Decode the differece between GNSS and barometric altitude

    Args:
        msg (string): 28 bytes hexadecimal message string, TC=19

    Returns:
        int: Altitude difference in ft. Negative value indicates GNSS altitude
            below barometric altitude.

def altitude_diff(msg):
    """Decode the differece between GNSS and barometric altitude

    Args:
        msg (string): 28 bytes hexadecimal message string, TC=19

    Returns:
        int: Altitude difference in ft. Negative value indicates GNSS altitude
            below barometric altitude.
    """
    tc = common.typecode(msg)

    if tc != 19:
        raise RuntimeError("%s: Not a airborne velocity message, expecting TC=19" % msg)

    msgbin = common.hex2bin(msg)
    sign = -1 if int(msgbin[80]) else 1
    value = common.bin2int(msgbin[81:88])

    if value == 0 or value == 127:
        return None
    else:
        return sign * (value - 1) * 25