TeamRJJO
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Posts posted by TeamRJJO
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friendly bump
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http://www.geocaching.com/seek/cache_detai...76-d1fd50fc1e15
We hope to meet some new cachers as we get our newly formed NoVaGo off the ground!
See you there!
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NoVaGo is the new Northern Virginia Geocaching Organization and we are still in the organizing phase. We have a Yahoo discussion group that you can join and post the same request there
Here is a link to our Fall event. We would love to have you join us!
http://www.geocaching.com/seek/cache_detai...bf-215578f4b556
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We are definitely in for at least 5 coins.
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Great coin! We would like to be put on the presale list for two.
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We would be happy to sponsor a cache near Manassas Battlefield. I will send you a separate email.
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Would you go and see a replica of "The Last Supper" or other works of art?
I think you might have just made the point....not having ever been to Italy, a replica is the only version of "The Last Supper" that I have ever seen. Not the same as the real thing, certainly. But if someone along the way had said, "sorry, no replicas allowed," then I NEVER would have seen or enjoyed the painting.
Peace,
Ron
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Just to be clear, "Virtual" TB's as well as "Virtual" coins are not allowed. "Copy" coins, while disappointing to some, are allowed.
Sorry for any confusion....I was using the terms interchangably in my above post. By "virtual" geocoin, I mean a laminated card with a picture of the coin and the code used to track it on the web site.
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I'd suggest if someone's in an area where there are plenty of coins in caches, then don't bother with copies. However, if someone's in a place where there practically aren't any coins at all for anyone to find and real coins just disappear like dew on a sunny day, it really wouldn't hurt anything to experiment a little and put out a nicely done copy of one or two of your favorite coins and just see for yourself what happens. It's true that the copies aren't nearly as good to find as the real thing but they are better than nothing to find at all. Generally the people reading and posting on this forum have become so accustomed to real coins that they forget that the majority of cachers still have never seen a real coin (or even a copy) in a cache.
I think this post sums it up nicely. I my geographic area (Northern VA), real coins disappear quickly, unfortunately. Same problem last year with many White Jeep TB's that came through the area.
I think "virtual" geocoins are a great alternative. You can find something tangible that still travels from cache to cache, with little risk of theft. So, other cachers who may never find a geocoin -- not to mention a rare geocoin -- get to log the find and yes, get a cool icon. The bonus is that if the virtual disappears, it's easy to make a new one.
With a few guidelines, virtual coins seem to me to be a great idea.
Peace,
Ron (TeamRJJO)
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not compatable with current users GPSR units!!!If you think 2' of increesed accuracy is worth it, get ready to buy another unit!
Now china can guide the nuke missles to the united states!!!
The GPS IIF satellites which start launching this year, feature similar accuracy as the Galileo satellites.
Peace,
TeamRJJO
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How is DOP (GDOP, HDOP, PDOP, TDOP, and VDOP) calculated?
Alternatively, can someone point me to a good reference: book or website?
I am looking for the equation with elements.
Twinstars,
There is an article, "GPS Error Analysis", by Brad Parkinson, which shows in detail how to calculate GDOP. The article is in the book, "Global Positioning System: Theory and Applications: Volume 1". The calculations you want are illustrated on page 475.
It's a bit cumbersome to fully reproduce here, but a simplified example is that you take the 4 satellites that give you the best geometry. Using the Azimuth and Elevation of each, you set up a 4x4 matrix as follows:
cos(E1)sin(Az1) cos(E1)cos(Az1) sin(E1) 1
cos(E2)sin(Az2) cos(E2)cos(Az2) sin(E2) 1
cos(E3)sin(Az3) cos(E3)cos(Az3) sin(E3) 1
cos(E4)sin(Az4) cos(E4)cos(Az4) sin(E4) 1
If you multiply that matrix by it's transpose, then invert that resultant matrix, you end up with the "DOP matrix," where the DOP values can be obtained from the diagonal elements:
(East DOP)^2 - - -
- (North DOP)^2 - -
- - (Vertical DOP)^2 -
- - - (Time DOP)^2
Then, GDOP would eequal the square root of the sum of those diagonal elements.
I hope this quick explanation helps a bit.
Peace,
TeamRJJO
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So, it's very possible that Galileo will turn into a subscription-only service, where you wouldn't be able to access the signal without paying a fee.
This is what I have read in some of the early story about the Galileo system, you want'a play, you got'a pay. My thoughts are that the ESA is going to wind up holding the bag on this one. I just do not see a lot of peole paying to use a system when there is one the has no user fee. The GPS system is accurate enough, Galileo will be orbiting white elephant.
You would be surprised. A good portion of the European community doesn't want to have to rely on GPS since it's a US DoD system. There are a lot more regulations in place as to how Galileo can and can't be used, which makes some -- right or wrong -- view it as a more "stable" resource than GPS.
Also, national pride is a big factor.
But the bottom line is money. The Galileo business model makes the fundamental assumption that users worldwide will all want more satellites if they can get them. And if they have to pay a modest fee, then so be it.
Peace,
Ron
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Does anyone actually know if we will need new GPS receivers to make use of the *new* civilian signal this satellite provides?
