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This thread is privately moderated by Jack Crossfire, who may elect to delete unwanted replies. |
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Discussion
Death of GPS
GPS had a pretty big loss of performance recently & it seems due to the loss of satellite 5 on Mar 26. Your communist government doesn't have enough money to keep all 34 satellites going, so it's paired satellites which it thinks R about to die with newer satellites in the same plane, & that's 5 & 12, 3 & 6, 9 & 27, 7 & 25, 30 & 1. Unfortunately, 5 & 12 was the pair used during our normal flying time.
The effect is greater sensitivity to interference, degraded altitude precision & that means you're probably getting back the SCP1000. All the media is reporting is "an increased likelihood that in 2010, as old satellites begin to fail" & "the first replacement satellite is expected to be launched this November, some three years after the original launch date." Whatever it takes to keep your mortgages solvent, but we know they're already dying. http://www.pcworld.com/businesscente...in_a_year.html While Arrow shipping only took 1 day, the passives R still a week away, so instead, how about the highest resolution photos of the LISY300AL ever shown on the internet. |
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Last edited by Jack Crossfire; May 20, 2009 at 02:18 PM.
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Where are you getting this information from?
The GPS system only requires 24 satellites to be fully operational. The remaining satellites (there are a total of 32 normally, not 34) are there to be spares should any of the primary satellites fail. The loss of one satellite will not cause any loss of resolution or performance to the system beyond the time it takes to move a spare into position. Nowhere in the article you linked to have they said anything about the GPS system losing any performance, they only state that it is a possibility in the future if the air force doesn’t meet its schedule for launching replacement satellites. Also, there is no satellite #5, that’s just an arbitrary number assigned by your GPS and may represent a different satellite every time you turn the GPS on (i.e. the 5th satellite it found). GPS satellites are designated by SVN’s (space vehicle number). The current satellites are all ether Block II (SVN 13 through 21), Block IIA (SVN 22 through 40), or Block IIR/IIR-M (SVN 41 through 61). The US Naval Observatory shows 31 functioning satellites currently, 7 more than needed for the system to function at 100%. |
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There were a total of 34 in 2008. 32 were in mid earth orbits. 2 were in geostationary orbits. The geostationary ones are attached to Canadian TV satellites & broadcast WAAS corrections, & 2 additional navigation beacons. The current status is given here http://www.nstb.tc.faa.gov/Full_WaasSatelliteStatus.htm. This is the numbering convention on the Jack Crossfire blog. Once again, the current status is given here http://www.nstb.tc.faa.gov/Full_WaasSatelliteStatus.htm.
You don't get all 33 remaining satellites all the time. You get maybe 8 at a time. If 1 of the 8 is the one that's down, now you lost 12% of the data. If the one that's down is the one you need for altitude precision, now you have no data. 24 satellites are not enough to do the job. The signal path is too noisy. Let's say it again. 24 satellites are not enough to do the job. For your government to say any different is like saying Mission Accomplished in Iraq. The original 34 satellite constellation was barely enough for altitude sensing. Did you get that? The original 34 satellite constellation was barely enough for altitude sensing. You don't get 1000 pristene beacons every second from every satellite. You probably get 500 beacons passing the CRC check. Of those, none of them can give your true position. Your module computes a cloud of erroneous readings from the small number of noisy beacons that pass & averages them. If you take away 12% of those readings, the result is even more noise. |
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Last edited by Jack Crossfire; May 28, 2009 at 12:41 PM.
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It seems that what you qualify as GPS isn’t the same thing I do (which explains our difference in numbers). There are no GPS satellites in geostationary orbits; they all orbit in 12 hour intervals. Here’s a link to the U.S. Naval Observatory list of all the GPS Block II and later satellites in the GPS system, it shows which ones are no longer functioning ftp://tycho.usno.navy.mil/pub/gps/gpsb2.txt.
