Inside GNSS Media & Research

JUL-AUG 2018

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Page 48 of 59 J U L Y / A U G U S T 2 0 1 8 Inside GNSS 49 ment focused on CDMA techniques tar- geting single bands and simplifying the receiver design. Given all this, it is worth asking: Is there benefit in GLONASS FDMA signal processing? What are the advantages of a navigation system pro- cessing GLONASS FDMA signals? Previous research highlights feasibil- ity and effectiveness of cheap jammers on radio navigation signals (T. Kraus et alia; A. D. Fonzo et alia). It can be specu- lated then that a system moving toward a single band navigation system by means of CDMA exploitation is also extremely susceptible to commercial off-the-shelf jammers and Personal Privacy Devices (PPD). An analysis of commercial off- the-shelf units and PPDs showed how these devices can turn a wide range of CDMA signals completely unusable in their presence (T. Kraus et alia). Interest- ingly enough, of the seven devices stud- ied, only 10% were capable of blocking the bands where GLONASS and its fre- quency channels operate ( Tables 1 and 2 ). Taking advantage of the processing of GLONASS FDMA signals is not a direct anti-jamming or anti-spoofing technique, but use of these signals does make it harder to harm the system. Another interesting case was the recently reported episodes of GPS spoof- ing happening in the Black Sea (S. Goff). Given the resources available, receivers can no longer simply rely on one single constellation or the other. The future lies in the design of receivers capable of mixing solutions from multiple constel- lations in a wide range of frequencies. Maybe the presence of a GLONASS capable receiver would have avoided the spoofing by allowing the system to eliminate the compromised GPS mea- surements and perform navigation with the aid of the GLONASS FDMA signals, assuming of course that the latest signals were not also spoofed in the area during those episodes. is work presents a new signal addi- tion to the Global Navigation Satellite System Soware Defined Radio (GNSS- SDR) platform, making the receiver more robust and diverse. Wit h t he GLONASS L1 C/A signal addition, the GNSS-SDR soware receiver is available for a new set of applications based on the use of the GLONASS FDMA signals. Given the points made earlier, it seems that receivers taking advantage of a new signal could be better prepared against malicious attacks of any kind. is work is not of course the first implementation of the GLONASS L1 C/A signal and there are multiple implementations by commercial off-the-shelf receivers that support this signal without major issues. Such commercial receivers come with a high price tag that could be a barrier in low funded applications or studies. Some open source versions also support the GLONASS L1 C/A signal, like GNSS- SDRLIB, but its soware implementa- tion is limited to only the Windows 64 bits platform, does not takes advantage of the latest Intel Advanced Vector Extensions (AVX), and does not sup- port its exportation to available embed- ded platforms (T. Suzuki and N. Tubo). GNSS Software Receivers Since the advent of the Soware Defined Radio (SDR), an increasing number of GNSS sof tware receivers have been developed by members of the naviga- tion community. Typical implementa- tions will use low level programming languages like C or C++ in order to achieve a real time receiver running in a general purpose computer or an embed- ded platform. Another design approach implements the receiver in a high level programming language like MATLAB or Py thon, and develops a software receiver ideal for post-processing appli- cations. Popular and open source imple- mentations include GNSS-SDR, GNSS- SDRLIB, and SoGNSS. GNSS-SDRLIB, mainly developed by Taro Suzuki was developed in C and uses the RTKLib navigation engine for computation of the position solution (T. Suzuki and N. Kubo). e soware receiver supports all major constellations and signals provid- ing acquisition, tracking, and pseudor- ange metrics for post-processing analy- sis. is soware receiver, however, is no longer under active development. On the other hand, soGNSS is a soware receiver for post processing analysis developed in MATLAB, source code of the receiver is provided with the book that introduces it (K. Borre et alia), and it provides code for a GPS L1/CA signal receiver. Even though multiple efforts have been directed towards extending the receiver capability with the addition of new signals and features, its MAT- LAB implementation does not make this soware receiver adequate for real time signal processing. Another impor- tant contribution to the community comes with the soware receiver devel- oped by omas Pany and his book on GNSS signal processing (see Additional Resources), which introduces key con- cepts of GNSS software processing in personal computers and discusses sever- al common techniques that can be used to achieve real time processing. In the author's opinion, a major contribution of this book was the usage of assembly language to accelerate common opera- tions on GNSS signal processing. There are also commercially avail- able software receivers for use in the community. e business approach for this concept mostly includes a combina- Type Center Freq. Band- width Ppeak dBm I 1.57475 GHz 0.9 kHz -12.1 II 1.57507 GHz 11.82 kHz -14.4 II 1.58824 GHz 44.9 kHz -9.6 I 1.57444 GHz 0.92 kHz -25.6 III 1.57130 GHz 10.02 kHz -19.3 IV 1.57317 GHz 11.31 kHz -9.5 II 1.57194 GHz 10.72 kHz -30.8 Table 1 PPD Characteristics (T. Kraus et alia) Parameter GLONASS L1 C/A Center Freq. 1602 MHz + k*562.5 kHz Access Technique FDMA Spreading Mod. BPSK Code Freq. 0.511 MHz Code Length 511 Meander Sequence 100 Hz Data Rate 50 bps Table 2 GLONASS Signal Parameters

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