Garmin 396 Weather avoidance..

Roger wrote:

On 9 Jun 2006 17:04:28 -0700, "Robert M. Gary"
wrote:


The 396 has some advantages and disadvantages when compared to
airborne RADAR. Starting with the good, it does not have the blind
spots you will find from time to time in airborne RADAR caused by
absorption in heavy precipitation which can hide some nasty stuff.
OTOH if you keep in mind that the display is probably 5 minutes old or
a tad more AND you have been following it you can pick your course.

All systems have their limitations, including airborne weather radar.
That is the reason that the prudent operation of airborne weather radar
requires minimim avoidance distances, depending upon altitude and
weather the outside air temp is above freezing.

The limitation you cite indeed exists but can be avoided through use of
distance-to-avoid parameters and not pushing the envelope to get the
mission accomplished, so to speak. The EAL wind shear crash at JFK, the
Delta L-1011 wind shear crash at DFW, and the Soutern Airways DC-9 crash
in southern Georgia all happaned when penetration rather than avoidance
was attempted..

The ideal setup in high-end aircraft today is airborne radar with the
largest feasible antenna and piped in weather radar for planning
purposes. The latter doesn't work in much of the world, though, just
like the 396 won't provide weather outside the 48 states.

Dan wrote:
For those of you with a Garmin 396, how do you avoid dangerous weather,
avoid yellow and steer clear of the lightning strike indications? I am
considering the purchase of one and am wondering how to use the info
safely, but yet with the maximum utility.

I just want to add to those who suggested getting your feet wet a little at a
time and using the 396 information conservatively until you have some experience
with it:

The 396 I think still comes with the automobile kit, so on a day with some
active weather you can stick the 396 in your car and drive around looking for
interesting conditions. Its' a lot cheaper and safer to experiment in your car.

Dave

Dave Butler wrote:


The 396 I think still comes with the automobile kit, so on a day with
some active weather you can stick the 396 in your car and drive around
looking for interesting conditions. Its' a lot cheaper and safer to
experiment in your car.

Sure does. The 396 is basically a 296 with weather option. It does
planes, cars, and boats.

Several posters have stated that NEXRAD images displayed on the Garmin 396
are five or more minutes old because that's how long it takes for the images
to be processed before they're sent to the XM satellite.

What kind of processing could possibly take five minutes? I suspect
whomever's in control of the processing is introducing the delay so they can
sell seconds old data as a premium service.

Jon

Jon Woellhaf wrote:

Several posters have stated that NEXRAD images displayed on the Garmin 396
are five or more minutes old because that's how long it takes for the images
to be processed before they're sent to the XM satellite.

What kind of processing could possibly take five minutes? I suspect
whomever's in control of the processing is introducing the delay so they can
sell seconds old data as a premium service.

As far as I can figure out, the Nexrad updates on the www.nws.noaa.gov
occur every few minutes.

Wouldn't it require a whole lot more bandwidth to see a Nexrad site near
real time?

Guess I should have remembered that. But then he should not have inserted
his comments about what he sees at a terminal facility into a discussion of
delays in cockpit weather/WARP. His radar doesn't feed NOAAPORT, so his " I
get updated every six seconds" is irrelevant.

Bob

"Sam Spade" wrote in message
news:QFdjg.178854$bm6.100188@fed1read04...
Bob Gardner wrote:
OK. Apples and oranges. Way back at the beginning of this thread we were
talking about using an [airborne] 396 weather display. The information
sent to cockpits, installed or handheld, is NEXRAD-based, and I jumped in
to emphasize the point that real-time weather does not exist in the
cockpit unless you have airborne weather radar, which few have. Now, all
of a sudden, you chime in with what you see at a terminal facility?
Without disclosing the kind of facility you work at?
\

My recollection is that he has told the group more than once that he works
at Billings TRACON.

The NEXRAD site, in order to deliver composite reflectivity, must complete a
full scan. That is, one rotation at each elevation of the transmitted beam.
That's what takes the time. Why the WARP system is further delayed is pretty
much a function of collecting scans from all of the relevant NEXRAD sites
and combining them into a mosaic, and I have no idea how much time that
takes.

Bob

"Jon Woellhaf" wrote in message
news
Several posters have stated that NEXRAD images displayed on the Garmin 396
are five or more minutes old because that's how long it takes for the
images to be processed before they're sent to the XM satellite.

What kind of processing could possibly take five minutes? I suspect
whomever's in control of the processing is introducing the delay so they
can sell seconds old data as a premium service.

Jon

Bob Gardner wrote:

This is from WSI's web page. Note the "near real-time" qualifier and
"mosaic."

Thanks, Bob. I am aware of the limitations of this and any downlinked
RADAR mosaic. In addition to Flight Service/Flight Watch, ATC weather
radar (where applicable), and eyeballs, I do use WSI for tactical (big
picture) weather avoidance. It is because of this that I was interested in
reading of the details of this particular accident.

There seem to be some vague details in the explanation of this accident
that I would like to explore. The fact that the accident aircraft was a
C172 (TAS 125 knots) and that it flew into an area of previously
convection-free activity suggests that the downlinked RADAR data were very
stale.

I certainly don't have the experience you have, but I have seen areas go
from no precipitation to level 5 precipitation in about 15-20 minutes. Is
it possible for a strong thunderstorm cell to mature quicker than that?

WSI advertises (at least when I first bought the receiver) that the radar
download would never be more than 4 minutes old. However, add to that the
"pre-processing" that they apply to the data and perhaps the picture is up
to ten minutes old.

