If you wish to contribute or participate in the discussions about articles you are invited to contact the Editor

System Design Details: Difference between revisions

From Navipedia
Jump to navigation Jump to search
No edit summary
m (Included editor logo.)
 
(16 intermediate revisions by 5 users not shown)
Line 1: Line 1:
{{Article Infobox2
{{Article Infobox2
|Category=Receivers
|Category=Receivers
|Title={{PAGENAME}}
|Editors=GMV
|Authors=GMV
|Level=Intermediate
|Level=Medium
|YearOfPublication=2011
|YearOfPublication=2011
|Logo=GMV
|Logo=GMV
|Title={{PAGENAME}}
}}
}}
In order to process the [[GNSS signal|L-band signals]] transmitted by the satellites and compute a navigation solution, GNSS receivers can be designed to target different [[GNSS Applications|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.
[[File:Rec_design.png|center|thumb|750px|'''''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==
[[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).


In order to process the [[GNSS signal|L-band signals]] transmitted from the satellites and compute the navigation solution, a GNSS receiver can be designed to target different [[GNSS Applications|applications]], markets, and solutions. From single or multi-frequency, single or multi-constellation, to survey or automotive applications, system specification details extend through a broad range of decisions and trade-offs, in order to achieve the best performance desired. The following sections tackle some considerations at a GNSS receiver system design level.
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.


==Overview==
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.
Most of the current GNSS receiver systems gather (at least) the blocks depicted in Figure 1, although some architecture variations might be present to accommodate different solutions. Besides these blocks, other common receiver components are the power unit (e.g. batteries) or the enclosure (e.g. for ruggedization). All such components and blocks are carefully chosen when a GNSS receiver is designed for a target application, and different considerations are made on the choices and trade-offs involved.


Furthermore, in order for a GNSS receiver to be able to provide the required solution, the specification team should have a clear knowledge of the [[GNSS Measurements Modelling|system]] as a whole, with special focus on the space segment (satellites, RF signals, modulations and bandwidths) and user segment (hardware, receivers and applications). At system design level, it is the interface between these two segments that is targeted, and a receiver is tailored not only to provide PVT (or other) solutions, but also to take full advantage of the characteristics of the signals received and their respective transmitting satellite constellation(s).
Another important parameter is phase stability and repeatability in applications that use carrier phase measurements to provide a navigation solution, e.g. [[Real Time Kinematics|RTK]].


==Block diagram==
Further information on GNSS Antennas can be found [[Antennas|here]].
The figure shows the main blocks inside a GNSS receiver system, as they represent most of the dimensioning and engineering work involved in a receiver system specification and design. These different subsets, from a functional point of view, can be categorized as [[Antennas|antenna]], [[Front End|front end]], [[Baseband Processing|baseband processing]] and [[Applications Processing|applications processing]], and are shortly described as<ref>For further details, refer to their corresponding articles (links provided in the "Related articles" section).</ref>:


[[File:Rec_design.png|center|thumb|750px|'''''Figure 1:''''' Block diagram of a typical GNSS receiver, illustrating the different parallel processing channels.]]
==Front End==
 
The [[Front End|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<ref>[[wikipedia:Superheterodyne receiver|Superheterodyne receiver in Wikipedia]]</ref> 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<ref name="Dierendonck">A.J. Van Dierendonck, <i>“Global Positioning System: Theory and Applications”</i>, Volume I, Chapter GPS Receivers, edited by B. Parkinson and J. Spilker Jr, Published by the American Institute of Aeronautics and Astronautics, Inc.</ref>:
 
*<b>Local Oscillator (LO)</b>: short-term and long-term stability and phase noise. Although most commercial applications can cope with a low-cost crystal oscillator, other applications such as military may require atomic oscillators (e.g. rubidium).


*<b>Antenna</b>: L-band antenna for capturing GNSS signals, with the associated amplification and filtering. It represents the entry point from the space segment to the user segment, and 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).
*<b>Frequency synthesizer</b>: the design of the receiver frequency plan takes into account not only 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).


*<b>Front End</b>: 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<ref>http://en.wikipedia.org/wiki/Superheterodyne_receiver</ref> 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.
Further information on Front Ends can be found [[Front End|here]].


*<b>Baseband processing</b>: All acquisition and tracking [[Digital Signal Processing|signal processing]] tasks are performed in the baseband processing blocks, where the core functions that enable GNSS signal tracking occur, such as correlations, delay/frequency/phase lock loops, filtering, and others. Receivers typically process several channels in parallel, where different signals are tracked, and produce observables like code delay, carrier phase, and Doppler frequency, as measurement data.
==Baseband Processing==


*<b>Applications Processing</b>: Since the target application for a receiver can vary, the applications processing block is designed to extract the GNSS measurements, observables, and navigation data, and compute the desired solutions - for instance, the position of the user on Earth. For this purpose, the receiver combines the outputs of the signal processing algorithms in order to extract the navigation message and other information necessary for PVT computation.
The [[Baseband Processing|baseband processing]] block is responsible for the signal processing tasks, such as acquisition and tracking of each signal. The input of this block is typically a down-converted digital signal.


