The transfer of precise time via satellite has as its largest potential error source the propagation delays of the Earth’s neutral atmosphere and the ionosphere. In this section these range delays will be outlined and various techniques for compensating for them will be presented and discussed. The range delays of the troposphere and of the ionosphere differ in several important aspects. The range delay of the earth’s troposphere is not dispersive; that is, it is not a function of frequency, at least not over the normal radio frequency range used for ranging to artificial Earth satellites. The range delay of the ionosphere, on the other hand, is dispersive; it varies inversely with frequency. Thus, by measuring the relative range delay at two, suitably spaced, frequencies the absolute range delay can be computed directly along the satellite to user path. The range delay of the Earth’s troposphere cannot be measured directly, but several models, or indirect measurement techniques can be used to infer the tropospheric time delay contribution for time transfer via satellite to high accuracy.