This program is a software-defined receiver which is able to process (that is, to perform detection, synchronization, demodulation and decoding of the navigation message, computation of observables and, finally, computation of position fixes) the following Global Navigation Satellite System's signals:
GNSS-SDR provides interfaces for a wide range of radio frequency front-ends and raw sample file formats, generates processing outputs in standard formats, allows for the full inspection of the whole signal processing chain, and offers a framework for the development of new features. Please visit [https://gnss-sdr.org](https://gnss-sdr.org "GNSS-SDR website") for more information about this open source software-defined GNSS receiver.
This section describes how to set up the compilation environment in GNU/Linux or [macOS / Mac OS X](#macosx), and to build GNSS-SDR. See also our [build and install page](https://gnss-sdr.org/build-and-install/ "GNSS-SDR's Build and Install").
Before building GNSS-SDR, you need to install all the required dependencies. There are two alternatives here: through software packages or building them from the source code. It is in general not a good idea to mix both approaches.
### Alternative 1: Install dependencies using software packages
If you want to start building and running GNSS-SDR as quick and easy as possible, the best option is to install all the required dependencies as binary packages.
Please note that the required files from `libgtest-dev` were moved to `googletest` in Debian 9 "stretch" and Ubuntu 18.04 "bionic", and moved back again to `libgtest-dev` in Debian 10 "buster" and Ubuntu 18.10 "cosmic" (and above).
**Note for Ubuntu 14.04 LTS "trusty" users:** you will need to build from source and install GNU Radio manually, as explained below, since GNSS-SDR requires `gnuradio-dev` >= 3.7.3, and Ubuntu 14.04 came with 3.7.2. Install all the packages above BUT EXCEPT `libuhd-dev`, `gnuradio-dev` and `gr-osmosdr` (and remove them if they are already installed in your machine), and install those dependencies using PyBOMBS. The same applies to `libmatio-dev`: Ubuntu 14.04 came with 1.5.2 and the minimum required version is 1.5.3. Please do not install the `libmatio-dev` package and install `libtool`, `automake` and `libhdf5-dev` instead. A recent version of the library will be downloaded and built automatically if CMake does not find it installed.
This option is adequate if you are interested in development, in working with the most recent versions of software dependencies, want more fine tuning on the installed versions, or simply in building everything from the scratch just for the fun of it. In such cases, we recommend to use [PyBOMBS](https://github.com/gnuradio/pybombs "Python Build Overlay Managed Bundle System") (Python Build Overlay Managed Bundle System), GNU Radio's meta-package manager tool that installs software from source, or whatever the local package manager is, that automatically does all the work for you. Please take a look at the configuration options and general PyBOMBS usage at https://github.com/gnuradio/pybombs. Here we provide a quick step-by-step tutorial.
Download, build and install GNU Radio, related drivers and some other extra modules into the directory `/path/to/prefix` (replace this path by your preferred one, for instance `$HOME/sdr`):
This will perform a local installation of the dependencies under `/path/to/prefix`, so they will not be visible when opening a new terminal. In order to make them available, you will need to set up the adequate environment variables:
Now you are ready to use GNU Radio and to jump into building GNSS-SDR after installing a few other dependencies. Actually, those are steps that PyBOMBS can do for you as well:
By default, PyBOMBS installs the ‘next’ branch of GNSS-SDR development, which is the most recent version of the source code. This behaviour can be modified by altering the corresponding recipe at `$HOME/.pybombs/recipes/gr-recipes/gnss-sdr.lwr`
In case you do not want to use PyBOMBS and prefer to build and install GNSS-SDR step by step (i.e., cloning the repository and doing the usual `cmake .. && make && make install` dance), Armadillo, GFlags, Glog and GnuTLS can be installed either by using PyBOMBS:
or manually as explained below, and then please follow instructions on how to [download the source code and build GNSS-SDR](#download-and-build-linux).
