double eraPap	(	double	a[3],
		double	b[3]
	)

Definition at line 3 of file pap.c.

References eraPdp(), eraPm(), eraPmp(), eraPn(), and eraPxp().

Referenced by palDpav(), and t_pap().

{
   double am, au[3], bm, st, ct, xa, ya, za, eta[3], xi[3], a2b[3], pa;

/* Modulus and direction of the a vector. */
   eraPn(a, &am, au);

/* Modulus of the b vector. */
   bm = eraPm(b);

/* Deal with the case of a null vector. */
   if ((am == 0.0) || (bm == 0.0)) {
      st = 0.0;
      ct = 1.0;
   } else {

   /* The "north" axis tangential from a (arbitrary length). */
      xa = a[0];
      ya = a[1];
      za = a[2];
      eta[0] = -xa * za;
      eta[1] = -ya * za;
      eta[2] =  xa*xa + ya*ya;

   /* The "east" axis tangential from a (same length). */
      eraPxp(eta, au, xi);

   /* The vector from a to b. */
      eraPmp(b, a, a2b);

   /* Resolve into components along the north and east axes. */
      st = eraPdp(a2b, xi);
      ct = eraPdp(a2b, eta);

   /* Deal with degenerate cases. */
      if ((st == 0.0) && (ct == 0.0)) ct = 1.0;
   }

/* Position angle. */
   pa = atan2(st, ct);

   return pa;

}

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