[amsat-bb] LHCP or RHCP?

On the premise that the only dumb question is the one that goes unasked, here goes.

If I'm building an antenna of circular polarization for use with the birds what determines if it should be LHCP or RHCP? I've read the ARRL Satellite Handbook and "sort of" searched the AMSAT-BB archives but found nothing about this question.

Thanks. Tom - K0TW