The full stop separated from `cmake` by a space is important. [CMake](https://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 build or install** Google Test. Every user needs to compile tests using the same compiler flags used to compile the Google Test libraries; otherwise he or she may run into undefined behaviors (_i.e._, the tests can behave strangely and may even crash for no obvious reasons). The explanation is that C++ has 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, GNSS-SDR does not make use of pre-compiled Google Test libraries. Instead, it compiles Google Test's source code 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 manages the compilation and linking of Google Test's source code to its own tests; it is only required that you tell the system where the Google Test folder that you downloaded resides. Just type in your terminal (or add it to your `$HOME/.bashrc` file for a permanent solution) the following line:
changing `/home/username/googletest-1.10.x` by the actual path where you unpacked Google Test. If the CMake script does not find that folder, or the environment variable is not defined, or the source code is not installed by a package, then it will download a fresh copy of the Google Test source code and will compile and link it for you.
#### Install [Protocol Buffers](https://developers.google.com/protocol-buffers/ "Protocol Buffers' Homepage"), a portable mechanism for serialization of structured data:
GNSS-SDR requires Protocol Buffers v3.0.0 or later. If the packages that come with your distribution are older than that (_e.g._, Ubuntu 16.04 Xenial and Debian 8 Jessie came with older versions), then you will need to install it manually. First, install the dependencies:
By default, you will be in the 'master' branch of the Git repository, which corresponds to the latest stable release. If you want to try the latest developments, you can use the 'next' branch by going to the newly created gnss-sdr folder doing:
More information about GNSS-SDR-specific Git usage and pointers to further readings can be found at our [Git tutorial](https://gnss-sdr.org/docs/tutorials/using-git/ "Using Git").
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 an 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:
This will create four executables at gnss-sdr/install, namely `gnss-sdr`, `run_tests`, `front-end-cal` and `volk_gnsssdr_profile`. 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:
This will also 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.
from the gnss-sdr/build folder. This will generate HTML documentation that can be retrieved pointing your browser of preference to build/docs/html/index.html.
GNSS-SDR comes with a library which is a module of the Vector-Optimized Library of Kernels (so called [VOLK_GNSSSDR](./src/algorithms/libs/volk_gnsssdr_module/volk_gnsssdr/README.md)) and a profiler that will build a config file for the best SIMD architecture for your processor. Run `volk_gnsssdr_profile` that is installed into `$PREFIX/bin`. This program tests all known VOLK kernels for each architecture supported by the processor. When finished, it will write to `$HOME/.volk_gnsssdr/volk_gnsssdr_config` the best architecture for the VOLK function. This file is read when using a function to know the best version of the function to execute. It mimics GNU Radio's [VOLK](http://libvolk.org/) library, so if you still have not run `volk_profile`, this is a good moment to do so.
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 gr-gn3s/firmware/GN3S_v2/bin/gn3s_firmware.ihx. Please copy this file to the application path.
With `Fmcomms2_Signal_Source` you can use any SDR hardware based on [FMCOMMS2](https://wiki.analog.com/resources/eval/user-guides/ad-fmcomms2-ebz), including the ADALM-PLUTO (PlutoSdr) by configuring correctly the .conf file. The `Plutosdr_Signal_Source` offers a simpler manner to use the ADALM-PLUTO because implements only a subset of FMCOMMS2's parameters valid for those devices.
In order to enable the building of blocks that use CUDA, NVIDIA's parallel programming model that enables graphics processing unit (GPU) acceleration for data-parallel computations, first you need to install the CUDA Toolkit from [NVIDIA Developers Download page](https://developer.nvidia.com/cuda-downloads "CUDA Downloads"). Make sure that the SDK samples build well. Then, build GNSS-SDR by doing:
In order to build an executable that not depends on the specific SIMD instruction set that is present in the processor of the compiling machine, so other users can execute it in other machines without those particular sets, use:
Using this option, all SIMD instructions are exclusively accessed via VOLK, which automatically includes versions of each function for different SIMD instruction sets, then detects at runtime which to use, or if there are none, substitutes a generic, non-SIMD implementation.
More details can be found in our tutorial about [GNSS-SDR configuration options at building time](https://gnss-sdr.org/docs/tutorials/using-git/ "Configuration options at building time").
GNSS-SDR can be built on MacOS or Mac OS X, starting from 10.9 (Mavericks) and including 10.14 (Mojave). If you still have not installed [Xcode](https://developer.apple.com/xcode/ "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:
First, [install Macports](https://www.macports.org/install.php). If you are upgrading from a previous installation, please follow the [migration rules](https://trac.macports.org/wiki/Migration).
