List of my papers with passages on orthogonality, useful references, and more:
Šprlák M, Novák P, Pitoňák M (2016) Spherical Harmonic Analysis of Gravitational Curvatures and Its Implications for Future Satellite Missions. Surveys in Geophysics, 37(3), pp. 681-700, https://doi.org/10.1007/s10712-016-9368-0.
Šprlák M, Tangdamrongsub N (2018) Vertical and Horizontal Spheroidal Boundary-Value Problems. Journal of Geodesy, 92(7), pp. 811-826, https://doi.org/10.1007/s00190-017-1096-9.
Šprlák M (2023) Comments and Corrections to: "Ellipsoidal spectral properties of the Earth's gravitational potential and its first and second derivatives" by Bölling and Grafarend (2005) in J.Geod. 79(6-7):300-330. Journal of Geodesy, 97(11), 101, https://doi.org/10.1007/s00190-023-01799-x.