Satnav User Equipment Architecture

Satnav user equipment as described in this section has the architecture shown in Figure 2.7; it comprises an antenna and associated antenna electronics, a receiver, other sensors that augment or aid the receiver, and interfaces to a user or host system; route finding is optional and is not addressed further in this chapter.

The receive antenna must be able to receive signals from satellites over the upper hemisphere. The simplest way to accomplish this is with an antenna that has an approximately hemispherical gain pattern, known as a fixed reception pattern antenna (FRPA).

Figure 2.7. Satnav user equipment

The antenna electronics for an FRPA are relatively simple - protection from lightning or other over voltages, filtering to suppress strong out-of band interference, and amplification. An alternative to an FRPA is to use an array of elements whose outputs are weighted in gain and phase and then summed to decrease the gain in the direction of the undesired signals (interference or jamming) while retaining the gain in the direction of the desired satellite signals.

This is called a controlled reception pattern antenna (CRPA), and its antenna electronics is a far more complicated set of analog electronics (e.g. filters, amplifiers, phase shifters, summers) usually combined with digital electronics (analog-to- digital converters (ADCs), processors).

While there are many fundamentally different receiver architectures used for different applications, a generic processing flow is shown in Figure 2.8. The antenna and antenna electronics output RF waveforms that contain the satnav signals of interest, along with noise and interference.

Figure 2.8. Conventional receiver processing flow. Source: Reproduced with permission of IEEE

These RF signals are conditioned by filtering and amplification within the receiver, then downconverted (shifted in frequency) to a lower, intermediate frequency (IF) or to baseband, changing the carrier frequency but retaining the other characteristics of the waveforms. IF conditioning then includes additional filtering and amplification, along with ADC, although more modern receiver architectures can perform ADC after RF conditioning rather than on the IF waveforms.

Signal processing involves the extraction of needed information from the signals, as described subsequently. Navigation processing uses this information to produce PVT. An oscillator drives a frequency synthesizer providing frequency references and timing signals used throughout the receiver.