The script explains what it will do, and then it pauses before doing it. There are more installation options [here](https://docs.brew.sh/Installation.html).
This will create three executables at gnss-sdr/install, namely `gnss-sdr`, `run_tests` and `volk_gnsssdr_profile`. You can install the software receiver on your system by doing:
GNSS-SDR comes with a library which is a module of the Vector-Optimized Library of Kernels (so called [VOLK_GNSSSDR](./src/algorithms/libs/volk_gnsssdr_module/volk_gnsssdr/README.md)) and a profiler that will build a config file for the best SIMD architecture for your processor. Run `volk_gnsssdr_profile` that is installed into `$PREFIX/bin`. This program tests all known VOLK kernels for each architecture supported by the processor. When finished, it will write to `$HOME/.volk_gnsssdr/volk_gnsssdr_config` the best architecture for the VOLK function. This file is read when using a function to know the best version of the function to execute. It mimics GNU Radio's [VOLK](http://libvolk.org/) library, so if you still have not run `volk_profile`, this is a good moment to do so.
GNU Radio and other dependencies can also be installed using other package managers than Macports, such as [Fink](http://www.finkproject.org/ "Fink") or [Homebrew](https://brew.sh/ "Homebrew"). Since the version of Python that ships with OS X is great for learning but it is not good for development, you could have another Python executable in a non-standard location. If that is the case, you need to inform GNSS-SDR's configuration system by defining the `PYTHON_EXECUTABLE` variable as:
The CMake script will create Makefiles that download, build and link Armadillo, Gflags, Glog, Matio, Protocol Buffers, PugiXML and Google Test on the fly at compile time if they are not detected in your machine.
* **Docker image**: A technology providing operating-system-level virtualization to build, ship, and run distributed applications, whether on laptops, data center VMs, or the cloud. Visit [https://github.com/carlesfernandez/docker-gnsssdr](https://github.com/carlesfernandez/docker-gnsssdr) or [https://github.com/carlesfernandez/docker-pybombs-gnsssdr](https://github.com/carlesfernandez/docker-pybombs-gnsssdr) for instructions.
* **Snap package**: [Snaps](https://snapcraft.io) are universal Linux packages aimed to work on any distribution or device, from IoT devices to servers, desktops to mobile devices. Visit [https://github.com/carlesfernandez/snapcraft-sandbox](https://github.com/carlesfernandez/snapcraft-sandbox) for instructions, or directly [get the software from the Snap Store](https://snapcraft.io/gnss-sdr-next):
<ahref="https://snapcraft.io/gnss-sdr-next"><imgsrc="https://snapcraft.io/static/images/badges/en/snap-store-white.svg"alt="Get GNSS-SDR from the Snap Store"></a>
* **GNSS-SDR in embedded platforms**: we provide a Software Development Kit (SDK) based on [OpenEmbedded](http://www.openembedded.org/wiki/Main_Page) for cross-compiling GNSS-SDR in your desktop computer and for producing executables that can run in embedded platforms, such as a Zedboard or a Raspberry Pi 3. Visit [Cross-compiling GNSS-SDR](https://gnss-sdr.org/docs/tutorials/cross-compiling/) for instructions.
If you cloned or forked GNSS-SDR some time ago, it is possible that some developer has updated files at the Git repository. If you still have not done so, add the `upstream` repository to the list of remotes:
If you are interested in contributing to the development of GNSS-SDR, please check out [how to do it](https://gnss-sdr.org/contribute/ "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 in our [Git Tutorial](https://gnss-sdr.org/docs/tutorials/using-git/ "Using Git").
1. After building the code, you will find the `gnss-sdr` executable file at gnss-sdr/install. You can make it available everywhere else by `sudo make install`. Run the profilers `volk_profile` and `volk_gnsssdr_profile` for testing all available VOLK kernels for each architecture supported by your processor. This only has to be done once.
2. You will need a GPS active antenna, a [USRP](https://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 underruns 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 an RF front-end, you can download a sample raw data file (that contains GPS and Galileo signals) from [here](https://sourceforge.net/projects/gnss-sdr/files/data/).
3. The configuration file has in-line documentation, you can try to tune the number of channels and several receiver parameters. Store your .conf file in some working directory of your choice.
