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SDRs as a Reference and Common Clock Source for GNSS Timing Apps

This article appeared in Microwaves and RF and is published here with permission.

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What you will learn:

  • What SDRs are and why they are important for GNSS synchronization systems.
  • How the SDR clock distribution ensures that the various functions of the GNSS system are properly synchronized.
  • Integration of ground stations, satellites and SDR for precise tracking and synchronization.

Software-defined radio (SDR) is a universal radio communication system that uses reconfigurable software-based components to process digitized signals. The SDR paradigm offers high flexibility, facilitates upgrades or updates of radio communication systems at a low cost due to the functionality and components implemented as a software embedded system. Some of these components include filters, amplifiers, decoders and mixers. In addition, SDRs offer superior performance suitable for a wide range of industrial applications.

The SDR usually consists of two parts: a front radio and a digital rear end. In a typical SDR receiver, the RF end is an RF tuner that converts the incoming RF frequency signal into an intermediate frequency (IF) signal. The IF signal is then fed to an analog-to-digital converter (ADC), which converts it to a digital signal. The digitized signal is then routed to a digital rear end for processing.

In the case of a typical SDR transmitter, the front end consists of an RF converter and a power amplifier. The RF converter receives an analog IF signal from a digital-to-analog converter (DAC) and converts it into an RF signal that is amplified by a power amplifier before being fed to a transmitting antenna. Unlike conventional radio communication systems, SDR is capable of processing signals in a wide frequency band. The highest bandwidth SDRs can control signals in the bandwidth up to 18 GHz.

The digital rear end of the SDR system has a field-programmable array of gates (FPGAs) with built-in digital signal processing (DSP) capabilities. This board processes digital signals and consists of various reconfigurable components, including mixers, filters, modulators and demodulators. It also provides tools for implementing various application-specific functionalities, ranging from triggering and queuing for radar or MRI, to implementing each wireless communication standard / protocol in the chip itself. DSP operations also appear as a block in the FPGA, including computational tasks such as up-conversion, down-conversion, modulation, demodulation, and error control.

The versatility, reconfigurability, interoperability and superior SDR features make them the right choice for use in a wide range of industrial applications. This includes critical applications such as radar, measurement and measurement systems, spectrum monitoring and recording, MRI / NMR systems, network security systems and GNSS systems. An example of a high-performance SDR platform suitable for various critical applications is developed by On vices (Fig. 1).

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Global Navigation Satellite System

The Global Navigation Satellite System (GNSS), which has worldwide coverage, serves synchronization and / or geospatial positioning applications. Some of today’s fully functional GNSSs include the Global Positioning System (GPS), the BeiDou Navigation Satellite System (BDS), the Global Navigation Satellite System (GLONASS) and Galileo. GNSS uses precision clocks to provide the time and position accuracy required by today’s applications.

GNSS satellites use on-board atomic clocks, while ground stations use crystal oscillators in SDR or external reference clocks to provide very accurate synchronization and positioning. The superior performance of SDR clock systems makes them a popular choice for GNSS synchronization applications.

Most of today’s financial institutions, radar stations and data networks consist of many nodes that form a distributed network. Such distributed systems require accurate time to ensure that all transactions are accurate and synchronized. Accurate GNSS synchronization makes it a good choice for synchronizing nodes in distributed systems.

Some applications such as precision farming, autonomous driving, port automation, commercial aircraft tracking, and Google Maps require accurate location data. In addition, GNSS provides accurate location data and is used in a wide range of industries, including agriculture, defense, mining and construction.

Using SDR clock systems in GNSS synchronization applications

The SDR time board provides clock distribution for all ADC / DAC of the receiver and transmitter and FPGA for synchronous operation. This internal timing board consists of an oven-controlled crystal generator (OCXO) that delivers a very stable (5 parts / billion) and accurate 10-MHz signal. Such oscillators also offer the high frequency stability, low noise and low phase noise needed to accurately synchronize and synchronize functions. The unsurpassed frequency stability of this generator makes the SDR watch ideal for applications with strict synchronization requirements, such as GNSS systems.

