Pilot Study on the Validation of the Software-Defined Receiver for TWSTFT

International audience

The International Bureau of Weights and Measures and the Consultative Committee for Time and Frequency Working Group on Two-Way Satellite Time and Frequency Transfer (TWSTFT) have stimulated a pilot study on using software defined radio (SDR) receivers for TWSTFT in the framework of the realization of Coordinated Universal Time. The SDR receivers based on a software developed by TL have been set up at PTB, OP and NIST during the summer and fall of 2016. Continuous SDR TWSTFT measurements have been established for the OP/PTB, NIST/OP and NIST/PTB links. From the SDR TWSTFT clock comparison results (TWSTFT difference), we observed that the commonly present but undesired diurnal pattern in the OP/PTB SDR TWSTFT difference is greatly reduced compared to the diurnal pattern in the difference obtained from the regular (nonSDR) TWSTFT equipment. However, the diurnal reduction in the NIST/OP and NIST/PTB SDR TWSTFT differences is not as significant as in the OP/PTB link. This is a strong indication that the application of SDR receivers has the ability to significantly improve the stability of some TWSTFT links, but is not a solution for all links. In this paper, we will analyze the diurnal reduction using the SDR receivers for the three links and study the dominant causes for the diurnal effect in the EuropeEurope and transatlantic TWSTFT links.

Two-way satellite time and frequency transfer (TWSTFT) is a primary technique for the generation of coordinated universal time (UTC). About 20 timing laboratories around the world continuously operate TWSTFT using satellite time and ranging equipment (SATRE) modems for remote time and frequency comparisons in this context. The precision of the SATRE TWSTFT as observed today is limited by an apparent daily variation pattern (diurnal) in the TWSTFT results. The observed peak-to-peak variation have been found as high as 2 ns in some cases. Investigations into the origins of the diurnals have so far provided no complete understanding about the cause of the diurnals. One major contributor to the diurnals, however, could be related to properties of the receive part in the modem. In 2014 and 2015, it was demonstrated that bypassing the receive part and the use of software-defined radio (SDR) receivers in TWSTFT ground stations (SDR TWSTFT) instead could considerably reduce both the diurnals and the measurement noise.In 2016, the International Bureau of Weights and Measures (BIPM) and the Consultative Committee for Time and Frequency (CCTF) working group (WG) on TWSTFT launched a pilot study on the application of SDR receivers in the TWSTFT network for UTC computation.The first results of the pilot study were reported to the CCTF WG on TWSTFT annual meeting in May 2017, demonstrating that SDR TWSTFT shows superior performance compared to that of SATRE TWSTFT for practically all links between participating stations. In particular, for continental TWSTFT links, in which the strongest diurnals appear, the use of SDR TWSTFT results in a significant suppression of the diurnals by a factor of between two and three. For the very long inter-continental links, e.g. the Europe-to-USA links where the diurnals are less pronounced, SDR TWSTFT achieved a smaller but still significant gain of 30%. These findings are supported by an evaluation of some of the links with an alternate technique based on GPS signals (GPS IPPP) as reported in this paper.Stimulated by these results, the WG on TWSTFT prepared a recommendation for the 21st CCTF meeting, which proposed the introduction of SDR TWSTFT in UTC generation. With CCTF approval of the recommendation, a roadmap was developed for the implementation of SDR TWSTFT in UTC generation. In accordance with the roadmap, most of the stations that participated in the pilot study have updated the SDR TWSTFT settings to facilitate the use of SDR TWSTFT data in UTC generation. In addition, the BIPM conducted a final evaluation to validate the long-term stability of SDR TWSTFT links, made test runs using the BIPM standard software for the calculation of UTC, now including SDR TWSTFT data, and started to calculate SDR TWSTFT time links as backup from October 2017. The use of SDR TWSTFT in UTC generation will begin in 2018.

The object of this study is the process of determining the coordinates of an air object. The main hypothesis of the study assumed that the use of a network of two Software-Defined Radio (SDR) receivers would make it possible to determine the coordinates of an air object. The determined coordinates could be used as preliminary target designation for the radar. A method for determining the coordinates of an air object by a network of two SDR receivers has been improved, which, unlike the known ones, involves: – using signals from the airborne systems of the air object; – using SDR receivers as network elements; – using the triangulation method for determining the coordinates of an air object. The accuracy of determining the coordinates of an air object by a network of two SDR receivers has been assessed. It was found that: – the accuracy of measuring the coordinates of an air object decreases sharply as the polar angle of observation from the middle of the base approaches 0 or π; – the smallest coordinate measurement error can be ensured when the air object is located on the traverse to the middle of the base and when the distance to the air object is close to the base size; – with small bases, the non-uniformity of the dependence of the coordinate determination error on the position of the air object relative to the SDR receivers is more pronounced than with large ones; – at a long range, the error values for small bases grow rapidly, which is primarily due to the small angle of intersection of the bearing lines; – it is advisable to place SDR receivers at a sufficiently large distance from each other (recommended value: (1–3) times the distance to the air object); – the errors of measuring the coordinates of the air object have a value of (250–350) m in a sufficiently wide range of directions; – with a decrease in the base size, the errors grow rapidly (reaching a value of more than 1000 m) when the observation angle deviates from the 90° direction