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 and decode at least 4 satellites, you will get PVT fixes. The program will write .kml, .geojson and RINEX files in the folder from which `gnss-sdr` was run. 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 your own receiver, work with captured raw data or from an 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 (or at /usr/local/share/gnss-sdr/conf if you installed the program). By default, the executable `gnss-sdr` will read the configuration available at `gnss-sdr/conf/gnss-sdr.conf` (or at (usr/local/share/gnss-sdr/conf/default.conf if you installed the program). 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 generated by invoking gnss-sdr with the `--config_file` flag pointing to your configuration file:
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](https://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 producing 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 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 of the program that uses it.
Internally, GNSS-SDR makes use of the complex data types defined by [VOLK](http://libvolk.org/ "Vector-Optimized Library of Kernels home"). They are fundamental for handling sample streams in which samples are complex numbers with real and imaginary components of 8, 16 or 32 bits, common formats delivered by GNSS (and generic SDR) radio frequency front-ends. The following list shows the data type names that GNSS-SDR exposes through the configuration file:
- **`short`**: Signed integer, 16-bit two's complement number ranging from -32768 to 32767. C++ type name: `int16_t` .
- **`float`**: Defines numbers with fractional parts, can represent values ranging from approx. 1.5e-45 to 3.4e+38 with a precision of 7 digits (32 bits). C++ type name: `float`.
- **`ibyte`**: Interleaved (I&Q) stream of samples of type `byte`. C++ type name: `int8_t`.
- **`ishort`**: Interleaved (I&Q) stream of samples of type `short`. C++ type name: `int16_t`.
- **`cbyte`**: Complex samples, with real and imaginary parts of type `byte`. C++ type name: `lv_8sc_t`.
- **`cshort`**: Complex samples, with real and imaginary parts of type `short`. C++ type name: `lv_16sc_t`.
- **`gr_complex`**: Complex samples, with real and imaginary parts of type `float`. C++ type name: `std::complex<float>`.
More information about the available processing blocks and their configuration parameters can be found at the [Signal Processing Blocks documentation page](https://gnss-sdr.org/docs/sp-blocks/).
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 reading 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.
Type `gr_complex` refers to a GNU Radio typedef equivalent to `std::complex<float>`. In order to save some storage space, you might want 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:
Sometimes, samples are stored in files with a format which is not in the list of _native_ types supported by the `File_Signal_Source` implementation (i.e, it is not among `byte`, `ibyte`, `short`, `ishort`, `float` or `gr_complex`). This is the case of 2-bit samples, which is a common format delivered by GNSS RF front-ends. The `Two_Bit_Packed_File_Signal_Source` implementation allows reading two-bit length samples from a file. The data is assumed to be packed as bytes `item_type=byte` or shorts `item_type=short` so that there are 4 two bit samples in each byte. The two bit values are assumed to have the following interpretation:
Within a byte the samples may be packed in big endian `big_endian_bytes=true` (if the most significant byte value is stored at the memory location with the lowest address, the next byte value in significance is stored at the following memory location, and so on) or little endian `big_endian_bytes=false` (if the least significant byte value is at the lowest address, and the other bytes follow in increasing order of significance). If the order is big endian then the most significant two bits will form the first sample output, otherwise the least significant two bits will be used.
Additionally, the samples may be either real `sample_type=real`, or complex. If the sample type is complex, then the samples are either stored in the order: real, imag, real, imag, ... `sample_type=iq` or in the order: imag, real, imag, real, ... `sample_type=qi`.
Finally, if the data is stored as shorts `item_type=short`, then it may be stored in either big endian `big_endian_items=true` or little endian `big_endian_items=false`. If the shorts are big endian then the 2nd byte in each short is output first.
