عنوان مقاله [English]
نویسنده [English]چکیده [English]
The Global Positioning System (GPS) is the world's leading position-determining means around the world at all the hours of the day, in different atmospheric, space, sea and land conditions.
Currently, there are 10 satellites of the GPS’s block 1 in the orbit for testing the system and land receivers. Many of these receivers are operating beyond their predicted lifetime. Two satellites from this group are out of activity and one is about ending its mission. The Challenger Spaceship Crash postponed the full deployment of the satellites of Block 2 for nearly two years. According to the latest decision, the first launch of block 2 satellites was carried out in the fourth quarter of 1988 (Jewkes, 1987), and, according to a predetermined schedule, complete deployment of the
block 2 satellites was completed by the end of 1991.
The initial design of the system was based on a 24-satellite system, which was later reduced to 18 active and 3 reserve satellites due to financial issues. The calculations show that the recent system (18 satellites), despite all the efforts made to give it the best geometric status, leads at each moment to the creation of four areas on the ground in which the act of position determining takes place with poor quality. These areas are called “Outage”. The extent, location and time of this state vary as a function of time, but can be precisely predicted. In areas where this situation is created, temporary use of other navigation devices is recommended.