With SA turned off now, why still so inaccurate?

September 20, 2003

I could understand inaccuracies in the GPS system back when SA was active, but now that it's turned off, why is the system still so inaccurate? 3 meters is acceptable for caching I guess but I don't understand with the technology today why units are still as inaccurate as they are. Even the 3 meter accuracy is only using a unit that works with WAAS. I commonly find my receiver grabbing singals from up to 8 different satellites at once. With all those bird in the air and no SA ruining our picnics, shouldn't we be able to get better accuracy? Is it the satellites? the receivers & softwares? or maybe just me? (I'm a little bit of a perfectionist sometimes)

I've heard there are "commercial units" available that are accurate to the centemeter. What that all about? If they can get CM accuracy, why can't I?

2.7k · September 20, 2003

Well, if the coords were originally calculated and posted by a surveyer, the accuracy might improve.

We generally use "consumer grade" instruments. (Because most cachers are not filthy rich)

We have to double the error because both hider and seeker use cheap gpsr's.

To place yourself, repeatably, within 50ft anywhere on the face of the earth is phenominal for a hundred bucks!

The system used to brag "6 inches" accuracy for military uses. I wonder if that is an exaggeration.

Doesn't really matter.

"Almost don't count except in horseshoes and hand grenades."

Military use is "hand grenades".

"Freedom is a two-way street."

GDAE, Dave

September 20, 2003

I wonder how many cachers don't realize they have to take multiple readings when placing a cache? I take 6 to 8 readings by walking towards the cache from different directions, then average the #s. The best accuracy I've had on my eTrex is 16 feet, but it's usually 25 to 30 feet, which is AMAZING accuracy considering how high the satellites are. The Earth is about 8000 miles in diameter. 20 feet accuracy is pretty stunning.

September 20, 2003

So i am assuming, antenna issues aside, it has to do with the processor speed of the individual GPS unit; calculating speed if GPS' don't have CPUs. ??

September 20, 2003

quote:
Originally posted by jaedend:
So i am assuming, antenna issues aside, it has to do with the processor speed of the individual GPS unit; calculating speed if GPS' don't have CPUs. ??

Basically, Yes. GPSr's do have computer circuitry to process the signals from it's multiple receivers, and the electronics vary in different brands.

1.3k · September 20, 2003

I normally get accuracy of 3 or 4 meters (bah - 12 to 15 feet) with WAAS off. I've noticed no difference with WAAS on and I've seen quite a few people mention the same thing.

I make sure my GPS has an accuracy of 15 and no worse than 20 when I mark my waypoint. The only time my coordinates have been 'off' was when my accuracy was around 50 feet due to an EXTREMELY thick canopy. You don't HAVE to take several readings to get an accurate waypoint. I'm not saying it hurts (as long as you're not averaging BAD data).

If you need more accuracy than a consumer priced GPSr provides, you probably should contact a professional surveyor. Out of curiousity, why would you need accuracy of 6 inches?

You may be a perfectionist, but Earth is quite large - 20 feet is pretty d@mn accurate if you ask me

sd

southdeltan

"Man can counterfeit everything except silence". - William Faulkner

September 20, 2003

hey, i learned that it was due to the Department of Home defense. the military will make satelites off for certain times for an example the iraqi war, they made them off so many meters becuase of targets.... but now they should be back to normal....

September 20, 2003

First off, the SA can be regionally turned on/off so the war in Iraq would not affect us here. They turned it on in the theater of war, namely the Middle East. (I was in the Navy - Never Again Volunteer Yourself )

The EPE your reading on the GPS is not accurate. Just because your not seeing an improvement on EPE readings with WAAS on or off, doesn't mean your not getting a more accurate reading. Comon, if your GPSr knew it was "12 ft off" then WHY DOES IT TELL YOU THE WRONG COORDINATES? Things to make you go hmmmm...

And what is "consumer grade"? Doesn't it all crunch the same signal? The processor speed and features are pretty much the difference.

In the Navy, the GPSr I used were the same accuracy, just different features and much more rugged. It doesn't use a "Professional Grade" number crunching system Why the missles are so accurate is because it gets in the general area by GPS signals, then it uses a camera and terrain recognition software to put it on the target.