The user may prefer to use a [UHD](https://files.ettus.com/manual/)-compatible RF front-end and try real-time processing. For instance, for a USRP1 + DBSRX daughterboard, use:
[OsmoSDR](http://sdr.osmocom.org/trac) is a small form-factor, inexpensive software defined radio project. It provides a driver for several front-ends, such as [RTL-based dongles](https://www.rtl-sdr.com/tag/v3/), [HackRF](https://greatscottgadgets.com/hackrf/), [bladeRF](https://www.nuand.com/), [LimeSDR](https://myriadrf.org/projects/limesdr/), [etc](https://github.com/osmocom/gr-osmosdr/blob/master/README). Note that not all the OsmoSDR-compatible devices can work as radio frequency front-ends for proper GNSS signal reception, please check the specifications. For suitable RF front-ends, you can use:
In case of using a Zarlink's RTL2832 based DVB-T receiver, you can even use the `rtl_tcp` I/Q server in order to use the USB dongle remotely. In a terminal, type:
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 you need to adapt some aspect of your signal, you can enable the Signal Conditioner and configure three internal blocks: a data type adapter, an input signal and a resampler.
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:
This block 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](https://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 response on those bands, and the weight given to the error in those bands.
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 to improve 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.
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.
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 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. For a GPS L1 C/A receiver:
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 [cpu_multicorrelator.h](./src/algorithms/tracking/libs/cpu_multicorrelator.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. For instance, for GPS l1 channels:
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 and 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.
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:
In case you are configuring a multi-system receiver, you will need to decimate the one with the fastest code rate in order to get both data streams synchronized. For instance, for hybrid GPS L1 / Galileo E1B receivers:
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 position fixes (stored in GIS-friendly formats such as [GeoJSON](https://tools.ietf.org/html/rfc7946), [GPX](http://www.topografix.com/gpx.asp) and [KML](http://www.opengeospatial.org/standards/kml), or transmitted via serial port as [NMEA 0183](https://en.wikipedia.org/wiki/NMEA_0183) messages), and leaves room for more sophisticated positioning methods by storing observables and navigation data in [RINEX](https://en.wikipedia.org/wiki/RINEX) files (v2.11 or v3.02), and generating [RTCM](http://www.rtcm.org "Radio Technical Commission for Maritime Services") 3.2 messages that can be disseminated through the Internet in real time.
* **GeoJSON** is a geospatial data interchange format based on JavaScript Object Notation (JSON) supported by numerous mapping and GIS software packages, including [OpenLayers](https://openlayers.org), [Leaflet](https://leafletjs.com), [MapServer](http://www.mapserver.org), [GeoServer](http://geoserver.org), [GeoDjango](https://www.djangoproject.com), [GDAL](http://www.gdal.org), and [CartoDB](https://cartodb.com). It is also possible to use GeoJSON with [PostGIS](https://postgis.net/) and [Mapnik](http://mapnik.org), both of which handle the format via the GDAL OGR conversion library. The [Google Maps Javascript API](https://developers.google.com/maps/documentation/javascript/) v3 directly supports the [integration of GeoJSON data layers](https://developers.google.com/maps/documentation/javascript/examples/layer-data-simple), and [GitHub also supports GeoJSON rendering](https://github.com/blog/1528-there-s-a-map-for-that).
* **KML** (Keyhole Markup Language) is an XML grammar used to encode and transport representations of geographic data for display in an earth browser. KML is an open standard officially named the OpenGIS KML Encoding Standard (OGC KML), and it is maintained by the Open Geospatial Consortium, Inc. (OGC). KML files can be displayed in geobrowsers such as [Google Earth](https://www.google.com/earth/), [Marble](https://marble.kde.org), [osgEarth](http://osgearth.org), or used with the [NASA World Wind SDK for Java](https://worldwind.arc.nasa.gov/java/).
* **GPX** (the GPS Exchange Format) is a light-weight XML data format for the interchange of GPS data (waypoints, routes, and tracks) between applications and Web services on the Internet. The format is open and can be used without the need to pay license fees, and it is supported by a [large list of software tools](http://www.topografix.com/gpx_resources.asp).
* **NMEA 0183** is a combined electrical and data specification for communication between marine electronics such as echo sounder, sonars, anemometer, gyrocompass, autopilot, GPS receivers and many other types of instruments. It has been defined by, and is controlled by, the U.S. [National Marine Electronics Association](http://www.nmea.org/). The NMEA 0183 standard uses a simple ASCII, serial communications protocol that defines how data are transmitted in a *sentence* from one *talker* to multiple *listeners* at a time. Through the use of intermediate expanders, a talker can have a unidirectional conversation with a nearly unlimited number of listeners, and using multiplexers, multiple sensors can talk to a single computer port. At the application layer, the standard also defines the contents of each sentence (message) type, so that all listeners can parse messages accurately. Those messages can be sent through the serial port (that could be for instance a Bluetooth link) and be used/displayed by a number of software applications such as [gpsd](http://www.catb.org/gpsd/ "The UNIX GPS daemon"), [JOSM](https://josm.openstreetmap.de/ "The Java OpenStreetMap Editor"), [OpenCPN](https://opencpn.org/ "Open Chart Plotter Navigator"), and many others (and maybe running on other devices).
