gnss-sdr/docs/doxygen/other/main_page.dox

# Copyright (C) 2012-2014  (see AUTHORS file for a list of contributors)
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/*! \mainpage

\image html gnss-sdr_logo.png
\image latex gnss-sdr_logo.png "GNSS-SDR logo" width=2.5cm

Welcome to GNSS-SDR!

GNSS-SDR is an open-source <a href="http://en.wikipedia.org/wiki/Software_GNSS_receiver" target="_blank">GNSS software receiver</a> freely available to the research community. This project provides a common framework for GNSS signal processing which can operate in a variety of computer platforms. This tool is intended to foster collaboration, increase awareness, and reduce development costs in the field of GNSS receiver design and customized use of GNSS signals.

For details about GNSS-SDR and using it, please see the <a
href="http://gnss-sdr.org" target="_blank"><b>main project page</b></a> or browse the code at the <a
href="http://sourceforge.net/p/gnss-sdr/cttc/ci/master/tree/" target="_blank"><b>Sourceforge project page</b></a>. You could be also interested in
<a href="http://lists.sourceforge.net/lists/listinfo/gnss-sdr-developers" target="_blank"><b>subscribing to the mailing list</b></a>.

\section toc Contents
\li \ref overview
\li \ref build
\li \ref using_gnss-sdr
\li \ref control_plane
\li \ref signal_processing
\li \ref license
\li \ref publications
\li \ref now_what


More details on GNSS-SDR signal processing blocks:

\li \ref signal_source
\li \ref signal_conditioner
\li \ref channel
    <ul>
      <li> \ref acquisition </li>
      <li> \ref tracking</li>
      <li> \ref decoding
    </ul>
\li \ref observables
\li \ref pvt
\li \ref output_filter


\section overview Overview
GNSS-SDR provides an interface to different suitable RF front-ends and implements all the receiver chain up to the navigation solution. 
Its design allows any kind of customization, including interchangeability of signal sources, signal processing algorithms, 
interoperability with other systems, output formats, and offers interfaces to all the intermediate signals, parameters and variables. 
The goal is to write efficient and truly reusable code, easy to read and maintain, with fewer bugs, and producing highly optimized executables 
in a variety of hardware platforms and operating systems. In that sense, the challenge consists of defining a gentle balance within level 
of abstraction and performance. GNSS-SDR runs in a personal computer and provides interfaces through USB and Ethernet 
buses to a variety of either commercially available or custom-made RF front-ends, adapting the processing algorithms to different sampling frequencies, intermediate
 frequencies and sample resolutions. This makes possible rapid prototyping of specific receivers intended, for instance, to geodetic applications, 
 observation of the ionospheric impact on navigation signals, GNSS reflectometry, signal quality monitoring, or carrier-phase based navigation techniques. 

\image html overview.png
\image latex overview.png "Overview" width=12cm

As signal inputs, it accepts:
\li Raw data file captured with a data grabber (digitized at some intermediate frequency or directly at baseband).
\li Any suitable RF configuration that can be driven by the Universal Software Radio Peripheral Hardware Driver (<a href="http://files.ettus.com/uhd_docs/manual/html/" target="_blank">UHD</a>).
This includes all current and future <a href="http://www.ettus.com/">Ettus Research</a> products. The USRP1 + DBSRX 2.2 daughterboard is an example of working configuration for GPS L1 C/A and Galileo E1B and E1C signals.
\li The <a href="http://gnss-sdr.org/documentation/sige-gn3s-sampler-v2-usb-front-end" target="blank">GN3S v2 USB dongle</a> (GN3S v3 might work with small modifications).
\li Experimentally, with some <a href="http://gnss-sdr.org/documentation/gnss-sdr-operation-realtek-rtl2832u-usb-dongle-dvb-t-receiver" target="_blank">USB DVB-T dongles based on the Realtek RTL2832U chipset</a>.
\li For mathematical representations of the targeted signals, check out the \ref the_signal_model page.

As outputs, it provides:
\li Dump of intermediate signals (configurable by the user)
\li The processing is logged at a  system temporary folder (usually, <tt>/tmp</tt>)
\li Observables in form of RINEX file (experimental)
\li Navigation message data in form of RINEX file
\li Position, Velocity and Time solution in KML format and NMEA


\section build Building GNSS-SDR  

In principle, GNSS-SDR can be built in any Unix-like system. In practice, it depends on being able to install all the required dependencies. See the <a href="http://gnss-sdr.org/documentation/building-guide" target="_blank">building guide</a> page for details about the project's
dependencies and build process. Mainly, it consists on installing <a href="http://gnuradio.org/" target="_blank">GNU Radio</a> plus some few more libraries:

\li <a href="http://code.google.com/p/gflags/" target="_blank">Gflags</a>, a library that implements commandline flags processing,
\li <a href="http://code.google.com/p/google-glog/" target="_blank">Glog</a>, a library that implements application-level logging, 
\li <a href="http://arma.sourceforge.net/" target="_blank">Armadillo</a>, a C++ linear algebra library,
\li <a href="http://code.google.com/p/googletest/" target="_blank">Googletest</a>, Google's framework for writing C++ tests (requires definition of the GTEST_DIR variable),

and, optionally,
\li <a href="http://code.google.com/p/gperftools/" target="_blank">Gperftools</a>, which provides fast, multi-threaded malloc() and performance analysis tools.

After all dependencies are installed, clone the GNSS-SDR repository:
\verbatim
$ git clone git://git.code.sf.net/p/gnss-sdr/cttc gnss-sdr 
\endverbatim

This will create a folder named gnss-sdr with the following structure:
\verbatim
 |-gnss-sdr
 |---build       <- where gnss-sdr is built
 |---cmake       <- CMake-related files
 |---conf        <- Configuration files. Each file represents one receiver.
 |---data        <- Populate this folder with your captured data.
 |---docs        <- Contains documentation-related files
 |---drivers     <- Drivers for some RF front-ends
 |---firmware    <- Firmware for some front-ends
 |---install     <- Executables 
 |---src         <- Source code folder
 |-----algorithms
 |-------PVT
 |-------acquisition
 |-------channel
 |-------conditioner
 |-------data_type_adapter
 |-------input_filter
 |-------libs
 |-------observables
 |-------output_filter
 |-------resampler
 |-------signal_source
 |-------telemetry_decoder
 |-------tracking
 |-----core
 |-------interfaces
 |-------libs
 |-------receiver
 |-------system_parameters
 |-----main
 |-----tests
 |-----utils     <- some utilities (e.g. Matlab scripts)
\endverbatim

You are now ready to build GNSS-SDR by using <a href="http://www.cmake.org/" target="_blank">CMake</a> as building tool:
\verbatim
$ cd gnss-sdr/build
$ cmake ../
$ make
\endverbatim

If everything goes well, three new executables will be created at <tt>gnss-sdr/install</tt>, namely <tt>gnss-sdr</tt>, <tt>volk_gnsssdr_profile</tt> and <tt>run_tests</tt>.
You can run them from that folder, but if you prefer to install gnss-sdr on your system and have it available anywhere else, do:
\verbatim
$ sudo make install
\endverbatim

This will make a copy of the conf/ folder into /usr/local/share/gnss-sdr/conf for your reference. 
We suggest to create a working directory at your preferred location and store your own configuration and data files there. 


You can create the documentation by doing:
\verbatim
$ make doc
\endverbatim

from the <tt>gnss-sdr/build</tt> folder. In both cases, <a href="http://www.stack.nl/~dimitri/doxygen/" target="_blank">Doxygen</a> will generate HTML documentation that can be 
retrieved pointing your browser of preference to <tt>gnss-sdr/docs/html/index.html</tt>.

There are two more extra targets available. From the <tt>gnss-sdr/build</tt> folder:
\verbatim
$ make doc-clean
\endverbatim
will remove the content of previously-generated documentation and, if a LaTeX installation is detected in your system,
\verbatim
$ make pdfmanual
\endverbatim
will create a PDF manual at <tt>gnss-sdr/docs/GNSS-SDR_manual.pdf</tt>. Please note that the PDF generation requires some fonts to be installed on the host system.
In Ubuntu 12.10, those fonts do not come by default. You can install them by doing:
\verbatim
$ sudo apt-get install texlive-fonts-recommended
\endverbatim
and then run <tt> cmake ../</tt> and <tt>make pdfmanual</tt> again.

\subsection debug_and_release Debug and Release builds

By default, CMake will build the Release version, meaning that the compiler will generate a faster, optimized executable. This is the recommended build type when using
a RF front-end and you need to attain real time. If working with a file (and thus without real-time constraints), you may want to obtain more information about
the internals of the receiver, as well as more fine-grained logging. This can be done by building the Debug version, by doing:
\verbatim
$ cd gnss-sdr/build
$ cmake -DCMAKE_BUILD_TYPE=Debug ../
$ make
$ sudo make install
\endverbatim

\subsection updating_gnss-sdr Updating GNSS-SDR
If you checked out GNSS-SDR some days ago, it is possible that some developer had updated files at the Git repository. You can update your local copy by doing:
\verbatim
$ git checkout next
$ git pull origin next
\endverbatim
Before rebuiling the source code, it is safe (and recommended) to remove the remainders of old builds:
\verbatim
$ cd gnss-sdr/build
$ sudo make uninstall
$ rm -rf *
\endverbatim

You can also check <a href="http://git-scm.com/book" target="_blank">The Git Book</a> for more information about Git usage.

