This is a javascript decoder for the GlobalSat LT-20 LORAWAN GPS tracker. The decoder is built to work with TheThingsNetwork/Stack V3

lt-20_payload_decoder.js

/*
This is a javascript decoder for the GlobalSat LT-20 LORAWAN GPS tracker.
The decoder is built to work with TheThingsNetwork/Stack V3
Special thanks to Gul-Dukat, AH and akenza.io for initial work.
The script was found on the Forum of TheThingsNetwork. But it had some flaws, where
".toString(16)" would remove the leading zero of a hex-string, so sometimes it would happen to 
get the wrong latitude/longitude.
*/


//Functions Convert HEX to Bits
function hexToBits(hex) {
    var out = "";
    for (var c of hex) {
        switch (c) {
            case '0': out += "0000"; break;
            case '1': out += "0001"; break;
            case '2': out += "0010"; break;
            case '3': out += "0011"; break;
            case '4': out += "0100"; break;
            case '5': out += "0101"; break;
            case '6': out += "0110"; break;
            case '7': out += "0111"; break;
            case '8': out += "1000"; break;
            case '9': out += "1001"; break;
            case 'a': out += "1010"; break;
            case 'b': out += "1011"; break;
            case 'c': out += "1100"; break;
            case 'd': out += "1101"; break;
            case 'e': out += "1110"; break;
            case 'f': out += "1111"; break;
            default: return "";
        }
    }
    return out;
}
//Convert Bits to Signed INT
function bitsToSigned(bits) {
    var value = parseInt(bits, 2);
    var limit = 1 << (bits.length - 1);

    if (value >= limit) {
        // Value is negative; calculate two's complement.
        value = value - limit - limit;
    }
    return value;
}
function bitsToUnsigned(bits) {
    return parseInt(bits, 2);
}
function toLittleEndianSigned(hex) {
    // Creating little endian hex DCBA
    var hexArray = [];
    while (hex.length >= 2) {
        hexArray.push(hex.substring(0, 2));
        hex = hex.substring(2, hex.length);
    }
    // seems to already be reversed so not needed 
    //hexArray.reverse();
    hex = hexArray.join('');

    // Hex to Bits
    var hex2bits = hexToBits(hex);

    // To signed int
    var signedInt = bitsToSigned(hex2bits);

    return signedInt;
}
// Add leading zero if toString(16) removed it
function toPaddedHexString(number) {
    hex = number.toString(16).toLowerCase();
    if (hex.length % 2 !== 0) {
        hex = '0' + hex;
    }
    return hex;
}
function decodeUplink(input) {
    var data = {};
    //protocol version
    //data.protocolVersion = (input.bytes[0] << 8);
    //command id
    //data.commandID = (input.bytes[1] << 8);
    //longitude
    longitudeInteger = (input.bytes[2]) + (input.bytes[3] << 8) + (input.bytes[4] << 16);
    data.longitude = ((toLittleEndianSigned(toPaddedHexString(longitudeInteger))) * 215) / 10 * 0.000001;
    //latitude
    latitudeInteger = (input.bytes[5]) + (input.bytes[6] << 8) + (input.bytes[7] << 16);
    data.latitude = ((toLittleEndianSigned(toPaddedHexString(latitudeInteger))) * 108) / 10 * 0.000001;
    //gps fix status and report type
    //data.reportType = (input.bytes[8] << 8);
    var gps = Math.round((bitsToUnsigned(hexToBits(toPaddedHexString(input.bytes[8] << 8))) / 256) / 32);
    if (gps === 0) {
        data.gps = "No fix";
    } else if (gps == 1) {
        data.gps = "2D fix";
    } else if (gps == 2) {
        data.gps = "3D fix";
    }
    //battery capacity
    data.batterypercent = bitsToUnsigned(hexToBits(toPaddedHexString(input.bytes[9] << 8))) / 256;
    //preserved
    //data.preserved = (input.bytes[10] << 8);
    var warnings = [];
    /*  if (data.battery < 30) {
        warnings.push("low battery");
      } */
    return {
        data: data,
        warnings: warnings
    };
}