Lots - for values of lots including "practically all" - of ships use the AIS (Automatic Identification System), which broadcasts various details on radio. For an example application, try www.aisliverpool.org.uk

On Fri, Apr 11, 2008 at 6:44 AM, <Valdis.Kletnieks@vt.edu> wrote:
On Tue, 08 Apr 2008 22:16:57 PDT, Joel Jaeggli said:

> spot can generally deliver an image within 1 day in 60kmx60km blocks
> assuming no contention for the slot. 20m resolution is more than
> adequate to pick up ships underway at sea. ikonos can deliver 11x11km
> swaths.

And ikonos can do a lot better than 20m resolution.  We had ikonos target
the campus for a special event a few months ago:

http://glovis.geog.vt.edu/hokiesthanktheworld/

"The full extent of the satellite image is approximately 100-square kilometers,
stretching from Brush Mountain (upper left) across Blacksburg (center) to
Ellett Valley (lower right). The Virginia Tech Drillfield is located near the
center of this November 17, 2007 scene from the GeoEye IKONOS satellite. This
true color rendering by Peter Sforza combines red, green, and blue wavelengths
(3.7-meter pixels) into a RGB color image, and sharpened using the panchromatic
band (0.92-meter pixels). Note the sun's angle of elevation was 32 degrees and
the azimuth was 164.1 degrees (north being zero)."

1-meter pixels sounds about right - if you look at the right place on the
full-scale image, you can find (and tell the difference between) my dark green
Camry, and the immediately adjacent dark grey Nissan my neighbor drives, and
still see 3 feet worth of parking lot pavement in between them too...

Not bad for 423 miles up and moving at 4.7 miles per second....