If you have questions about GNSS-SDR, please [subscribe to the gnss-sdr-developers mailing list](http://lists.sourceforge.net/lists/listinfo/gnss-sdr-developers "Subscribe to the gnss-sdr-developers mailing list" ) and post your questions there.
This section describes how to set up the compilation environment in GNU/Linux or [Mac OS X](#macosx), and to build GNSS-SDR. See also [our Building Guide](http://gnss-sdr.org/documentation/building-guide "GNSS-SDR's Building Guide").
Tested distributions: Ubuntu 12.04, 12.10, 13.04, 13.10 and 14.04 (32 and 64 bits), Debian 6.0.6 and 7.2, Fedora 18, 19 and 20 and openSUSE 13.1 (newer versions should work, too). You will need GCC 4.7 or newer.
Downloading, building and installing [GNU Radio](http://gnuradio.org/redmine/projects/gnuradio/wiki "GNU Radio's Homepage") and all its dependencies is not a simple task. We recommend to use [PyBOMBS](http://gnuradio.org/redmine/projects/pybombs/wiki) (Python Build Overlay Managed Bundle System), the GNU Radio install management system that automatically does all the work for you. In a terminal, type:
You can safely accept the default options but for ```prefix```. We recommend to put ```/usr/local``` there. After the configuration, you should get something similar to:
Then, you are ready to download and install [UHD](http://files.ettus.com/uhd_docs/manual/html/) (the Universal Hardware Driver), GNU Radio and all their required dependencies by doing:
This can take some time (up to two hours to complete, depending on your system), and installs the latest versions of the Universal Hardware Driver (UHD) and GNU Radio in your system, including all their dependencies.
In case you do not want to use PyBOMBS and prefer to build and install GNU Radio manually from source, follow instructions at the [GNU Radio Build Guide](http://gnuradio.org/redmine/projects/gnuradio/wiki/BuildGuide).
The full stop separated from ```cmake``` by a space is important. [CMake](http://www.cmake.org/ "CMake's Homepage") will figure out what other libraries are currently installed and will modify Armadillo's configuration correspondingly. CMake will also generate a run-time armadillo library, which is a combined alias for all the relevant libraries present on your system (eg. BLAS, LAPACK and ATLAS).
Please **DO NOT install** gtest (do *not* type ```sudo make install```). Every user needs to compile his tests using the same compiler flags used to compile the installed Google Test libraries; otherwise he may run into undefined behaviors (i.e. the tests can behave strangely and may even crash for no obvious reasons). The reason is that C++ has this thing called the One-Definition Rule: if two C++ source files contain different definitions of the same class/function/variable, and you link them together, you violate the rule. The linker may or may not catch the error (in many cases it is not required by the C++ standard to catch the violation). If it does not, you get strange run-time behaviors that are unexpected and hard to debug. If you compile Google Test and your test code using different compiler flags, they may see different definitions of the same class/function/variable (e.g. due to the use of ```#if``` in Google Test). Therefore, for your sanity, we recommend to avoid installing pre-compiled Google Test libraries. Instead, each project should compile Google Test itself such that it can be sure that the same flags are used for both Google Test and the tests. The building system of GNSS-SDR does the compilation and linking of gtest its own tests; it is only required that you tell the system where the gtest folder that you downloaded resides. Just add to your ```$HOME/.bashrc``` file the following line:
By default, CMake will build the Release version, meaning that the compiler will generate a fast, 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:
from the gnss-sdr/build folder. This will generate HTML documentation that can be retrieved pointing your browser of preference to gnss-sdr/docs/html/index.html.
If a LaTeX installation is detected in your system,
In order to gain access to USB ports, gnss-sdr should be used as root. In addition, the driver requires access to the GN3S firmware binary file. It should be available in the same path where the application is called.
GNSS-SDR comes with a pre-compiled custom GN3S firmware available at gnss-sdr/firmware/GN3S_v2/bin/gn3s_firmware.ihx. Please copy this file to the application path. The GNSS-SDR default path is gnss-sdr/install
If you still have not installed [Xcode](http://developer.apple.com/xcode/), do it now from the App Store (it's free). You will also need the Xcode Command Line Tools. Launch the Terminal, found in /Applications/Utilities/, and type:
Then, [install Macports](http://www.macports.org/install.php). If you are upgrading from a previous installation, please follow the [migration rules](http://trac.macports.org/wiki/Migration).
If you still have not installed [Xcode](http://developer.apple.com/xcode/), do it now from the App Store (it's free). Once installed, download and install the command line tools:
The libraries [Gflags](http://code.google.com/p/gflags/ "Gflags' Homepage") and [Glog](http://code.google.com/p/google-glog/ "Glog's Homepage") should be installed manually, and in that particular order (same steps as above). If they are not already installed
when building GNSS-SDR, [CMake](http://www.cmake.org/ "CMake's Homepage") will download, build and link them statically for you when doing ```make```, but they will not remain installed in the system.
