Have the Feds reduced GPS accuracy?

[muskybuck]

Last year my GPS was able to put me right back on the same branch of a brush pile crib that I had lost lures on the year before. This year I cannot get within 15 to 20 feet of it. If I set the GPS down and leave my tracking feature going it will will lay down tracks like I am walking around with it. There is no problem with my unit as I have used others to test it out and they all do it. Back when these were first available the Gov had what they called a dither on the satelites to prevent them from being so accurate. They dropped that after awhile but now with the war on terror in full swing I am thinking the GPS accuracy has been reduced. Have you people noticed this also?

[M.T. Bucket]

I'm not having any problems at all. Not to be the "is it out of gas" guy, but are you still using the same datum? If you've switched from say WGS 84 to NAD 83 you might notice a little variance. Also, it sounds like the coordinates you are using are quite old. I would imagine that the coordinate quality "deteriorates a little over time due to the movement of the earth, satellites, plate tectonics, whatever...but I might be COMPLETELY off base on that.

What unit are you using? Did you turn off the WAAS system, knowingly or unknowingly? Did you set it in power save mode?

These are just some ideas, but there could be dozens of reasons for the reduction in accuracy.

[muskybuck]

It is the same Rino 120 I used last year with the same coordinates and we tested out other units. These also wandered all about. You can turn go to a spot, start over with a new goto and it will tell you a spot 20 feet away. I am sure that there is a new and stronger dither in place. Not that theres anything wrong with that.

[LMITOUT]

Turn "pinning" on, if you have that option with your GPS. That should help eliminate the wandering you see.

[Crappieman]

I never heard of pinning. What is it? I have a Garmin 76 map. I would if it has it?

Ken

[fishermn]

Selective Availability was turned off as of May 1, 2000. I did some searching and cant find any evidence of it going back on.

However, I have noticed the same thing a couple times in the last year.

[Ruegs23]

I am on Camp Ripley about twice a week and I asked the guys up there this same question. I had the same problem awhile back. Their answer was that the govt can throw off the consumer gps units, but this is only done when the terrorist level is Red. Which is the highest level of awareness.

[LMITOUT]

POSITION PINNING

When using a GPS receiver at extremely low speeds, it can have trouble

determining your course over ground, or direction you’re travelling. This is

due in large part to SA, or selective availability. SA is small inaccuracies

purposefully put into the GPS satellite’s signal by the government. This

cause wide variations in the track display and other navigation displays

when using the unit at slow speeds.

If you’re using this receiver without DGPS and stop, the position pinning

feature locks the present position indicator on the plotter until you’ve moved

a short distance or exceed a very slow speed. This prevents the “wandering”

plot trail seen when you’re stopped with position pinning turned off.

This also affects the navigational displays.

The easiest way to see the effects of S/A is to stand still with the GPS

receiver turned on and watch your plot trail with position pinning turned

off. You’ll see the present position change, speed increase and decrease,

and a random plot trail on the plotter’s screen.

[Juan Grande]

You also have to consider that the hand held GPS units have varying accuracies. You could easily be off 5-10 feet at any one time, depending on the quality of your GPS unit.

[McGurk]

I have a garmin etrex legend, and the roads and rec pack and am very pleased with it. The way most of these units work means that standing still will be inaccurate. Moving allows the unit to gather more info about your position, and then it calculates the bearing and speed using the distance, both north/south and east/west, changed over the time recorded. As the unit stops, it only has the info arriving to that point, and a rough guess at where you are at (15-50 feet).

As the satelites move and the planet moves there are large changes in the distance and the random codes sent to the unit, causing you to see movement even as you are standing still. The more satelites' signals being received, the more accurate and therefore smaller change in you will see.

As you travel to an entered or stored waypoint, you can easily get a steady bearing when you are hundreds of feet away. It doesn't seem to change even though the data is indeed changing but not drastically enough to see. As you get closer those changes really start to show. Just as your compass using the satelites for reference doesn't work at all while standing still. Check it sometime against a magnetic compass.

Trying to confuse less, McGurk

[beaverlakeman]

I just bought the explorist 200 about 2 months ago. I have the same problem with mine except mine is way worse. On the box and the guy who sold it to me say it is accurate upto 3 meters. Since i have had it, I have been from fargo to duluth, to two harbors, northome, ect.. I have yet to see the accuracy go lower than 47 feet. On my unit it shows what the accuracy is at that time without marking a spot. It changes every blink, but never has it gone lower than 47 feet. Deer season i was walking to my stand through the woods. I knew where it was but wanted to test out my unit. I got to within 10 yards of the tree and my unit said the accuracy was 250 feet.. Something is wrong with the sats or something. brand new unit shouldnt be like this.

[McGurk]

I can only state what I've read and what I've seen myself on my legend, but I routinely get sub-35' accuracy with relatively open sky. On the ice, unless I'm on the southern shore with tree cover, I usually get 15-17 feet. Tree cover and a obstructed southern view will significantly cut accuracy. I'd try getting on a lake while we've got ice and try it, check your accuracy and the number of located and locked satelites. Without 4 strong locked satelites you won't have much for accuracy. Good Luck! McGurk

[LMITOUT]

The only way that I know of that you can achieve a better accuracy (<30 ft) is with a WAAS GPS receiver. Most of the new ones have it and you must have it enabled on the handset. My Globalmap 100 was pre-WAAS and shows an error of 60' consistantly.

To achieve the best waypoint accuracy, leave the unit sit still in the desired spot as long as possible and then punch in the waypoint. I know that isn't always possible, but thats something I read back when I first got into GPS's.

