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Accuracy of GPS in Garmin 430/530
Is there any way to have the Garmin 430/530 put its current display accuracy
on the primary display as an ongoing statistic, based on the number of satellites in view? How, in general, do the Garmin units notify you of situations where GPS accuracy has been compromised to a level that makes it unsafe to use the Garmin for a GPS approach? I got an interesting lesson in GPS recently while traveling with a handheld GPS as the passenger in a plane. The GPS showed us landing about two miles east of the airport. I figured out only later that the position of the antenna was such that many satellites were blocked, so the accuracy of the GPS signal was greatly diminished. The particular software I was using didn't display its current accuracy on the primary display. Based on that event, I realize I cannot just trust a GPS display without first understanding the current accuracy of the signal. What would be really nice is if the primary display would show vertical and horizontal accuracy as two separate numbers, based on some high confidence interval (99.99+%). Knowing that the current display reading is accurate to 10 ft vertical and 15 ft horizontal, for example, might make you a lot more comfortable in following a GPS approach than a display where the 99.99% confidence interval is 2000 ft vertical/horizontal (i.e., GPS reliability is completely compromised by virtue of blocked satellites, bad GPS antenna, etc). -- Will |
#2
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Accuracy of GPS in Garmin 430/530
Will wrote:
Is there any way to have the Garmin 430/530 put its current display accuracy on the primary display as an ongoing statistic, based on the number of satellites in view? How, in general, do the Garmin units notify you of situations where GPS accuracy has been compromised to a level that makes it unsafe to use the Garmin for a GPS approach? I got an interesting lesson in GPS recently while traveling with a handheld GPS as the passenger in a plane. The GPS showed us landing about two miles east of the airport. I figured out only later that the position of the antenna was such that many satellites were blocked, so the accuracy of the GPS signal was greatly diminished. The particular software I was using didn't display its current accuracy on the primary display. Based on that event, I realize I cannot just trust a GPS display without first understanding the current accuracy of the signal. What would be really nice is if the primary display would show vertical and horizontal accuracy as two separate numbers, based on some high confidence interval (99.99+%). Knowing that the current display reading is accurate to 10 ft vertical and 15 ft horizontal, for example, might make you a lot more comfortable in following a GPS approach than a display where the 99.99% confidence interval is 2000 ft vertical/horizontal (i.e., GPS reliability is completely compromised by virtue of blocked satellites, bad GPS antenna, etc). Do some geocaching in pine forests. You'll see the effect of the trees. They just cut off the signal and no updates happen for awhile. You keep on the indicated heading and then all of a sudden the gps catch a few bits and the arrow flips to a completely new direction! The gps jocks I've talk to state things like "gps is within 10 meters 95% of the time". I presume that means the gps has a signal. When it's not getting a signal, I don't think there is much you can say. |
#3
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Accuracy of GPS in Garmin 430/530
In a previous article, "Will" said:
Is there any way to have the Garmin 430/530 put its current display accuracy on the primary display as an ongoing statistic, based on the number of satellites in view? How, in general, do the Garmin units notify you of situations where GPS accuracy has been compromised to a level that makes it unsafe to use the Garmin for a GPS approach? Google up the term "RAIM warning". All approach certified GPSes have to warn you if the accuracy is degraded. -- Paul Tomblin http://xcski.com/blogs/pt/ "You are a human being, capable of emotions and rational thought. A computer is only capable of floating point math and crude malice." http://www.hamsterrepublic.com/james/technomancy/ |
#4
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Accuracy of GPS in Garmin 430/530
Another poster correctly pointed you to the real answer: RAIM.
I just wanted to point out that the accuracy of the GPS signal was never the problem The problem was that you had insufficent signals for a good solution. RAIM would have detected this and alerted you to the problem. Thus GPS was never at fault...your user equipment (including antenna) were not sufficient for the job. Ron Lee "Will" wrote: Is there any way to have the Garmin 430/530 put its current display accuracy on the primary display as an ongoing statistic, based on the number of satellites in view? How, in general, do the Garmin units notify you of situations where GPS accuracy has been compromised to a level that makes it unsafe to use the Garmin for a GPS approach? I got an interesting lesson in GPS recently while traveling with a handheld GPS as the passenger in a plane. The GPS showed us landing about two miles east of the airport. I figured out only later that the position of the antenna was such that many satellites were blocked, so the accuracy of the GPS signal was greatly diminished. The particular software I was using didn't display its current accuracy on the primary display. Based on that event, I realize I cannot just trust a GPS display without first understanding the current accuracy of the signal. What would be really nice is if the primary display would show vertical and horizontal accuracy as two separate numbers, based on some high confidence interval (99.99+%). Knowing that the current display reading is accurate to 10 ft vertical and 15 ft horizontal, for example, might make you a lot more comfortable in following a GPS approach than a display where the 99.99% confidence interval is 2000 ft vertical/horizontal (i.e., GPS reliability is completely compromised by virtue of blocked satellites, bad GPS antenna, etc). -- Will |
#5
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Accuracy of GPS in Garmin 430/530
"Paul Tomblin" wrote in message
