Hindawi Wireless Communications and Mobile Computing Volume 27, Article ID 749273, 4 pages https://doi.org/.55/27/749273 Research Article Simulation and Performance Evaluations of the New GPS and L Signals Tahir Saleem, Mohammad Usman, Atif Elahi, and Noor Gul Department of Electronic, International Islamic University, H-, Islamabad 44, Pakistan Correspondence should be addressed to Tahir Saleem; tahir.phdee4@iiu.edu.pk Received 27 June 26; Revised 9 November 26; Accepted 5 December 26; Published 7 January 27 Academic Editor: Dajana Cassioli Copyright 27 Tahir Saleem et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The Global Positioning System (GPS) signals are used for navigation and positioning purposes by a diverse set of users. As a part of GPS modernization effort has been recently introduced for better accuracy and availability service. This paper intends to study and simulate the GPS L/ signal in order to fulfill the following two objectives. The first aim is to point out some important features/differences between current L (whose characteristics have been fairly known and documented) and new GPS signal for performance evaluation purpose. The second aim is to facilitate receiver development, which will be designed and assembled later for the actual acquisition of GPS data. Simulation has been carried out for evaluation of correlation properties and link budgeting for both L and signals. The necessary programming is performed in Matlab.. Introduction The GPS (Global Positioning System) is satellite system operated by the United States of America (USA) defense department. Its services (Location, Navigation, and Time) can be accessed all the time by anyone having necessary GPS receiver. The GPS system has total 32 satellites out of which 24 satellites are operational. These operational satellites are arranged in 6 orbits. GPS satellites being connected to ground stations revolve around the earth with a distance of 2, km from the surface of earth. Initially GPS had started its operation with two signals L and L2. L is transmitted at 575.42 MHz frequency and L2 at 227.6 MHz. These GPS signals include two ranging codes. C/A (Carrier Acquisition) code and P (Y) or Precision code. The first code is used for civilian purpose, while the second one is restricted to military use only. These ranges codes are utilized for measurement of distance to the satellite as well as identifying uniquely the navigation message []. Although the GPS system has almost reached its full operational capability, due to the increasing demand for better service and advances in technology, modernization and implementation of a new GPS system have recently started. The addition of GPS signal is one of the modernization efforts being taken by US Department of Defense. signal which is being transmitted on 76.45 MHz frequency is also known as Safety of Life signal in the GPS community. With higher transmission power and improved signal design as compared to other GPS signals (L or L2) it is believed that will enhance the existing performance of the GPS system. Due to wide bandwidth and comparatively longer spreading codes, the signal is expected to give a high processing gain. For signal transmission the Aeronautical Radio Navigation Service frequency band has been reserved which is easily accessible around the world. One of the unique features in proposed signal is the inclusion of both separate carrier and quadrature data modulation component. Separate PN codesareusedforthemodulationofbothcomponentswith PN chip clock rates of.23 Mcps and periods of 23 chips or ms [2 4]. In this paper we tried to point out some important features like power levels, data encoding, correlation (auto and cross), and power budget analysis between L and signal.bothlandsignalshavebeensimulatedand comparison is done on the basis of obtained results.