\section using_gnss-sdr Using GNSS-SDR

With GNSS-SDR, you can define you own receiver, work with captured raw data or from a RF front-end, dump into files intermediate signals, or tune every single algorithm used in the \ref signal_processing. All the configuration
is done in a single file. Those configuration files reside at the <tt>gnss-sdr/conf</tt> folder. By default, the executable <tt>gnss-sdr</tt> will read the configuration
available at <tt>gnss-sdr/conf/gnss-sdr.conf</tt>. You can edit that file to fit your needs, or even better, define a new <tt>my_receiver.conf</tt> file with your own configuration. 
This new receiver can be done by invoking gnss-sdr with the <tt>--config_file</tt> flag pointing to your configuration file:
\verbatim
$ gnss-sdr --config_file=../conf/my_receiver.conf
\endverbatim

You can see a guide of available implementations at <tt>gnss-sdr/conf/master.conf</tt>. That folder contains other working examples as well. If you have a working
configuration and want to share it will others, please email it to the <a href="http://lists.sourceforge.net/lists/listinfo/gnss-sdr-developers" target="_blank"><b>GNSS-SDR developers mailing list</b></a>
and we will be happy to upload it to the server.

You can use a single configuration file for processing
different data files, specifying the file to be processed with the <tt>--signal_source</tt> flag:
\verbatim
$ gnss-sdr --config_file=../conf/my_receiver.conf --signal_source=../data/my_captured_data.dat
\endverbatim
This will override the <tt>SignalSource.filename</tt> specified in the configuration file.

You can get a complete list of available commandline flags by doing: 
\verbatim
$ gnss-sdr --help
\endverbatim
For general usage of commandline flags, see <a href="http://google-gflags.googlecode.com/svn/trunk/doc/gflags.html" target="_blank">how to use Google Commandline Flags</a>.

\section control_plane Control plane
GNSS-SDR's main method initializes the logging library, processes the command line flags, if any, provided by the user and instantiates a ControlThread object. 
Its constructor reads the configuration file, creates a control queue and creates a flowgraph according to the configuration. Then, the program's main method 
calls the run() method of the instantiated object, an action that connects the flowgraph and starts running it. After that, and until a stop message is received, 
it reads control messages sent by the receiver's modules through a safe-thread queue and processes them. Finally, when a stop message is received, the main 
method executes the destructor of the ControlThread object, which deallocates memory, does other cleanup and exits the program.

The GNSSFlowgraph class is responsible for preparing the graph of blocks according to the configuration, running it, modifying it during run-time and stopping it. 
Blocks are identified by its role. This class knows which roles it has to instantiate and how to connect them. 
It relies on the configuration to get the correct instances of the roles it needs and then it applies the connections between GNU Radio blocks to make the 
graph ready to be started. The complexity related to managing the blocks and the data stream is handled by GNU Radio's <tt>gr::top_block</tt> class. GNSSFlowgraph wraps 
the <tt>gr::top_block</tt> instance so we can take advantage of the \ref gnss_block_factory, the configuration system and the processing blocks. This class is also responsible 
for applying changes to the configuration of the flowgraph during run-time, dynamically reconfiguring channels: it selects the strategy for selecting satellites. 
This can range from a sequential search over all the satellites' ID to smarter approaches that determine what are the satellites most likely in-view based on rough 
estimations of the receiver position in order to avoid searching satellites in the other side of the Earth.

The Control Plane is in charge of creating a flowgraph according to the configuration and then managing the modules. Configuration allows users to define in an easy way their own
custom receiver by specifying the flowgraph (type of signal source, number of channels, algorithms to be used for each channel and each module, strategies for 
satellite selection, type of output format, etc.). Since it is difficult to foresee what future module implementations will be needed in terms of configuration, 
we used a very simple approach that can be extended without a major impact in the code. This can be achieved by simply mapping the names of the variables in the 
modules with the names of the parameters in the configuration.

\subsection configuration Configuration

Properties are passed around within the program using the ConfigurationInterface class. There are two implementations of this interface: FileConfiguration and 
InMemoryConfiguration. FileConfiguration reads the properties (pairs of property name and value) from a file and stores them internally. InMemoryConfiguration does 
not read from a file; it remains empty after instantiation and property values and names are set using the set property method. FileConfiguration is intended to be 
used in the actual GNSS-SDR application whereas InMemoryConfiguration is intended to be used in tests to avoid file-dependency in the file system. Classes that 
need to read configuration parameters will receive instances of ConfigurationInterface from where they will fetch the values. For instance, parameters related 
to SignalSource should look like this:

\verbatim
SignalSource.parameter1=value1
SignalSource.parameter2=value2
\endverbatim

The name of these parameters can be anything but one reserved word: implementation. This parameter indicates in its value the name of the class that has to be instantiated 
by the factory for that role. For instance, if our signal source is providing data already at baseband and thus we want to use the implementation Pass_Through for module SignalConditioner, the corresponding line in the 
configuration file would be

\verbatim
SignalConditioner.implementation=Pass_Through
\endverbatim

Since the configuration is just a set of property names and values without any meaning or syntax, the system is very versatile and easily extendable. Adding new 
properties to the system only implies modifications in the classes that will make use of these properties. In addition, the configuration files are not checked 
against any strict syntax so it is always in a correct status (as long as it contains pairs of property names and values in <a href="http://en.wikipedia.org/wiki/INI_file" target="_blank">INI format</a>).

