Performance Analysis of Doppler Aided Tracking Loops in Modernized GPS Receivers Sana Ullah Qaisar School of Surveying and Spatial Information Systems, University of New South Wales, Australia BIOGRAPHY having the pilot channel and relatively slower dynamics, is used as aiding source in the considered arrangement. The Sana Ullah Qaisar received his Masters degree in L2C PLL, operated with a wide loop bandwidth, absorbs Telecommunication Engineering from the University of the dynamic stress and provides Doppler estimates to the New South Wales (UNSW), Australia in 2003. He has L1 PLL (affected by wide-band interference) operating at gained experience in the telecommunications industry and narrow-band. The tracking error associated with such served as faculty member at National University of collaboration between two tracking loops is analyzed. It is Computer & Emerging Sciences, Pakistan. He is currently shown that a 7 dB-Hz RFI margin (equivalent to that pursuing his PhD at School of Surveying & Spatial achieved with a moderate quality inertial sensor), can be Information Systems, UNSW. His research interests achieved through the local Doppler-assistance. include synchronization algorithms for baseband processing and FPGA-based receiver design for INTRODUCTION modernized GNSS signals. In a modernized GPS receiver, multiple civil signals can ABSTRACT be simultaneously tracked. Conventionally, each of the civil signals will be tracked in an independent channel. The GPS modernization program deploys new generations However, collaboration across multiple channels can be of satellites designated Block IIR-M, Block IIF and Block established to improve the tracking performance of III-A, equipped to transmit multiple civil signals. The co- individual channels as well as to enhance the overall existence of multiple civilian signals in a GPS receiver has robustness of the receiver. The aim of this paper is to been a focus in recent research activities. In the literature develop and evaluate such collaboration for „carrier to date, correlator outputs of individual channels are tracking‟, while each channel is tracking a different carrier combined in ways either to increase the power available frequency. for processing or to reduce the error in tracking measurements. These combinations are typically made at For channels operating on the same carrier frequency (e.g. the discriminator level, after the discriminator or after the data and pilot channels in L2C and L5 GPS signals), the loop filter. The combinations have been considered both output of individual channels has been combined in the for channels operating on the same carrier frequency (e.g. following ways. Spilker & Van Dierendonck (1999) data and pilot channels of L5) as well as channels across proposed to combine (non-coherently) the coherent different carrier frequencies. In a modernized GPS integration output of L5 data and pilot channels at the receiver, tracking multiple civil signals, the relationship code discriminator. Muthuraman et al. (2006) proposed between Doppler frequencies (proportional to the differential combinations of L2C data and pilot channels corresponding carrier frequencies) of all signals is known at the discriminator level. Both of the above combination and therefore Doppler estimates of one PLL can be schemes aim to increase the signal energy available for utilized by another PLL tracking a different frequency. tracking. In another approach, the measurements of While such benefits are conventionally achieved through independent data and pilot channels are combined to ultra-tight GPS/INS coupling, this paper presents reduce the tracking error. For example, Hegarty (1999) performance analysis of a Doppler-aided tracking loop proposed to run independent discriminators on data and architecture, where the Doppler assistance is obtained pilot channels and then combine their outputs using from another loop in the same receiver. L1 C/A and L2C weights that are inversely proportional to their output are the two civil signals selected for the analysis, because variances, in the context of the L5 signal. Also, Tran et al. this pair of signals will be available long before any other (2002) applied the above optimal weights technique for combination. The analysis, however, is relevant to any tracking the L2C signal. For any tracking combination pair of signals from a single satellite. The L2C carrier, across different carrier frequencies, the ionospheric and Doppler