Tire global Positioning System (NAVSTAR/GPS) should reach full 3 D capability in about 1992. Due to budget constraints, funds were cut back from a level which would provide a 24 satellite constellation to the present level which will maintain only 18 satellites plus three fully functioning on-orbit spares. As a consequence, there will be continuously some areas of temporarily reduced accuracy, requiring those who are using such data to periodically adjust their operations as consequence. One way of doing that is to calculate the changes in system measurements accuracy factors, mainly the geometric dilution of precision (GDOP) which is a measure of how satellite geometry degrades accuracy. This is very important for users relying only on four satellites GPS navigation such as our case. The used algorithm gives the user at Cairo city the changes of the GDOP with that of the satellites'' geometry and consequently with time.