\subsection gnss_block_factory GNSS block factory
Hence, the application defines a simple accessor class to fetch the configuration pairs of values and passes them to a factory class called GNSSBlockFactory. 
This factory decides, according to the configuration, which class needs to be instantiated and which parameters should be passed to the constructor. Hence, the factory 
encapsulates the complexity of blocks' instantiation. With that approach, adding a new block that requires new parameters will be as simple as adding the block 
class and modifying the factory to be able to instantiate it. This loose coupling between the blocks' implementations and the syntax of the configuration 
enables extending the application capacities in a high degree. It also allows to produce fully customized receivers, for instance a testbed for acquisition 
algorithms, and to place observers at any point of the receiver chain.



\section signal_processing Signal Processing plane

GNU Radio's class <tt>gr::basic_block</tt> is the abstract base class for all signal processing blocks, a bare abstraction of an entity that has a name and a set of 
inputs and outputs. It is never instantiated directly; rather, this is the abstract parent class of both <tt>gr::hier_block2</tt>, which is a recursive container that 
adds or removes processing or hierarchical blocks to the internal graph, and <tt>gr::block</tt>, which is the abstract base class for all the processing blocks. 


\image html ClassHierarchy.png
\image latex ClassHierarchy.png "Class hierarchy of signal processing blocks" width=12cm

A signal processing flow is constructed by creating a tree of hierarchical blocks, which at any level may also contain terminal nodes that actually implement signal 
processing functions.

Class <tt>gr::top_block</tt> is the top-level hierarchical block representing a flowgraph. It defines GNU Radio runtime functions used during the execution of the 
program: run(), start(), stop(), wait(), etc. A a subclass called GNSSBlockInterface is the common interface for all the GNSS-SDR modules. It defines pure virtual 
methods, that are required to be implemented by a derived class.

Subclassing GNSSBlockInterface, we defined interfaces for the GNSS receiver blocks depicted in the figure above. This hierarchy provides the definition of different 
algorithms and different implementations, which will be instantiated according to the configuration. This strategy allows 
multiple implementations sharing a common interface, achieving the objective of decoupling interfaces from implementations: it defines a family of algorithms, encapsulates each one, 
and makes them interchangeable. Hence, we let the algorithm vary independently from the program that uses it.

\subsection signal_source Signal Source

The input of a software receiver are the raw bits that come out from the front-end's analog-to-digital converter (ADC). 
Those bits can be read from a file stored in the hard disk or directly in real-time from a hardware device through USB or Ethernet buses. 

The Signal Source module is in charge of implementing the hardware driver, that is, the portion of the code that communicates with the RF front-end and receives 
the samples coming from the ADC. This communication is usually performed through USB or Ethernet buses. Since real-time processing requires a highly optimized 
implementation of the whole receiver, this module also allows to read samples from a file stored in a hard disk, and thus processing without time constraints. 
Relevant parameters of those samples are the intermediate frequency (or baseband I&Q components), the sampling rate and number of bits per sample, that must be 
specified by the user in the configuration file.

This module also performs bit-depth adaptation, since most of the existing RF front-ends provide samples quantized with 2 or 3 bits, while operations inside
 the processor are performed on 32- or 64-bit words, depending on its architecture. Although there are implementations of the most intensive computational 
 processes (mainly correlation) that take advantage of specific data types and architectures for the sake of 
 efficiency, the approach is processor-specific and hardly portable. We suggest to keep signal samples in standard data types and letting the compiler 
 select the best library version (implemented using SIMD or any other processor-specific technology) of the required routines for a given processor.
 
Example: FileSignalSource

The user can configure the receiver for reading from a file, setting in the configuration file the data file location, sample format, 
and the sampling frequency and intermediate frequency at what the signal was originally captured.
 
\verbatim
;######### SIGNAL_SOURCE CONFIG ############
SignalSource.implementation=File_Signal_Source
SignalSource.filename=/home/user/gnss-sdr/data/my_capture.dat
SignalSource.item_type=gr_complex
SignalSource.sampling_frequency=4000000 ; Sampling frequency in [Hz]
SignalSource.freq=1575420000 ; RF front-end center frequency in [Hz] 
\endverbatim 

Example: UhdSignalSource

The user may prefer to use a UHD-compatible RF front-end and try real-time processing. For instance, for a USRP1 + DBSRX daughterboard, use:
\verbatim
;######### SIGNAL_SOURCE CONFIG ############
SignalSource.implementation=UHD_Signal_Source
SignalSource.item_type=gr_complex
SignalSource.sampling_frequency=4000000 ; Sampling frequency in [Hz]
SignalSource.freq=1575420000 ; RF front-end center frequency in [Hz] 
SignalSource.gain=60 ; Front-end gain in dB
SignalSource.subdevice=B:0 ; UHD subdevice specification (for USRP1 use A:0 or B:0)
\endverbatim

Other examples are available at <tt>gnss-sdr/conf</tt>.

