As shown in the study "ATM Strategy for the years 2000s" made by EUROCONTROL, the satellite-based positioning and navigation plays a fundamental role in the framework of ATM integrated services thanks to its capability of allowing for an efficient functionality of monitoring and control.
The use of satellites as powerful and global radio-navigation beacons for navigation purposes on the earth was experimented since the 1960s. The possibility to exploit satellite signals for positioning was initially suggested by the observed Doppler effect on the ground-satellite data link signal that allowed for trajectory tracking of satellites (see Parkinson and Spilker 1996, vol. 1, pp. 3-28, and (Kayton and Fried, p. 178). At first marine applications (position updates of ships) and later, in the early 1970s, aircraft navigation was approached by satellite systems. Subsequently, miniaturization of electronics has led to a wide diffusion of compact and reduced cost receivers such that, at present, also almost all general aviation aircraft have a satellite navigation receiver on board that allows for the determination of the aircraft's time and three dimensional position. The International Civil Aviation Organization (ICAO) has defined a general system that includes each existing and future satellite system (GPS, GLONASS, GALILEO, etc.) together with geostationary overlay satellites, and has named it Global Navigation Satellite System (GNSS). A GNSS receiver is a device for positioning and navigation that is based on the integration of many satellite-based positioning services. A GNSS receiver only receives and does not transmit (it is a passive sensor) and is based on the measurement of range and change of range between a satellite and the receiver. The major advantage of GNSS systems is that they provide an all- weather worldwide navigation capability. The disadvantage is that the satellite- receiver link is long-distance; hence the power of the received signal is low. This turns into a high vulnerability to interferences.
The ranging capability is related to the propagation time, since the speed of radio-signals is assumed constant and equal to the speed of light in vacuum. In particular, the receiver measurements are based on arrival time of the satellite radio-signals, and for this reason the availability of satellite systems as navigation aids relied on the possibility to install sufficiently stable clocks on board vehicles at relatively low cost.
The advantage of satellite navigation systems with respect to ground based radio-navigation systems is to provide a worldwide navigation solution with a typically-better accuracy and less dependent on atmospheric conditions. On the other hand, the main disadvantages are:

• strong dependence of the overall performance on signal-in-space interference;
• unavailability of the navigation solution owing to the lack of a sufficient number of tracked satellite signals. (This problem can arise either because of signal masking or because of a too low number of in-view satellites);
• necessity of external augmentation equipment in order to fulfill the accuracy, integrity and availability requirements of particular applications.

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