[muskybuck]

Like I said in my first post was last year this unit would put me exactly on target. I mean the ice hole was in exactly the same spot. To get this accuracy I would have to set the unit down on a pail and it would zero right in. It would tell me say 5 feet NE. I would move it until it said zero. Then it would never waver. I did not do this just one time. I think the Feds have secretly screwed up the accuracy. Its OK with me as I can still find what I need but its more like the old days using the auger to fine tune.

[M.T. Bucket]

In Minnesota there is a daily window of reduced accuracy between ~11:30 am and 2:30 pm because of the positioning of satellites.

[computerboy]

I’m new to the boards and figured I’d dive right in on my first post...

Assuming the government is not altering the satellite signals, you may actually have a defective unit. To verify, you should test the baseline accuracy of your GPS receiver by doing the following:

Upload your waypoints, routes, etc to your computer and save them

If you aren't running the current software for your GPS, download the latest version from the manufacturer and load it on to your GPS

Clear all the memory on your GPS (waypoints, routes, settings, etc)

Re-initialize your GPS

Conduct a side-by-side accuracy test with a buddy who has a comparable unit (i.e. WAAS vs. non-WAAS) by marking 3 test locations in the shape of a triangle appx 500 feet between each spot

When you conduct your accuracy test, make sure you have locked on to at least 4 satellites. If you have a WAAS enabled unit, make sure your location has also been corrected with WAAS. Mark your test spots and attempt to return to them. If you are having problems returning to the test spots and you're buddy isn't, you most likely have a defective unit. If you both experience the same problems, it would be reasonable to conclude that the government may be altering the signals. I wouldn’t be all that surprised to see this happen during major military offensives in Iraq and Afghanistan. After all, GPS was developed for military applications and is controlled by the government.

If your GPS unit does pass the test, download your old waypoints from your computer and attempt to return to that brush pile crib. If your GPS doesn't put you within the range that it's capable of (10 feet for WAAS, 50 feet for non-WAAS), locate the crib and mark the "new" location. You may need to re-mark all of your old waypoints if you find the same discrepancy.

Hope that helps.

[Juan Grande]

Musky Buck,

I'm not a GPS expert by any means, but I have spent a lot of hours at my job doing natural resource inventories with a higher end Trimble GPS unit.

I've never owned a hand held, but I'm assuming they work the same way. I used a back pack unit that had sub-meter accuracy and cost about 10K. Before I'd head out each day I'd put the backpack on and go outside to collect the "almanac." I could then come back inside and download the daily almanac to the GPS software on my computer. I could print out a report showing when the best times to go out would be based on the number of satellites available and their distribution in the sky (PDOP error).

When collecting point data, I'd often stand in one spot for 20-30 seconds. The unit would actually beep after 15 seconds to indicate I was there long enough for it to register the point. Obviously, the more satellites you're reading, the better your accuracy. Collecting in an open area such as a lake where you have full access to all horizons is going to yield better results than collecting in a forest, even with leaves off.

The unit I was using wouldn't collect data if it was reading less than 4 satellites and if the PDOP was higher than 7 (I think). I'm not sure how sophisticated the hand held units are, but if they collect data regardless of PDOP and # of satellites then that could cause your readings to be way off.

There's no consistent time of day you should avoid in MN. It changes daily as there are 24 satellites orbiting so you'll be picking up different combinations of them at any one time.

Sorry to get so long winded.

[Uncle Grump]

MB

I have a Garmin Etrex (yellow one). Have had it for about 3 yrs. Most often (clear signal), I get 19ft accuracy. I've not noticed it to "wander" as you say (watching the track log).

UG

[jas]

Try this. Change the units to a format with high precision (I use decimal degrees), then go to your waypoint list and write the coordinates down on a piece of paper. These coordinates are for a very small spot. Go to the realtime screen (where your current position is shown) and navigate yourself to your spot. this is far more accurate than using the go to command because the minimum tolerance is not as accurate in "go to". This method has never failed me, and it is impossible to be as accurate doing it the other way.

[Ely Lake Expert]

This is kinda off this topic, but you guys might still know the answer. I am also on an atv site all the time and people say if you truely want to know the top speed of your atv, use a gps. But someone also said that they are not entirely accurate when it comes to speed. Do you guys how accurate a gps is when it comes to speed. Is it right on or a few mph off. Thanks for the help if anyone knows.

Ryan

[FreeFlySpy]

Does accuracy very if the unit is in the battery save mode?

[M.T. Bucket]

Yes, because WAAS is turned off.

BTW, I've used GPS units with and w/o WAAS, and here in MN, I don't think it's that great a feature. I rarely turn it on. It just uses more batteries, and it's virtually useless if there is any canopy around at all.

On the coasts it would be much more useful.

[M.T. Bucket]

I've checked several vehicles' speeds using a Garmin III+ and a Garmin 72 and have found them to be extremely accurate.

[muskybuck]

Thank you everyone. I still think theres something going on though.

[mudman]

Quote:

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The only way that I know of that you can achieve a better accuracy (<30 ft) is with a WAAS GPS receiver. Most of the new ones have it and you must have it enabled on the handset. My Globalmap 100 was pre-WAAS and shows an error of 60' consistantly.

To achieve the best waypoint accuracy, leave the unit sit still in the desired spot as long as possible and then punch in the waypoint. I know that isn't always possible, but thats something I read back when I first got into GPS's.

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I too have the global map 100 (1999-2000)and is there a way to update it? I love the mapping feature and I am too cheap to spend 200-300 for a new one