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?

Why would you want that information? In single-pilot operations,
especially, looking at those data constitutes information overload.
That is what RAIM is all about, to keep it simple. RAIM is much more
robust for the final approach segment than for terminal mode. You
simply aren't going to have issues with an IFR-certified GPS (properly
installed) that you will have with a hand-held.

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.

That large of an error was probably due to the substantial altitude
change of the airliner while your GPS was staggering along in 2-D mode.

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.

As others have told you, the portable does not have RAIM. It is a VFR
device. It was not designed to be robust through a cabin window of an
airliner. Some owners, who are savvy on this still, install an external
antenna on their aircraft for their hand-held GPS. It will never have
the problems you experienced with an external antenna.

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).


Again, RAIM and proper IFR installation procedures mitigate your
concerns to the point of being irrelevant.

There is different, higher level of accuracy, integrity, and continuity
than "plain vanilla" TSO-C129 IFR GPS. That is an IFR-approved RNP
platform, which is a quandum leap in RNAV integrity. RNP platforms have
enough information to make you happy in your quest. But, the displays
and software are presently heavy iron stuff, and huge overkill for most
IFR operations today.

"Will" wrote:
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.

I disagree. We already have information overload. A binary "go/no-go" is
exactly what you want. If I told you that the SNR from satellite 17 was
down 6dB, what would you do with that information?

RAIM factors in signal strength as well as satellite geometry. To get a
good fix, you need to be getting a good signal from 4 satellites positioned
appropriately in both azimuth and elevation. Figuring out if the signal
strength and geometry is "good enough" is not the kind of problem people
can do in their heads.

"Will" wrote:

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.

A handheld GPS used by a hiker in the woods is working under completely
different environmental conditions than one on an airplane. The biggest
reason for a hiker's GPS to get poor signal is because of nearby terrain or
overhead foliage blocking line of site to the sky.

By the time an airplane's view of the sky is blocked by overhead foliage,
they've probably got bigger problems than not having a good GPS signal.

Roy Smith wrote:
"Will" wrote:

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.


I disagree. We already have information overload. A binary "go/no-go" is
exactly what you want. If I told you that the SNR from satellite 17 was
down 6dB, what would you do with that information?

RAIM factors in signal strength as well as satellite geometry. To get a
good fix, you need to be getting a good signal from 4 satellites positioned
appropriately in both azimuth and elevation. Figuring out if the signal
strength and geometry is "good enough" is not the kind of problem people
can do in their heads.
I'm somewhat surprised that no one has mentioned that both the Garmin
and Lowrance units that I'm familiar with have a page for satelitte
signal strength. I've never felt the need to run thru the 430/530 pages
to find a similar page but would not be surprised to find it buried in
there somewhere. To answer the OP's question it's there you just need
to read the manual to find which sub-menu it's on. If he's using a
non-aviation unit then all bets are off but again I would think it would
be there somewhere. Also on the units I use regularly the airplane icon
flashs on the main display when the signal is lost ala a pseudo RAIM
indicator

John Theune wrote:
..

I'm somewhat surprised that no one has mentioned that both the Garmin
and Lowrance units that I'm familiar with have a page for satelitte
signal strength.

Page 4 of the nav section.

On 04/25/06 05:03, John Theune wrote:
Roy Smith wrote:
"Will" wrote:

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.


I disagree. We already have information overload. A binary "go/no-go" is
exactly what you want. If I told you that the SNR from satellite 17 was
down 6dB, what would you do with that information?

RAIM factors in signal strength as well as satellite geometry. To get a
good fix, you need to be getting a good signal from 4 satellites positioned
appropriately in both azimuth and elevation. Figuring out if the signal
strength and geometry is "good enough" is not the kind of problem people
can do in their heads.
I'm somewhat surprised that no one has mentioned that both the Garmin
and Lowrance units that I'm familiar with have a page for satelitte
signal strength. I've never felt the need to run thru the 430/530 pages
to find a similar page but would not be surprised to find it buried in
there somewhere. To answer the OP's question it's there you just need
to read the manual to find which sub-menu it's on. If he's using a
non-aviation unit then all bets are off but again I would think it would
be there somewhere. Also on the units I use regularly the airplane icon
flashs on the main display when the signal is lost ala a pseudo RAIM
indicator

The OP was asking why this can't be displayed on the main page...

You snipped it from your response. Here it is:

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?




--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

"* 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."

In an airplane, your immediate surroundings should have no bearing on
your GPS accuracy unless you're flying under bridges, trees, through
tunnels...you get the idea. Keep in mind that the GPS constellation is
constantly moving so there are no dead areas, like you might have with
a VOR signal. I'm not sure what you mean by deteriorating
conditions...how is your meter going to predict ionospheric activity or
a satellite going off line? It's not like bars on a cell phone.

Regarding standards for handheld GPS, who would enforce the standards?
What if a particular receiver did meet the standards?

