NMEA checksum calculation

Question

I have a problem with calculating the checksum for NMEA sentences. I am using the following java code:

private static String getSum(String in) {
    int checksum = 0;
    if (in.startsWith("$")) {
        in = in.substring(1, in.length());
    }

    int end = in.indexOf('*');
    if (end == -1)
        end = in.length();
    for (int i = 0; i < end; i++) {
        checksum = checksum ^ in.charAt(i);
    }
    String hex = Integer.toHexString(checksum);
    if (hex.length() == 1)
        hex = "0" + hex;
    return hex.toUpperCase();
}

This code is similar to many other examples around the internet and everything works fine until I try a sentence like this..

$PSRF101,-2686700,-4304200,3851624,96000,497260,921,12,3*1C

This sentence is from the NMEA Reference Manual and so I assume the checksum will be correct. But when I calculate it, I get *2F as the checksum and not 1C.

I think this is because of the negative values in the sentence, but I have no clue how to deal with them. Does anybody have a suggestion?

The - sign makes no difference: the checksum would still be 2F — David Grant
Incorrect, every character contributes. If, however, you remove BOTH of the dashes you are in effect XORing twice... which gets you back where you were. Removing one or the other absolutely does change the result. — Anders8

Aki Suihkonen Aki Suihkonen · Accepted Answer · 2012-10-04T10:33:31

The difference of the assumed and the calculated checksums equals to omitting (or having an extra character '3'); so I'd be tempted to believe in error in the NMEA Reference Manual.

You can try some online NMEA calculator to verify the results.
e.g. http://www.hhhh.org/wiml/proj/nmeaxor.html

NMEA checksum calculation

1 Answers