There is no mapping software on the GPSr's I used in the Navy as well. This is to force the user to use a map with the coordinates for position location. GPS information must always be backed up with a map (not software, the paper kind) in the Navy because of accuracy concerns.

So just because a $1000 GPSr says it has better EPE than the eTrex, doesn't mean jack. EPE is misunderstood by most.

[This message was edited by FlashMaster on September 20, 2003 at 10:31 PM.]

[This message was edited by FlashMaster on September 20, 2003 at 10:41 PM.]

September 20, 2003

Well, I don't NEED it that accurate. I'd just LIKE it that accurate Mostly I'm just curious as to what the differences are in consumer GPSr's and commercial ones (besides price). Sounds like the electronics and signal processing speeds.

Also, I'm using a Magellan 315 right now. Does anyone know of a way to turn averaging off? I've heard it's not useful anymore because of SA being deactivated. I figure turning averaging off will free up the processor somewhat, speeding up the positioning calculations, and maybe give me some inmprovement.

1.4k · September 20, 2003

hey, i learned that it was due to the Department of Home defense.

You learned this, huh?

Where did you learn this?

I hate to break it to you, but our accuracy has not been degraded... by the way, what is the Department of Home Defense... it that where you guard your house against intruders? Did you mean the Department of Homeland Security? Here's the link:

http://www.dhs.gov

I can't seem to find anything on the site that says they have the ability to affect the accuracy of the GPS system....

Jeff

http://www.StarsFellOnAlabama.com

http://www.NotAChance.com

If you hide it, they will come....

1.1k · September 21, 2003

So many misconceptions with accuracy but understanding some of the basics then one wouldn't be questioning 3 metres but I question 3 metres as one certainly shouldn't expect that all the time either. There's no such thing as a absolute (perfect) navigation system but all things considered even 10 metres should be considered a blessing considered the mechanics of the system.

As for CM accuracy it's not so much the receiver (but that certainly does help) but the way the equipment is used. Sure there's receivers that are capable of CM accuracy but this type of accuracy can be limited by distance and/or time. A so called $20,000 survey grade receiver is no more accurate when used in the same manner as a $200 handheld. It's more how $20,000 receivers are used that matters.

Comparative military receivers do not do any better than many of the receivers used here and the Iraq war had no affect on normal GPS operations except probably be a little more stable as maintainence was kept to a minimum.

But give it a few more years and dual freq recreational type handhelds will have an accuracy of around 1.5m, just that we have to wait for the civil dual freq signals to get into space.

Cheers, Kerry.

I never get lost everybody keeps telling me where to go

38.2k · September 21, 2003

quote:
I wonder how many cachers don't realize they have to take multiple readings when placing a cache? I take 6 to 8 readings by walking towards the cache from different directions, then average the #s.

Not to stray too far off the topic, but I find doing this to be a complete waste of time and it doesn't provide better coordinates...and sometimes it provides worse.

"You can't make a man by standing a sheep on his hind legs. But by standing a flock of sheep in that position, you can make a crowd of men" - Max Beerbohm

3.8k · September 21, 2003

This is the info you were looking for with regards to satellites being de-tuned during war, for testing, etc.

http://www.schriever.af.mil/GpsSupportCenter/index.htm

~robert

www.CacheGear.com

September 21, 2003

At its simplest, if the 50th percentile was X, half of the recorded values were greater than X and the other half were less than X. The 95th percentile is similar except that, if the 95th percentile was Y, only 5% of the recorded values were greater than Y

........uhhhhhhhhhh.................

1.3k · September 21, 2003

quote:
Originally posted by FlashMaster:
The EPE your reading on the GPS is not accurate. Just because your not seeing an improvement on EPE readings with WAAS on or off, doesn't mean your not getting a more accurate reading. Comon, if your GPSr knew it was "12 ft off" then WHY DOES IT TELL YOU THE WRONG COORDINATES? Things to make you go hmmmm...

Let me clarify: I have noticed NO difference in how close I am to a cache with it on or off. Yes, I get the same 'accuracy reading' with WAAS on or off (actually it's often better with it off). That's not my point - I can tell no REAL WORLD difference with them on or off. I'm generally 20-40 feet off regardless.

