GNSS Receivers General Introduction

Receivers
Title	GNSS Receivers General Introduction
Author(s)	GMV
Level	Basic
Year of Publication	2011

Overview

The main application for a GNSS receiver is to determine the user position, velocity, and precise time (PVT) by processing the signals broadcasted by satellites. Because the satellites are always in motion, the receiver has to continuously acquire and track the signals from the satellites in view (or line of sight, LOS), in order to compute an uninterrupted solution, as desired in most applications. Any navigation solution provided by a GNSS receiver is based on the computation of its distance to a set of LOS satellites, by means of extracting the delay of the incoming signals traveling through space at the speed of light. A rough measurement of this distance is called "pseudorange", and is can be seen as an estimated distance to each satellite, given by [math]\displaystyle{ R = c \times dt }[/math]. A GNSS receiver is then responsible for refining this pseudorange, by computing and extracting the different signal propagation errors, and the navigation data transmitted by each satellite is used to compute a PVT solution. Figure 1.1 shows this principle for two LOS satellites and pseudoranges [math]\displaystyle{ R_1 }[/math] and [math]\displaystyle{ R_2 }[/math] - for details on determining the position solution, see this approach to GNSS positioning.

Figure 1.1: General concept of GNSS receivers: the user determines its PVT solution by processing the signals transmitted by a set of satellites in view.

Finding and tracking satellites

Most of the Global Navigation Satellite Systems use Code Division Multiple Access (CDMA) techniques to multiplex several satellite signals onto the same frequency. The basic concept behind the CDMA schemes 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 and difficult to identify. In order to search for LOS satellites, and track these signals in space (SIS), the GNSS receiver must have prior knowledge of each satellite's PRN code (e.g. through the relevant SIS ICD), and generates local replicas of them. Since these PRN codes have properties such that their autocorrelation function is at a maximum when they are completely aligned, a GNSS receiver correlates the incoming signals with the stored code replicas, and determine if a given satellite is visible or not.

In their most common architecture, GNSS receivers assign a dedicated channel to each signal being tracked and, for the case of multi-frequency receivers, each signal from each satellite can be processed independently. In order to ensure tracking of the signals in each processing channel, receivers are continuously estimating and correcting two parameters:

• The code delay: quantifies the misalignment between the local PRN code replica and the incoming signal.

• The carrier phase (or its instantaneous value, the Doppler frequency): 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, and continuously track the incoming satellite signal, in order to generate the code and carrier phase measurements. Each estimate of the code delay and the carrier phase is used to modulate the local PRN code replica, which is then correlated again 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. After synchronization with the incoming signal, the receiver is able to determine pseudoranges to each satellite, and to compute a navigation solution following techniques described in GNSS Fundamentals.

Receivers in history and trends

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 devices, including integration in most mobile phones. In fact, GNSS receivers run in a wide variety of platforms, and the 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 hardware implemented receivers, providing the flexibility required for some user applications.

Following future trends, with 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.

Related articles

For a description of a generic GNSS receiver, please visit the following link:

• Generic Receiver Description