The distribution of the SDR clock ensures that the various functions of the GNSS system are properly synchronized. In addition to sampling functions, this clock distribution system can synchronize other processing operations such as up and down conversion functions.

One can use an SDR watch in master or slave mode. In basic mode, the internal clock is used within the SDR platform. Using an SDR clock in slave mode requires an external reference clock. This gives engineers the flexibility to use a clock system that best meets the synchronization requirements of an application. Applying SDR as the main clock for your GNSS system improves the flexibility of your systems and saves you time and resources.

The SDR-based clock distribution system offers high flexibility and allows engineers to implement new features and protocols without the need to change or modify existing hardware. In addition, because the software components are reusable, the technology reduces the time and resources required to prototype and test advanced clock distribution systems.

The SDR watch system is suitable for a wide range of applications that require precise synchronization. With the SDR platform, you don’t need to build a separate clock distribution network for your application. Figure 2 illustrates all integrated components.

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Having a built-in clock system allows engineers to save time and resources, thus reducing the overall cost of deploying, testing, and deploying complex systems. In addition, the SDR clock system offers impressive synchronization accuracy and its performance is comparable or even better than completely external clock systems.

SDR-based GNSS ground station

A ground control station is a critical segment of any GNSS system that requires extremely accurate clocks (Fig. 3). Some of the key functions of this station include tracking satellites, calculating the exact locations of satellites, monitoring the drift in the satellites and adjusting their direction to ensure that they remain in the right orbital plane, while monitoring the integrity of the satellite and resolves satellite anomalies.

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At the heart of any GPS / GNSS ground station is a reference clock that ensures that the entire system is accurately synchronized. Due to the excellent performance of the SDR clock, it is very suitable for use as a reference clock on a GNSS ground station. For such applications, the SDR clock is used in main mode.

The accuracy of the synchronization signals provided by GNSS strongly depends on the transmission and reception of the signal. GNSS uses bands L1 and L2 to communicate signals between ground stations and satellites. Ground segments use atomic clocks to generate system time, while receivers use clock oscillators to maintain their time. Although these two time scales are highly accurate, their time measurements have little deviation. As such, the time signal from the satellite received by the user needs to be traceable to a reference time scale.

The system time scales used by GNSS are based on Coordinated Universal Time (UTC). This subsequently processed timeline is generated and disseminated by the International Bureau of Weights and Measures (BIPM). The UTC time scale provides a reference for measuring the time of a wide range of synchronization systems, including those for GNSS.

Institutes and research centers that support UTC implementations, known as UTC (k) time scales, use high-quality clocks and time transmission equipment. Precise synchronization systems, such as GNSS synchronization systems, must ensure that their time measurements are reliable and traceable to regional or international standards.

The efficiency of the SDR clock system also makes it ideal for various DSP operations, including decoding, encoding, modulation and demodulation. In addition, the reconfigurability of SDR platforms allows engineers to prototype and test new features and protocols without modifying existing hardware. This significantly reduces the overall cost of maintaining the GNSS radio communication system.

Conclusion

Conventional radio communication systems are implemented using special hardware components. This application approach consumes large amounts of resources and yields systems that are rigid and highly susceptible to aging. Such cabling systems are difficult to upgrade because the process involves modifying or replacing existing hardware.

SDR’s configurable architecture allows engineers to deploy new features and protocols without modifying existing hardware suitable for synchronizing applications in GNSS systems. The flexibility of these platforms allows engineers to deploy universal systems that are interoperable with different GNSS constellations. The SDR paradigm also allows for the deployment of scalable systems capable of accepting current and future GNSS constellations.

Thanks to the limited synchronization capabilities of SDR clocks, these platforms can meet the stringent synchronization requirements of today’s GNSS synchronization applications. In addition, reusing SDR reconfiguration modules saves the time, money, and resources required to develop and test new GNSS control earth stations. As such, SDR platforms are generally recognized as the most suitable radios for GPS / GNSS test and simulation.

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