def vtas2cas(tas, h):
    """ tas2cas conversion both m/s """
    p, rho, T = vatmos(h)
    qdyn = p*((1.+rho*tas*tas/(7.*p))**3.5-1.)
    cas = np.sqrt(7.*p0/rho0*((qdyn/p0+1.)**(2./7.)-1.))

    # cope with negative speed
    cas = np.where(tas<0, -1*cas, cas)
    return cas
tas2cas conversion both m/s