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{{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}}
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In the first years of GNSS, when only GPS was available for public use, a receiver was not expected to perform as the navigation devices we see today. In fact, a first-generation GPS receiver could be designed to only process 4 or 5 signals at any given time, and it was deemed suitable for positioning applications. Today, with the increased availability and potential of different GNSS signals and constellations, any receiver is expected to support at least 10 or 12 channels up to hundreds, resulting in a solution with improved performance. However, at their core - and despite the many differences in target applications or implementation philosophy - most receivers still share common control and processing flow management properties, resulting in common operations<ref><i>Borre, K. et al, "A Software-Defined GPS and Galileo Receiver - A Single-Frequency Approach", Birkhäuser, chapter 5</i>.</ref> performed. The following sections describe these common operation modes in which a receiver functions. 


In the first years of GNSS, when only GPS was available for public use, a receiver was not expected to perform as the navigation devices we see today. In fact, a first-generation GPS receiver could be designed to only process 4 or 5 signals at any given time, and it was deemed suitable for positioning applications. Today, with the increased availability and potencial of different GNSS signals and constellations, any receiver is expected to track at least 10 or 12 signals in parallel channels, and up to hundreds, resulting in a more accurate solution. However, at their core - and despite the many differences in target applications or implementation philosophy - most receivers still share common control and processing flow management properties, such as:
==Overview==
 
One thing most GNSS receivers have in common is how they operate in terms of processing chain, from reception of signals to solution outputs. Although their [[Receiver Types|types]], [[System Design Details|architectures]], and applications may vary, the operations (illustrated in Figure 1) apply transversely.
*<b>Channel management</b>: each channel is managed depending on the opeartion mode and current status, and resources are allocated as needed. For every step of the computations, each channel is seamlessly controlled in the receiver.
 
*<b>Signal processing</b>: from signal capture at the antenna up to successfull tracking and data demodulation, different processing tasks are performed for each receiver channel for [[Baseband Processing|baseband processing]]. Examples are the [[Correlators|correlation operations]] or the [[Tracking Loops|tracking loops]].
 
*<b>Navigation solution computation</b>: with the observables from [[Digital Signal Processing|signal processing]] stages, together with [[GNSS_signal|almanac and ephemeris]] from the data messages, the receiver periodically computes the different outputs, such as the PVT solution.
 
The following sections detail several operation modes controlled and managed within a receiver, that fall under the above categories. 
 
==Background==
A GNSS receiver main objective is to determine its distance to a set of visible satellites, by tracking their transmitted signals. These signals have and underlying periodic code modulation that is precisely time-tagged by each satellite. Since the satellite time reference is [[An intuitive approach to the GNSS positioning|very accurate and the same]] for all satellites, a receiver can determine the time it took for each signal to arrive, thus determining its relative position to each satellite. This process translates into a <b>pseudorange</b> measurement, which is a rough estimate of the user-satellite distance (not compensated for signal transmission delays and clock offsets).
 
Besides the pseudorange, other measurements are extracted in successful tracking operations: the <b>carrier phase</b>, defined as the phase difference between the received and the internally generated carrier (codeless), is a measurement of the cycle turns (accumulated wavelengths) tracked by the receiver. This observation is very precise, but ambiguous, since the total number of integer wave cycles from transmission to reception is unknown. Also, the <b>Doppler frequency</b> seen by the receiver, caused by the dynamics of the satellite-user system, represents the difference between received and internally generated signals, and is a good representation of the relative velocity between satellite and receiver.
 
The describel realm of measurements are the basics for which a GNSS signal receiver operates, and a receiver typicaly processes and refines these measurements in different stages of operation.
 
==Operation modes==
One thing most GNSS receivers have in common is how they operate in terms of processing chain, from reception of signals to solution outputs. Alhough their [[Receiver Types|types]], [[System Design Details|architectures]], and [[Applications Processing|applications]] may vary, the operations of receivers apply transversly:
 
*<b>RF stage</b>: the first stage of a receiver comprises the antenna and front end, responsible for the RF processing chain.
 
*<b>Start-up</b>: a receiver starts processing the samples from the RF stage in different ways, depending on the available <i>a priori</i> information.
 
*<b>Acquisition</b>: the first step, in many cases, is the search for visible satellites that can be tracked and used in the computed solution.


*<b>Tracking</b>: after the initial determination of the satellites to track, several signal processing algorithms are use to continuously track the satellite's motion relative to the user.
[[File:Receiver_operations.png|center|thumb|650px|'''''Figure 1:''''' GNSS receiver operations diagram.]]


