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/

"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

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

"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.

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

"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