... Google up the term "RAIM warning". All approach certified GPSes have to warn you if the accuracy is degraded. I assumed as much, which is why I wanted to know how that condition is displayed on the Garmin 430/530. It may just be personal preference, but I see a lot of value in user interfaces that make the data quality a primary display attribute at all times. That way I not only know I have a GPS signal, but I can quickly assess the quality of the signal. I see value in making this more than just a binary state ("good enough for the FAA" GPS signal quality / "not good enough for the FAA" GPS signal quality). Possibly that data could be colored or made to blink in situations where integrity is compromised sufficiently. -- Will |
#6
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Accuracy of GPS in Garmin 430/530
I guess my wording was not very precise, but I think I already understood
what you are saying here. I was stressing that the GPS' *accuracy* was compromised, and as you correctly point out the underlying technical reason for that lowering of accuracy was probably that the number of satellites that could be seen had lowered. I think signal quality can also factor into things however. Some of the newer GPS technologies like SiRFXTrac have the ability to enumerate high-quality signals, low-quality signals, and remove the lower quality signals from the satellites used in calculations. I've used XTrac for hiking under forest cover, and it's a marvelous technology that lets you get a signal reading in situations that would leave most GPS antennas baffled. XTrac has potential for both lower accuracy and higher accuracy, since it gives a way to add low-quality signals into the mix of satellites used in calculations. In reviewing the few GPS applications I own, I'm pretty surprised that none of them tackles the issue of data quality head on in a way that makes it immediately obvious to the user how much accuracy they can expect out of the current signal reading. Even when you are hiking that is important. But when you look at the number of PDA and notebook GPS applications that are being developed for aviation use, it's life-or-death potentially for someone to understand how much they can rely on the software's display about current position. I'm no GPS expert, but I have to assume it's straightforward for a GPS to calculate the number of meters of accuracy to which it can currently display, given any number of satellites at specific positions and given specific signal quality for each satellite. Knowing the exact number of meters of accuracy is for my taste a critical piece of information. I'm not sure I feel comfortable letting the GPS just take care of my safety by deducing its conclusions about GPS data quality, even if it is an FAA approved appliance like the Garmin. Showing accuracy of a displayed position using a common measure like number of meters for a 99.xx% confidence interval would give all software packages that display GPS positions a means of being quickly understood by almost any user who cares about such things. -- Will "Ron Lee" wrote in message ... Another poster correctly pointed you to the real answer: RAIM. I just wanted to point out that the accuracy of the GPS signal was never the problem The problem was that you had insufficent signals for a good solution. RAIM would have detected this and alerted you to the problem. Thus GPS was never at fault...your user equipment (including antenna) were not sufficient for the job. Ron Lee |
#7
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Accuracy of GPS in Garmin 430/530
Once the 430/530's are upgraded for WAAS, they will have built-in
integrity monitoring and no need for RAIM. Divorce yourself from the notion that an installed, certified receiver is the same thing as a handheld gps, pda, whatever. What you're suggesting regarding horizontal and vertical accuracy is simply unnecessary. The receiver either meets or does not meet the required tolerances specified by TSO C129 or TSO C145/146. If it doesn't sufficient accuracy, it will flag itself. Why do you want something else to monitor? Do you have a signal strength meter for your nav receiver? You mentioned that you were surprised gps software for pda's and pc's did not have accuracy monitoring and that this was possibly a life-and-death scenario. This is not the case...non certified software is for use as a situational awareness aid or for VFR...hardly life or death. If you're solely using non-approved gps equipment for safety of life operations, you're seriously acting in a careless and wreckless manner. Brad |
#8
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Accuracy of GPS in Garmin 430/530
Will wrote:
Possibly that data could be colored or made to blink in situations where integrity is compromised sufficiently. It does.. as previously stated... you get a RAIM warning. All you need to know is "is the signal performing to a legal/safe standard" or not. "Good enough for the FAA" is what counts when you are considering the purpose of a GPS guided instrument approach. Adding the percentage parameters you envision is just the sort of thing to add workload to a single pilot IFR approach to minimums. One more thing to monitor.. Instead, I think the Feds got it right.. either its GOOD or its BAD. Period. Dave |
#9
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Accuracy of GPS in Garmin 430/530
It's nice that a certified instrument flags an unsafe condition. I would
still like to know the current level of GPS accuracy on a certified instrument, for many reasons: * It helps to educate me about GPS and conditions in my immediate surroundings that might affect accuracy of the technology. * It helps to alert me about possibly deteriorating conditions, before I get into a situation where I needed to rely on the instrument and suddenly I cannot. I wouldn't mind having an indicator about Nav or ILS signal strength either, by the way, but usually you get that implicitly by noticing unstable behavior in the Nav needle. What makes GPS a bit more dangerous is the digital computer display that creates an illusion that everything is perfectly ordered and working, when in fact the degree of accuracy is constantly changing. Unlike the analog needle, I don't see any wavering or other clues about an imminent failure with most GPS applications. I don't take comfort from knowing that I could get into a GPS approach in bad weather in a valley with high mountains on all sides and then have the GPS suddenly announce that it's no longer good enough for the FAA. What if my exit was a missed approach that is GPS based? The GPS is no longer approved for the approach, so