*<b>Lock detection</b>: a receiver constantly performes decisions to assess the quality of the observables in terms of noise or uncertainty, and these decisions determine the probability of a given tracked signal being "lost". This decision can yield a loss of lock, and the receiver channel is free to process other signals (e.g. start acquisition for other satellites).
After start-up procedures (e.g. loading necessary data, initializations and resource allocations), the antenna and the front-end blocks start providing digitized data continuously to the signal processing channels, also referred to as [[Baseband Processing|baseband processing]].


*<b>Solution computation</b>: when tracking is successfull for a large enough period of time, and for enough satellites, the navigation message can be decoded from each signal, and the extracted information is used to compute the best solution needed - in most cases, the user's [[GNSS Receivers General Introduction|PVT solution]].
==Channel Level==
A typical GNSS receiver controls independent channels that are assigned to a specific signal from a specific satellite. Each channel can be in two modes: acquisition or tracking. In acquisition mode, each channel conducts a rough [[Generic Receiver Description|2D search]] (code delay and Doppler frequency) of the signal in order to assess whether the signal is present or not. The channel remains in acquisition mode until a signal is detected and a first estimate of the code delay and Doppler frequency is computed. At that point, the channel goes into tracking mode where the estimates of the code delay and the Doppler frequency are continuously refined and the navigation message is decoded.


===RF stage===
In tracking mode, the channel monitors the quality of the tracking results in order to assess if the signal is present and is being correctly tracked. If the [[Lock Detectors|lock detection]] indicators fall below a threshold, the signal is considered lost and the channel goes back to acquisition mode and re-starts the whole process. The signal can be lost due to several factors such as shadowing of the satellite, cycle slips on the signal, or simply due to noisy initial estimates provided by the acquisition mode.
GNSS receivers operate in three


===Start-up===
==Receiver Level==
GNSS receivers operate in three
At receiver level, the outputs of the channels (measurements and navigation message) are processed into usable solutions. Depending on the application, a GNSS receiver may need measurements from a minimum of three satellites (e.g. for 2D positioning), four satellites (e.g. for 3D positioning) or more satellites (e.g. for [[Criticality of GNSS Applications|liability critical applications]]).
Therefore at global level, if the receiver has enough measurements to provide application data it outputs a solution, otherwise it remains in idle mode. Some receivers might cope with instantaneous lack of measurements by extrapolating from previous measurements or even [[Receiver Types|using information from external sources]] to support this extrapolation. An example is the possibility to use measurements from an Inertial Measurement Unit (IMU) in order to continue estimating positioning, even in degraded mode.
Furthermore, the receiver may also use measurements from a satellite with bad quality indicators just to provide a solution, even if degraded, e.g. in case availability is more important than accuracy for its specific application. This is often the case for receivers that provide 2D solutions when only measurements from three satellites are available, e.g. in [[:Category:Autonomous Applications|car applications]] where it is reasonable to assume that the user positioning can be map matched to a given road and therefore availability is the key performance factor.


===Acquisition===
==Advanced Operations==
GNSS receivers operate in three
On top of the basic operations at channel and receiver level, the overall receiver can further benefit from information either collected from past operations or recently decoded. For example, the receiver can use the information conveyed in the navigation message of a given satellite – e.g. [[GNSS signal|almanacs]] from other satellites – in order to estimate which satellites are visible and therefore assign channels to those satellites that have a higher probability to be in view.
Following these principles, acquisition is often sub-categorized in: cold, warm and hot start mode.


===Tracking===
*<b>In cold start</b>, the receiver has no prior information about its own position or which satellites are in view. As a consequence, each channel has to perform extensive search of all possible signals / satellites over all possible code delay / Doppler frequency pairs. It is only when (at least) one full navigation message is decoded that the receiver can infer the remaining satellites in view (through almanac information), and may redirect the other channels to follow them.
GNSS receivers operate in three


===Lock detection===
*<b>In warm start</b>, the receiver has access to rough initial position (e.g. last position before powering down the receiver), and to almanacs relatively up to date (e.g. via external sources or stored at power down). With this information, the receiver is able to predict which satellites are most likely to be in view, and estimate their rough code delay and Doppler frequency, hence being able to narrow down the acquisition search space.
GNSS receivers operate in three


===Solution computation===
*<b>In hot start</b>, the receiver has access not only to its rough initial position, but also to the ephemerides of the satellites, hence greatly reducing the acquisition search space, improving the time to acquire and therefore the final Time To First Fix (TTFF).
GNSS receivers operate in three


==Related articles==
==Related articles==
For further details on GNSS receivers, please visit the following links:
*[[Generic Receiver Description]]
*[[Generic Receiver Description]]
*[[Receiver Types]]
*[[Receiver Types]]
*[[Applications Processing]]