With my WSI install, I have experienced numerous downlink outages (which
were attributable to both WSI system outages and installation problems at
my end) and significant delays in the data of up to 25 minutes, so I
learned long ago that this tool could never be used as a replacement to
active, on-board radar for navigating through a convective line.

It is worth mentioning that when the system was working and refreshing once
every four minutes, I did find the precipitation levels seem to match that
through which I was currently flying or that which I could see in the
distance.


--
Peter

On Mon, 12 Jun 2006 09:27:06 -0600, "Jon Woellhaf"
wrote:

Several posters have stated that NEXRAD images displayed on the Garmin 396
are five or more minutes old because that's how long it takes for the images
to be processed before they're sent to the XM satellite.

What kind of processing could possibly take five minutes? I suspect

Bob said it better than I, but it takes multiple scans, the
application of a number of algorithms, and then combining.

For an explanation as to how NEXTRAD works try
http://www.desktopdoppler.com/help/nws-nexrad.htm

We have to be careful when we refer to real time as with NEXRAD real
time does not exist. When we see the words "real time" as applied to
NEXRAD "I believe" they are referring to the time when the processing
is completed to the time you receive the image. On my systems I see
updates every 5 to 6 minutes during severe weather. Given that the
RADAR composite image takes 5 to 6 minutes to compose and it's updated
as soon as the image is completed on the commercial, subscription
sites what you know is the information is going to be older than 5
minutes. However the time from the first scan until you see the newest
image is going to be 5 to 6 minutes minimum. IOW the storm can change
substantially while the RADAR image is being generated.

There is such a thing as real time RADAR, but it's not NEXTRAD.
NEXTRAD is a very complex *system* that includes more than a simple
reflection. It includes radial winds, Rainfall accumulation, and about
5 to 10 other parameters including several modes of which I'd have to
go to the site to read.

You also have to remember that what NEXTRAD shows close in to the
station is not the same as it shows farther out. Close in images
extend from near ground level up to some specific height/altitude. 50
miles out those images start around 3000 AGL and may extend up almost
twice as high as the ones close in. Near the outer edges of the
coverage area the images may only extend down to about 5000 AGL.

NEXTRAD images composed from *Doppler* RADAR need to be processed to
find speed, intensity, and direction at each of the various levels and
then those images combined. It is much more than simply combining the
reflected signals at the various levels. Currently I believe for
complete processing it takes 5 to 6 minutes for NEXTRAD to update when
in the base reflectivity mode.

I subscribe to two RADAR services which provide essentially the same
information. It's rare to see both systems down at the same time. If
the problem is at the NWS end then both get behind. I typically have
more complete and up-to-date information including storm track
prediction than they have at the local EOC, but like interpreting the
information on the 396 there is even more to interpret on the screens
here.

Also I base my statements on what the researcher said about the next
generation RADAR versus the current as well as conversations with NWS
people at Sky Warn training sessions as well as descriptions as to how
NEXTRAD works. IE, it updates about every 10 minutes in clear air
mode and every 5 to 6 minutes in the base reflectivity mode.

whomever's in control of the processing is introducing the delay so they can
sell seconds old data as a premium service.

As far as I know the processing is all done by the NWS with only the
inherent processing delay. There is the processing delay and then
there is the delay that is introduced to those images that are
provided free and has nothing to do with the processing delay. IE they
just aren't put up as soon as the ones we pay for.

I already pay that premium as does the satellite service for the
images down linked to the 396.

The important thing to remember is that NEXTRAD "does not produce real
time images!" Period. For confirmation of that just go to the NWS
site for the explanation as to how NEXTRAD works.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

Jon

On Mon, 12 Jun 2006 06:16:21 -0700, Sam Spade
wrote:

Roger wrote:

On 9 Jun 2006 17:04:28 -0700, "Robert M. Gary"
wrote:


The 396 has some advantages and disadvantages when compared to
airborne RADAR. Starting with the good, it does not have the blind
spots you will find from time to time in airborne RADAR caused by
absorption in heavy precipitation which can hide some nasty stuff.
OTOH if you keep in mind that the display is probably 5 minutes old or
a tad more AND you have been following it you can pick your course.

All systems have their limitations, including airborne weather radar.
That is the reason that the prudent operation of airborne weather radar
requires minimim avoidance distances, depending upon altitude and
weather the outside air temp is above freezing.

The limitation you cite indeed exists but can be avoided through use of
distance-to-avoid parameters and not pushing the envelope to get the

But again in the context of the OP it takes experience to realize
these things exist.

When you see a line and particularly a bow that starts out green on
your side, then yellow and then red followed by nothing it's time to
go some where else. That is no guarantee that sever weather exists
behind that line but it's a good indicator.

Like you and others have said, being conservative, using all available
information, and education are the important items.

mission accomplished, so to speak. The EAL wind shear crash at JFK, the
Delta L-1011 wind shear crash at DFW, and the Soutern Airways DC-9 crash
in southern Georgia all happaned when penetration rather than avoidance
was attempted..

When this stuff can take the "big boys" down the smaller stuff should
be some where else entirely.


The ideal setup in high-end aircraft today is airborne radar with the
largest feasible antenna and piped in weather radar for planning
purposes. The latter doesn't work in much of the world, though, just
like the 396 won't provide weather outside the 48 states.

There are areas where it won't do that good a job inside the US
either, but for the most part it can be a very useful tool,
particularly when used in conjunction with other available
information.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com