Fishery harvesting information compressing and transmitting method based on Beidou short message

Abstract: Getting fishery-harvesting real-time information can provide data for investigating the present situation of the South China Sea fishery resources, and provide decision support for the conservation of the South China Sea fishery resources. Most of the vessels fishing in the South China Sea have been installed BDG-MF-05 ship terminal by BDStar. Beidou satellite system capable of timing, positioning can also send and receive short message. Our team has developed fishery-harvesting information collecting terminal based on S3C2440. This terminal, sharing the same channel with BDStar ship terminal, communicates real-time fishery-harvesting information with the land data center by Beidou short message, but the Beidou system's civil message service has the problems such as packet loss, limited communication frequency and packet length. Through analyzing the characteristics of the fishery-harvesting information and the short message space, one strategy of the byte-space allocation for fishery-harvesting information message has been proposed. The strategy was that packet number took up 1 byte, vessel type took up 4 bit, longitude and latitude took up 36 bit, net-hauling time and work hours took up 24 bit, and terminal number took up 1 byte. The total was 11 bytes. According to BDStar ship terminal user manual, short message communication limit is 41 Chinese characters/time. If the bytes number associated with fish yield data, according to a certain GB2312 encoding strategy: fish yield data plus 11, divided by 3 , than multiplied 4, its value is less than or equal to 41 times 2, and fish yield data plus 11 should be a multiple of 3, the maximum value of fish yield data was 49. It meant that the number of bytes related to fish yield data was up to 49. Each type of vessel focused on 31 different kinds of economic fishes, and every kind of fish yield was allocated 2 bytes. Under the condition of 49 bytes space limitation, how to compress fish yield data and transmit one-time, was the key problem to solve. Analysis result showed that mainstream lossless compression algorithms such as RLE (run-length encoding), LZW (Lempel-Ziv-Welch Encoding), Huffman coding and arithmetic coding was not suitable for compressing the fish yield data. So, in this paper, we put forward two kinds of fish yield data compressing methods: fish-code mode and index-code table mode. The fish-code mode could only be used to describe less than or equal to 16 kinds of fishes yield data at a time. When more than 16, data could be divided to second transmission. Index-table mode completely had the ability to adapt to describing less than or equal to 20 kinds of fishes yield data at a time. When meeting some conditions, index-table mode could be used to describe 21 to 31 kinds of fishes yield data at a time, and fish-code mode had not such a possibility. In the index-code table mode, assuming to describe X kinds of fishes yield data at a time, it must meet that there were no more than 41-X kinds of fishes yield data containing two non-zero bytes. When describing less than or equal to 8 kinds of fishes yield data, the ability of the two modes was almost the same. When describing more than 8 kinds of fishes yield data, index-code table mode needed fewer numbers of bytes than fish-code mode. In order to further enhance compressing ability of the index-code table mode, in this paper, we discussed the method of transforming the fixed-length index-code table to the variable-length index-code table based on Huffman coding. Although that might save the byte space more, the effect was not obvious, and at the same time that would increase the encoding and decoding complexity. Therefore, in practice, fixed-length index-code table mode was adopted, and the experiment proved that the method could satisfy the fishery harvesting information real-time communication based on Beidou short message.