've read is that the selective availability is unaffected by the war in Iraq. Instead, the Air Force broadcast "GPS Noise" to confuse signals in the war theater.

Forbes Story on GPS

What I have noticed is that my unit seems to provide accurate longitude and latitude, but altitude is off by 40 to 60 feet. Most of the time it says I'm under water! Not sure if that's the system or the signal, but as long as I'm not gurgling, I'm happy. :grin:

 

[STGRule]

Pete: We noticed the GPS was off a little, but the altitude was off A LOT. Went from 250 feet above sea level to 40 below in about 100 feet of Columbia River. This at a true altitude of about 30 feet.

 

[Pete]

Oh Ruler of space and dimension! Bring me to the surface! :grin:

 

[Wright Angle]

Pete, I called Garmin right after our Government turned off S.A. in may of 2000 and ask why the Lat/Lon was right on but the altitude still seemed to wander like S.A. was still on, the tech person told me that the altitude would never be very accurate because the GPS needs one satellite below the horizon to have good enough geometry for accurate altitude. Our antennas can't see below the horizon :hoboy: . Most manufactures are now building units with an actual Barometric altimeter.

[ 06-11-2003, 10:17 PM: Message edited by: Wright Angle ]

 

[ctflyfish]

I don't think that we will ever see the system intentionally degraded again. There are thousands of ships and planes which requite it, and Loran C has been scheduled for phase out in hhe next few years. I think that the existing accuracy standard is for 95% of fixes to fall within 100 meters of actual.

 

[Miss B Haven]

Altitude is the hardest thing to get right on GPS. It's all trig and the altitude is the lowest angle, hardest to get an accurate fix. Salty Dogs don't have to worry to much about this :grin: . If you have WAAS, it's pretty dang accurate. Not sure if DGPS helps altitude or not, it was meant for Navigation at Sea, not flight (I think). WAAS is the best of all worlds without the built in error by the gov. and I have not heard anything about the gov. re-enabling SA. :cheers:

 

[Pete]

Here's the official word on SA degrading ... looks like they have other ways to deal with localized conflicts without hurting civilian use.

March 21, 2003 Interagency GPS Board Press Release.

 

[Jet Drifter]

We never had high accuracy :wink:

I started traversing timber sale units in the woods in 1988 during selective availability. Now, I use a survey grade 12 channel ProXR GPS Unit at work when I need to survey my road construction, landings and harvest units in the woods to make timber sale maps and compute acreage for logging appraisals and expand timber cruises from time to time, (good work if you can get it).

I adjust my Elevation Mask to 30 degrees, Position Dilution of Precision PDOP to 8.4 and Signal-to-Noise Ratio SNR to 4.0 and maneuver the antenna in between trees and branches to record at least 4 satellites per "hit" to obtain 3-D coverage from satellites which I can "hear clearly". Heck, I’m even wobbling trying to collect a hit on 50% slope with rain dripping down my collar, …was that a spider? I want to filter out bad data before I store it and move on. I’ve got to collect at least 30 3-D hits to make 1 raw position.

I download my roving raw data into a computer with some GPS software and differentially correct it against a GPS Base Station which collected GPS hits from the same satellites during the same time, but from a fixed location with a surveyed position, Portland State University maintains a GPS Base Station, and are online for this purpose.

Sometimes I look at the raw data of a fixed position, and yes, there is wobble, but I have a lot of trees, limbs and needles or leaves in the way, deflecting the signals. Anyway, so I compare my rover files to the Base Station, and that's what I call Differential Correction. Kind of like a science experiment between a variable and a control. The Base Station will also indicate a wobble, but with a clear view of the constellation of satellites. The Differential Correction process computes out the differences between the two wobbles to identify correct data.

From the book, several factors affect accuracy even with Selective Availability turned off. Briefly, occupation time at a point, number of Satellites visible, satellite elevations, Signal-to-Noise Ratio, PDOP, Multipath or signals bouncing off of stuff, and the distance between base station and the rover receiver.

My corrected data for my last survey work in timber last March had errors. A brass cap section corner way upslope of the North Fork Reservoir with 1/2 view of blue sky with about 50 hits from at least 4 satellites had a standard deviation of 13 feet, 95% Precision Horizontal 3 feet and vertical 6 feet. The next day, mapping a spot on the Hillockburn Road in wet timber with sheets of hail with about 60 hits corrected to a standard deviation of 7 feet, 95% Precision horizontal 6 feet and vertical under 13 feet. This is using very expensive equipment, and logging about 30 to 60 hits to establish 1 raw point, downloading the raw data, downloading the base station data, and subtracting out the differences between wobbles with a high end computer, to establish just one point on a map with some reasonable confidence.

I’ve compared my GPS survey work to information collected by “Recreation Grade GPS Units” in the woods without Differentially Correction capability, and the differences will be at best, 200 horizontal feet apart 95% of the time. I don’t worry about vertical GPS differences on a 2 dimensional map in the woods, its all relative. No doubt accuracy is best out in the clear to pick-up the most number of satellites, like out on the water.

By the way, my first personal “Recreational Grade” handheld GPS unit should arrive in the mailbox this weekend :dance:

[ 06-13-2003, 12:12 AM: Message edited by: Jet Drifter ]

 

[Miss B Haven]

Jet Drifter - ya, ok, but where's the feesh? :grin: :wink:

 

[OneLastCast]

Pretty good explanation JD.

Another reason for poor vertical numbers is that the gps is not really measuring elevation. It is measuring a mathematical approximation of the earths surface for that Lat and Long. This is often called the geoid model. This model is updated and “improved” every two to four years to better match the earths shape (more of an ellipse than a sphere) and the variations of gravitational pull. That is one reason you see so many horizontal coordinate datums offered in the gps unit. Datums that are a fairly good approximation for one area will be completely out of wack for another.

Survey grade gps (or differential gps, utilizing two or more receivers with at least one receiver on known coordinates) requires accurate differential leveling to exterior control points to verify and establish a vertical control grid that the geoid model can then be adjusted to. Then while working within the control network the elevations should be accurate to the datum adjusted to. For survey accuracy on coordinates the correct elevation is required.

 

[lost_sailor]

Last week on the Deschutes I tried to see how close it (Magellan meridian handheld) would get me to the EXACT waypoint ... it was off by at least 6 feet.

Which is really close enough for my purposes -