 \subsection signal_conditioner Signal Conditioner
 The signal conditioner is in charge of resampling the signal and delivering a reference sample rate to the downstream processing blocks, acting as
  a facade between the signal source and the synchronization channels, providing a simplified interface to the input signal. 
 In case of multiband front-ends, this module would be in charge of providing a separated data stream for each band. 


\subsection channel Channel
A channel encapsulates all signal processing devoted to a single satellite. Thus, it is a large composite object which encapsulates the \ref acquisition,
 \ref tracking and \ref decoding modules. As a composite object, it can be treated as a single entity, meaning that it can be easily replicated. Since the number of
  channels is selectable by the user in the configuration file, this approach helps improving the scalability and maintainability of the receiver.

This module is also in charge of managing the interplay between acquisition and tracking. Acquisition can be initialized in several ways, depending on
the prior information available (called cold start when the receiver has no information about its position nor the satellites almanac; warm start when 
a rough location and the approximate time of day are available, and the receiver has a recently recorded almanac broadcast; or hot start when the receiver 
was tracking a satellite and the signal line of sight broke for a short period of time, but the ephemeris and almanac data is still valid, or this information
is provided by other means), and an acquisition process can finish deciding that the satellite is not present, that longer integration is needed in order to 
confirm the presence of the satellite, or declaring the satellite present. In the latter case, acquisition process should stop and trigger the tracking module
with coarse estimations of the synchronization parameters. The mathematical abstraction used to design this logic is known as finite state machine (FSM), that is
a behavior model composed of a finite number of states, transitions between those states, and actions. For the implementation, we used the 
<a href="http://www.boost.org/libs/statechart/doc/tutorial.html" target="_blank">Boost.Statechart library</a>, 
which provides desirable features such as support for asynchronous state machines, multi-threading, type-safety, error handling and compile-time validation.
     
The abstract class ChannelInterface represents an interface to a channel GNSS block. Check Channel for an actual implementation.
     
\subsubsection acquisition Acquisition
The first task of a GNSS receiver is to detect the presence or absence of in-view satellites. This is done by the acquisition system process, which also provides a coarse estimation of two signal parameters: the frequency shift 
 with respect to the nominal IF frequency, and a delay term which allows the receiver to create a local code aligned with the incoming code.
AcquisitionInterface is the common interface for all the acquisition algorithms and their corresponding implementations. Algorithms' interface, that may vary 
depending on the use of information external to the receiver, such as in Assisted GNSS, is defined in classes referred to as <i>adapters</i>. 
These adapters wrap the GNU Radio blocks interface into a compatible interface expected by AcquisitionInterface. This allows the use of existing GNU Radio blocks 
derived from <tt>gr::block</tt>, and ensures that newly developed implementations will also be reusable in other GNU Radio-based applications. 
Moreover, it adds still another layer of abstraction, since each given acquisition algorithm can have different implementations (for instance using
 different numerical libraries). In such a way, implementations can be continuously improved without having any impact neither on the algorithm interface nor the general acquisition interface.

Check GpsL1CaPcpsAcquisition and GalileoE1PcpsAmbiguousAcquisition for examples of adapters from a Parallel Code Phase Search (PCPS) acquisition block, and
 pcps_acquisition_cc for an example of a block implementation. The source code of all the available acquisition algorithms is located at:
 
 \verbatim
  |-gnss-sdr
  |---src
  |-----algorithms
  |-------acquisition
  |---------adapters          <- Adapters of the processing blocks to an AcquisitionInterface
  |---------gnuradio_blocks   <- Signal processing blocks implementation
 \endverbatim

The user can select a given implementation for the algorithm to be used in each receiver channel, as well as their parameters, in the configuration file:
\verbatim
;######### ACQUISITION GLOBAL CONFIG ############

;#dump: Enable or disable the acquisition internal data file logging [true] or [false] 
Acquisition.dump=false
;#filename: Log path and filename
Acquisition.dump_filename=./acq_dump.dat
;#item_type: Type and resolution for each of the signal samples. Use only gr_complex in this version.
Acquisition.item_type=gr_complex
;#if: Signal intermediate frequency in [Hz] 
Acquisition.if=0
;#sampled_ms: Signal block duration for the acquisition signal detection [ms]
Acquisition.sampled_ms=1
;#implementation: Acquisition algorithm selection for this channel:
Acquisition.implementation=GPS_L1_CA_PCPS_Acquisition
;#threshold: Acquisition threshold
Acquisition.threshold=0.005
;#pfa: Acquisition false alarm probability. This option overrides the threshold option. 
;Only use with implementations: [GPS_L1_CA_PCPS_Acquisition] or [Galileo_E1_PCPS_Ambiguous_Acquisition] 
Acquisition.pfa=0.0001
;#doppler_max: Maximum expected Doppler shift [Hz]
Acquisition.doppler_max=10000
;#doppler_max: Doppler step in the grid search [Hz]
Acquisition.doppler_step=500

;######### ACQUISITION CHANNELS CONFIG ######
;#The following options are specific to each channel and overwrite the generic options 


