Howdy! Holger,
On the ARRL Grid Squares page
http://www.arrl.org/grid-squares

I found several links on their page that was of interest to Me

The first link is to a Program
http://w1ghz.org/new/bd_2004.zip

Here is the second link
http://www.levinecentral.com/ham/grid_square....
I removed the grid_square.php in the Address and found
a link to His Ham page at:
http://www.levinecentral.com/ham/index.html
You may want to look at the other links on His page

Here is the third link
http://stevemorse.org/jcal/latlon.php
He also has other links

The fourth link is to a AMSAT page 
http://www.amsat.org/amsat-new/tools/grids.ph...

I really liked the seventh link to the Summits on the Air page
http://www.sotamaps.org/

On the SOTA page
Click the Map Grid Options button at top center of page
Then check the Display Grid check box and select one of the two options
Locator Grid
   or
Latitude/Longitude Grid

There is another button for displaying if the area is in Daylight or Darkness

That page has it ALL!

Hope these links are of interest to You.

73 de Ed W9ODR .    .

... As I said before... I never repeat myself.
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