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GNSS Receivers General Introduction
| Receivers | |
|---|---|
| Title | GNSS Receivers General Introduction |
| Author(s) | GMV |
| Level | Basic |
| Year of Publication | 2011 |
GNSS receivers are used to determine the user’s position, velocity, and precise time with the use of the signals broadcasted by satellites. As the satellites are always in motion, the receiver has to continuously track the satellite signal to generate an uninterrupted solution, as desired in most applications.
Any navigation solution provided by a GNSS receiver is based on the computation of the distance to a given satellite by means of extracting the delay of the incoming signal due to the signal’s travel through space. In their most common architecture, GNSS receivers assign a dedicated channel for each signal tracked (in case of multi-frequency receivers, each signal from each satellite can be processed independently).
Most of the Global Navigation Satellite Systems use Code Division Multiple Access to multiplex the existence of several satellite signals in the same frequency. The basic concept behind the CDMA scheme is that each satellite is assigned with a Pseudo-Random Noise (PRN) code that modulates the transmitted signal. The use of these PRN codes spread the signal over the spectrum making it look like noise. In order to track the signals in space, the GNSS receiver has prior knowledge of each satellite PRN code (e.g. through the relevant SIS ICD) and replicates them locally. These PRN codes have properties such that their autocorrelation function is maximal when they are completely aligned.
GNSS receivers are continuously estimating and correcting two parameters:
- The code delay, the misalignment between the local PRN code and the incoming signal
- The carrier phase (or its instantaneous value, the Doppler frequency) which reflects the relative motion between the satellite and the user
In order to determine these parameters, the receiver puts in place (code and carrier) tracking loops that form the core of the signal processing so as to continuously track the incoming satellite signal in order to generate the code and carrier phase measurements. The current estimates of the code delay and the carrier phase are used to modulate the local PRN replica which is then correlated with the incoming signal. The result of this operation is then re-assessed at the receiver to further estimate these parameters, in a continuous loop.
Back in the 1970s, receivers were large analog equipments built for the military domain. Nowadays, GNSS receivers have been widely expanded to miniaturized platforms, chipsets, microprocessors, Integrated Chips (IC), DSP, FPGA, handheld units and integrated in most mobile phones. In fact, GNSS receivers run in a wide variety of platforms, whose choice results from a trade-off of parameters such as receiver performance, cost, power consumption and autonomy. Furthermore, the increasing capabilities of microprocessors have enabled the emergence of software receivers with performances comparable to full-hardware receivers, providing the flexibility required for some user applications.
In the context of the emergence of multiple satellite navigation systems (both regional and global), multi-constellation receivers are increasingly available. This has been encouraged at system design level by working towards interoperability and compatibility among all systems. From the receiver perspective, multi-constellation brings a key added value to solution availability, especially in urban canyon environments.
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