What is the alternative to android orientation sensor?

There is no actual “orientation” sensor – this is (was) actually a composite sensor, generated from a combination of the accelerometer and the magnometer.

From http://developer.android.com/reference/android/hardware/SensorEvent.html

“This sensor type exists for legacy reasons, please use getRotationMatrix() in conjunction with remapCoordinateSystem() and getOrientation() to compute these values instead.”

public class OrientationTestActivity extends Activity implements SensorEventListener
{
    private SensorManager mSensorManager;
    private Sensor mAccelerometer;
    private Sensor mMagnetometer;

    private float[] mLastAccelerometer = new float[3];
    private float[] mLastMagnetometer = new float[3];
    private boolean mLastAccelerometerSet = false;
    private boolean mLastMagnetometerSet = false;

    private float[] mR = new float[9];
    private float[] mOrientation = new float[3];

    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        mSensorManager = (SensorManager)getSystemService(SENSOR_SERVICE);
        mAccelerometer = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
        mMagnetometer = mSensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD);
    }

    protected void onResume() {
        super.onResume();
        mLastAccelerometerSet = false;
        mLastMagnetometerSet = false;
        mSensorManager.registerListener(this, mAccelerometer, SensorManager.SENSOR_DELAY_NORMAL);
        mSensorManager.registerListener(this, mMagnetometer, SensorManager.SENSOR_DELAY_NORMAL);
    }

    protected void onPause() {
        super.onPause();
        mSensorManager.unregisterListener(this);
    }

    public void onAccuracyChanged(Sensor sensor, int accuracy) {
    }

    @Override
    public void onSensorChanged(SensorEvent event) {
        if (event.sensor == mAccelerometer) {
            System.arraycopy(event.values, 0, mLastAccelerometer, 0, event.values.length);
            mLastAccelerometerSet = true;
        } else if (event.sensor == mMagnetometer) {
            System.arraycopy(event.values, 0, mLastMagnetometer, 0, event.values.length);
            mLastMagnetometerSet = true;
        }
        if (mLastAccelerometerSet && mLastMagnetometerSet) {
            SensorManager.getRotationMatrix(mR, null, mLastAccelerometer, mLastMagnetometer);
            SensorManager.getOrientation(mR, mOrientation);
            Log.i("OrientationTestActivity", String.format("Orientation: %f, %f, %f",
                                                           mOrientation[0], mOrientation[1], mOrientation[2]));
        }
    }
}