Provided two diagonal real matrix which has positive entries, $V$ and $U$. Find a real matrix A, satisfying $A^TA=a^2I$ for some scalar $a$, to minimise

$\left|A^TVA-U\right|$

where the matrix norm could be an induced one, such as $|M|^2_{L^2}=\mathrm{tr}(M^TM)$.

I believe the problem is quite useful, however I am not sure where I can find the related materials. A numerical approach is also welcome.