effects must be addressed carefully. Gernot et al. 20-ms (2008) proposed a combined L1/L2C tracking scheme L2 CM code (511.5 Kcps) L2 CM DATA where the relationship between phase variations (due to ionosphere) in the two signals is utilized for combining the tracking measurements. Ries et al. (2002) discuss the The L2C code (1.023 Mcps) possibility of L1/L5 joint tracking by aiding the L1 L2 CL tracking with L5 carrier NCO, assuming the ionospheric L2 CL code drift to be less than 50 Hz. (511.5 Kcps) 1.5-s This paper presents a Doppler-aided carrier tracking architecture for the modernized GPS receivers. The GPS civil signals transmitted by Block IIR-M satellites, i.e. L1 Fig. 1. The L2C code structure and L2C, are selected for evaluating the performance of the proposed architecture. The slower (relative to L1 GPS signal) dynamics and the pilot channel (where coherent 1. CM 0 discriminators with an extended linearity region can be used), make the L2C PLL a natural choice for operating at wider loop bandwidth to look after the carrier Doppler, CM CM 2. while the L1 loop bandwidth can be set as tight as 0.5 Hz to mitigate the wide-band noise. The Doppler estimates from the L2C PLL, after appropriate scaling, are fed to the Fig. 2. Choices of local replica code for tracking the L2C data channel (only two chips are shown) L1 tracking loop. The aim of this paper is to evaluate the error introduced by this local-aiding and analyze what RFI return-to-zero (RZ) CM code. In the second option, each margin can be achieved through such locally Doppler- CM chip is extended to the duration of two chips to make aided carrier tracking loops. The results of this research it a non-return-to-zero (NRZ) CM code. The E-L DLL reveal that for an L1 PLL, without Doppler aiding, 22 dB- discriminator (0.5 chip correlator spacing) response, Hz is the minimum C/No that can be tracked within the multipath envelope and cross-correlation performance of acceptable error threshold, while it can go down to 15 dB- the two options are compared in Fig. 3 through Fig. 5 Hz in the Doppler-aided mode thus offering a 7 dB-Hz respectively, suggesting RZ CM code as the obvious RFI margin. The C/No refers to effective carrier to noise choice for tracking as it is better in all three of these ratio throughout this paper. criteria. Moreover, the correlation noise with NRZ CM code will be twice as much as with the RZ CM code The paper is organized as follows. The next section (Qaisar et al. 2008). For all experiments conducted in this describes the structure of the L2C signal and the choices research, RZ CM code is therefore used for tracking the of local replica code for tracking the L2C signal. The PLL L2C data channel. The same arguments apply to the pilot performance of the L1 and L2C signals is then discussed. channel where RZ CL code is used for tracking. Aided tracking architectures are then evaluated and finally, some concluding remarks are given. TRACKING LOOP PERFORMANCE THE L2C SIGNAL STRUCTURE In a phase locked loop implementation, illustrated in Fig. 6, the IF carrier is matched with the NCO generated The L2C signal is composed of two codes, namely L2 CM replica carrier and the difference in phases of the two and L2 CL. The L2 CM code is 20 milliseconds long and carriers is determined by the discriminator, while the loop has 10230 chips while the L2 CL code is 1.5 seconds long filter is responsible for removing noise in the phase and has 767250 chips. The CM code is modulo-2 added to measurements as well as for looking after the signal data (i.e. it modulates the data) and the resultant sequence dynamics. The performance of a PLL is measured as of chips (data channel) is time-multiplexed with the CL variance of the total phase jitter, contributed by the code (pilot channel) on a chip-by-chip basis. The thermal noise, clock errors, platform vibration and individual CM and CL codes are clocked at 511.5 KHz dynamic stress. The total phase jitter for a two-quadrant while the composite L2C code has a frequency of 1.023 arctangent discriminator is given as (Kaplan 1996, Van MHz. This time-multiplexed L2C sequence, modulates the Dierendonck 1997): L2 (1227.6 M Hz) carrier (IS-GPS-200D, 2006). Fig. 1 illustrates the code structure with corresponding chipping rates. (1) With the new L2C code structure, two basic options can where be used for tracking the data channel (M. Tran 2002, Fontana et al. 2001). As shown in Fig. 2, the two options is the 1-sigma thermal