GPS/XM Weather Question

"Ron Natalie" wrote:


I use XM NEXRAD tactically all the time. If there's a growing cell 15
miles ahead, I will use the NEXRAD picture to decide whether to go left
or right around it based on its history and the location of other cells
beyond it. That's tactical, to me, and it's the best thing about having
satellite weather aboard.


That's strategy not tactics.

You have your definition; I have mine.

--
Dan
C172RG at BFM

".Blueskies." wrote:

: Nexrad XM datalink could be as much as 10 minutes old. So, no, you
: cannot use it to buzz between cells. It is a strategic tool, not a
: tactical tool. Flying 50 miles thataway is a much better plan.
:


Is there some sort of time marker on the screen to indicate the time the
last data was updated, or to indicate how old
it is?

Yes.

".Blueskies." wrote:

Yes the lightening strikes tell the tale. What is the difference between
Stormscope strikes and ones displayed from XM?

Luggage left on the ramp tells the lightening tale (sorry)

Lightning depicted on XM is triangulated from ground sensors. It is very
accurate, but it is not real-time like spherics and does not display all
cloud discharges.

A stormscope--properly installed and interpreted--may tip you off about a
cell that is getting convective a bit before XM will. On the other hand, XM
lightning display is not subject to spurious signals or radial spread.

In practice I have found the XM lightning feature quite useful. It would be
nice to have both, but if I have to pick just one, I prefer the XM.

--
Dan
C172RG at BFM

..Blueskies. wrote:
"Andrew Sarangan" wrote in message oups.com...
:
: Nexrad XM datalink could be as much as 10 minutes old. So, no, you
: cannot use it to buzz between cells. It is a strategic tool, not a
: tactical tool. Flying 50 miles thataway is a much better plan.
:


Is there some sort of time marker on the screen to indicate the time the last data was updated, or to indicate how old
it is?


On the MX20 XM FIS display the time is on the right side bottom of the
display for NEXAD (and if you have it overlaid, the METAR flags).

In article ,
".Blueskies." wrote:

"Jim Burns" wrote in message
...
: Agreed. I also think that it's much more beneficial in IFR to have a
: second
: source of weather such as a Stormscope or such to help delineate between
: rain and actual thunderstorms.
: Jim
:
:

Yes the lightening strikes tell the tale. What is the difference between
Stormscope strikes and ones displayed from XM?

I hate to disagree with Dan, who otherwise provides excellent comments
on XM weather, but in my personal experience the Lightning product is
not that impressive. I have a StrikeFinder and a Garmin 396, and the
StrikeFinder MOST DEFINITELY picks up quite a bit that the Lightning
product misses. I'm not just talking about stuff that's building, I'm
talking about established cells which show as RED on NEXRAD but with no
XM lightning depiction.

The comments about real-time versus delayed data are also accurate. If
I recall correctly, the StrikeFinder cost me around $4500 installed
several years ago. Today, I'm not sure that I'd spend that money on
sferics now that I have XM weather, based upon the type of flying that I
do. However, having both devices, it has been quite illuminating as to
how much the XM lightning product misses.



JKG

For those of you who have XM weather integrated with your GPS.

Do you use the weather information to paint a big picture of what's going on
around you. I.E. "If I fly 50 miles thataway, it looks like I'll be able to
circumvent this line of storms."

Mary and I are very new to XM, but have already used it on several
x-country flights, including one today that highlighted some weaknesses
in the system.

We flew from Racine, WI to Iowa City, IA, about a 1.5 hour flight,
depending on winds. A cold front was approaching from the north, with
a very juicy airmass in place to the south. Flight service indicated
good VFR all the way, leaving Racine around 7 PM. This would put us on
the ground right around sunset, which was our goal.

Using the airport's computer, the satellite pic showed no cloud-cover
over our route of flight at all. This, of course, was contradicted by
our Mark V eyeballs, and METARs that indicated a broken to overcast
layer at around 2800 feet, all the way from the Mississippi east to our
position on the western shore of Lake Michigan. Visibilities ranged
from 7 to 10 miles along the route of flight when we launched.

The XM weather on the 496 takes around ten minutes to upload, which
meant that we were already airborne by the time we were getting useful
weather. Although this is something we will learn to work around, I
find this time lag to be a bother. (I know, much wants more!) We're
going to have to get in the habit of turning on the 496 before engine
start, to allow time for downloading.

We were soon buzzing along under the overcast in smooth air but really
crappy visibility (especially when the sun would occasionally break
through and hit that moist, tropical air), and we were really glad when
the METARs finally downloaded into the Garmin.

We've mounted the 496 on the co-pilot's yoke, so Mary was working the
GPS for the first time, but had only minimal difficulties navigating
Garmin's excellent menus. She laughed when she was able to look at the
"live" satellite photo, which (like the one in the airport) showed
nothing but clear, blue skies ahead, while we were obviously under a
pretty thick overcast.

Strike one for XM. Apparently with a thin-but-solid overcast, the
satellite photo data is worse than useless. In fact, it shows perfectly
clear skies where none exist. (Given the same faulty presentation on
the airport's computer, I'm prepared to accept that this is not a flaw
in XM weather -- but it *is* a flaw, nonetheless.)

She then started checking METAR data ahead, to make sure that things
weren't falling apart along our route of flight. Having this data on
board is priceless, IMHO, and it showed that conditions were stable
until the Mississippi, and then improved dramatically to the west.
Obtaining this data is as easy as running your cursor over a little
triangle next to each reporting station, and having it in the plane was
a major reason for purchasing the 496.

