code: def _at(self, t):
          
          pos, vel = terra(self.latitude.radians, self.longitude.radians,
                           self.elevation.au, t.gast)
          pos = einsum('ij...,j...->i...', t.MT, pos)
          vel = einsum('ij...,j...->i...', t.MT, vel)
          if self.x:
              R = rot_y(self.x * ASEC2RAD)
              pos = einsum('ij...,j...->i...', R, pos)
          if self.y:
              R = rot_x(self.y * ASEC2RAD)
              pos = einsum('ij...,j...->i...', R, pos)
          # TODO: also rotate velocity
  
          return pos, vel, pos, None
docs: Compute the GCRS position and velocity of this Topos at time `t`.