noise in degrees differ in the choice of alternate chips. In the first option, is the 1-sigma vibration-induced oscillator jitter in the local code alternates between CM chips and zeros, the degrees 1 IF Carrier I&D NRZ CM RZ CM Discri- minator 0.5 DLL discriminator output Loop NCO Filter 0 Fig. 6. Generic PLL implementation -0.5 (2) -1 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 Code offset (chips) where is the single-sided loop filter bandwidth, is Fig. 3. E-L DLL discriminator (0.5 chip correlator spacing) response for the coherent observation interval, is the single-sided different replica codes in L2C data channel noise power spectral density and is the signal power. It is clear from (1) that minimizing leads to more noise filtering and thus the resulting phase jitter is reduced. 800 NRZ CM There is a limit, however, to which the can be reduced 600 RZ CM without affecting the total phase jitter, as discussed later in 400 the paper. Tracking error (chips) 200 The equation for vibration (when the oscillator is installed 0 in an environment where it is subjected to mechanical vibrations) induced oscillator jitter is (Kaplan 1996): -200 -400 -600 (3) -800 0 0.5 1.0 1.5 2.0 2.5 Multipath delay (chips) where Fig. 4. Multipath envelopes of L2C signal for different replica codes. The correlator spacing is set to 0.5 chips. is the input frequency in Hz is the oscillator vibration sensitivity 1 is random vibration modulation frequency in Hz = power curve of random vibration in g2/Hz 0.8 Cumulative probability For a third order loop (considered in this paper), the Allan 0.6 phase jitter is (Kaplan 1996): 0.4 NRZ CM (4) RZ CM 0.2 where is the Allan deviation and is the short term stability gate time for Allan variance measurement. 0 -70 -60 -50 -40 -30 Relative cross-correlation power (dB) The dynamic stress refers to the phase jitter introduced due to abrupt platform motion such as a step, acceleration Fig. 5. Cross-correlation performance of the L2C signal with different or a jerk in the input phase or frequency. For a third order replica codes. The RZ CM code has a 3dB relative improvement PLL, the phase error due to dynamic stress is given by (Kaplan 1996): is the Allan variance-induced oscillator jitter in degrees is the dynamic stress error in the PLL tracking (5) loop where is the maximum value of jerk The variance of thermal noise jitter is given by experienced by the GPS receiver. (Kaplan 1996, Dierendonck 1997): 18Hz, 20ms, 10g/s 10Hz, 20ms, 1g/s 2Hz, 20ms, 0.01g/s 20 50 Data channel Total RMS phase jitter (degrees) 40 Dataless channel RMS phase jitter (degrees) Optimal combination 15 30 20 10 10 5 15 20 25 30 35 0 C/No (dBHz) 15 20 25 30 35 C/No (dBHz) Fig. 7. Total phase jitter for a 3rd order carrier tracking loop for the given Fig. 8. Performance (phase jitter) of different PLL implementations loop bandwidth, coherent integration time and dynamic stress. Oscillator specifications are given in Table-I TABLE-I L2C carrier from Oscillator specifications data channel Allan Vibration Discri- Vibration I&D α deviation sensitivity minator 2 .005 g /Hz Loop 1 1×10-11 1×10-9 (200- D Filter NCO 2000)Hz Discri- I&D minator β o The 15 threshold is compliant to model a PLL as a linear feedback system (Dierendonck 1997, Egziabher et al. 2003). For phase errors above 150, the navigation signals L2C carrier from might still be tracked and decoded, at the expense of pilot channel performance degradation. Fig.7 illustrates the total phase jitter as a function of C/No for a third order PLL (tracking Fig. 9. The optimal linear-combination implementation of data and pilot L1 carrier frequency: 1575.42 MHz), including all of the channels in L2C PLL above mentioned effects. A coherent integration of 20 milliseconds is used (throughout this paper) while the where is the combined output, and are the oscillator specifications are given in Table-I (Kaplan individual outputs of data and pilot discriminators 1996): respectively while and are the weighting coefficients. The optimal choice for values of these coefficients that The L2C Signal Tracking minimizes the variance, with the constraint are: One of the key objectives of the L2C signal was to offer a (8) better solution for weak signal tracking (Fontana et al., 2001). The pilot channel incorporated in the L2C signal allows for a coherent discriminator, where the thermal (9) phase jitter is expressed as: (6) and hence the total phase variance becomes: Furthermore, the co-existence of