I notice no difference with WAAS on or off. If I could only find a way to turn canopy on/off. I'll just have to wait for cooler weather

southdeltan

"Man can counterfeit everything except silence". - William Faulkner

September 21, 2003

southdeltan,

Thanks for the clarification. I haven't noticed any real world differences either. My buddy I go caching with has an eTrex Camo w/o WAAS. I have WAAS on my vista, it's a crapshoot who comes closer to the cache.

October 8, 2003

I generally have accuracy of about 3-5 feet. I have an option of using a separate DGPS antenna to receive Coast Guard beacon signals, but with the kind of accuracy I have now (even with very heavy canopy) I don't feel the extra bucks is worth it.

The reason why WAAS doesn't always work well is because the satellites that transmit WAAS signals are only the satellites that are in geo-syncronous orbit around the equator. WAAS was designed for airplane use, not for ground use.

1.3k · October 8, 2003

I thought that a GPSr receiving from 12 satellites still only used 4 to calculate position... with some clever calculations to calculate the best 4.

As for averaging when placing a cache... what's the point? Sure if you average enough you may get the exact co-ordinates (even better than the best accuracy your reciever can manage... but how useful is that to the finder? They don't know the point to keep walking too and from, and they probably have 15-20ft of accuracy at best, so the average doesn;t help them. We tend to do the walking back and forth taking readings, then walk away, turn off the GPSr, restart it, walk back and then use the closest waypoint for the cache page. This is as good as averaging, but needs no calculations... and we've had no complaints of co-ords being out... so far.

Cheers,

Emily & Neil

The welfare of the forest and it's wildlife must come before the game.

5.3k · October 8, 2003

I don't think I've ever seen a thread with as much bad information posted as this one. Kerry (who tends to know what he's talking about) aside, I wonder what it is that compels people who don't know the answer to reply anyway?

Short answer: the accuracy of your receiver is dominated by physics, including ephemeris uncertainty and the uncertainties in the propagation delay due to the ionosphere and troposphere. Long answer: I recommend reading the articles at Joe Mehaffy's excellent website gpsinformation.net for more detailed answers.

One especially useful page explaining the sources of some errors is here.

1.6k · October 8, 2003

You mean it isn't the aluminum foil hat I put on my GPS (that used to be on my head)??

October 8, 2003

quote:
Originally posted by ChurchCampDave:
We have to double the error because both hider and seeker use cheap gpsr's.

No. Each GPS unit has a random position error due to the effects listed in fizzymagic's link above. The placer's error was 5-10m and your error is 5-10m as well, they may add constructively (10-20m error, yeah right) or destructively (0m error, yeah right) or anywhere in between. Take a look at my avatar, it's a global cluster, but it could also be a plot of GPS positions taken at a fixed location. If you add two of those together, so to speak, you get another one that looks just it.

... and thank you fizzymagic for clearing up the haze.

2.5k · October 8, 2003

quote:
Originally posted by fizzymagic:
I don't think I've ever seen a thread with as much bad information posted as this one.

When I can't find the cache, I usually just give my de-garminizer a spin. I then add that number to the coordinates in my Magellan, and it takes me right to the cache.

October 8, 2003

From WWW.Howstuffworks.com

How A GPS Works.

quote:
At a particular time (let's say midnight), the satellite begins transmitting a long, digital pattern called a pseudo-random code. The receiver begins running the same digital pattern also exactly at midnight. When the satellite's signal reaches the receiver, its transmission of the pattern will lag a bit behind the receiver's playing of the pattern.

The length of the delay is equal to the signal's travel time. The receiver multiplies this time by the speed of light to determine how far the signal traveled. Assuming the signal traveled in a straight line, this is the distance from receiver to satellite.

In order to make this measurement, the receiver and satellite both need clocks that can be synchronized down to the nanosecond. To make a satellite positioning system using only synchronized clocks, you would need to have atomic clocks not only on all the satellites, but also in the receiver itself. But atomic clocks cost somewhere between $50,000 and $100,000, which makes them a just a bit too expensive for everyday consumer use.