The object of this study is the process of determining the coordinates of low-visible aerial objects. The main hypothesis of the research assumed that the signals emitted by airborne systems of airborne objects that are not visible to radar stations have a greater power than the signal reflected from the airborne object. This, in turn, could improve the signal/noise ratio and, accordingly, the accuracy of determining the coordinates of low-visible aerial objects. It is suggested to use Software-Defined Radio receivers to receive such signals emitted by on-board systems of low-visible aerial objects. It was established that the main sources of signals for Software-Defined Radio receivers are signals of command, telemetry, target channels, manual control channels, and satellite navigation. It was established that an additional distinguishing feature when determining the coordinates of low-visible aerial objects is the uniqueness of their spectra and spectrograms. The method of determining the coordinates of low-visible aerial objects when using Software-Defined Radio receivers has been improved, which, unlike the known ones, involves: – the use as signals for Software-Defined Radio of signal receivers of on-board equipment of low-visible aerial objects; – the use of a priori coordinate values of a low-visible aerial object; – conducting additional spectral analysis of signals of on-board systems of low-visible aerial objects. The spectra and spectrograms of signals of on-board systems of aerial objects when using non-directional and directional antennas were experimentally determined. The experimental studies confirm the possibility of using the Software-Defined Radio receiver to receive signals from airborne equipment and improve the signal-to-noise ratio. The accuracy of determining the coordinates of aerial objects when using Software-Defined Radio receivers was evaluated. A decrease in the error of determining plane coordinates by the Software-Defined Radio system of receivers compared to the accuracy of determining coordinates by the P-19 MA radar station was established by an average of 1.88–2.47 times, depending on the distance to the aerial object

Time synchronization between ground and satellites is a key technology for satellite navigation system. With dual-channel satellite, a method called Two-Way Common-View (TWCV) satellite time transfer for Compass system is proposed, which combines both characteristics of satellite common-view and two-way satellite-ground time transfer. By satellite-ground two-way pseudo-range differencing and two stations common-view differencing, this TWCV method can completely eliminate the influence of common errors, such as satellite clock offset, ephemeris errors, troposphere delay and station coordinates errors. At the same time, ionosphere delay related to signal frequency is also weakened significantly. So the precision of time transfer is improved much more greatly than before. In this paper, the basic principle is introduced in detail, the effect of major errors is analyzed and the practical calculation model in the Earth-fixed coordinate system for this new method is provided. Finally, experiment analysis is conducted with actual Compass observing data. The results show that the deviation and the stability of the satellite dual channel can be better than 0.1 ns, and the accuracy of the two-way common-view satellite time transfer can achieve 0.4 ns. All these results have verified the correctness of this TWCV method and model. In addition, we compare this TWCV satellite time transfer with the independent C-band TWSTFT (Two-Way Satellite Time and Frequency Transfer). It shows that the result of the TWCV satellite time transfer is in accordance with the C-band TWSTFT result, which further suggests that the TWCV method is a remote high precision time transfer technique. The research results in this paper are very important references for the development and application of Compass satellite navigation system.

During the design and implementation of digital circuits for various applications, mapping of algorithms to different hardware components is a central task to achieve best in class performance (e.g., low power consumption per position fix, position accuracy and position deviation for satellite navigation receivers). During the last decade software defined radio receivers (SDRs) and Field Programmable Gate Array (FPGA) based receivers gain more and more importance during the prototyping phase and for receivers with an expected low volume production. These receivers are also of special interest during the introduction of new signals and frequency bands. FPGA based receivers allow a flexible design and the implementation of highly parallel digital logic and software based processing at the same time. The fundamental drawback of FPGA based GNSS receiver solutions is the high design complexity and the enlarged design space. In this context the design space describes the number of parameters the designer has to consider, to optimize the final design for a given application. Successful navigation in this design space, requires experience in various disciplines (e.g. Radio Frequency (RF) engineering, signal processing, GNSS algorithm design). The main idea of this paper is to ease the design, simulation and cost optimization of Software Defined Radio (SDR) GNSS receivers, implemented on standard PCs, Graphical Processing Units (GPUs) and FPGAs. The idea is to describe the receiver architecture and the receiver specifications at a very high and thus understandable level. During the next step a compiler maps the different receiver signal processing blocks to the existing hardware (i.e. General Purpose Computer (GPC), FPGA or GPU). Using this approach, the most time consuming parts (i.e. design description and specification) have to be done only once and the design is based on a single code base.

In this work, a based in FPGA acquisition system destined to Software Defined Radio (SDR) receivers is presented. The design includes a LEON3 embedded microprocessor which use the operative system RTEMS to execute the tasks that control the acquisition system and the communication interfaces. The system can be used in multiantenna receivers due to it allows acquiring samples from multiple inputs. A Direct Memory Access (DMA) interface over AMBA bus transfers the data to the RAM memory without the intervention of the processor. On the other hand, a client software, which runs in a PC, was developed to control, monitor and download the acquired samples. The design was tested using a Global Position System (GPS) receiver and the results obtained were satisfactory.