data and pilot channels in the L2C signal has been exploited to reduce the phase (10) jitter. Tran and Hegarty (2002) proposed to run independent discriminators on the data and pilot channel The performance of data, dataless and combined and then combine their outputs using weights that are implementations is illustrated in Fig. 8. The optimal inversely proportional to their output variances. The error combination offers the best performance followed by the variance of a carrier tracking loop operating on both data dataless and data channels respectively. The combined and pilot channel is given as: option, however, cannot be implemented because at low C/No levels and large dynamics the phase error on data (7) channel will grow outside its linearity region ( ) and 18Hz, 20ms, 10g/s 10Hz, 20ms, 1g/s 2Hz, 20ms, 0.01g/s 1400 20 L1 Total RMS phase jitter (degrees) 1200 L2C Carrier Doppler (Hertz) 15 1000 800 L2C 10 L1 600 400 5 5 10 15 20 25 15 20 25 30 35 C/No (dB Hz) time (minutes) Fig. 10. The carrier Doppler in L1 and L2C signals, computed from Fig. 11. Comparison of the total phase jitter in 3rd order PLL of L1 and almanac data (PRN-17), showing that the L2C has a slower rate of L2C signals change of Doppler than L1. C/No=15dBHz C/No=25dBHz C/No=35dBHz 40 consequently the data discriminator will not be able to 30 RMS phase jitter (degrees) correctly assess the phase error. To avoid the use of any 20 15 irrelevant information from the data discriminator, Julian et al. (2004) proposed a method that checks the 10 L1 consistency between the data and pilot discriminator L2C outputs as: If 1 0.1 1 10 (11) Single-sided PLL bandwidth (Hz) otherwise Fig. 12. Loop bandwidth vs. total phase jitter for L1 and L2C carrier tracking loops (12) However, it should be noted that the phase jitter of the two where the value of is chosen to be tight such as in signals shown here are with reference to their Julian et al. (2004). Fig. 9 illustrates this implementation corresponding loops. This is not to say that L2 for the L2C signal, followed in this paper, where D measurements are more accurate than L1 but to show that denotes the decision block described in (11) and (12). L2C PLL is more robust than L1 PLL. It can be observed from Fig. 11 that the difference in the phase jitter of the Performance Comparison of L1 & L2C Tracking two signals diverges as the C/No level drops, suggesting L2C is a better candidate for the aiding signal. Similarly, The carrier Doppler in L1 and L2C signals is proportional Fig.12 indicates that an L2C PLL would be a better choice to their carrier frequencies and hence the „rate of change for operating at wider bandwidth. Fig. 12 also informs that of Doppler‟ in the two carriers is different. Fig. 10 shows for a given carrier to noise ratio, there is a bandwidth for the carrier Doppler of L1 and L2C signals, computed from which the phase jitter is a minimum and as the loop noise a recent almanac data, for the PRN-17 Block IIR-M bandwidth is decreased below or increased above this satellite, indicating that the rate of change of Doppler in minimum value, the total phase jitter increases because the the L2C signal is slower than that in L1. contribution to total phase jitter by the various error sources is changed (Egziabher et al. 2003). It is also Also, it can be observed from equations (3) through (5) shown by Egziabher et al. (2003) that the 1-sigma that all stochastic components of total PLL jitter are standard deviation in the Doppler estimates, , is directly proportional to the carrier frequency; hence the smaller for L2 than for L1 and their relationship is given individual jitter contributions and the total phase jitter in as: the L2C signal would be smaller than that of the L1 signal. Fig 11 and Fig. 12 compare the total PLL jitter in the two signals for the specifications given in Table-I, as a (13) function of C/No and PLL bandwidth respectively. L2 Carrier I&D L2 Carrier I&D Discri- Discri- minator minator Loop Loop NCO Filter NCO Filter L1/L2 L1/L2 Loop NCO NCO Filter Discri- minator L1 Carrier I&D L1 Carrier I&D Fig. 13. L2C PLL feeding the L1 tracking Fig. 15. L2C PLL aiding the L1 PLL 4000 4000 Prompt correlator output (absolute value) L2C pilot Prompt correlator output (absolute value) L2C pilot 3000 L2C data 3000 L2C data L1 L1 2000 2000 1000 1000 0 0 -1000 -1000 -2000 -2000 -3000 -3000 -4000 -4000 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 time (milliseconds) time (milliseconds) Fig. 