In the previous chapter, positioning was examined from a historical perspective, recognizing that in many parts of the world, such data are not just useful, they are frequently the only data available. But in many areas, the case for extending the limits of the continental shelf will be dependent on the acquisition of new data, and for the most part, this will mean the use of satellite navigation systems. Therefore, this chapter deals in some detail with current and future satellite navigation and positioning systems. The first generation of satellite navigation systems used the principle of the Doppler shift of transmissions from satellites to provide measurements of a user's position. The Doppler shift of an emitted signal is related to the relative velocity between the source of the signal and the point at which it is received. The apparent frequency of the received signal is increased when the emitter is moving toward the receiver, and decreased when it is moving away. This phenomenon can often be observed in everyday situations, such as when a vehicle drives past a pedestrian. The pitch of the sound from the vehicle appears to drop as it passes the pedestrian, due to the transition from increased to decreased frequency of the sound. In satellite Doppler systems, measurements of the Doppler shift of signals from the satellites are combined with knowledge of the satellite's position and velocity (its ephemeris), to give an indication of the receiver's position. TRANSIT was the first operational satellite navigation system (see chapter 7). Data-processing techniques were developed which allowed a receiver to be located with respect to another at a known location, to an accuracy of the order of 1 m. TRANSIT ceased operation as a position and timing system at the end of 1996. A similar system to TRANSIT was developed by the Soviet Navy in 1965. The system, known as TSIKADA, is still operational today (2000). Since satellite Doppler systems rely on the accumulation of measurements over a period of time to provide a useful measure of a receiver's position, they could not be used as true real-time satellite navigation systems (see chapter 7).
Fog Computing-Based Differential Positioning Method for BDS
