subroutine fairway( vsurface, alpha,  t0,dt, s0,ds, scan,nt,ns)
integer it,is,                                           nt,ns
real	t,s,vel,    vsurface, alpha,  t0,dt, s0,ds, scan(nt,ns)
do is= 1, ns {	s= s0 + ds*(is-1)
do it= 1, nt {	t= t0 + dt*(it-1) 

	vel= vsurface * exp( alpha * t )	# default alpha=.156

	if( s > 1./vel +.1)	scan(it,is) = 0.
	if( s < 1./vel -.1)	scan(it,is) = 0.
	if( s > 1./1.6    )	scan(it,is) = 0.
	}}
return;	end