Question

我正在使用AIS（自动识别系统）数据来定位船只。我能够遵循this guide来成功解码几乎所有信息（与在线解码here相比）。

但是，我在经度部分遇到了问题。我认为与十进制值为负有关，但是我无法弄清楚要正确修改代码中的内容。

TLDR版本：如何从二进制字符串1101001000001001001110010000转换为十进制值-48196720（或48196720）？

完整版本：

玩具数据：

library(dplyr)
library(tidyr)
library(stringr)

# choose an example - two strings are provided.  
# The first string shows the issue with the longitude, 
# whereas the second string (where longitude is positive) has no issue
s <- "15E3tB001;J@BLPaK5j7qFA406;d" 
# s <- "133m@ogP00PD;88MD5MTDww@2D7k"
### for input into the online decoder - use these: 
# !AIVDM,1,1,,A,14eG;o@034o8sd<L9i:a;WF>062D,0*7D
# !AIVDM,1,1,,A,133m@ogP00PD;88MD5MTDww@2D7k,0*46

temp <- data.frame(V6 = s) %>%
    # splitting the AIS info into separate characters
    mutate(char_split = str_split(V6,pattern=""))
temp$Text <- apply(temp, 1, function(x) paste(unlist(x$char_split), collapse = ","))        

temp <- temp %>%
    select(-char_split) %>% 
    # and then into separate columns    
    separate(Text, into = paste0("v", 1:43, sep = ""), sep = ",", fill = "right") 

ASCII <- temp %>%
    select(v1:v43)
# translating to ASCII
ASCII <- apply(ASCII, c(1, 2), function(x) utf8ToInt(x))
# translating to 6-bit
ASCII <- apply(ASCII, c(1, 2), function(x) ifelse(x <= 88, x - 48, x - 48 - 8))

一旦数据以ASCII格式，则需要转换为二进制

# making binary
Binary <- apply(ASCII, c(1, 2), function(x){ paste(rev(as.integer(intToBits(x))[1:6]), collapse = "")})
# pasting all the binary info into a single string
temp$Binary <- apply(Binary, 1, function(x) paste(x, collapse = "" ))
temp <- temp %>%
    select(V6, Binary) %>%
    # selecting bits of the single binary string, 
    #and translating back to decimal, as per the guide
    mutate(MMSI = strtoi(substr(Binary, 9, 38), base = 2),
            SOG = strtoi(substr(Binary, 50, 60), base = 2)/10,
            Accuracy = strtoi(substr(Binary, 61, 61), base = 2),
            Lon = strtoi(substr(Binary, 62, 89), base = 2)/600000,
            Lat = strtoi(substr(Binary, 90, 116), base = 2)/600000,
            COG = strtoi(substr(Binary, 117, 128), base = 2)/10,
            Heading = strtoi(substr(Binary, 129, 137), base = 2))

输出：

select(temp, -Binary, -V6)

当我与在线解码器进行比较时，除了经度之外，其他所有东西都匹配。在解码器中，结果是80.3278667（尽管实际上是-80.3278667），而我的是367.0646。为了对此进行反向工程，我看了temp$Binary的相关子字符串：

mine <- substr(temp$Binary, 62, 89)
RevEng <- -80.3278667 * 600000
binaryLogic:::as.binary(as.integer(RevEng), signed = FALSE) 
mine

所以看起来RevEng值匹配我的二进制字符串的右尾，但是我不知道为什么它不匹配完整的二进制字符串，或者从这里开始怎么做...

Answer 1

与博客文章告诉您的相反，经度是一个有符号的整数。但是，它仅使用28位，而R内部使用32位。因此，您必须自己处理two's compliment conversion。对于设置了最高位的任何数字，您都必须减去2^28，例如：

mine <- "1101001000001001001110010000"
strtoi(mine, base = 2) - 2^28
#> [1] -48196720

您可以使用二进制字符串上的substr或通过查找数字>= 2^27来标识这些数字。

顺便说一句，对纬度的修改也是如此，因为它仅使用27位。

R二进制/十进制转换混乱-AIS数据

1 个答案: