U-blox7 UBX Checksum in Python

def ubx_checksum(packet_data):
    ck_a = 0
    ck_b = 0
    for i in packet_data[2::]:
        ck_a = (ck_a + i) & 0xff
        ck_b = (ck_b + ck_a) &0xff
    return (ck_a, ck_b)

 Usage:

packet_data = [0xB5, 0x62, 0x06, 0x04, 0x04, 0x00, 0x00, 0x00,0x08, 0x00]
print "0x%x, 0x%x" % ubx_checksum(packet_data)

Result:

 0x16, 0x74