Construction of the normal equations matrix for modeling of local gravitational field

We consider the method of constructing the local gravity field using nonorthogonal basic functions, which are solution of the Laplace equation in spherical cap or spherical segment. This approach involves using of associated Legendre functions of integer degree and noninteger order. These functions form two sets of functions. They are mutually orthogonal over the spherical cap in each set. Thus, for using both of these sets of functions it is traditionally used least squares method. However, for higher orders it is quite difficult to compute eigenvalues of these functions. In such case it is possible to project the initial data on the hemisphere and to use associated Legendre functions of integer degree and integer order. The properties of these functions are similar to properties of functions on the spherical cap. Traditionally, initial data is selected in the nodes of grid, especially for fast computations. There are many kinds of uniform grids, which allow to speed up the process of computation the unknown harmonic coefficients. Among these grids it is possible to allocate the geographical grid, Gauss grid and others. Thus, grid is developed to accommodate the initial data and is defined its basic properties in the segment of sphere and hemisphere . Using the properties of grid technique for computing the matrix of normal equations is obtained, which leads to a time reducing procedure. Also formulas for computations of unknown coefficients are obtained which allow to switch from the inversion of matrix with order α² to matrix with order α. The proposed algorithm for the constructing of the normal equations matrix and determination of harmonic coefficients of the local gravitational field leads to a time reducing procedure without degradation of accuracy.