"Roy Smith" wrote in message
...
I disagree. We already have information overload. A binary "go/no-go" is
exactly what you want. If I told you that the SNR from satellite 17 was
down 6dB, what would you do with that information?

That's not useful information the way you present it. I want conclusions
and not data. Specifically I want to know the number of feet/meters of
accuracy of my current position, that's all. If my current accuracy is 10
ft vertical versus 100 ft vertical versus 1000 ft vertical, that means
something to me about how much trust I should put in the GPS display.

--
Will

"John Theune" wrote in message
news:Udo3g.5009$bU6.3635@trnddc06...
I'm somewhat surprised that no one has mentioned that both the Garmin
and Lowrance units that I'm familiar with have a page for satelitte
signal strength. I've never felt the need to run thru the 430/530 pages
to find a similar page but would not be surprised to find it buried in
there somewhere. To answer the OP's question it's there you just need
to read the manual to find which sub-menu it's on. If he's using a
non-aviation unit then all bets are off but again I would think it would
be there somewhere. Also on the units I use regularly the airplane icon
flashs on the main display when the signal is lost ala a pseudo RAIM
indicator

You are right all GPS software usually implements a satellite signal page.
It's not in any way shape or form what I asked for.

I want the GPS to take all of the inputs for number of satellites and signal
strength and derive from that just two integers:

1) Number of feet/meters of horizontal accuracy, within some confidence
interval (e.g., 99.95%)

2) Number of feet/meters of vertical accuracy, within some confidence
interval (e.g., 99.95%)

Those two numbers could become optional numbers for the primary display.
No one is forcing anyone to use them. If you want to simply trust the
instrument to give you a go-nogo decision, it's your life and if you feel
that is safe it's a free world (as long as you follow FAA rules ) so be
my guest.

For my personal taste, I understand that a GPS display is always an illusion
subject to different levels of inaccuracy. I am sensitive to the
difference between a display that is showing me accuracy to 10 ft, 100 ft,
or 1000 ft. In the original posted example the GPS was off target by more
than 5000 ft. Nothing on the original display gave me any clue that this
was the case. The two numbers I am asking for would communicate quite
succinctly that no one should rely on the display for anything other than
the most gross kind of positioning.

While I would love to see the feature I am looking for in any FAA-compliant
instrument like a Garmin 530, I think the feature becomes most critical in
non-FAA compliant GPS devices/software. The authors of such packages
cannot control the quality of the satellite antenna, or mounting, and
substandard GPS reception is probably a routine thing for PDA based GPS
devices/software. So finding a succinct way to communicate the accuracy of
the current signal in numbers that mean something to any user becomes quite
important. Making people look at satellite maps and signal strength seems
like a pure engineering exercise, and it doesn't collapse the input data
into a useful form.

--
Will

"Roy Smith" wrote in message
...
A handheld GPS used by a hiker in the woods is working under completely
different environmental conditions than one on an airplane. The biggest
reason for a hiker's GPS to get poor signal is because of nearby terrain
or
overhead foliage blocking line of site to the sky.

By the time an airplane's view of the sky is blocked by overhead foliage,
they've probably got bigger problems than not having a good GPS signal.

For an FAA-approved device, properly installed, I think you are right
probably 99% of the time. Of course even then you could imagine cases like
what happens if the GPS antenna starts to slowly go bad? You don't want to
learn about that when it reaches a critical failure point in the final part
of an approach. Better to see the accuracy start out at 20 ft accuracy and
slip over time to 50 ft, 80 ft, 100 ft, etc. Over many flights even an
inobservant person might catch the deterioration and do something about it
when there is time.

For a non-FAA approved device, I think you are wrong. The problem here is
that the GPS software has no way to guarantee the integrity of the satellite
antenna, and very importantly it cannot guarantee the integrity of the
antenna's placement within the cockpit. If the user accidentally selects
XTrac mode without understanding the implications of that, places the
antenna out of view of most satellites within the cockpit, etc, the software
happily displays an aircraft position. And it never tells you that your
current position is only accurate to 10,000 ft horizontal!

The point is that in a non-approved device, the GPS software creates an
illusion that you are on a 2D map position, at a spacial coordinate, but
most of this software gives you no immediate way to determine if that
reading is accurate to 10 ft or 10K ft. Knowing in advance that you are
accurate to only 10K ft would probably give most pilots a reason to
investigate why, maybe resulting in a better position for the antenna, for
example. Maybe the user would find out that they had accidentally left the
unit in XTrac mode and they need to switch over to a more accurate mode.
Better to make these discoveries and tinker with such things in a calm
environment. You don't want to make the discovery that the "backup" GPS
display is worthless on the day you lose all your primary instruments in IFR
conditions.

I just don't understand the issue about pilot overload. I'm asking for two
integers, and you don't need to look at them ever if you don't want to.

--
Will