==References==
==References==

Latest revision as of 16:34, 18 September 2014


ReceiversReceivers
Title Receiver Operations
Edited by GMV
Level Intermediate
Year of Publication 2011
Logo GMV.png

In the first years of GNSS, when only GPS was available for public use, a receiver was not expected to perform as the navigation devices we see today. In fact, a first-generation GPS receiver could be designed to only process 4 or 5 signals at any given time, and it was deemed suitable for positioning applications. Today, with the increased availability and potential of different GNSS signals and constellations, any receiver is expected to support at least 10 or 12 channels up to hundreds, resulting in a solution with improved performance. However, at their core - and despite the many differences in target applications or implementation philosophy - most receivers still share common control and processing flow management properties, resulting in common operations[1] performed. The following sections describe these common operation modes in which a receiver functions.

Overview

One thing most GNSS receivers have in common is how they operate in terms of processing chain, from reception of signals to solution outputs. Although their types, architectures, and applications may vary, the operations (illustrated in Figure 1) apply transversely.

Figure 1: GNSS receiver operations diagram.

After start-up procedures (e.g. loading necessary data, initializations and resource allocations), the antenna and the front-end blocks start providing digitized data continuously to the signal processing channels, also referred to as baseband processing.

Channel Level

A typical GNSS receiver controls independent channels that are assigned to a specific signal from a specific satellite. Each channel can be in two modes: acquisition or tracking. In acquisition mode, each channel conducts a rough 2D search (code delay and Doppler frequency) of the signal in order to assess whether the signal is present or not. The channel remains in acquisition mode until a signal is detected and a first estimate of the code delay and Doppler frequency is computed. At that point, the channel goes into tracking mode where the estimates of the code delay and the Doppler frequency are continuously refined and the navigation message is decoded.

In tracking mode, the channel monitors the quality of the tracking results in order to assess if the signal is present and is being correctly tracked. If the lock detection indicators fall below a threshold, the signal is considered lost and the channel goes back to acquisition mode and re-starts the whole process. The signal can be lost due to several factors such as shadowing of the satellite, cycle slips on the signal, or simply due to noisy initial estimates provided by the acquisition mode.

Receiver Level

At receiver level, the outputs of the channels (measurements and navigation message) are processed into usable solutions. Depending on the application, a GNSS receiver may need measurements from a minimum of three satellites (e.g. for 2D positioning), four satellites (e.g. for 3D positioning) or more satellites (e.g. for liability critical applications). Therefore at global level, if the receiver has enough measurements to provide application data it outputs a solution, otherwise it remains in idle mode. Some receivers might cope with instantaneous lack of measurements by extrapolating from previous measurements or even using information from external sources to support this extrapolation. An example is the possibility to use measurements from an Inertial Measurement Unit (IMU) in order to continue estimating positioning, even in degraded mode. Furthermore, the receiver may also use measurements from a satellite with bad quality indicators just to provide a solution, even if degraded, e.g. in case availability is more important than accuracy for its specific application. This is often the case for receivers that provide 2D solutions when only measurements from three satellites are available, e.g. in car applications where it is reasonable to assume that the user positioning can be map matched to a given road and therefore availability is the key performance factor.

Advanced Operations

On top of the basic operations at channel and receiver level, the overall receiver can further benefit from information either collected from past operations or recently decoded. For example, the receiver can use the information conveyed in the navigation message of a given satellite – e.g. almanacs from other satellites – in order to estimate which satellites are visible and therefore assign channels to those satellites that have a higher probability to be in view. Following these principles, acquisition is often sub-categorized in: cold, warm and hot start mode.

  • In cold start, the receiver has no prior information about its own position or which satellites are in view. As a consequence, each channel has to perform extensive search of all possible signals / satellites over all possible code delay / Doppler frequency pairs. It is only when (at least) one full navigation message is decoded that the receiver can infer the remaining satellites in view (through almanac information), and may redirect the other channels to follow them.
  • In warm start, the receiver has access to rough initial position (e.g. last position before powering down the receiver), and to almanacs relatively up to date (e.g. via external sources or stored at power down). With this information, the receiver is able to predict which satellites are most likely to be in view, and estimate their rough code delay and Doppler frequency, hence being able to narrow down the acquisition search space.
  • In hot start, the receiver has access not only to its rough initial position, but also to the ephemerides of the satellites, hence greatly reducing the acquisition search space, improving the time to acquire and therefore the final Time To First Fix (TTFF).

Related articles

References

  1. ^ Borre, K. et al, "A Software-Defined GPS and Galileo Receiver - A Single-Frequency Approach", Birkhäuser, chapter 5.