14. Tracking output from L1 and L2C in-phase prompt correlators Fig. 16. Tracking output from L1 and L2C in-phase prompt correlators for a real Block IIR-M signal (PRN-17), collected by the Namuru GPS for a real Block IIR-M signal (PRN-17), collected by the Namuru GPS receiver, following the architecture shown in Fig. 13 receiver, following the architecture shown in Fig. 15 where and are the L-band carrier frequencies. All of Directly Fed Carrier Tracking Loops the above comparisons assess L2C as a promising choice for aiding signal. In this research, L2C PLL should The conventional aiding approach, such as proposed by therefore be set into aiding mode while the L1 as the aided Ries et al. (2005) suggests one PLL to track both of the mode. signals for reducing the receiver complexity. Fig. 13 illustrates this architecture for tracking both L1 and L2C AIDED PLL ARCHITECTURE carriers from the L2C PLL. However, in such a direct feeding, phases of the two local carriers will be identical The concept of Doppler aiding is not new. With Doppler while they should be independently synchronized with aiding, platform dynamic stress of the aided tracking loop those of the corresponding incoming carriers. As a result, is removed and its loop bandwidth is reduced to mitigate the L1 local carrier will have a periodic phase offset with the wide-band interference (Akos et al. 2004). However, its incoming signal leading to a periodic residual Doppler aiding across different carrier tracking loops frequency error. This effect can be observed in Fig. 15 hosted on the same receiver platform has not been which shows the in-phase prompt correlator outputs of the evaluated. For the considered collaboration between L1 real L1 and L2C signals simultaneously tracked from the and L2C carrier tracking loops, one of the two PLL should “Namuru” GPS Receiver (Qaisar and Dempster 2007). be set in aiding mode while other one goes to the aided The effect observed here is due to the fact that the total mode. electron count encountered on the signal path through There are two basic options for coupling the carrier ionosphere is not changed over the time (Gernot et al. tracking loops, discussed as follows. 2008) otherwise a random behavior of L1 in-phase prompt Discri- where represents the noise variance of the optimal L1 carrier I&D minator linear combination of data and pilot channels, shown in Loop Fig. 8. Filter Similarly, L1 PLL jitter can be given as: NCO (15) L1/L2 L2C carrier from Doppler estimates The tracking errors from L2C PLL, fed to the L1 PLL can data channel from L2C PLL be expressed as: Discri- I&D minator α (16) Loop D Filter NCO where is the error in L1 PLL, coming from the Discri- L2C PLL. All of the error components from L2C PLL I&D minator β except are correlated, as shown in equations (3) through (5). The error in aided L1 tracking loop can therefore be classified into correlated and un-correlated L2C carrier from error as: pilot channel Fig. 17. L2C PLL data/pilot combination, aiding the L1 PLL (17) and correlator output will be observed. (18) Aided Carrier Tracking Loops respectively. The total error in the aided L1 PLL can now In order to resolve this ionosphere problem, L2C can be be given as: used to aid L1 carrier with an architecture shown in Fig. 15. In this configuration, each of the two loops is fully operational and the Doppler estimates from the L2C PLL and fed to the L1 PLL as aiding. The tracking results shown in Fig. 16 verify that ionospheric phase shift (19) problem is resolved with this architecture and that the phase of L1 signal is tracked. L2C-Aided L1 Carrier Tracking Loops where and are the correlation coefficients, defined In order to take full advantage of the L2C tracking as: capabilities, the architecture shown in Fig. 17 is implemented. The purpose of this tracking loop collaboration is to remove the dynamic stress load (20) (dominant source of error at smaller loop bandwidths) from a narrow-band PLL, thus allowing it to keep the tracking lock in weak signal conditions in the event of (21) abrupt platform maneuvers. However, the tracking error introduced by this architecture must be evaluated for Note that the dynamic stress of the L1 PLL has been acceptable performance level. removed in (19). i. The Tracking Error Analysis The Performance Evaluation The L2C PLL jitter can be expressed as: To evaluate the tracking performance of Doppler-aided architecture, in this paper, the L2C PLL is set to operate at nominal loop bandwidth of 10 Hz while a range of 0.1 to (14) 40 Hz is tested for the optimal bandwidth of the aided L1 PLL under a dynamic stress of 0.25 g/s. Three levels of L1 C/No (15 dB-Hz, 25 dB-Hz and 35 dB-Hz) were trialed while the L2C C/No level is set to 45 dB-Hz to ensure a higher quality of Doppler estimates as the concern here is 40 to examine the scenarios where L1 is affected by the 30 interference but L2C is not. The performance of L1 PLL is 20 RMS phase jitter (degrees) assessed as a function of loop bandwidth and C/No, 15 respectively. 