Unable get the proper altitude from gps satellite in android Device -

i using android in small spy android helicopter getting proper latitude , longitude when above 500ft ground ,i doing landing of helicopter based on height calculated altitude .but getting altitude of times wrong due helicopter crashing in ground have idea how proper altitude gps satellite.and using getaltitude() method getting altitude , using in menifest

<uses-permission android:name="android.permission.access_fine_location"></uses-permission> <uses-permission android:name="android.permission.access_mock_location" /> 

using gps satellites landing bad idea. android using ellipsoid earth approximation. in location give me 40 meters in data i'm receiving getaltidude(). don't forget inaccuracy of gps signal can add next more 10 meters.