Dave, WW2R/G4FRE kindly pointed me in the direction of a really interesting article, dating back to 1998, written by Dr Tony Phillips on the NASA website. Rather than using the GRAVES transmitter in France, this is based on the NAVSPASUR radar in in Texas. This radar operates at 216.98MHz as opposed to the 143.049MHz at GRAVES.
The article shows some great illustrations of reflections AND satellite echoes which is something I was really curious about.
Enjoy the article here
I'd be interested to hear from any North American readers if they have any success listening to this system.