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

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.

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/

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

"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

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

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

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

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

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
|
|