XProfan

/**
* Method to convert octal data into an altitude with 100 foot resolution
*
* <p>Based on algorithm from Andy at Jetvision.de
*
* @param a an integer representing the first 3 bits
* @param b an integer representing the second 3 bits
* @param c an integer representing the third 3 bits
* @param d an integer representing the fourth 3 bits
* @return an int representing the altitude in feet with 100 foot resolution
*             or -9999 for error
*/

public int altitudeDecode(int a, int b, int c, int d) {

    int alt = 0;
    int dab = grayToBinary((d << 6) + (a << 3) + b);
    boolean i = (dab & 0x01) == 0;
    dab = (dab * 500) - 1000;

    if (i) {

        switch (c) {

            case 4:
            alt = dab + 200;
            break;
            case 6:
            alt = dab + 100;
            break;
            case 2:
            alt = dab;
            break;
            case 3:
            alt = dab - 100;
            break;
            case 1:
            alt = dab - 200;
            break;
            default:
            alt = -9999;

        }

    } else {

        switch (c) {

            case 4:
            alt = dab - 200;
            break;
            case 6:
            alt = dab - 100;
            break;
            case 2:
            alt = dab;
            break;
            case 3:
            alt = dab + 100;
            break;
            case 1:
            alt = dab + 200;
            break;
            default:
            alt = -9999;

        }

    }

    return alt;

}

/**
* Method to calculate altitude from Mode-C transponder type reply
* B[i] = XOR(B[i+1], G[i]) to convert Gray to binary
*
* <p>Based on an algorithm from Andy at Jetvision.de
*
* @param g an integer representing a gray code
* @return an integer representing the binary value of the given gray code
*/

private int grayToBinary(int g) {

    int val = g & 0x80;
    val = val | ((g & 0x40) ^ ((val & 0x80) >>> 1));
    val = val | ((g & 0x20) ^ ((val & 0x40) >>> 1));
    val = val | ((g & 0x10) ^ ((val & 0x20) >>> 1));
    val = val | ((g & 0x08) ^ ((val & 0x10) >>> 1));
    val = val | ((g & 0x04) ^ ((val & 0x08) >>> 1));
    val = val | ((g & 0x02) ^ ((val & 0x04) >>> 1));
    val = val | ((g & 0x01) ^ ((val & 0x02) >>> 1));
    return val;

}