10 Loop Bandwidth Fig. 18 shows the L1 PLL performance as a function of C/No=15dBHz loop bandwidth in the presence of dynamic stress, without C/No=25dBHz C/No=35dBHz Doppler aiding. The minimum loop bandwidth, as already 1 discussed in the paper, is dependent on the carrier-to-noise 0.1 1 10 Single-sided PLL bandwidth (Hz) ratio. However, it can be observed that for C/No levels below 25 dB-Hz, the tracking error grows above the 15 Fig. 18. Loop bandwidth vs. total phase jitter in the non-aided L1PLL degrees threshold. In other words, C/No levels below 25 with dynamic stress included dB-Hz cannot be tracked in this arrangement. If the dynamic stress is excluded (i.e. if perfect Doppler C/No=15dBHz C/No=25dBHz C/No=35dBHz estimates or static receiver conditions are assumed), the 40 PLL performance should be improved as shown in Fig. 30 19. This Figure also indicates the quality of Doppler 20 RMS phase jitter (degrees) estimates from the L2C PLL. As compared to the perfect 15 Doppler estimate (dynamic stress excluded) case, the L2C 10 PLL estimates are good enough for weak C/No levels (which is of more interest here) than for higher values of C/No. This is because at higher C/No levels, the noise from L2C PLL has more impact on the low noise in the aided L1 PLL and hence the effective tracking error in the Dynamic stress excluded aided L1 PLL is significantly increased. Similarly at low Dynamic stress removed through aiding 1 C/No levels, the increase in effective tracking error is 0.1 1 10 Single-sided PLL bandwidth (Hz) relatively small. Also, the minimum C/No level that can be tracked in the aided L1 PLL is 15 dB-Hz, at a loop Fig. 19. Loop bandwidth vs. total phase jitter in the L1 PLL with bandwidth of approximately 0.5 Hz. This means that the dynamic stress excluded and removed through Doppler assistance from locally-aided loop is able to continue tracking at as small a L2C PLL loop bandwidth as 0.5 Hz. Effective Carrier to Noise Ratio 40 30 The gain achieved in Doppler-aided PLL (reported above) 20 RMS phase jitter (degrees) appears more convincing when the results are arranged in 15 the form of Fig. 20 through Fig. 22 (i.e. as a function of 10 C/No). Fig. 20 illustrates the L1 PLL performance in the stand-alone mode, i.e. without any Doppler aid but including its dynamic stress. At 10 Hz loop bandwidth, BL=1Hz approximately 25 dB-Hz signal is tracked with acceptable BL=2Hz BL=5Hz level of performance (the figures match with those shown BL=10Hz in Fig. 18). 1 10 20 30 40 50 When the dynamic stress on the L1 PLL is excluded, the C/N0 (dBHz) PLL performance should be enhanced as shown in Fig. 21. However, the performance of L1 PLL aided by L2C PLL Fig. 20. Carrier-to-noise ratio vs. total phase jitter in the non-aided L1 and including the dynamic stress, shown in Fig. 22, is of PLL with dynamic stress included more interest here. In this case, it can be observed that the 15o threshold is exceeded when the C/No has dropped Again a higher quality of Doppler estimates can be below 15 dB-Hz, thus gaining a 7 dB-Hz RFI margin. observed for lower C/No levels for reasons explained Also, comparing Fig. 20 and Fig. 21 gives an indication of above. It is suggested that the L2C PLL discussed here the quality of Doppler estimates (difference in the perfect can be used for aiding both L1 and L2C code tracking Doppler estimates shown in Fig. 21 and the Doppler loops as well for improved code tracking performance and estimates provided by the L2C PLL) from the L2C PLL. enhanced receiver robustness. guidance and technical discussions carried out in this 40 work. 30 20 REFERENCES RMS phase jitter (degrees) 15 10 Spilker, J. J. and A. J. Van Dierendonck (1999), Proposed New Civil GPS Signal at 1176.45 MHz, ION GPS 1999 Nashville, TN, pp 1717-1725. BL=1Hz BL=2Hz Spilker, J. J. and A. J. Van Dierendonck (2001), Proposed BL=5Hz New L5 Civil GPS Codes, Navigation: Journal of The BL=10Hz Institute of Navigation, Vol.48, No.3, Fall 2001. 