If you cloned GNSS-SDR some days ago, it is possible that some developer has updated files at the Git repository. You can update your working copy by doing:
If you are interested in contributing to the development of GNSS-SDR, please check out [how to do it](http://gnss-sdr.org/documentation/how-contribute-source-code "How to contribute to GNSS-SDR source code").
There is a more controlled way to upgrade your repository, which is to use the Git commands ```fetch``` and ```merge```, as described [here](http://gnss-sdr.org/source-code).
1. The signal file can be easily recorded using the GNU Radio file sink in ```gr_complex<float>``` mode.
2. You will need a GPS active antenna, a [USRP](http://www.ettus.com/product) and a suitable USRP daughter board to receive GPS L1 C/A signals. GNSS-SDR require to have at least 2 MHz of bandwidth in 1.57542 GHz. (remember to enable the DC bias with the daughter board jumper).
3. The easiest way to capture a signal file is to use the GNU Radio Companion GUI. Only two blocks are needed: a USRP signal source connected to complex float file sink. You need to tune the USRP central frequency and decimation factor using USRP signal source properties box. We suggest using a decimation factor of 20 if you use the USRP2. This will give you 100/20 = 5 MSPS which will be enough to receive GPS L1 C/A signals. The front-end gain should also be configured. In our test with the DBSRX2 we obtained good results with ```G=50```.
4. Capture at least 80 seconds of signal in open sky conditions. During the process, be aware of USRP driver buffer underuns messages. If your hard disk is not fast enough to write data at this speed you can capture to a virtual RAM drive. 80 seconds of signal at 5 MSPS occupies less than 3 Gbytes using ```gr_complex<float>```.
5. If you have no access to a RF front-end, you can download a sample raw data file (that contains GPS and Galileo signals) from [here](http://sourceforge.net/projects/gnss-sdr/files/data/).
4. Run the receiver from the install directory. The program reports the current status in text mode, directly to the terminal window. If all goes well, and GNSS-SDR is able to successfully track an decode at least 4 satellites, you will get PVT fixes. The program will write a .kml file and RINEX (yet experimental) files in the install directory. In addition to the console output, GNSS-SDR also writes log files at /tmp/ (configurable with the commandline flag ```./gnss-sdr --log_dir=/path/to/log```).
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 signal processing. All the configuration is done in a single file. Those configuration files reside at the [gnss-sdr/conf/](./conf/) folder. By default, the executable ```gnss-sdr``` will read the configuration available at ```gnss-sdr/conf/gnss-sdr.conf```. You can edit that file to fit your needs, or even better, define a new ```my_receiver.conf``` file with your own configuration. This new receiver can be done by going to the ```gnss-sdr/install``` folder:
You can see a guide of available implementations at ```gnss-sdr/conf/master.conf```. That folder contains other working examples as well. If you have a working configuration and want to share it will others, please tell us and we will be happy to upload it to the server.
GNSS-SDR's main method initializes the logging library, processes the command line flags, if any, provided by the user and instantiates a [ControlThread](./src/core/receiver/control_thread.h) 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](./src/core/receiver/gnss_flowgraph.h) 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 ```gr::top_block``` class. GNSSFlowgraph wraps the ```gr::top_block``` instance so we can take advantage of the ```gnss_block_factory``` (see below), 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 other more efficient approaches.
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.
Properties are passed around within the program using the [ConfigurationInterface](./src/core/interfaces/configuration_interface.h) class. There are two implementations of this interface: [FileConfiguration](./src/core/receiver/file_configuration.h) and [InMemoryConfiguration](./src/core/receiver/in_memory_configuration.h). 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:
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](./src/algorithms/libs/pass_through.h) for module SignalConditioner, the corresponding line in the configuration file would be
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 the [INI format](http://en.wikipedia.org/wiki/INI_file)).
Hence, the application defines a simple accessor class to fetch the configuration pairs of values and passes them to a factory class called [GNSSBlockFactory](./src/core/receiver/gnss_block_factory.h). 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.
GNU Radio's class ```gr::basic_block``` 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 ```gr::hier_block2```, which is a recursive container that adds or removes processing or hierarchical blocks to the internal graph, and ```gr::block```, which is the abstract base class for all the processing blocks.
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 ```gr::top_block``` 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](./src/core/interfaces/gnss_block_interface.h) 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.
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.
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.