Unless your GPSr is able to "tune" to the new frequency then I'd say yes. You'll need to buy a new GPSr to take advantage of new functionabliliy associated with the new signal. The article says it is on a new frequency. I assume the new frequency is for increased accuracy (i.e. Your GPSr can compare the solutions for both frequencies and determine an average) and resistance to interference (i.e. if something is blocking or reducing the signal on one frequency your GPSr can "switch" to the other freq). The higher-power portion should be usable right away. Although I wouldn't think it would be very noticeable untill they more than just one satellite up there.
The second civil frequency allows for real-time correction of ionospheric delay. The receivers 99.9% of us are using -- since they are single-frequency -- must use pre-programed models to make these corrections. These models aren't as accurate as the dual-frequency correction method.
Everyone should check the documentation that came with their receiver, but it's highly doubtful that any of us have a receiver that will make use of this second signal at this point.
To answer another post -- it's generally 3 weeks from launch to payload turn-on, so the new satellite won't yet be making a difference in anyone's accuracy. That three weeks may be extended in this case due to expanded on-orbit testing of the new satellite features.
Peace,
TeamRJJO
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What are the big differences (if any)? Are Garmin or Magellan planning on GPSrs that will make use of both systems?
In terms of the satellite technology, the Galileo satellites are supposed to be on par with the GPS IIF satellites which will begin launching next year. They will have similar clock technology, accuracy requirements, etc.
The main difference is that Galileo is a program funded by a consortium of several governments, and is expected to ultimately be a revenue generator. (GPS doesn't have this problem as it's still a military system, even though lots of private infrastructure has popped up around it). So, it's very possible that Galileo will turn into a subscription-only service, where you wouldn't be able to access the signal without paying a fee.
Peace,
TeamRJJO
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I don't know if this is true but I read somewhere that while you are tracking 7 or 8 satellites, your GPS only uses 4 of them at one time. The 3 strongest for a lat/long fix and then the 4th strongest for the elevation. Please correct me if I am wrong. The GPS will constantly change which 4 they use based on signal strength.
Three satellites are need to generate a position, the fourth is needed to resolve receiver clock bias.
Peace,
TeamRJJO
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We burn these units in when new to calculate and store calibration constants across the entire rated temperature range of the product.
So, as I said, the accuracy of a unit is tied to internal clock, and the internal temperature sensor.
Unfortunately, what you just quoted shows that your original claim, that a GPS unit can get "out of calibration" without failing entirely, was incorrect.
If the temperature compensation tables get screwed up, your GPS is broken, not "out of calibration."
Broken, yes. But also still able to generate a navigation solution in some cases. I think that's what the original post was asking.
Peace,
TeamRJJO
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I don't think any of that's correct. As I understand the nature of the way GPSrs work, they have accurate time without regard to the accuracy of their internal oscillator (clock). After four satellites are obtained they calculate the time exceedingly accurately using the satellites themselves.
The original poster was correct. The quartz ocillator in your receiver does indeed flywheel off GPS time. If you had a bad crystal in your receiver, you would notice a large fluctuation in your calculated position.
Peace,
TeamRJJO
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Solar flares already being discussed here.
The other thread posed a slightly different question.
The answer to whether the solar flare will affect GPS satellites is: not likely. The old GPS Block I satellites, launched in the late 70s and early 80s did not have hardened components, and were far more susceptible to the space environment. None of these Block I satellites are still operational.
The satellites currently on orbit are hardened against radiation, and are therefore much less likely to experience disturbances from solar flares. If a flare did cause a flipped bit in the broadcast navigation message, the correct message will refresh from the navigation processor's protected memory within 6 seconds.
Peace,
TeamRJJO
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Good point. So you're saying that the monitoring station would regularly lose lock on all the satellites at once from scintillation during ionospheric storms? Like I said, I couldn't find any good information about that out there...
Fizzy,
It would be very sporadic. During scintillation season (~Oct to Jan for monitor stations near the equator), at local sunset, we would see signal strength on the receiver channels fluctuate wildly. Sometimes lock would drop altogether, although since the monitor station receiver channels are hooked up to an atomic frequency standard, the receiver would regain P-code lock fairly quickly. Still, the disruption would be enough that the orbit determination software would typically discard the measurements received during scintillation.
We could also check the received navigation message against the uploaded message as an integrity check. As I mentioned before, we would see massive errors in the received message, as a result of scintillaiton. This was a real pain because it could mask a "real" problem on board the satellite, i.e. a processor error or a real broadcast error caused by bit flips due to space environmental effects, etc.
Peace,
TeamRJJO
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All,
I'm sure many of you in the NoVA area have noticed, but it seems like in the last 2-3 weeks, someone is grabbing all the White Jeeps without logging them. So far, the following have gone missing:
I realize this sort of thing is going to happen, and there's not a heck of a lot to be done about it. But if you're in the area and thinking about trying to get a WJ grab or entering the contest, you may want to hook up with a friendly cacher who has one and arrange a private swap.
Peace,
TeamRJJO
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I think you may be greatly overestimating the impact of scintillation on modern, 12-channel GPS receivers.
