Determination of the Earth's gravitational field

In all three principal methods of measuring the earth's gravitational field—gravimetry, satellite orbit perturbations, and astrogeodetic networks—both mathematical theory and observational techniques have been developed in recent years to more than sufficient accuracy to define and determine the irregularities of the field significant on a planetary scale. The common defect of all three methods is inadequate distribution of observations. At present, the different methods agree only for major features such as the Indian Ocean minimum, and the normalized spherical harmonic coefficients of the potential are known to be of an order of magnitude of about ±1.3 × 10−6/(n - 1) with an uncertainty of about ±0.6 × 10−6 for the tesseral terms. Gradual improvement is anticipated with additional observations and the application of better statistical techniques made possible by modern computers.