SignalSource.sampling_frequency=4000000 ; Sampling frequency in [Hz]
SignalSource.freq=1575420000 ; RF front-end center frequency in [Hz]
~~~~~~
Type ```gr_complex``` refers to a GNU Radio typedef equivalent to ```std::complex<float>```. In order to save some storage space, you might wanted to store your signal in a more efficient format such as an I/Q interleaved ```short`` integer sample stream. In that case, change the corresponding line to:
~~~~~~
SignalSource.item_type=short
~~~~~~
In this latter case, you will need to convert the interleaved I/Q samples to a complex stream via Data Type Adapter block (see below).
***Example: UhdSignalSource***
The user may prefer to use a [UHD](http://code.ettus.com/redmine/ettus/projects/uhd/wiki)-compatible RF front-end and try real-time processing. For instance, for a USRP1 + DBSRX daughterboard, use:
~~~~~~
;######### 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, for USRP B210 use A:0)
~~~~~~
Other examples are available at [gnss-sdr/conf/](./conf/).
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.
If your signal source is providing baseband signal samples of type ```gr_complex``` at 4 Msps, you can bypass the Signal Conditioner block by:
~~~~~~
SignalConditioner.implementation=Pass_Through
~~~~~~
If you need to adapt some aspect of you signal, you can enable the Signal Conditioner and configure three internal blocks: a data type adpater, an input signal and a resampler.
~~~~~~
;#[Signal_Conditioner] enables this block. Then you have to configure [DataTypeAdapter], [InputFilter] and [Resampler] blocks
This block changes the type of input data samples. If your signal source delivers data samples of type ```short```, you can use this block to convert them to ```gr_complex``` like this:
~~~~~~
;######### DATA_TYPE_ADAPTER CONFIG ############
;#implementation: [Pass_Through] disables this block
This blocks filters the input data. It can be combined with frequency translation for IF signals. The computation of the filter taps is based on parameters of GNU Radio's function [pm_remez](http://gnuradio.org/doc/doxygen/pm__remez_8h.html), that calculates the optimal (in the Chebyshev/minimax sense) FIR filter impulse response given a set of band edges, the desired reponse on those bands, and the weight given to the error in those bands.
The block can be configured like this:
~~~~~~
;######### INPUT_FILTER CONFIG ############
;#implementation: Use [Pass_Through] or [Fir_Filter] or [Freq_Xlating_Fir_Filter]
;#[Pass_Through] disables this block
;#[Fir_Filter] enables a FIR Filter
;#[Freq_Xlating_Fir_Filter] enables FIR filter and a composite frequency translation that shifts IF down to zero Hz.
A channel encapsulates all signal processing devoted to a single satellite. Thus, it is a large composite object which encapsulates the acquisition, tracking and navigation data 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 use the [Boost.Statechart library](http://www.boost.org/libs/statechart/doc/tutorial.html), 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](./src/core/interfaces/channel_interface.h) represents an interface to a channel GNSS block. Check [Channel](./src/algorithms/channel/adapters/channel.h) for an actual implementation.
~~~~~~
;######### CHANNELS GLOBAL CONFIG ############
Channels.count=8 ; Number of available satellite channels.
Channels.in_acquisition=1 ; Number of channels simultaneously acquiring
Channel.system=GPS ; options: GPS, Galileo, SBAS
Channel.signal=1C ; options: "1C" for GPS L1 C/A or SBAS L1 C/A; "1B" for GALILEO E1 B (I/NAV OS/CS/SoL)
~~~~~~
You can optionally set specific parameters for different channels, overriding global parameters:
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](./src/core/interfaces/acquisition_interface.h) 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 *adapters*. 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 ```gr::block```, 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](./src/algorithms/acquisition/adapters/gps_l1_ca_pcps_acquisition.h) and [GalileoE1PcpsAmbiguousAcquisition](./src/algorithms/acquisition/adapters/galileo_e1_pcps_ambiguous_acquisition.h) for examples of adapters from a Parallel Code Phase Search (PCPS) acquisition block, and [pcps_acquisition_cc](./src/algorithms/acquisition/gnuradio_blocks/pcps_acquisition_cc.h) for an example of a block implementation. The source code of all the available acquisition algorithms is located at:
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:
~~~~~~
;######### ACQUISITION GLOBAL CONFIG ############
Acquisition.dump=false ; Enables internal data file logging [true] or [false]
Acquisition.dump_filename=./acq_dump.dat ; Log path and filename
Acquisition.item_type=gr_complex
Acquisition.if=0 ; Signal intermediate frequency in [Hz]
Acquisition.sampled_ms=1 ; Signal block duration for the acquisition signal detection [ms]
Acquisition.implementation=GPS_L1_CA_PCPS_Acquisition ; Acquisition algorithm selection for this channel
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](./src/core/interfaces/tracking_interface.h) class and subclasses implementing algorithms provides a way of testing different approaches, with full access to their parameters. Check [GpsL1CaDllPllTracking](./src/algorithms/tracking/adapters/gps_l1_ca_dll_pll_tracking.h) or [GalileoE1DllPllVemlTracking](./src/algorithms/tracking/adapters/galileo_e1_dll_pll_veml_tracking.h) for examples of adapters, and [Gps_L1_Ca_Dll_Pll_Tracking_cc](./src/algorithms/tracking/gnuradio_blocks/gps_l1_ca_dll_pll_tracking_cc.h) 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.h](./src/algorithms/tracking/libs/correlator.h), [lock_detectors.h](./src/algorithms/tracking/libs/lock_detectors.h), [tracking_discriminators.h](./src/algorithms/tracking/libs/tracking_discriminators.h) or [tracking_2nd_DLL_filter.h](./src/algorithms/tracking/libs/tracking_2nd_DLL_filter.h).
