![]() |
If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
|
#1
|
|||
|
|||
![]()
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 |
Thread Tools | |
Display Modes | |
|
|
![]() |
||||
Thread | Thread Starter | Forum | Replies | Last Post |
Amateur Review of the Garmin GPSMAP 296 GPS | Rhett | Piloting | 10 | March 23rd 05 01:16 AM |
Pirep: Garmin GPSMAP 296 versus 295. (very long) | Jon Woellhaf | Piloting | 12 | September 4th 04 11:55 PM |
Garmin 430 Terrain Avoidance | endre | Instrument Flight Rules | 5 | July 22nd 04 03:41 AM |
FA: WEATHER FLYING: A PRACTICAL BOOK ON FLYING | The Ink Company | Aviation Marketplace | 0 | November 5th 03 12:07 AM |