I'm left with maybe 30 seconds to go over to another missed approach based on different instruments? It's not making me feel any better to know that my pilot workload was a lot lower up until the FAA approved instrument started wailing that I'm in big trouble. If I had a choice, I would elect the additional pilot workload in order to maintain additional situation awareness so I can take action in a marginal situation sooner rather than later. Regarding non-certified GPS applications: half of these applications include user testimonials from VFR pilots who go in over their head and who profusely thank the author of the software for getting them out of a bad place. Many of these PDAs are being sold with terrain mapping features that clearly have IFR applications, and while they are never sold as primary instruments, you must be dreaming if you don't understand that pilots are buying these things as a last-resort backup instrument should they suffer catastrophic failures in the other instruments. And why not? The only time they get used in an IFR application is when all of the FAA certified instruments have failed and the pilot is in a truly bad way. Any situational awareness in a situation where you would otherwise have no situational awareness is better than the alternative. I would like to see some standards or at least common conventions develop about how accuracy is reported in the user interface of all types of GPS applications. And while I respect the right of other pilots to reduce their workloads by trusting FAA certified instruments and not worrying about accuracy issues, I personally don't like surprises. I don't see the harm in revealing the accuracy figures in a simple and easy to understand way for the pilot who would like to verify the accuracy levels at different phases of an approach. No one is trying to force it on you. I'm just interested in having an option. -- Will "Brad" wrote in message oups.com... Once the 430/530's are upgraded for WAAS, they will have built-in integrity monitoring and no need for RAIM. Divorce yourself from the notion that an installed, certified receiver is the same thing as a handheld gps, pda, whatever. What you're suggesting regarding horizontal and vertical accuracy is simply unnecessary. The receiver either meets or does not meet the required tolerances specified by TSO C129 or TSO C145/146. If it doesn't sufficient accuracy, it will flag itself. Why do you want something else to monitor? Do you have a signal strength meter for your nav receiver? You mentioned that you were surprised gps software for pda's and pc's did not have accuracy monitoring and that this was possibly a life-and-death scenario. This is not the case...non certified software is for use as a situational awareness aid or for VFR...hardly life or death. If you're solely using non-approved gps equipment for safety of life operations, you're seriously acting in a careless and wreckless manner. Brad |
#10
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Accuracy of GPS in Garmin 430/530
If you're in flight you will have nothing above your
antennas to block reception [unless you fly by GPS while refueling]. If you are, like a hiker shielded by trees, you'd better not be looking at your GPS, you have more serious troubles. RAIM does the job, you might read the section in the manual about RAIM, see the Garmin manual, on-line manual... -- James H. Macklin ATP,CFI,A&P -- The people think the Constitution protects their rights; But government sees it as an obstacle to be overcome. some support http://www.usdoj.gov/olc/secondamendment2.htm See http://www.fija.org/ more about your rights and duties. "Will" wrote in message ... |I guess my wording was not very precise, but I think I already understood | what you are saying here. I was stressing that the GPS' *accuracy* was | compromised, and as you correctly point out the underlying technical reason | for that lowering of accuracy was probably that the number of satellites | that could be seen had lowered. I think signal quality can also factor | into things however. Some of the newer GPS technologies like SiRFXTrac | have the ability to enumerate high-quality signals, low-quality signals, and | remove the lower quality signals from the satellites used in calculations. | | I've used XTrac for hiking under forest cover, and it's a marvelous | technology that lets you get a signal reading in situations that would leave | most GPS antennas baffled. XTrac has potential for both lower accuracy and | higher accuracy, since it gives a way to add low-quality signals into the | mix of satellites used in calculations. In reviewing the few GPS | applications I own, I'm pretty surprised that none of them tackles the issue | of data quality head on in a way that makes it immediately obvious to the | user how much accuracy they can expect out of the current signal reading. | Even when you are hiking that is important. But when you look at the | number of PDA and notebook GPS applications that are being developed for | aviation use, it's life-or-death potentially for someone to understand how | much they can rely on the software's display about current position. | | I'm no GPS expert, but I have to assume it's straightforward for a GPS to | calculate the number of meters of accuracy to which it can currently | display, given any number of satellites at specific positions and given | specific signal quality for each satellite. Knowing the exact number of | meters of accuracy is for my taste a critical piece of information. I'm | not sure I feel comfortable letting the GPS just take care of my safety by | deducing its conclusions about GPS data quality, even if it is an FAA | approved appliance like the Garmin. Showing accuracy of a displayed | position using a common measure like number of meters for a 99.xx% | confidence interval would give all software packages that display GPS | positions a means of being quickly understood by almost any user who cares | about such things. | | -- | Will | | | "Ron Lee" wrote in message | ... | Another poster correctly pointed you to the real answer: RAIM. | | I just wanted to point out that the accuracy of the GPS signal was | never the problem The problem was that you had insufficent signals | for a good solution. RAIM would have detected this and alerted you to | the problem. Thus GPS was never at fault...your user equipment | (including antenna) were not sufficient for the job. | | Ron Lee | | |
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