I think I can pretty much guarantee that nothing anyone here can buy commercially stacks up against what the Air Force uses to monitor the GPS constellation....
Peace,
TeamRJJO
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But I still disagree with you that large changes in TEC only cause small changes in the ionospheric delay.
Fizzy,
Again that's not what I'm trying to say. Referencing my original post, simply stated, in practice your receiver will drop lock long before you see a dramatic increase in error as a result of an increase in scintillation caused by a solar flare. For the same reason, WAAS wont be a help because you're going to have problems locking onto the WAAS signal as well.
The math you present is 100% sound, no question. But theory and practice are two different things. A doubling of the TEC would cause massive parity errors in your received navigation message, such that your receiver would NEVER use that satellite to generate a position solution. (Just for grins, if your receiver ignored the parity errors, the position solution generated would be completely nonsensical). That assumes that you could even continously track the GPS signal through such a disturbance, which you likely couldn't.
As far as citing a source, I will have to go back to some reports that we published circa '92-'94, and that will require some digging. But we used to see this all the time when I was working in the GPS master control station. It was nearly impossible to quantify errors caused by solar flares because the receivers would all lose carrier lock when scintillation started kicking up.
Peace,
TeamRJJO
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To expand on this....any error you might see as a result of TEC content (before your receiver simply lost lock altogether) might be on the order of a couple NANOseconds more delay. That equates to a few feet more error than you might otherwise see.
Hmm. I thought you agreed with me last time. I'm sorry, but from everything I have read, you are incorrect.
Let's start with the Wikipedia entry:
The Ionosphere is one of the leading causes of GPS error. The speed of light varies due to atmospheric conditions. As a result, errors greater than 10 metres may arise. To compensate for these errors, the second frequency band L2 was provided. By comparing the phase difference between the L1 and L2 signals, the error caused by the ionosphere can be calculated and eliminated.And from here:
Other Bias Error sources;Unmodeled ionosphere delays: 10 meters. The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionized air. The transmitted model can only remove about half of the possible 70 ns of delay leaving a ten meter un-modeled residual.
And this.
Also, read this.
Garmin mentions it.
From this page:
Since SA (selective availability) is deactivated, the largest single source of error in GPS position determination is the signal delay in the ionosphere. Being able to correct these errors significantly increases the accuracy of every GPS receiver that is able to process WAAS/EGNOS data.Now it's certainly possible that everything I have read is wrong and that the ionospheric corrections are only on the order of a nanosecond or so. But in order for me to accept that, I'd need to see some evidence. And probably some explanation for the existence of dual-frequency receivers, too.
Until then, since I know that the TEC can vary by a large amount during solar storms, I will continue to claim that the ionospheric corrections from WAAS help position accuracy significantly during solar storms.
Fizzy,
I think you're confusing two different things and misreading my earlier post.
First, you are absolutely correct that iono correction is one of the biggest contributors to positional error in single-channel receivers. No argument there. And you are also correct that this error is one of the main things WAAS helps fix.
Where I'm saying you are incorrect is in what happens during periods of increased solar activity, i.e. a solar flare. The main effect this will have is an increase in ionospheric scintillation or in other words, an increase in TEC. Scintillation causes massive bit flipping in the GPS navigation message, such that your receiver will discard the satellite entirely from it's position calculation. Further, if the scintilllation is bad enough, you won't be able to track the satellite at all.
There is a small chance that if the scintillation is light enough, you might see a few extra nanoseconds of delay, but that would be about it before your receiver just stopped using the satellite all together.
Peace,
TeamRJJO
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The WAAS carrier frequency is that same as that of the GPS signal, so if the flare disrupts GPS signals (which is unlikely, by the way), WAAS will be disrupted as well.
Huh?
The reason for using WAAS during solar storms is that WAAS provides near-real-time ionospheric delay corrections. The ionosphere changes significantly during solar storms, which means that a non-WAAS GPS unit can experience significant position errors.
UPDATE: I found a really cool page that has a downloadable movie of the ionospheric Total Electron Content (TEC) during a solar storm. (The TEC is what causes GPS position errors, because the signals propagate more slowly through high-TEC regions than through low-TEC regions).
Fizzymagic,
As I recall, we had a similar discussion about 2 months ago. Here's what I posted then, and it still holds:
"What you would see during scintillation would NOT likely be navigation errors. The bit errors that scintillation causes also make the received navigation message fail the built-in parity checks. So, if your user set is designed in accordance with the GPS payload-to-user specification, your receiver wouldn't use that satellite's signal at all. In reality, your receiver would likely have a tough time maintaining signal lock on the satellites, especially those at lower elevation angles. But again, this is a very localized effect."
To expand on this....any error you might see as a result of TEC content (before your receiver simply lost lock altogether) might be on the order of a couple NANOseconds more delay. That equates to a few feet more error than you might otherwise see.
Peace,
TeamRJJO
not compatable with current users GPSR units!!!
Accuracy question.....MPH
in GPS technology and devices
Posted
The GPS System Spec calls for 0.1 m/s accurracy with 95% certainty.
Peace,
Ron (TeamRJJO)