def airspeed_voltage(VFR_HUD, ratio=None):
    '''back-calculate the voltage the airspeed sensor must have seen'''
    import mavutil
    mav = mavutil.mavfile_global
    if ratio is None:
        ratio = 1.9936 # APM default
    if 'ARSPD_RATIO' in mav.params:
        used_ratio = mav.params['ARSPD_RATIO']
    else:
        used_ratio = ratio
    if 'ARSPD_OFFSET' in mav.params:
        offset = mav.params['ARSPD_OFFSET']
    else:
        return -1
    airspeed_pressure = (pow(VFR_HUD.airspeed,2)) / used_ratio
    raw = airspeed_pressure + offset
    SCALING_OLD_CALIBRATION = 204.8
    voltage = 5.0 * raw / 4096
    return voltage
back-calculate the voltage the airspeed sensor must have seen