Garmin 396 Weather avoidance..

On Sun, 11 Jun 2006 18:24:20 -0700, "Bob Gardner"
wrote:

http://www.faa.gov/aua/weather/warp/

Read the first paragraph.

Which if I understand right is about 5 minutes old by the time the
images are processed. This is the same "real time" stuff I get in my
subscription service. "Real Time" in this context meaning as soon as
they get it processed.

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

Bob

"Newps" wrote in message
...
I'm not aware of ATC displaying NEXRAD although I suppose it's possible.
We display real time weather in the TRACON up to six seconds old.

Ronnie wrote:

Bob,

Was this a center or approach control facility and more
specifically, was the weather info display primary or
NEXRAD? If NEXRAD, did they discuss the amount
of delay in the weather info on the controller's display? I'm
wondering if it is significantly shorter? I know the satellite
broadcast distribution adds some additional delay, but as I
undersrand it, NEXRAD radar systems take a few minutes
to build the composite view from several sweeps. Point is,
the controller's display may be a few minutes old as well.

Also, as you point out, the weather chances quickly and
the radar is only showing precip. Thus, it is not showing
the radid air currents of a developing storm. Until you have
experienced being sucked up into a radily developing convective
storm by flying through an area that your StrikeFinder and ATC
agreed was the best path, you don't fully appreciate how danerous
this type of weather can be.

I second your recommendation to stay well clear of this type of
weather.

Ronnie


"Bob Gardner" wrote in message
. ..

The first thing you need to understand is that the weather you see on the
396 is several minutes old...possibly as much as eleven minutes. So you
need to watch the display and figure out which way the echoes are going,
and plan on passing them on the upwind side. Then you need to realize
that radar reflects only decent size rain droplets, not vertical
currents, not the tops of clouds, not turbulence. Bottom line is: Stay
well clear of anything green...yellow and red go without saying. If you
have to get into the green, make the incursion as short as possible.

Saw a revealing presentation at a NATCA controller's conference. They
showed a slide with several large cells being displayed, and indicated
the position of a 172 when the scenario began. The pilot told the
controller that if he could have a certain heading, he would be clear of
the cell he saw on his screen and could proceed to his destination.
Unfortunately, by the time he got to the geographical area which was nice
and clear on his cockpit display at the time he asked for the heading,
the cell had moved directly into his path with predictable results. The
message to the controllers in attendance was "What the pilot sees in the
cockpit and real life are two different things."

Bob Gardner

"Dan" wrote in message
legroups.com...

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.

Dan

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?

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

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