Abstract: As the GPS signal travels from the GPS satellite to the receiver, it propagates through the atmosphere of the earth, where it is retarded and its path changed from a straight line to a curved one. If we take the simplified mathematical model for the observable to be one in which the sig nal is assumed to be propagating in a straight line and at the speed of light in vacuum, then the "atmospheric propag ation delay" is defined to be the difference between the true electrical path length and this assumed straig ht line length, and may be implemented in the GAM IT softw are. The present paper deals with the generation and effects of the atmospheric propagation delay on the GPS measurements, controls of the GAM IT softw are on the atmospheric propag ation delay, and methods for reducing and eliminating the multipath errors of the atmospheric propagation delay.