The Global Positioning System has a clever, effective solution to this problem. Every satellite contains an expensive atomic clock, but the receiver itself uses an ordinary quartz clock, which it constantly resets. In a nutshell, the receiver looks at incoming signals from four or more satellites and gauges its own inaccuracy.

When you measure the distance to four located satellites, you can draw four spheres that all intersect at one point. Three spheres will intersect even if your numbers are way off, but four spheres will not intersect at one point if you've measured incorrectly. Since the receiver makes all its distance measurements using its own built-in clock, the distances will all be proportionally incorrect.

The receiver can easily calculate the necessary adjustment that will cause the four spheres to intersect at one point. Based on this, it resets its clock to be in sync with the satellite's atomic clock. The receiver does this constantly whenever it's on, which means it is nearly as accurate as the expensive atomic clocks in the satellites.

In order for the distance information to be of any use, the receiver also has to know where the satellites actually are. This isn't particularly difficult because the satellites travel in very high and predictable orbits. The GPS receiver simply stores an almanac that tells it where every satellite should be at any given time. Things like the pull of the moon and the sun do change the satellites' orbits very slightly, but the Department of Defense constantly monitors their exact positions and transmits any adjustments to all GPS receivers as part of the satellites' signals.

This system works pretty well, but inaccuracies do pop up. For one thing, this method assumes the radio signals will make their way through the atmosphere at a consistent speed (the speed of light). In fact, the Earth's atmosphere slows the electromagnetic energy down somewhat, particularly as it goes through the ionosphere and troposphere. The delay varies depending on where you are on Earth, which means it's difficult to accurately factor this into the distance calculations. Problems can also occur when radio signals bounce off large objects, such as skyscrapers, giving a receiver the impression that a satellite is farther away than it actually is. On top of all that, satellites sometimes just send out bad almanac data, misreporting their own position

October 8, 2003

quote:
With all those bird in the air and no SA ruining our picnics, shouldn't we be able to get better accuracy? Is it the satellites? the receivers & softwares? or maybe just me? (I'm a little bit of a perfectionist sometimes)

I've heard there are "commercial units" available that are accurate to the centemeter. What that all about? If they can get CM accuracy, why can't I?

You are dealing with all kinds of issues here, from both physics and mathematics. Godel's Incompleteness Theorem comes to mind. You are dealing with very cheap electronics (they have to be able to make a profit at $100 retail), software errors, mathematical errors - remember, the software guys usually have a budget and a deadline - and they have to think about the limitations of their hardware when they are doing embedded software, so sometimes you have to round a number when you don't want to, or kludge in something that meets the minimum requirements as opposed to being perfect (and offer firmware upgrades later).

Put that fweakin' sandwich down!

9.5k · October 8, 2003

To help understand even the surveyor's GPS reading requirements which I discovered upon researching a Center of Population (CoP) benchmark...

When the benchmarks are put in place, particularly in the CoP's case. the surveyors don't go out and take a 5 minute reading and then walk away. With the CoP benchmark placement, the requirement called for 3 separate 5.5 hour readings. Two of those could be done on the same day, end-to-end, that is 11 hours worth of continuous readings. The third reading was required to be done on a separate day, and, with this particular setting, also needed to be validated by another benchmark which could not be more than 50KM away in distance. This is all done with industrial grade GPS receiving and related equipment.

So when you go out and can't get a 0/0 reading on your commercial grade GPS, just remember if you want absolute, you'll need to spend more than just a few minutes time and a heckuva lot more equipment to work with.

Cheers!

TL

5.3k · October 8, 2003

I saw an interesting quote in this article that said "receiving the [GPS] signal is somewhat equivalent to seeing a 100-watt lightbulb 12,000 miles out in space"

9.5k · October 8, 2003

Then it has improved

About a year ago I had read 60 watts from 20,000 miles. LOL

Cheers!

TL

October 8, 2003

That just cracks me up. "Industrial Grade" gps just means it's just built to take years of abuse, tougher casing etc. What algorithm do they change in calculating position?

I had a Panasonic Duramax phone that was "Industrial Grade". It's the Industrial Grade version of Panasonic Promax. It may have different features but it still makes phone calls the same way.

