The acceleration due to gravity at a height `1km` above the earth is the same as at a depth `d` below the surface of earth. Then :

Explanation

`g' = g (1- (d)/(R )) = g (1- (2h)/(R ))`
or `(d)/(R ) = (2h)/(R )` or `h = (d)/(2) = (160)/(2) = 80 km`