## 3 Materials and Methods
### Proposed Shipboard Data Compression Method
These properties are constant and are thereby gathered in the data item table, prepared and recorded in advance. Figure 12a shows the table of data items used on board TS _Fukae Maru_, where one line defines the properties of one data item. The structure of such data line is illustrated in Figure 12b on the example of a data item representing the COG. As shown in Figure 12b, each line contains the following information: the data item number, the number of digits after the decimal point, the data item name, the unit, the value range ID, and the minimum and the maximum value of the data item range. Commas separate these line elements. Figure 11: Proposed format of one data record. Figure 10: Lengths of binary fields composed of six-bit words and the corresponding data value ranges. 20,0,\" GPS Time(hhmmss)\",\"\",3,000000,235959,
21,0,\" GPS Time(msec)\",\"\",2,000,999,
22,0,\"GPS Status\",\"\",2,65,86,
23,1,\"GPS Time Difference\",\"hours\",-4,-12.5,12.5,
24,0,\"GPS(UTC) Date[YYYYMMDD]\",\"\",4,20000101,20301231,
25,2,\"GPS(UTC) Time[hhmmss.xx]\",\"\",4,00000.00,235959.99,
26,0,\"GPS Latitude(deg)\",\"deg\",-2,-90,90,
27,6,\"GPS Latitude(min)\",\"min\",5,00.000000,59.99999,
28,0,\"GPS Longitude(deg)\",\"deg\",-2,-180,180,
29,6,\"GPS Longitude(min)\",\"min\",5,00.000000,59.99999,
30,1,\"GPS COG\",\"deg\",2,000.0,360.0,
31,1,\"GPS SOG\",\"kts\",-2,-99.9,99.9,
32,1,\"GPS Mag\",\"deg\",2,000.0,360.0,
33,1,\"GPS Variation\",\"deg\",-2,-90.0,90.0,
34,0,\"GPS Altitude\",\"m\",-3,-99,999,
35,0,\"GPS No. of Satellite\",\"\",1,0,50,
36,1,\"GPS HDOP\",\"\",2,0.0,20.0,  
...  
(a)  
30,1,\"GPS COG\",\"deg\",2,000.0,360.0,  
|| || || || maximum value  
|| || || minimum value  
|| || || value range ID  
|| || unit  
|| name  
|| number of digits after decimal point  
data item number  
(b)  
The binary patterns resulting from binary data encoding are converted to ASCII text, where an ASCII character is assigned to each group of six bits. Thus, obtained text forms one data record. If the obtained text is longer than 60 characters, it is divided into multiple lines. Since six-bit words are used, a total of 64 ASCII characters is required for data encoding. The characters used by the proposed encoding technique are shown in Table 2 as the shaded area of the ASCII code table. As shown in Table 2, the proposed technique utilizes 64 consecutive ASCII characters, thus enabling the binary values to be converted into hexadecimal by simply adding a value of 0 \(\times\) 30. Therefore, the proposed conversion is more straightforward than the one employed by the AIS data encoding. Table 3 shows the proposed six-bit binary field conversion table. The proposed data record format also includes a two-digit hexadecimal checksum appended to the message. The checksum is computed by performing the exclusive or (XOR) operation on all ASCII code characters in the message following the \"1\" character. Finally, one line of the data record in the proposed format starts with the \"1\" character and includes the timestamp (HHMMSssss) denoting hours, minutes, seconds, and milliseconds, the serial number used when the transmitted data text is divided into several lines, the text obtained by the six-bit data encoding, \"\" character, and the checksum. Listed data line elements, except the checksum, are separated by comma (\",\") delimiters. The flowchart of the proposed data encoding method and the resulting format of the transmitted data are shown in Figure 13.  
Figure 12: Data items definition: (**a**) Table of data items for TS _Fukae Maru_; (**b**) Structure of a data line defining properties of one data item.