==Design considerations==
Receivers guarantee several channels that process each signal (e.g. a given frequency from a given satellite), which 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 (DLL)|Delay Lock Loop (DLL)]] and [[Phase Lock Loop (PLL)|Phase Lock Loop (PLL)]], to track code and phase delays respectively.
From a system design perspective, there isn't a real set of "rules of thumb" to chose a given approach for a GNSS receiver design. In fact, the requirements are strongly influenced by the application itself, whether in terms of architecture or performance. Below are a few examples<ref>For another example on receiver design considerations, see <i>Kaplan, E.D. et al, "Understanding GPS: Principles and Applications", second edition, chapter 3, section 3.4</i>.</ref> of the requirements, analysis and tradeoffs involved:


*<b>Environmental Constraints</b>: Factors such as temperature, humidity, water resistance, shock and vibration ranges depend on the application. Furthermore, the surrounding environment can also impact the design in terms of performance, e.g. urban environments can originate several multipath signals, where the reflection of GNSS signals on surrounding surfaces generates a non-line-of-sight ray. In the latter case, multipath mitigation signal-processing techniques may be required.
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.


*<b>Precision and Accuracy</b>: Higher precision is required, for instance, in survey or military operations, whereas automotive or mobile applications may not require centimeter-level precision. As another example, the solution accuracy, precision, and rate are surely different for aircraft precision approach, than for marine or automotive navigation.
Further information on Baseband processing can be found [[Baseband Processing|here]].


*<b>Single/Multi Frequency/Constellation</b>: An increasing range of GNSS frequencies, bands, modulations and constellations enable a multitude of possibilities in receiver design, from single frequency / single constellation (e.g. the common GPS L1 C/A devices on current PND's or smartphones) to multi-frequency / multi-constellation (e.g. L-band GPS/Galileo/GLONASS capable high-end survey receivers).
==Applications Processing==


*<b>Assisting Sources</b>: Several external or assistance information sources can be taken into account at system design, since there is a straightforward impact on resources and solution computation. If the use of augmentation (SBAS) or differential (DGPS) data is envisaged, the receiver must be prepared to interface and use the information provided from such systems. Other external interfaces and aiding sensors can be coupled to a receiver, such as inertial sensors<ref>http://en.wikipedia.org/wiki/Inertial_measurement_unit</ref> (IMU and INS), or even the means to get external aiding information (e.g. accessing the internet through WiFi or GPRS/UMTS).
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.


*<b>Hardware Platform</b>: On a physical platform point of view, several design considerations are made in terms of the hardware host platform, user interface design, storage capability, power/battery consumption, electromagnetic compatibility, or portability. The underlying platform for a GNSS receiver can be an ASIC, FPGA, DSP, CPU, hardware/software defined, or a combination of these and other technologies. Table 1<ref>Hein, G., Pany, T., Wallner, S., Won, J., <i>"Platforms for a future GNSS receiver - A discussion of ASIC, FPGA, and DSP technologies"</i>, Working Papers, InsideGNSS, March 2006.</ref> below shows an assessment of the advantages and disadvantages of each approach.
The most common raw information provided by a GNSS receiver is Position, Velocity and Timing (PVT) information, but other information may still be used such as computed atmospheric delays, which can be useful for [[:Category:Scientific Applications|scientific applications]]. Receivers may still process the computed results for specific target applications, such as<ref name="Dierendonck" />:


*Time and frequency transfer
*Static and kinematic surveying
*Ionospheric parameters monitoring
*Differential GNSS reference stations
*GNSS signal integrity monitoring


{| border="1" align="center" cellpadding="5" style="text-align: center; font-size: 11px;"
This way, the main idea behind the receiver system design is that a given GNSS receiver does not satisfy the requirements for all possible applications, and therefore it is of paramount importance to have the target application requirements in mind when designing a receiver.
|+align="bottom" style="font-size:12px;"|'''''Table 1:''''' GNSS technology comparison, from (++) major advantage to (--) major disadvantage.
|-
!Technology||Development Costs||Performance||Power Consumption||Single Unit Costs||Flexibility
|-
| '''ASIC''' || -- || ++ || ++ || ++ || --
|-
| '''FPGA''' || - || ++ || + || - || +
|-
| '''DSP / CPU''' || ++ || + / ++ || + / -- || + / - || ++
|-
| '''Hybrid FPGA / CPU''' || + || ++ || + || - || +
|}


==Related articles==
==Related articles==
Line 62: Line 72:
*[[Front End]]
*[[Front End]]
*[[Baseband Processing]]
*[[Baseband Processing]]
*[[Applications Processing]]
*[[Receiver Characteristics]]


==References==
==References==

Latest revision as of 16:34, 18 September 2014


ReceiversReceivers
Title System Design Details
Edited by GMV
Level Intermediate
Year of Publication 2011
Logo GMV.png

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.

Further information on GNSS Antennas can be found here.

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[1] 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[2]:

  • Local Oscillator (LO): short-term and long-term stability and phase noise. Although most commercial applications can cope with a low-cost crystal 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 not only 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).

Further information on Front Ends can be found here.

Baseband Processing

The baseband processing block is responsible for the signal processing tasks, such as 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), which 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 (DLL) and Phase Lock Loop (PLL), 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 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.

Further information on Baseband processing can be found here.

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 (PVT) information, but other information may still be used such as computed atmospheric delays, which can be useful for scientific applications. Receivers may still process the computed results for specific target applications, such as[2]:

  • Time and frequency transfer
  • Static and kinematic surveying
  • Ionospheric parameters monitoring
  • Differential GNSS reference stations
  • GNSS signal integrity monitoring

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

Related articles

References

  1. ^ Superheterodyne receiver in Wikipedia
  2. ^ a b A.J. Van Dierendonck, “Global Positioning System: Theory and Applications”, Volume I, Chapter GPS Receivers, edited by B. Parkinson and J. Spilker Jr, Published by the American Institute of Aeronautics and Astronautics, Inc.
Retrieved from "https://gssc.esa.int/navipedia/index.php?title=System_Design_Details&oldid=13219"