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