What you linked to isn’t GPS, its WAAS (Wide Area Augmentation System) which was developed by the FAA (GPS is a DOD system) to augment the GPS system to improve accuracy, especially for altitude. It is a separate system only available in North America (GPS is worldwide) that broadcasts supplement information from geostationary satellites to improve GPS accuracy for receivers designed to receive the extra correction signals (most arn't). They used to do the same thing utilizing AM broadcast stations (Differential GPS) instead of geostationary satellites to send the supplemental information. It certainty improves the accuracy of GPS (especially when SA was employed), but it’s not GPS and I know of no GPS used in R/C or automotive that can take advantage of it. Heck, even sport aviation class GPS’s don’t offer WAAS, only the professional class avionic GPS units utilize it. The GPS system has always employed spare satellites. The system originally operated using a 21 active satellite constellation and 3 spares (24 total) and operated actually as an 18 active satellite constellation with 3 spares as a cost saving measure until 1987. What we know today as GPS is given a birth date of April, 1995 and consisted until last year of the 24 Block II satellite constellation with (ideally) 8 spares so dropping one satellites wouldn’t compromising performance. After doing a little reading, I found that as of March, 2008, the DOD reconfigured the system to have 31 active satellites and (back then) 2 spares instead of 24 active satellites. This changed the configuration of the constellation to a non-uniformed arrangement from its prior uniformed orbits. Part of the reason for doing this was to improve the reliability and availability of the system in the event of a multiple satellite failure. It also resulted is some improved precision, but even if they’re forced to go back to a 24 satellite consolation were no worse off than we were before March of last year. Do you really think they failed do design any redundancy into the system? The accuracy of the system doesn’t have a linear relationship with how many satellites are locked. Loosing 1 out of 8 satellites locked doesn’t drop your accuracy by 12%, it probably wouldn’t even affect it by more than 1 or 2 percent. Once you get 4 satellites locked, you have a 3D position that is about 85% as accurate as you can get. Lock up 6 satellites and the accuracy goes up to about 97% as accurate as it will ever be. Locking up more satellites then that is nice of course, but not so much for any improved accuracy (although there is some), but more to have an already locked satellite to replace any that might drop out of view. I understand and agree with your comments about the poor accuracy for altitude with GPS, but you must remember that do to the geometry (or trigonometry) of how the system works, altitude will always have roughly 1/3rd the resolution that latitude and longitude have. In addition, the system wasn’t originally envisioned to provide precision aviation navigation; it was more a replacement for the aging Loran C system, altitude was just a bonus. Don’t get me wrong, there is great reason for concern in the near future as the air force is way behind on the launching of new GPS satellites and the current birds are getting very old. But the system was designed with redundancy (the spare satellites) so that losing one bird won’t compromise the system. Even if the resent defunct satellite had no spare, your statement of “GPS had a pretty big loss of performance recently” is a gross exaggeration of the impact it would have. We would have to lose 7 or 8 more satellites before we would be any worse off then we were last year when they changed over from the 24 satellite constellation. |
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Our only concern is the use of GPS for flying VikaCopter, not the normal consumer application. VikaCopter can't fly with less than 6 satellites & is marginal below 10, although over 11 satellites doesn't make much difference. The loss of satellite #5 in the 12am hour really killed us. The system has no redundancy as far as VikaCopter is concerned. She needed every satellite working to stay in the air. If you take off with 5 satellites as we have done, you'll end up in a tree. The green squares on the WAAS status page & the GPS constellation R 1 in the same. Those are the actual satellite positions & status. The geostationary satellites are considered part of GPS. They broadcast all the codes & are used in the navigation solutions, although all our GPS modules from the EM406, EB-85, uBlox4, & uBlox5 have used WAAS corrections.
Don't think Google can legally provide search results for a system which is affected by the loss of 1 satellite. It would be like searching for mp3. |
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Last edited by Jack Crossfire; May 28, 2009 at 11:06 PM.
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Quote:
Also, the system isn’t affected by the loss of one satellite except for the time it takes to move a spare into position. That’s what I’ve been trying to explain, there are spare satellites in the system for just such an occurrence. Information showing this is available all over the place. The spare satellites are asleep and aren’t shown by your GPS RX or tracking sites. The whole WAAS/GPS discussion has taken us off point in respects to whether or not the current GPS system, however you wish to define it, has had a reduction in performance. The only reference I could find about a GPS satellite in March was a new satellite (PRN01/SVN49) launched on the 24th,, nothing about a lost satellite. The original article you linked to said nothing about the current state of the system having a loss in performance or a lost satellite and the FAA’s site you linked to earlier seams to conform that there has been no resent degradation of the system as seen on the chart at http://www.nstb.tc.faa.gov/reports/R.../2009%2004.pdf titled “Civil Report Card On GPS Performance April 2009”. All of the performance numbers for March and April are better than those for the 2008 calendar year. The best source for information on the status of the system is from Schriever Air Force Base at http://www.schriever.af.mil/gpsoc/. This is where the Air Force Space Command maintains the system from. They show no advisories issued in March at all. The system has always had spares, satellites that are asleep and not visible to a GPS RX or generally shown in tracking programs. It is designed for the loss of some satellites without the system degrading. Perhaps you’re experiencing some form of interference with your GPS receiver on the VikaCopter that can explain your lack on sufficient satellite locks. One last thought and I will stop polluting your blog; I’m starting to think that the information in my last post about the change from 24 to 31 satellites last year may not be correct. That was from Wikipedia and when I tried to follow the reference link, it only brought me to a old notice advisory to NAVSTAR users announcing the decommission of a satellite. I have found no other information discussing a 31 satellite constellation and I believe it is still operating as a 24 sat constellation which makes sense as a 31 sat configuration would mean that you wouldn’t see the same sat at the same time each day do to the non-uniformed arrangement it required. |
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