1 10 20 30 40 50 C/N0 (dBHz) Muthuraman, K., S.K. Shanmugam and G. Lachapelle (2007), Evaluation of Data/Pilot Tracking Algorithms Fig. 21. Carrier-to-noise ratio vs. total phase jitter in the non-aided L1 for GPS L2C Signals Using Software Receiver, in PLL with dynamic stress excluded Proceedings of GNSS 2007, 25-28 September, Fort Worth TX, Institute of Navigation. 40 30 Hegarty, C. (1999), Evaluation of the Proposed Signal Structure for the New Civil GPS Signal at 1176.45 20 RMS phase jitter (degrees) 15 MHz, WN99W0000034, The MITRE Corporation. 10 Tran, M., and C. Hegarty (2002), Receiver Algorithms for the New Civil GPS Signals, ION NTM 2002, 28-30 BL=1Hz January 2002, San Diego, CA, pp. 778-789. BL=2Hz BL=5Hz Gernot, C, Kyle O‟Keefe and Gerard Lachapelle, BL=10Hz Combined L1 / L2C Tracking Scheme for Weak Signal 1 Environments, Proceedings of ION GNSS 2008, Session 10 20 30 40 50 C/N0 (dBHz) C4, Savannah, GA, 16-19 September 2008 Fig. 22. Carrier-to-noise ratio vs. total phase jitter in the L1 PLL with Ries, L., Macabiau, C. O. Nouvel, Q. Jeandel, W. Vigneau dynamic stress removed through L2C PLL and V. Calmettes (2002), A Software Receiver for GPS- IIF L5 Signal, ION-GPS/GNSS 2002, 24-27 Sep 2002, Portland, OR, pp. 1540-1553. CONCLUSIONS NAVSTAR Global Positioning System Interface In a modernized GPS receiver, tracking multiple civil Specification IS-GPS-200 revision D, 7 March 2006. signals, collaboration across multiple carrier tracking loops can be established to combat un-usual tracking Fontana, R., W. Cheung, P. Novak, T. Stansell (2001), conditions such as both interference and dynamics. A The New L2 Civil Signal, Proceedings of US. Institute of tracking architecture for such collaboration between L1 Navigation (Salt Lake City, UT, Sept. 11-14), pp. 617- and L2C carrier tracking loops is evaluated. The robust 631. L2C PLL is set to operate at wider-band to provide Doppler estimates to the narrow-band L1 PLL under the Qaisar, S. and A. G. Dempster, Evaluating the wide-band interference and dynamic stress conditions. acquisition potential of GPS L1 and L2C codes for From the results reported in this paper it appears that a 7 weak signal environments, IEEE Transactions on dB-Hz margin against RFI can be achieved through the Aerospace and Electronic Systems, 2009, submitted for Doppler-aided tracking loop architecture. Future work will review. investigate the inclusion of additional signals in the collaboration and alteration of the loop bandwidth in real Qaisar S., N. C. Shivaramaiah and A. G. Dempster, time. Exploiting the spectrum envelope for GPS L2C signal acquisition, proc. of European Navigation Conference, ACKNOWLEDGEMENTS April 2008. Toulouse, France, 2008. This research work is supported by the Australian Van Dierendonck, A.J. (1997), GPS Receivers in Global Research Council Discovery Project DP0556848. The Positioning System: Theory and Applications, Volume I, author would like to thank Associate Professor Andrew G. AIAA. Dempster, University of New South Wales, for his Kaplan, E. (1996), Understanding GPS: Principles and Applications, Artech House. Egziabher, D., A. Razavi, P. Enge, D. Akos, S. Pullen (2003), Doppler Aided Tracking Loops for SRGPS Integrity Monitoring, in Proceedings of ION GPS/GNSS 2003 9-12 September, Portland OR. Chiou T. S. Alban, D. S. Atwater, J. Gautier, S. Pullen, P. Enge, D. Akos, B. Egziabher, B. Pervan (2004), Performance Analysis and Experimental Validation of a Doppler-Aided GPS/INS Receiver for JPALS Applications, On Proceedings of ION GNSS 17th International Technical Meeting of the Satellite Division, 21-24 Sept. Long Beach CA. Julien, O., G. Lachapelle and M.E. Cannon (2004), A New Multipath and Noise Mitigation Technique Using Data/Dataless Navigation Signals, Proceedings of GNSS 2004 (Session A1, Long Beach, CA, 21-24 September), The Institute of Navigation, Fairfax, VA Dempster A. G., Correlators for L2C: Some Considerations, Inside GNSS Oct. 2006, pp32-37. Qaisar S. and A. G. Dempster, Cross-correlation Performance Comparison of L1 and L2C GPS Codes for Weak Signal Acquisition, Proc. of International Symposium on GPS/GNSS, November 2008, pp 692-700. Qaisar S. and A. G. Dempster, Receiving the L2C signal with ‘Namuru' GPS L1 receiver, Proc. of IGNSS2007 Symp. on GPS/GNSS, Sydney, Australia, December 2007, paper 53. Julien O. (2005), Carrier Phase Tracking of Future Data/Pilot Signals, ION GNSS 2005, Long Beach, CA, September 13-16, 2005.