;######### ACQUISITION CH 0 CONFIG ############
;Acquisition0.implementation=GPS_L1_CA_PCPS_Acquisition
;Acquisition0.threshold=0.005
;Acquisition0.pfa=0.001
;Acquisition0.doppler_max=10000
;Acquisition0.doppler_step=250

;#repeat_satellite: Use only jointly with the satellite PRN ID option. The default value is false
;Acquisition0.repeat_satellite = false

;######### ACQUISITION CH 1 CONFIG ############
;Acquisition1.implementation=GPS_L1_CA_PCPS_Acquisition
;Acquisition1.threshold=0.005
;Acquisition1.pfa=0.001
;Acquisition1.doppler_max=10000
;Acquisition1.doppler_step=250
;Acquisition1.repeat_satellite = false
\endverbatim


\subsubsection tracking Tracking
When a satellite is declared present, the parameters estimated by the acquisition module are then fed to the receiver tracking module, which represents the
 second stage of the signal processing unit, aiming to perform a local search for accurate estimates of code delay and carrier phase, and following their eventual
  variations. 

Again, a class hierarchy consisting of a TrackingInterface class and subclasses implementing algorithms provides a way of testing different approaches, 
with full access to their parameters. Check GpsL1CaDllPllTracking or GalileoE1DllPllVemlTracking for examples of adapters, and Gps_L1_Ca_Dll_Pll_Tracking_cc for an example 
of a signal processing block implementation. There are also available some useful classes and functions for signal tracking; take a look at Correlator, lock_detectors.h, tracking_discriminators.h or
tracking_2nd_DLL_filter.h. 

The source code of all the available tracking algorithms is located at:
 \verbatim
  |-gnss-sdr
  |---src
  |-----algorithms
  |-------tracking
  |---------adapters          <- Adapters of the processing blocks to a TrackingInterface
  |---------gnuradio_blocks   <- Signal processing blocks implementation
  |---------libs              <- libraries of tracking objects (e.g. correlators, discriminators, and so on)
 \endverbatim

The user can select a given implementation for the algorithm to be used in all the tracking blocks, as well as its parameters, in the configuration file:
\verbatim
;######### TRACKING GLOBAL CONFIG ############

;#implementation: Selected tracking algorithm
Tracking.implementation=GPS_L1_CA_DLL_PLL_Tracking
;#item_type: Type and resolution for each of the signal samples. Use only [gr_complex] in this version.
Tracking.item_type=gr_complex

;#sampling_frequency: Signal Intermediate Frequency in [Hz] 
Tracking.if=0

;#dump: Enable or disable the Tracking internal binary data file logging [true] or [false] 
Tracking.dump=false

;#dump_filename: Log path and filename. Notice that the tracking channel will add "x.dat" where x is the channel number.
Tracking.dump_filename=./tracking_ch_

;#pll_bw_hz: PLL loop filter bandwidth [Hz]
Tracking.pll_bw_hz=50.0;

;#dll_bw_hz: DLL loop filter bandwidth [Hz]
Tracking.dll_bw_hz=2.0;

;#fll_bw_hz: FLL loop filter bandwidth [Hz]
Tracking.fll_bw_hz=10.0;

;#order: PLL/DLL loop filter order [2] or [3]
Tracking.order=3;

;#early_late_space_chips: correlator early-late space [chips]. Use [0.5]
Tracking.early_late_space_chips=0.5;
\endverbatim

\subsubsection decoding Decoding of the navigation message
Most of GNSS signal links are modulated by a navigation message containing the time the message was transmitted, orbital parameters of satellites 
(also known as ephemeris) and an almanac (information about the general system health, rough orbits of all satellites in the network as well as data related to 
error correction). Navigation data bits are structured in words, pages, subframes, frames and superframes. Sometimes, bits corresponding to a single parameter are 
spread over different words, and values extracted from different frames are required for proper decoding. Some words are for synchronization purposes, others for 
error control an others contain actual information. There are also error control mechanisms, from parity checks to forward error correction (FEC) encoding and 
interleaving, depending on the system. All this decoding complexity is managed by a finite state machine implemented with the <a href="http://www.boost.org/libs/statechart/doc/tutorial.html" target="_blank">Boost.Statechart library</a>.

The common interface is TelemetryDecoderInterface. Check GpsL1CaTelemetryDecoder for an example of the GPS L1 NAV message decoding adapter, and gps_l1_ca_telemetry_decoder_cc
for an actual implementation of a signal processing block. Configuration example:

\verbatim
;######### TELEMETRY DECODER CONFIG ############
TelemetryDecoder.implementation=GPS_L1_CA_Telemetry_Decoder
TelemetryDecoder.dump=false
\endverbatim

See the \ref reference_docs for more information about the signal format.