2 Wireless Communications and Mobile Computing 2. Performance Evaluation Parameters 2.. Autocorrelation. In satellite navigation applications, autocorrelation function has great importance. It basically refers to the integration and multiplication of a signal with its delayed copy. The general formula for the autocorrelation functionasgivenin[,5]canbewrittenas T R (τ) = lim T 2T f i (t) f i (t τ) dt, () T where f i (t) represents signal with time t for ith satellite, T is time period, and τ is delay in time. The autocorrelation properties are utilized to detect a GPS signal in a noisy environment. The C/A codes of GPS signals exhibit greater autocorrelation peak and low crosscorrelation. For better detection of a weak signal, it is necessary that autocorrelation peak of the weak signal must be greater than the cross-correlation peak of the strong signal. As C/A codes are near to orthogonal therefore cross-correlation value will approach to a smaller value. The autocorrelation function of a maximum length C/A code consists of an infinite sequence of triangular function, as shown in Figures and 2 for both L and, respectively. The peaks in the figures show high correlation value, which can be defined mathematically for L and as follows: R (τ) =,23T CA t=23 t= R 5 (τ) =,23T CA t=23 t= f z (t) f z (t τ) dt, f z (t) f z (t τ) dt, where f z (t) is C/A code with time t for zth satellite, T CA is single C/A chipping period (L = 977.5 nsec and = 97.75 nsec), and τ is delay in time. 2.2. Power Budget Analysis. To check suitability of reflected L and GPS signal with the aim of its utilization in remote sensing, power budget analysis is needed. Power budget analysis of reflected GPS signals has been elaborated in [6 8]wherethereflectedsignalswereusedforpassiveimaging and target detection. The analysis is first accomplished for L andthenforsignal. Let P t represent the power of transmitter (GPS satellite), G t gain of transmitter, and σ target cross section, R shows range (distance) from GPS satellites to target, and R 2 is range from target to receiver; then the received power can be calculated as [9] P r = (2) P t G t 4πR 2 4πR 2 2 σa ef, (3) where A ef represents receiving antenna effective area calculatedbyfollowingmathematicalformulawhengpssignal wavelength and receiver gain is known: A ef = λ2 G r 4π. (4) /23 /23 R (τ) 977.5 9 3 Figure : L autocorrelation function. R 5 (τ) τ (sec) 3 97.75 9 τ (sec) Figure 2: autocorrelation function. The receiver antenna SNR (Signal to Noise ratio) can be computed as P SNR = t G t G r λ 2 σ (4π) 3, R 2 R 2 (5) 2 KTB n where KTB n represent noise of receiver while λ is wavelength of GPS signal. Due to wide bandwidth, addition of Neuman-Hoffman codes in modulation, and comparatively longer spreading codes, the signal is expected, given a high processing gain which is evident from Figure 6. SNR comparison plots for L andshowninfigures5and6areinsection3. 2.3. Power Level. The L signal has a minimum signal strength of 58.5 dbw while has 54.9 dbw. It means that is 3.6 db better level as compared to L. Some other important difference parameters between L and are summarized in Table. 3. Results and Simulation Simulation is carried out in Matlab environment, for the autocorrelationofonesetofc/acodeforasatellitebroadcasting L signal, and the result is plotted in Figure 3. The amplitude peak value around 5 in the diagram represents autocorrelation of C/A code. Similarly Figure 4 depicts the autocorrelation of the simulated signal with autocorrelation value more than 4. It is evident from the diagram that autocorrelation value is greater than L. Hence provides better detection capability as compared to L signal. The secondary peaks in Figure 3 of the autocorrelation are significantly less than higher peak. Both Figures 3 and 4 clearly show that the cross-correlation values are very small, which enables the
Wireless Communications and Mobile Computing 3 Table : L and comparison parameters. Parameter L Bandwidth 2 MHz 24 MHz 575.42 MHz 76.45 MHz Center frequency Secondary codes Chip rate Remarks The higher bandwidth of can provide better accuracy in noisy environment N/A Neuman-Hoffman (NH) codes.23 MHz.23 MHz No Improved data encoding with Parity & Cyclic Redundancy Check Data encoding 3 SNR (db) Amplitude 4 2 6 4 C oa 2 rse f requ enc y.5.5 C o de delay 2 4 The addition of NH codes in signal: () Improved spectral line component spacing (2) Reduced effect of narrow band interference (3) Resulted in low cross-correlation (4) Provided better synchronization at bit level Increasing chipping rate of : () provides greater bandwidth performance, (2) low signal distortion, (3) provides greater accuracy Greater signal and data integrity can be achieved with advanced methods of encoding 5 5 2 25 3 35 4 45 5 55 6 65 7 75 8 Figure 3: The autocorrelation of L signal. 