r. king   june 1977  subroutine fermn
calculate interferometry observables for spots on moon or
artificial space probes in cislunar space with central body
either the earth or moon.
this is a revised version of old pep subroutine mnterf,
written originally by m. ash and r. preston, feb 1970, and
modified dec 1971 by r.king to calculate differential
n-count (quasi-vlbi) observable - either accumulated
differential cycle count (difnct) or differential delay
increment (ddi)
fermn is analogous to subroutine radmn

end of iteration loop for first site position
(also second site position if counted-cycle vlbi)
for counted-cycle observable determine send time
given receive time at second site  ( n = 2 )