Presented at NOAA WebShop '97, 23 Oct 1997, Silver Spring, MD

Presenting Interactive Satellite Image Loops using Java, IDL, Perl, C, and Fortran

Delivered by Lonnie Maynard of FSL

Index:

Available Products

FSL's Satellite Looper page shows the following products:

flow charts: small | large
Data arrives at FSL's GOES Local Groundstation System
- Applet currently shows only GOES-8 data
Converted to Lambert Conformal Conic Projection
- (standard projection used by NWS Workstations)
5-km resolution (for large national image)
Fast spatial bilinear interpolation technique
- developed by Tom Schlatter and implemented by Dan Birkenheuer
Uses navigation information from the satellite itself
Results in very little jitter and shifting
- (much less than raw feed seen on WeFax)
Fortran and C languages, output in netCDF format

Scripts written in Interactive Data Language (IDL) which allows rapid processing
Political boundaries added
- xor filter used to generate light lines on dark data and vice-versa
Color Coding added
Time and labels added
Output (now) in JPEG format
- 60% quality factor
- 8-10 times smaller than GIF
- quality indistinguishable from GIF
Images (except for large) sub-sampled down to 600 x 448 pixels, 45 Kilobytes

Four threads (in addition to system-generated threads)...

...interact with cgi scripts at FSL (in perl) that handle image indexing and usage logging.

Runs when requested by Monitor
Notifies the logging script
Downloads a list of available images from the image-index cgi script.
Compares to files already loaded
If a new image is available...
- pushes down other images
- loads new image(s)
- resets the time to wait for the next update
If unable to connect to server
- Notifies the user
- Checks again later
- (Thus, a user can disconnect from the internet, and still use the applet, and it will update whenever it can)

Runs when requested by Updater
Loads images in a separate (system) thread
Uses the MediaTracker class to notify when each image is completely loaded
Not bomb-proof
- If user decreases the number of images, there can be trouble
- If not enough memory, browser can crash

Runs when the applet is started and looping
- Started means when the applet is displayed or iconified. If the applet doesn't watch for this, we've committed a denial of service attack
Double-buffers images for speed
- so the applet doesn't have to re-render images when the applet window is uncovered by another window

Rapid feedback to the user via the 'time' and 'loaded' feedback window(s)
Loop flexibility
- Based on 15-years of FSL experience
JPEG format results in much faster loading than GIF with no visible degradation in quality
In-house remapping results in stable images
Auto-update(?)
- most users don't use
Ease of re-adaptibility
- Ronnie Holman's WEFAX/APT Weather Page
Intelligent logging

Memory management - browser can crash
Relative priority of threads is hard/impossible to control, particularly when images are loading
The browser cache is hard to deal with:
- files with unchanging names, such as contents.txt have to be supplied by a cgi script that includes Pragma: no-cache in the header, or they may not be reloaded
- No easy push for new versions into the browser's mysterious (to me) applet cache.
  - (We had to rename classes to force a new version to be used. Is there a better method?)
Different browsers behave slightly differently

Over 200 hits per day when the weather is interesting
Very helpful in-house, where loading is very rapid (a few seconds per image)
Has been re-adapted successfully elsewhere
Has been adapted to additional image types such as local wide-band NEXRAD radar.
Positive comments from users
(Code is freely available )