2 3 4 5 6 7 Range (distance in meters) 8 9 L Fre 5 que nc y.5 d C o de.5 elay 2 4 Figure 4: The autocorrelation of signal. GPS satellites to broadcast signals simultaneously at the same frequency using different C/A codes. From both figures it is evident that the secondary peaks in the autocorrelation diagram are significantly lower than the higher peak. This higher peak value helps the receiver in acquisition and tracking of the GPS signal. The simulation result of SNR versus range for reflected L and GPS signals shown in Figure 5 was carried out for to m range with target cross section of m2. SNR is calculated for different values of the range. At a range of m SNR (db) Amplitude Figure 5: SNR versus range plot without processing gain. 2 8 6 4 2 5 45 4 35 3 25 2 5 5 2 3 4 5 6 7 Range (distance in meters) 8 9 L Figure 6: SNR versus range plot with processing gain. SNR values of L and signals are, respectively, 5 db and 3 db, which are very low and detection of target is almost impossible in both cases. It is evident from Figure 5 that the
4 Wireless Communications and Mobile Computing SNR is very poor even at short distances; hence tracking of the GPS signals is almost impossible. For further SNR improvement the GPS signals were correlated for a longer period of time consequently better processing gain was achieved. The simulation results of SNR with processing gain of 43 db for GPS L and around 5 db for are shown in Figure 6. It is worth mentioning that the reflected GPS has 7 db more processing gain when compared with L signal. Since correlation peaks of signal are much larger than L signal, as shown in Figures 3 and 4, it can be deduced that signals have improved signal reliability and are more resistant to false acquisition problems. Moreover from the results it can be observed that, for same acquisition time, noise floor of signal is lower than L and therefore acquisition peaks are more prominent. This low noise floor decreases, susceptibility to wave form distortion, and offers better accuracy and processing gain than L signal. [7] B.Mojarrabi,J.Homer,K.Kubik,andI.D.Longstaff, Power budget study for passive target detection and imaging using secondary applications of GPS signals in bistatic radar systems, in Proceedings of the IEEE International Geoscience and Remote Sensing Symposium (IGARSS 2), vol., pp. 449 45, IEEE, Ontario, Canada, June 22. [8] M.Cherniakov,D.Nezlin,andK.Kubik, Airtargetdetection via bistatic radar based on LEOS communication signals, IEE Proceedings: Radar, Sonar and Navigation,vol.49,no.,pp.33 38, 22. [9] E. Glennon, A. Dempster, and C. Rizos, Feasibility of air target detection using GPS as a bistatic radar, Positioning, vol., no., 26. 4. Conclusion In this paper performance evaluation for the newly introduced and L GPS signals have been performed. Different evaluation parameters are considered and analyzed. Among these parameters simulation is carried out for correlation and power budget analysis. The results are recorded above. From the results it can be deduced that has superior detection characteristics as compared to L due to greater bandwidth, high correlation peaks, and better SNR. Hence better results can be achieved with GPS signal as compared to L signal. Competing Interests The authors declare that they have no competing interests regarding the publication of this paper. References [] E. Kaplan and C. Hegarty, Understanding GPS: Principles and Applications,ArtechHouse,25. [2] K. Krumvieda, C. Cloman, E. Olson et al., A complete IF software GPS receiver: a tutorial about the details, in Proceedings of the 4th International Technical Meeting of the Satellite Division of the Institute of Navigation (ION GPS ), pp. 789 829, Salt Lake City, Utah, USA, September 2. [3] A.Komjathy,J.A.Maslanik,V.U.Zavorotny,P.Axelrad,andS. J. Katzberg, Towards GPS surface reflection remote sensing of sea ice conditions, in Proceedings of the 6th International Conference on Remote Sensing for Marine and Coastal Environments, Charleston, SC, USA, May 2. [4] J.L.Garrison,A.Komjathy,V.U.Zavorotny,andS.J.Katzberg, Wind speed measurement using forward scattered GPS signals, IEEE Transactions on Geoscience and Remote Sensing,vol. 4,no.,pp.5 65,22. [5] B. Sklar, Digital Communications, Prentice Hall, Upper Saddle River, NJ, USA, 2. [6] V. Behar and C. Kabakchiev, Detectability of air targets using bistatic radar based on GPS signals, in Proceedings of the International Radar Symposium (IRS ), pp. 22 27, IEEE, Leipzig, Germany, September 2.
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