Generic Receiver Description

Receivers
Title	Generic Receiver Description
Author(s)	GMV
Level	Basic
Year of Publication	2011

GNSS receivers are responsible for processing the L-band Signals In Space (SIS) coming from the GNSS satellites. This way, a GNSS receiver can be seen as a radionaviagation user device that aims at tracking the GNSS signals in view, in order to correctly demodulate and extract the measurements and navigation information - one example is to decode the transmitted navigation message and calculate the user's position.

Receiver overview

Although each receiver architecture is tailored to the different GNSS systems applicable and to the target applications, the basic building blocks of a generic GNSS receiver are as shown in Figure 1:

• Antenna - L-band antenna, responsible for capturing the GNSS signals transmitted (as well as noise and possible interference).

• Front End - The hardware front-end typicaly downconverts, filters, amplifies, and digitizes the incoming signals.

• Baseband signal processing - Several signal processing routines are used to acquire and track the different signals.

• Receiver application - Depending on the envisaged task, the receiver performs different tasks with the resulting GNSS information, and provide meaningful results to the user.

Figure 1: Generic Receiver Architecture. See this block diagram for a detailed view.

In a typical receiver implementation, the Signals In Space (SIS) arriving at the antenna are down-converted, filtered, and digitized in the front end section. This process ultimately generates a baseband representation of the desired GNSS spectrum, yelding the samples as real and complex components, namelly I (In-Phase) and Q (Quadrature) components, in baseband.

Baseband signal processing is built around the principle of signal correlation: the incoming signal is repeatedly correlated with a replica of the expected signal minus the navigation message, which is unknown at this point. To extract valid significance from the correlation, the local replica is generated in the receiver taking into account the signal's carrier frequency, code delay, Doppler frequency, and spreading code (which is unique to each satellite/signal).

The correlation principle is first used to searh for the satellites in view. After a receiver starts operating, it first needs to know which satellites are in view and can be tracked for extracting measurements. This process is known as acquisition, and is based on a fast correlation between the incoming signal and multiple replicas of the possible "expected" signals, generated for different code delays and Doppler frequencies. If a correlation peak is observed for a given replica, this means that there is a good chance that the signal with the spreading code used is visible, and the code delay and Doppler frequency estimates are passed on to the tracking process as a first estimate of the signal's delay.

In the tracking loops, correlations are also used to refine the local replica generation, so as to match as best as possible the incoming signal. The correlation results are then used to aid diferent tracking loops in providing a measurement of tracking quality. Typicaly, the receiver tracks each signal using dedicated channels running in parallel, where each channel tracks one signal (i.e. for single frequency users, each channel tracks one satellite), providing pseudorange and phase measurements, as well as navigation data and additional signal information, such as carrier-to-noise ratio (C/N0). For details on the signal processing blocks, see the signal processing section.

In the application processing block, the receiver uses the information from the tracking loops for different purposes: from computing its own position and velocity, to performing time transfer, or simply collecting data to be post-processed in the ground stations. In addition to processing the SIS, GNSS receivers may also use aiding information to enhance their solution performance, as there are various architectural solutions to cope with aiding information. In fact, this information can be potentially used at any block of the receiver: as an example, when using Inertial Navigation Systems (INS), the sensor information is commonly used in the application processing block, although it could also be used as feedback to the baseband processing block for improved performance.For a wider discussion on application specifics, see GNSS Applications.

Trade-offs and limitations

GNSS Receiver manufacturers rely on each system’s SIS ICD (Signal In Space Interface Control Documents) to develop their solutions. The SIS ICDs define signal properties, transmitted codes and navigation messages contents that allow the receivers to process the SIS signals. For further information please refer to XXX. Pushed by the emergence of new services aimed at professional and safety of life users, standardization activities have been launched at European level (CEN, CENELEC and ETSI), at global level (e.g. ICAO standard and recommended practices) and at industry level (e.g. industry standards, RTCA and EUROCAE MOPS/ MASPS).

The design and selection of a receiver is tightly linked to the target user application: for example, a multi-constellation GNSS receiver will certainly improve solution availability (critical for example in urban environments), whereas if the user application is focused on improved accuracies, then the selected receiver will probably turn to carrier-based technologies or differential and augmented solutions. The type of assisting/aiding information to be used also focuses on the user application. On one hand, different technologies such as WiFi, UWB and INS can be used to improve solution availability and continuity, in environments where GNSS cannot guarantee the desired availability (e.g. mixed open/ indoor environments). On the other hand, this information can be used to improve indicators such as Time To First Fix (TTFF): as an example, downloading the navigation data through the internet will greatly improve this factor, since the receiver will not have to wait to demodulate the whole message to compute position.

Related articles

For further details of GNSS receivers and operations, please visit the following links:

• Receiver Types
• Receiver Operations
• Applications Processing