The GPSrs I used in the Military was much more rugged, different software but the accuracy was the same as my Etrex.

Sheesh

9.5k · October 8, 2003

Yah I hear you. Here's how I see it.

Commercial Grade=Cheap

Industrial Grade=Very Expensive

Military Grade=Hit Over The Back Of the Head Are You Crazy Prohibitively Expensive

Each has it's accuracy augmented by other means, be it by dead reckoning, compass and map, landmarks, benchmarks or AWACS.

Cheers!

TL

October 8, 2003

Ha, yeah.

You know, even if it is more accurate I'm not spending $10,000 on a rediculously huge "Industrial Grade" GPS and stand at my cache for 5.5 hours to get 10ft better accuracy. Just to have the Geocacher come along with a "Consumer Grade" GPSr which puts him/her 10 ft away anyway.....Too funny!

October 8, 2003

A couple of differences between the consumer grade gear we use for geo-caching and a survey grade unit are not simply skin deep. The SG unit actually has some pretty sophisticated on-board computqtional power and collects measurements beyond the CG L1. Actually each freq (L1 and L2) breakdown into code and carrier. Your typical Garmin only utilizes L1 code and then I assume they have a series of computations that help remove outliers and generate a reasonable estimated accuracy. They SG units also have extremely advanced multi-path rejection correlators and a bunch of super geek stuff that actually does make them different. Also to get the accuarcies down to the sub-cm even the mm range the data is differentially processed in a PC. I hope this helps.

GO Sox!

p

9.5k · October 8, 2003

Thanks vtpaddler. That does help.

Cheers!

TL

October 8, 2003

DOh, forgot about the ol' "Survey Grade". I wonder how my "Caching Grade" Etrex and "Driving Grade" iQue stacks up. Is there a definition for all these grades like for waterproofing? (IPX-0, IPX-1, IPX-2.. etc). Or is it marketing lingo by companies?

Is mm accuracy even possible considering all the variable factors involved? Sat Drift, Atmosphere conditions, etc.? A mm is pretty darn tiny. I though cm accuracy was pushing it. That's why even a 2 million dollar Tomahawk missle gets to the general area using GPS and switches to terrain recognition software for pinpoint accuracy. If mm accuracy is possible, I think it'd be GPS coordinates to all the way to the target. I'm not a Scientist in this area so I wouldn't know. Just thinking from common sense so I'm probably wrong.

[This message was edited by FlashMaster on October 08, 2003 at 05:03 PM.]

October 8, 2003

All of this technical stuff is going way over my head. My system to get with 1 inch of a cache is easy. I follow my cheap GPSr until the distance to the cache hits zero, and then tell my ten year old daughter to "find it, girl!"

It never fails.

October 8, 2003

pkgef,

I tried that with my dog and it didn't work! You've got it made, a "Survey Grade" cache locator that doesn't require 5.5 hours!

9.5k · October 8, 2003

quote:
Originally posted by FlashMaster:
DOh, forgot about the ol' "Survey Grade". I wonder how my "Caching Grade" Etrex and "Driving Grade" iQue stacks up. Is there a definition for all these grades like for waterproofing? (IPX-0, IPX-1, IPX-2.. etc). Or is it marketing lingo by companies?

Is mm accuracy even possible considering all the variable factors involved? Sat Drift, Atmosphere conditions, etc.? A mm is pretty darn tiny. I though cm accuracy was pushing it. That's why even a 2 million dollar Tomahawk missle gets to the general area using GPS and switches to terrain recognition software for pinpoint accuracy. If mm accuracy is possible, I think it'd be GPS coordinates to all the way to the target. I'm not a Scientist in this area so I wouldn't know. Just thinking from common sense so I'm probably wrong.

[This message was edited by FlashMaster on October 08, 2003 at 05:03 PM.]

You can go here for the IEC 529 standard

Cheers!

TL

October 8, 2003

quote:
Originally posted by FlashMaster:
DOh, forgot about the ol' "Survey Grade". I wonder how my "Caching Grade" Etrex and "Driving Grade" iQue stacks up. Is there a definition for all these grades like for waterproofing? (IPX-0, IPX-1, IPX-2.. etc). Or is it marketing lingo by companies?