\subsection observables Observables
GNSS systems provide different kinds of observations. The most commonly used are the code observations, also called pseudoranges. The <i>pseudo</i> comes from 
the fact that on the receiver side the clock error is unknown and thus the measurement is not a pure range observation. High accuracy applications also use the
 carrier phase observations, which are based on measuring the difference between the carrier phase transmitted by the GNSS satellites and the phase of the carrier
 generated in the receiver. Both observables are computed from the outputs of the tracking module and the decoding of the navigation message.
This module collects all the data provided by every tracked channel, aligns all received data into a coherent set, and computes the observables.

The common interface is ObservablesInterface. 

Configuration example:
\verbatim
;######### OBSERVABLES CONFIG ############
;#implementation: Use [GPS_L1_CA_Observables] for GPS L1 C/A.
Observables.implementation=GPS_L1_CA_Observables

;#dump: Enable or disable the Observables internal binary data file logging [true] or [false] 
Observables.dump=false

;#dump_filename: Log path and filename.
Observables.dump_filename=./observables.dat
\endverbatim

\subsection pvt Computation of Position, Velocity and Time
Although data processing for obtaining high-accuracy PVT solutions is out of the scope of GNSS-SDR, we provide a module that can compute a simple least square
 solution and leaves room for more sophisticated positioning methods. The integration with libraries and software tools that are able to deal with multi-constellation
  data such as <a href="http://www.gpstk.org" target="_blank">GPSTk</a> or <a href="http://gage14.upc.es/gLAB/" target="_blank">gLAB</a> appears as a viable solution for high performance, completely customizable GNSS receivers.

The common interface is PvtInterface. For instance, in order to use the implementation GpsL1CaPvt, add to the configuration file:
\verbatim
;######### PVT CONFIG ############
PVT.implementation=GPS_L1_CA_PVT

;#nmea_dump_filename: NMEA log path and filename
PVT.nmea_dump_filename=./gnss_sdr_pvt.nmea;

;#flag_nmea_tty_port: Enable or disable the NMEA log to a serial TTY port (Can be used with real hardware or virtual one)
PVT.flag_nmea_tty_port=true;

;#nmea_dump_devname: serial device descriptor for NMEA logging
PVT.nmea_dump_devname=/dev/pts/4

;#dump: Enable or disable the PVT internal binary data file logging [true] or [false] 
PVT.dump=false
\endverbatim

This implementation allows tuning of the following parameters:
\verbatim
PVT.averaging_depth=10 ; Number of PVT observations in the moving average algorithm
PVT.flag_averaging=true ; Enables the PVT averaging between output intervals (arithmetic mean) [true] or [false] 
PVT.output_rate_ms=100 ; Period in [ms] between two PVT outputs
PVT.display_rate_ms=500 ; Position console print (std::out) interval [ms].
PVT.dump=false ; Enable or disable the PVT internal binary data file logging [true] or [false] 
PVT.dump_filename=./PVT ; Log path and filename without extension.
\endverbatim

\subsection output_filter Output filter

(To be completed)

\section license About the software license
GNSS-SDR is released under the <a href="http://www.gnu.org/licenses/gpl.html" target="_blank">General Public License (GPL) v3</a>, thus securing practical usability, inspection, 
and continuous improvement by the research community, allowing the discussion based on tangible code and the analysis of results obtained with real signals. 
The GPL implies that:

\li Copies may be distributed free of charge or for money, but the source code has to be shipped or provided free of charge (or at cost price) on demand. The receiver of the source code has the same rights meaning he can share copies free of charge or resell.
\li The licensed material may be analyzed or modified.
\li Modified material may be distributed under the same licensing terms but <b>do not</b> have to be distributed.

That means that modifications only have to be made available to the public if distribution happens. So it is perfectly fine to take the GNSS-SDR source code, modify it heavily and use it in a not distributed application / library. This is how companies like Google can run their own patched versions of Linux for example.

But what this also means is that non-GPL code cannot use GPL code. This means that you cannot modify / use GNSS-SDR, blend it with non-GPL code, and make money with the resulting software. You cannot distribute the resulting software under a non-disclosure agreement or contract. Distributors under the GPL also grant a license for any of their patents practiced by the software, to practice those patents in GPL software. You can sell a device that runs with GNSS-SDR, but if you distribute the code, it has to remain under GPL.

\section publications Publications and Credits
If you use GNSS-SDR to produce a research paper or Thesis, we would appreciate if you reference any of these articles to credit the GNSS-SDR project:

\li \anchor Navitec2012 C. Fern&aacute;ndez-Prades, J. Arribas, L. Esteve, D. Pubill, P. Closas, <a href="http://www.cttc.es/publication/an-open-source-galileo-e1-software-receiver/" target="_blank"><i>An Open Source Galileo E1 Software Receiver</i></a>, in Proc. of the 6th ESA Workshop on Satellite Navigation Technologies (NAVITEC 2012), ESTEC, Noordwijk, The Netherlands, Dec. 2012.
\li J. Arribas, <a href="http://theses.eurasip.org/theses/449/gnss-array-based-acquisition-theory-and/" target="_blank"><i>GNSS Array-based Acquisition: Theory and Implementation</i></a>, PhD Thesis, Universitat Polit&egrave;cnica de Catalunya, Barcelona, Spain, June 2012.
\li C. Fern&aacute;ndez-Prades, J. Arribas,  P. Closas, C. Avil&eacute;s, and L. Esteve, <a href="http://www.cttc.es/publication/gnss-sdr-an-open-source-tool-for-researchers-and-developers/" target="_blank"><i>GNSS-SDR: an open source tool for researchers and developers</i></a>, in Proc. of the ION GNSS 2011 Conference, Portland, Oregon, Sept. 19-23, 2011. 
\li C. Fern&aacute;ndez-Prades, C. Avil&eacute;s, L. Esteve, J. Arribas, and P. Closas, <a href="http://www.cttc.es/publication/design-patterns-for-gnss-software-receivers/" target="_blank"><i>Design patterns for GNSS software receivers</i></a>, in Proc. of the 5th ESA Workshop on Satellite Navigation Technologies (NAVITEC'2010), ESTEC, Noordwijk, The Netherlands, Dec. 2010. DOI:10.1109/NAVITEC.2010.5707981 

For LaTeX users, these are the BibTeX cites for your convenience:

\verbatim
@INPROCEEDINGS{GNSS-SDR12
  author = {C.~{Fern\'{a}ndez--Prades} and J.~Arribas and L.~Esteve and D.~Pubill and P.~Closas},
  title = {An Open Source {G}alileo {E1} Software Receiver},
  booktitle = {Proc. of the 6th ESA Workshop on Satellite Navigation Technologies (NAVITEC'2012)}, 
  year = {2012}, 
  address = {ESTEC, Noordwijk, The Netherlands}, 
  month = {Dec.} }
\endverbatim
 
\verbatim
@PHDTHESIS{Arribas12,
  author = {J.~Arribas},
  title = {{GNSS} Array-based Acquisition: Theory and Implementation},
  school = {Universitat Polit\`{e}cnica de Catalunya},
  year = {2012},
  address = {Barcelona, Spain},
  month = {June} }
\endverbatim
 
\verbatim
@INPROCEEDINGS{GNSS-SDR11, 
 AUTHOR = {C.~{Fern\'{a}ndez--Prades} and J.~Arribas and P.~Closas and C.~Avil\'{e}s and L.~Esteve}, 
 TITLE = {{GNSS-SDR}: An Open Source Tool For Researchers and Developers}, 
 BOOKTITLE = {Proc. of the ION GNSS 2011 Conference}, 
 YEAR = {2011}, 
 address = {Portland, Oregon}, 
 month = {Sept.} } 
\endverbatim
 
\verbatim
@INPROCEEDINGS{GNSS-SDR10, 
 AUTHOR = {C.~{Fern\'{a}ndez--Prades} and C.~Avil\'{e}s and L.~Esteve and J.~Arribas and P.~Closas}, 
 TITLE = {Design patterns for {GNSS} software receivers}, 
 BOOKTITLE = {Proc. of the 5th ESA Workshop on Satellite Navigation Technologies (NAVITEC'2010)}, 
 YEAR = {2010}, 
 address = {ESTEC, Noordwijk, The Netherlands}, 
 month = {Dec.}, 
 note = {DOI:10.1109/NAVITEC.2010.5707981} } 
\endverbatim

\section now_what Ok, now what?
In order to start using GNSS-SDR, you may want to populate <tt>gnss-sdr/data</tt> folder (or anywhere else on your system) with raw data files. By "raw data" we mean the output 
of a Radio Frequency front-end's Analog-to_Digital converter. GNSS-SDR needs signal samples already in baseband or in passband, at a suitable intemediate frequency (on the order of MHz). 
Prepare your configuration file, and then you are ready for going to the <tt> gnss-sdr/install</tt> folder, running <tt>./gnss-sdr</tt>, and see how the file is processed.
Please ask the Developer Team for a signal sample if you need one, and they will do their best ;-)

Another interesting option is working in real-time with a RF front-end. We provide drivers for UHD-compatible hardware (see \ref signal_source), for the GN3S v2 USB dongle
and for some DVB-T USB dongles. Start with a low number of channels and then increase it in order to test how many channels your processor can handle in real-time.

You can find more information at the <a href="http://gnss-sdr.org/documents" target="_blank"><b>GNSS-SDR Documentation page</b></a> or directly asking to the 
<a href="http://lists.sourceforge.net/lists/listinfo/gnss-sdr-developers" target="_blank"><b>GNSS-SDR Developers mailing list</b></a>. 
 
You are also very welcome to contribute to the project, there are many ways to <a href="http://gnss-sdr.org/participate" target="_blank"><b>participate in GNSS-SDR</b></a>.
If you need some special feature not yet implemented, the Developer Team would love to be hired for developing it. 
Please do not hesitate to <a href="http://gnss-sdr.org/contact-us" target="_blank"><b>contact them</b></a>.

Enjoy GNSS-SDR! 

The Developer Team.
*/