However, after 30 minutes in the air, she noticed that the METAR data
had not updated. The NEXRAD radar and satellite data were updating
every 6 minutes or so, but the METARs remained the same as when they
initially uploaded.

Obviously, in changing flight conditions this slow rate of change is
simply unacceptable, and we quickly reverted to listening to AWOS's
ahead on the radio.

Strike two for XM.

By the time we hit the Big Muddy, we were able to climb on top of the
layer, which rapidly diminished to a thick haze layer. The rest of the
flight was uneventful, and we didn't refer to the 496 again.

IMHO, the jury is still out on the unit. It's wonderful for
watching/avoiding precipitation and storms, to be sure, but it
certainly proved to be less than useful on this flight. And I can
assure you that we fly in these kinds of fuzzy VFR conditions FAR more
often than we do when thunderstorms are threatening.

I'm going to contact Garmin about the update rate on METARs, and hope
there's some setting or software update I can install to improve the
rate of change. If it can't be improved, the unit will end up being far
less useful than expected.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

"Jay Honeck" wrote in message
oups.com...
[...]
Strike one for XM. Apparently with a thin-but-solid overcast, the
satellite photo data is worse than useless. In fact, it shows perfectly
clear skies where none exist. (Given the same faulty presentation on
the airport's computer, I'm prepared to accept that this is not a flaw
in XM weather -- but it *is* a flaw, nonetheless.)

I'd say that's a pretty sure indication that the satellite imagery itself
was defective. I don't know what might have caused that, but normally it
should be reliable, and I don't think you can blame that on the XM weather
stuff. They can't give you information that's not available.

[...]
However, after 30 minutes in the air, she noticed that the METAR data
had not updated. The NEXRAD radar and satellite data were updating
every 6 minutes or so, but the METARs remained the same as when they
initially uploaded.

When you called the FSS on the radio to obtain the latest METARs, were you
able to get more recent information (by a significant time period) than was
available from the XM weather? As I'm sure you know, METARs are only
updated once an hour at airports with an ATIS (unless conditions change
significantly), and where an automated observation system is used, the
updates may or may not happen frequently.

If the FSS doesn't have information that is significantly more recent than
that available from XM weather, then again I don't think you can blame that
on XM weather.

In both of these cases, the obvious comparison is between the XM weather and
what the FSS can tell you. They have the same satellite imagery, so if
their computers show clouds when XM doesn't, that'd be a problem. Likewise,
if they have METARs more recent than XM does, that'd be a problem.

Otherwise, it seems to me that XM is giving you the best information
available, and the problem lies in what information is actually available.
The real value for the XM weather is to provide a nice graphical description
of the data that the FSS can provide, and without having to use the radio.
Not having the most up-to-date information would be the fault of the FSS,
NOAA, NWS, etc.

If you didn't call the FSS on the radio to compare the information, then it
must not have been that important to you in the first place and so I'd
wonder why it's such a problem for XM to not provide it.

Pete

Strike one for XM. Apparently with a thin-but-solid overcast, the
satellite photo data is worse than useless. In fact, it shows perfectly
clear skies where none exist. (Given the same faulty presentation on
the airport's computer, I'm prepared to accept that this is not a flaw
in XM weather -- but it *is* a flaw, nonetheless.)

I'd say that's a pretty sure indication that the satellite imagery itself
was defective. I don't know what might have caused that, but normally it
should be reliable

Actually, I've noticed this many times in the past. Satellite photos
will show clear skies while we are actually beneath a solid overcast.
I don't know if the operators can selectively set the sensitivity of
the camera to not show thin layers, or what, but this isn't the first
time that satellite imagery of cloud cover has been 100% wrong.

However, after 30 minutes in the air, she noticed that the METAR data
had not updated. The NEXRAD radar and satellite data were updating
every 6 minutes or so, but the METARs remained the same as when they
initially uploaded.

When you called the FSS on the radio to obtain the latest METARs, were you
able to get more recent information (by a significant time period) than was
available from the XM weather?

We reverted to listening to AWOS and ASOS reporting stations, which
update every minute or so. I don't expect the 496 to be THAT quick or
accurate, but I would hope that XM could update METARs more often than
hourly.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Peter Duniho wrote:


I'm not sure what "operators" you think are there. The data comes in from a
satellite as a photographic image, and that image is relayed as the
satellite photo. There's not some guy sitting there tweaking the brightness
and contrast or something.

There aren't operators intervening most likely, but the data is far from
a simple "photographic image." The thing is intensity adjusted, noised
filtered, and geometrically remapped before it comes out in the format
that you see it in weathermation or whatever.

The METARs are not within XM's control\

Yes, I'm not sure what the filter is, but there seems to be some
filter on the thing just watching it at our local field. The
weather gets listed on the external sites hourly unless something
changes a lot, but I haven't figured out what it is, and how long
it waits after it thinks things have changed it waits to see if
the change was transient or worth reporting.

In article .com,
"Jay Honeck" wrote:

Using the airport's computer, the satellite pic showed no cloud-cover
over our route of flight at all. This, of course, was contradicted by
our Mark V eyeballs, and METARs that indicated a broken to overcast
layer at around 2800 feet,

IIRC visible satellite images will show low clouds as very dark... the bright
clouds are higher. Are you sure it showed no cloud-cover?

--
Bob Noel
Looking for a sig the
lawyers will hate