The mm range is actually not that hard just a bunch of hoops to jump through. We regularly measured basleines (all gps surveying is differential) that exceeded 200km and resolved the vector to 1ppm. After you do the math it is pretty tight. If you are into bm hunting you'll once in awhile run into a stainless steel rod driven to refusal in a greased sleeve and that is a very high accuracy mark.

The yankees are the spawn of the satan. GO Sox.

p

Is mm accuracy even possible considering all the variable factors involved? Sat Drift, Atmosphere conditions, etc.? A mm is pretty darn tiny. I though cm accuracy was pushing it. That's why even a 2 million dollar Tomahawk missle gets to the general area using GPS and switches to terrain recognition software for pinpoint accuracy. If mm accuracy is possible, I think it'd be GPS coordinates to all the way to the target. I'm not a Scientist in this area so I wouldn't know. Just thinking from common sense so I'm probably wrong.

[This message was edited by FlashMaster on October 08, 2003 at 05:03 PM.]

October 8, 2003

I don't doubt it, I was just being funny because we are talking about 1 mm accuracy that we'll never see with our sub $1000 units for geocaching.

1.1k · October 8, 2003

So many differences but just where and how does one explain somethings that some don't/ wouldn't/couldn't understand.

From mm's to Tomahawks to FA18's landing on a carrier under full GPS control it's all explainable but somehow there will always be the distractors.

Really mm static accuracy might fade into old hat stuff (and that's been possible for years) if one considers that a piece of metal (FA18) closing on a carrier's deck at X hundred odd knots, with a carrier doing X knots, with an angled deck along with the pitch, yaw and roll of both the carrier and the 18 and a pilot sitting on his hands while a GPS assisted navigation system targets a hook through a 3 foot virtual box 14 feet off the deck to hit a 2" wire. Now that's variable.

As for Tomahawks being dead 1mm from the target point is still the same dead as being 30 metres from the target point. The results much the same, dead, but for many reasons Tomahawks or any of the JDAM's use a variety of navigation systems and simply don't rely on any one system.

Besides the biggest problem with hitting something is knowing where it actually is in the first place. If one doesn't know exactly where something is then all the accuracy in the world will never hit it, without other guidance.

As for so called "Survey Grade" stuff it's not so much the hardware (which is obviously different in several ways) but the way the stuff is used. The following is a SPS comparison between a $25,000 survey receiver and a $500 (lets call it) "Caching grade" receiver. The difference is really bugga all but then there's more to things than just SPS accuracy, with any "grade".

everybody keeps telling me where to go

[This message was edited by Kerry on October 08, 2003 at 08:24 PM.]

9.5k · October 8, 2003

Thank you, Kerry, for your patient explanation.

Cheers!

TL

October 8, 2003

looks like lowrance = red.....garmin= blue......maybe jus me.......Gg

October 9, 2003

Commercial vs consumer grade...it couldn't have anything to do with antenna size and/or highth, could it?

I mean - those things ain't stickin' way above the backpack (or tripod) for nothin'.

"...clear as mud?"

1.1k · October 9, 2003

quote:
Originally posted by StarshipTrooper:
Commercial vs consumer grade...it couldn't have anything to do with antenna size and/or highth, could it?

I mean - those things ain't stickin' way above the backpack (or tripod) for nothin'.

Not so much the size but basically more the design and purpose. Many high order receivers are dual frequency and that requires an antenna capable of receiving the 2 freq plus some other refinements.

Height has all to do with a clear view of the sky at all times without obstructions and this principle apllies equally to ALL GPS receivers/antenna's regardless of the purpose.

When there's no obstructions it would basically work just as well flat on the ground much like this

everybody keeps telling me where to go

9.4k · October 9, 2003

quote:
Originally posted by mikeh420:
The Earth is about 8000 miles in diameter. 20 feet accuracy is pretty stunning.

Especially when you consider the satellites are 11,500 miles up!

Took sun from sky, left world in eternal darkness

1.5k · October 9, 2003

If it wasn't for that blasted 20', I wouldn't keep trying to walk into the wrong house when I get home from work each night.

With SA turned off now, why still so inaccurate?

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