Accuracy of GPS in Garmin 430/530

J. Severyn wrote:

Well you get a real-time calculated number in the horizontal direction,

http://www.garmin.com/manuals/GNS430_PilotsGuide.pdf pages 44, 46
and
http://www.garmin.com/manuals/GNS530_PilotsGuide.pdf page 43

Garmin calculates the Estimated Position Error (in feet or meters
horizontally), but does not present an estimate of the vertical error. I
believe Garmin has done this on most if not all of their aviation units,
handheld and panel mount. I've got an old GPS95XL and it does the same
thing.

The vertical solution is not part of the IFR certification of an IFR
unit under TSOC-129. It simply is not good enough for IFR.

IFR-certified Baro VNAV and LPV are the only valid vertical solutions to
date.

What I was
describing doesn't require any new hardware, and it's just a matter of
calculating some accuracy numbers and representing them in the standard UI.

Hee hee. Those of us who develop software have all heard this one before. "It's
a simple matter of programming..."

Har har

Will,

I assumed as much, which is why I wanted to know how that condition is
displayed on the Garmin 430/530.


The airplane symbol changes (color? blinking? I forgot - the manual is
available online at garmin.com), should the number of satellites fall
below the required level. However, in opposition to your situation, the
antenna is outside the airplane on its top, so it is very unlikely to
have that situation. Also, the receiver uses not only RAIM, but RAIM
prediction for the route programmed into it, so it will analyze the
satellite constellation in advance and alert of problems. If RAIM isn't
sufficient, you'll get a warning and will have to use a different
approach.

All this is part of what makes the units "approach certified". Thus, a
pilot doesn't need to worry about PDOP values or some such, the box will
simply say GO or NO GO.

--
Thomas Borchert (EDDH)

Will,

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


What you want is in the values for PDOP, VDOP and HDOP - and EPE.

--
Thomas Borchert (EDDH)

Will,

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.


Again, for a pilot, that's not useful information. It might be nice to know,
but for a pilot, useful is only if the GPS works or not - to specifications
set by the FAA during certification.

--
Thomas Borchert (EDDH)

Will,

* 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.


Are you a pilot? Instrument rated? Trust the people here that a During an
approach you have other things to worry about.

--
Thomas Borchert (EDDH)

Will,

Knowing the exact number of
meters of accuracy is for my taste a critical piece of information.


Why? The distance to obstacles during approach is designed to be
sufficient for the worst case allowed under certification. If the
signal quality doesn't meet those criteria, the receiver will tell you.
Why should you need to know more?

--
Thomas Borchert (EDDH)

Will,

There
is no proof at all for the claim that some straightforward math calculations
in any GPS software is going to to double the cost of the GPS.


Again, the info you seek is in the values EPE, PDOP, VDOP and HDOP, which most
GPS receivers readily display. It's just not exactly the format you propose.
Who cares?

--
Thomas Borchert (EDDH)

Roy,

And tell me that pilots aren't already using those "for situational
awareness only" extended runway centerlines drawn on the moving maps of
VFR-only handhelds to cobble up their own instrument approaches?


If they do, and they come to harm - their problem.

--
Thomas Borchert (EDDH)

Will,

First, the pilot shouldn't be using the handheld in IMC


Wrong. In fact, completely wrong. It just shouldn't be his primary nav
source.

--
Thomas Borchert (EDDH)