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Navigation help for AO51 wkg Pacific Maritime Mobile

Hi Amsat folk.

I need some help to figure out a schedule for G4FVG who is at sea on the Pacific Ocean. When AO51 "Echo" presents a path  between the ship P & O Nedlloyd "Mercator" and any land mass that might contain a few of us guys trying to work AO51, he would like to work the satellite - or at least hear it! His call might be G4FVG/MM.

I have the suitable software (Satscape) to provide a list of pass predictions
for known times in the future. The problem is that this is a big powerful ship
that moves to a new lattitude and longitude between AO51 passes. This is
about what to do if the QTH is also moving!

I would much appreciate some pointers as to how to figure out great circle routes between known locations, so I can get a set of UTC times that correspond to his earth positions. I need a the formula, and maybe some usage procedure. Then I think it has to be done by creating a whole load of these (temporary) QTH positions in Satscape, and making it generate a few passpredictions for each.

I know he was at 41-40N  143-00E (Japan) at  19:09 GMT on 26th Sept 2004.
He is heading for 48-28N  125-20W (Vancouver) eta 15:24 GMT on 3rd October.
The route is a southerly dip great circle with a midway point 41-40N  175-00W , which would be reached at 00:16 on 30th September. I understand they try to keep the ship speed constant.

Hmm.. maybe time to dig out those old college math notes!

Thanks for any help.
Graham G4WNT
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