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:
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 [Boost.Statechart library](http://www.boost.org/libs/statechart/doc/tutorial.html).
The common interface is [TelemetryDecoderInterface](./src/core/interfaces/telemetry_decoder_interface.h). Check [GpsL1CaTelemetryDecoder](./src/algorithms/telemetry_decoder/adapters/gps_l1_ca_telemetry_decoder.h) for an example of the GPS L1 NAV message decoding adapter, and [gps_l1_ca_telemetry_decoder_cc](./src/algorithms/telemetry_decoder/gnuradio_blocks/gps_l1_ca_telemetry_decoder_cc.h) for an actual implementation of a signal processing block. Configuration example:
GNSS systems provide different kinds of observations. The most commonly used are the code observations, also called pseudoranges. The *pseudo* 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.
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 [GPSTk](http://www.gpstk.org), [RTKLIB](http://www.rtklib.com/) or [gLAB](http://gage14.upc.es/gLAB/) appear as viable solutions for high performance, completely customizable GNSS receivers.
The common interface is [PvtInterface](./src/core/interfaces/pvt_interface.h). For instance, in order to use the implementation GpsL1CaPvt, add to the configuration file:
~~~~~~
;######### PVT CONFIG ############
PVT.implementation=GPS_L1_CA_PVT
PVT.nmea_dump_filename=./gnss_sdr_pvt.nmea ; NMEA log path and filename
PVT.flag_nmea_tty_port=true ; Enable the NMEA log to a serial TTY port
PVT.nmea_dump_devname=/dev/pts/4 ; serial device descriptor for NMEA logging
PVT.dump=false ; Enables the PVT internal binary data file logging [true] or [false]
~~~~~~
This implementation allows tuning of the following parameters:
~~~~~~
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.
GNSS-SDR is released under the [General Public License (GPL) v3](http://www.gnu.org/licenses/gpl.html), 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:
1. 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.
2. The licensed material may be analyzed or modified.
3. Modified material may be distributed under the same licensing terms but *do not* 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.
* C. Fernández-Prades, J. Arribas, L. Esteve, D. Pubill, P. Closas, [An Open Source Galileo E1 Software Receiver](http://www.cttc.es/publication/an-open-source-galileo-e1-software-receiver/), in Proc. of the 6th ESA Workshop on Satellite Navigation Technologies (NAVITEC 2012), ESTEC, Noordwijk, The Netherlands, Dec. 2012.
* J. Arribas, [GNSS Array-based Acquisition: Theory and Implementation](http://theses.eurasip.org/theses/449/gnss-array-based-acquisition-theory-and/), PhD Thesis, Universitat Politècnica de Catalunya, Barcelona, Spain, June 2012.
* C. Fernández-Prades, J. Arribas, P. Closas, C. Avilés, and L. Esteve, [GNSS-SDR: an open source tool for researchers and developers](http://www.cttc.es/publication/gnss-sdr-an-open-source-tool-for-researchers-and-developers/), in Proc. of the ION GNSS 2011 Conference, Portland, Oregon, Sept. 19-23, 2011.
* C. Fernández-Prades, C. Avilés, L. Esteve, J. Arribas, and P. Closas, [Design patterns for GNSS software receivers](http://www.cttc.es/publication/design-patterns-for-gnss-software-receivers/), 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
In order to start using GNSS-SDR, you may want to populate ```gnss-sdr/data``` 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 ```gnss-sdr/install``` folder, running ```./gnss-sdr```, and seeing how the file is processed.
Another interesting option is working in real-time with a RF front-end. We provide drivers for UHD-compatible hardware such as the [USRP family](http://www.ettus.com/product), 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 [GNSS-SDR Documentation page](http://gnss-sdr.org/documents) or directly asking to the [GNSS-SDR Developers mailing list](http://lists.sourceforge.net/lists/listinfo/gnss-sdr-developers).
You are also very welcome to contribute to the project, there are many ways to [participate in GNSS-SDR](http://gnss-sdr.org/participate). 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 [contact them](http://gnss-sdr.org/contact-us).
