Satnav System Architecture

Figure 2.2 depicts the architecture of a satnav system. The space segment comprises a constellation of satellites that broadcast signals to the system’s service region - the part of Earth, the atmosphere, and outer space where the system is defined to provide given levels of PVT service.

These sig­nals are carefully designed and precisely transmitted, enabling receivers to make the number and quality of mea­surements needed to form the needed observables. The user segment consists of all devices that receive and process these broadcast satnav signals for any purpose. While many of these are used to estimate position, velocity, or time, others are used for remote sensing or other applications.

Figure 2.2 Satnav system architecture. Source: Reproduced with permission of IEEE

The ground segment monitors the broadcast signals for accuracy, quality, and integrity, controlling the satellite buses and payloads, and uplinking to the satellites data that is modulated onto the broadcast signals and used by the receivers.

While the satnav system operator designs and controls the space and ground segments and the signals, the opera­tors have little control, or even influence, over the user seg­ment. This situation is both a boon, enabling tremendous diversity and innovation in the use of satnav, and a concern, since user equipment may use signals in ways that are flawed or fragile with resulting problems blamed on the satnav system.

These components enable the functionality described in Sections 2.2.1, 2.2.2, and 2.2.3. However, the functions they actually perform involve many subtleties beyond the basics described in these previous sections. Without the many detailed steps that address these subtleties, positioning errors would be hundreds or even thousands of meters, with corresponding timing errors. The many steps needed to achieving meter-level or even centimeter-level position­ing, and correspondingly small timing errors, are outlined in subsequent sections of this chapter.