System Design Details

Receivers
Title	System Design Details
Author(s)	GMV
Level	Medium
Year of Publication	2011

In order to process the L-band signals transmitted by the satellites and compute a navigation solution, GNSS receivers can be designed to target different applications, markets, and solutions. From single or multi-frequency, single or multi-constellation, to survey or automotive applications, system design details extend through a broad range of decisions and trade-offs, in order to achieve the target performance.

Block Diagram

Most GNSS receivers have a similar block diagram, although some architecture variations might be present to accommodate different solutions. Figure 1 shows the main blocks of a GNSS receiver, as they represent most of the dimensioning and engineering work involved in a receiver system specification and design.

Figure 1: Block diagram of a typical GNSS receiver, illustrating the different parallel processing channels.

Besides these blocks, other common receiver components are the power unit (e.g. batteries) or the enclosure (e.g. for ruggedization). These components are designed and dimensioned to match each specific target application (e.g. a receiver designed for road applications may have less stringent power requirements than a receiver designed for outdoor environments). At system design level, the receiver is designed to take full advantage of the characteristics of the targeted GNSS signals: in fact, each architectural block is dimensioned to cope with the targeted signal bandwidth, modulation and code rate, in order to maximize performance.

Antennas

GNSS antennas are Right Hand Circularly Polarized (RHCP) and aim at capturing GNSS signals in the L-band, with the associated amplification and filtering. It is the entry point from the space segment to the user segment, as it receives the L-band signals to pre-process and feed as an analog electrical signal to the front end (still as a 1.2 - 1.6 GHz range RF signal).

When designing a GNSS antenna, the main objective is to maximize the antenna gain towards emitting satellites above a given elevation angle, while rejecting multipath signals (usually at lower elevation angles) and interference. The design of the antenna has to cope with the environmental conditions of the target application, while respecting mobility, power and size constraints. Usually GNSS antennas present hemispherical radiation patterns that can reject multipath coming from low elevation angles.

As far as interference is concerned, antenna arrays can be used to modify the radiation pattern so as to reject signals coming from the direction of the interferer. In addition, beam steering techniques are often employed to "follow" the signal from a given satellite with maximum gain.

Another important parameter is phase stability and repeatability in applications that use carrier phase measurements to provide a navigation solution, e.g. RTK.

Front End

The front end section receives the RF inputs from the antenna, and performs down-conversion, filtering / amplification, and sampling (digitizing) of the captured signals. Typically in a superheterodyne configuration, the front end converts the analog GNSS signals to digital data streams in an intermediate frequency (IF) spectrum (centered in the MHz range), and finally to a baseband digital signal in-phase (I) and quadrature (Q) components.

Additional parameters that affect the front-end are :

• Local Oscillator (LO): short-term and long-term stability and phase noise. Although most commercial applications can cope with a crystal low cost oscillator, other applications such as military may require atomic oscillators (e.g. rubidium)
• Frequency synthesizer: the design of the receiver frequency plan takes into account the target GNSS signals and their characteristics, but also sampling frequencies and intermediate frequencies that maximize overall performance (e.g. rejecting down-conversion harmonics, out of band interference and minimizing phase noise impact).

Baseband Processing

The baseband processing block is responsible for the signal processing tasks. These include acquisition and tracking of each signal. The input of this block is typically a down-converted digital signal. Receivers guarantee several channels that process each signal (e.g. a given frequency from a given satellite); and that are usually independent from each other. The main objective is to track code delay and carrier phase measurements in order to produce observables like code pseudorange, carrier phase measurements, and Doppler frequency. For that purpose, each channel ensures at least two lock loops: Delay Lock Loop and Phase Lock Loop to track code and phase delays respectively. Depending on the target application, the baseband processing block will also accommodate any dedicated algorithms, e.g. for multipath mitigation. The design of the tracking loops [Tracking Loops] is far from bearing a single solution and receivers may use several lock loops and use their information at will. For example, some receivers will aid the DLL with the PLL outputs. In addition, some receivers can be smart enough to dynamically change the configuration parameters of the loops, e.g. increase the PLL bandwidth when in high receiver dynamics conditions in order to avoid lock losses.

Applications Processing

The applications processing block extracts observables and navigation data from each channel of the baseband processing block, and combines this information to satisfy the requirements of a given application. The most common raw information provided by a GNSS receiver is Position, Velocity and Timing information, but other information may still be used such as computed atmospheric delays that can be useful for scientific applications [scientific applications]. Receivers may still process the computed for specific target applications, such as:

• Time and Frequency transfer
• Static and kinematic surveying
• Ionospheric parameters monitoring
• Differential GNSS reference stations
• GNSS Signal Integrity Monitoring

The main idea to retain is that a given GNSS receiver does not satisfy the requirements for all possible applications and therefore it is of paramount importance have the target application requirements in mind when designing a receiver.

Related articles

• Generic Receiver Description
• Receiver Types
• Antennas
• Front End
• Baseband Processing
• Applications Processing
• Receiver Characteristics