The most computationally intensive part of a software defined radio (SDR) receiver is the channelizer, which extracts multiple narrowband channels from a wideband input signal. In an SDR receiver, the compatibility of the channnelizer with different wireless communication standards is guaranteed by its reconfigurability. In this paper, we present the implementation of a reconfigurable channelizer based on tree-structured quadrature mirror filter (QMF) banks using Texas Instruments TMS320C6711 digital signal processor (DSP). Experimental results show that the proposed channelizer offers better adjacent channel interference attenuation and low complexity when compared with conventional Discrete Fourier Transform (DFT) based channelizer.

The most computationally demanding block in the digital front end of a software defined radio (SDR) receiver is the channelizer which operates at the highest sampling rate. Finite impulse response (FIR) filters are employed as channel filters in SDR receivers. These channel filters must be less complex and reconfigurable. A new reconfigurable architecture for the implementation of channel filters based on frequency response masking (FRM) technique is proposed in this paper. Our architecture offers multiple levels of reconfigurability in addition to the inherent low complexity features offered by the FRM technique. Design results show that our method offers an average adder reduction of 37% over binary subexpression elimination (BSE) technique and 57% over canonical signed digit (CSD) based techniques. © 2007 IEEE.

Two-way satellite time and frequency transfer has become an important technical component in the process of the realization of International Atomic Time and also for comparisons among the best primary frequency standards. This paper contains results of studies aimed at improving the knowledge of the uncertainty for true time transfer between remote sites. Our study addresses mostly the collection of so-called time transfer triplets between sites 1, 2, and 3. Systematic deviations from zero in the combination (1-2) + (2-3) + (3-1) would point to incomplete understanding and handling of systematic offsets in the individual time transfers. We show that such offsets exist, but that they are only in specific cases significant (up to about 2 ns), and we discuss possible causes.

The applications of unmanned aerial vehicles (UAVs) have increased dramatically in the past decade. Meanwhile, close-range UAV detection has been intriguing by many researchers for its great importance in privacy, security, and safety control. Positioning of the UAV controller (hub) is quite challenging but still difficult. In order to combat this emerging problem for public interest, we propose to utilize a software-defined radio (SDR) platform, namely HackRF One, to enable the UAV hub detection and localization. The SDR receiver can acquire the UAV source signals. The theoretical path-loss propagation model is adopted to predict the signal strength attenuation. Thus, the UAV hub location can be estimated using the modified multilateration approach by only three or more SDR receivers.

The Calibration Device is used to measure the time delay difference between the transmission and receiving paths inside Two-Way Satellite Time and Frequency Transfer (TWSTFT) earth station, it can improve the time comparison precision. A new calibration device for TWSTFT has been developed recently at Beijing Institute of Radio Metrology and Measurement (BIRMM). The signal paths inside calibration device is bidirectional which allow both the transmission and receiving signal passed, so the delay difference of the transmission and receiving inside calibration device almost be zero. The calibration device works on Ku-band. To evaluate the performance of the BIRMM calibration device, a local TWSTFT experiments was done with SATRE modem. The measurement results show that the time delay difference was quite small instability with standard deviation (1 σ) equal to 0.17ns.

In a manner different from the two-way satellite time and frequency transfer (TWSTFT) via geostationary satellites which has been employed for subnanosecond time transfer between widely separated earth stations, TWSTFT via non-geostationary satellites has a problem attaining subnanosecond accuracy because of satellite motion. The impact of satellite motion on the accuracy of TWSTFT via non-geostationary satellites is wholly attributed to the non-reciprocal geometric path. We have analysed the non-reciprocal geometric path caused by satellite motion in order to evaluate the accuracy of TWSTFT via non-geostationary satellites. Our analysis shows that the non-reciprocal geometric path introduces significant error to the accuracy of subnanosecond time transfer. Using a numerical model, we have demonstrated that ordinary orbit estimation can correct the non-reciprocal geometric path in TWSTFT via non-geostationary satellites to subnanosecond accuracy. Residuals of the non-reciprocal geometric path correction with respect to the orbit estimation accuracy and baseline length are presented. As an example of a non-geostationary satellite orbit, we employed an orbit of a quasi-zenith satellite orbiting the Earth in a period of one day, in synchronization with the Earth's rotation and with an orbital inclination of 45°.

In this paper we address the problem of designing a back-end for Software Defined Radio (SDR) receiver using only one low-rate Analog-to-Digital Converter (ADC). After the Intermediate Frequency (IF) down-conversion stage, the signals are sampled and all the other demodulation tasks are assigned to a Digital Signal Processor (DSP). In this paper we concentrate on only one Quadrature Amplitude Modulation (QAM) signal. After reduction of the communication chain to a linear model, the issue of proper sampling rate choice is addressed. It is shown how improper sampling rate can lead to a poorly conditioned problem that causes symbol cancelations and poor performances. In the analysis the role of the device noise at the input of the ADC is also addressed by computing autocorrelation and Power Spectral Density (PSD) after undersampling. Some simulations confirm the analysis.