* **RINEX** (Receiver Independent Exchange Format) is an interchange format for raw satellite navigation system data, covering observables and the information contained in the navigation message broadcast by GNSS satellites. This allows the user to post-process the received data to produce a more accurate result (usually with other data unknown to the original receiver, such as better models of the atmospheric conditions at time of measurement). RINEX files can be used by software packages such as [GPSTk](http://www.gpstk.org), [RTKLIB](http://www.rtklib.com/) and [gLAB](http://gage14.upc.es/gLAB/). GNSS-SDR by default generates RINEX version [3.02](https://igscb.jpl.nasa.gov/igscb/data/format/rinex302.pdf). If [2.11](https://igscb.jpl.nasa.gov/igscb/data/format/rinex211.txt) is needed, it can be requested through the `rinex_version` parameter in the configuration file:
* **RTCM SC-104** provides standards that define the data structure for differential GNSS correction information for a variety of differential correction applications. Developed by the Radio Technical Commission for Maritime Services ([RTCM](http://www.rtcm.org/differential-global-navigation-satellite--dgnss--standards.html "Radio Technical Commission for Maritime Services")), they have become an industry standard for communication of correction information. GNSS-SDR implements RTCM version 3.2, defined in the document *RTCM 10403.2, Differential GNSS (Global Navigation Satellite Systems) Services - Version 3* (February 1, 2013), which can be [purchased online](https://ssl29.pair.com/dmarkle/puborder.php?show=3 "RTCM Online Publication Order Form"). By default, the generated RTCM binary messages are dumped into a text file in hexadecimal format. However, GNSS-SDR is equipped with a TCP/IP server, acting as an NTRIP source that can feed an NTRIP server. NTRIP (Networked Transport of RTCM via Internet Protocol) is an open standard protocol that can be freely downloaded from [BKG](https://igs.bkg.bund.de/root_ftp/NTRIP/documentation/NtripDocumentation.pdf "Networked Transport of RTCM via Internet Protocol (Ntrip) Version 1.0"), and it is designed for disseminating differential correction data (*e.g.* in the RTCM-104 format) or other kinds of GNSS streaming data to stationary or mobile users over the Internet. The TCP/IP server can be enabled by setting `PVT.flag_rtcm_server=true` in the configuration file, and will be active during the execution of the software receiver. By default, the server will operate on port 2101 (which is the recommended port for RTCM services according to the Internet Assigned Numbers Authority, [IANA](https://www.iana.org/assignments/service-names-port-numbers/ "Service Name and Transport Protocol Port Number Registry")), and will identify the Reference Station with ID=1234. This behaviour can be changed in the configuration file:
In order to get well-formatted GeoJSON, KML and RINEX files, always terminate `gnss-sdr` execution by pressing key `q` and then key `ENTER`. Those files will be automatically deleted if no position fix have been obtained during the execution of the software receiver.
GNSS-SDR is released under the [General Public License (GPL) v3](https://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, 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 Proceedings of the 24th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS), Portland, Oregon, Sept. 19-23, 2011, pp. 780-794.
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 intermediate frequency (on the order of MHz). Prepare your configuration file, and then you are ready for running `gnss-sdr --config_file=your_configuration.conf`, and seeing how the file is processed.
Another interesting option is working in real-time with an RF front-end. We provide drivers for UHD-compatible hardware such as the [USRP family](https://www.ettus.com/product), for OsmoSDR and other front-ends (HackRF, bladeRF, LimeSDR), 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](https://gnss-sdr.org/docs/) or directly asking to the [GNSS-SDR Developers mailing list](https://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](https://gnss-sdr.org/contribute/). 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](https://gnss-sdr.org/team/).
