A Statistical Analysis of GPS L1, L2, and L5 Tracking Performance During Ionospheric Scintillation

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1 A Statistical Analysis of GPS L1, L2, and L5 Tracking Performance During Ionospheric Scintillation Susan H. Delay, Charles S. Carrano, Keith M. Groves, Patricia H. Doherty Institute for Scientific Research, Boston College, Chestnut Hill, MA BIOGRAPHY Susan H. Delay is a senior research analyst at Boston College s Institute for Scientific Research. She received a B.A. in Mathematics from Trinity College and an MA from Boston College. Her interests are space weather, satellite communications and the Global Positioning System. Charles S. Carrano is a senior research physicist at Boston College s Institute for Scientific Research. He received a B.S. in mechanical engineering from Cornell University and M.S. and Ph.D. degrees in aerospace engineering from The Pennsylvania State University. His research interests include ionospheric impacts on radar, satellite communications, and the Global Positioning System. Keith M. Groves is currently a Program Manager in the Space Vehicles Directorate of the Air Force Research Laboratory where he investigates ionospheric scintillation and its impact on satellite based communication and navigation systems. He has a Ph.D. in Space Physics from MIT and a B.S. in Physics from Andrews University. Patricia H. Doherty is the Director and a Senior Scientist of the Institute for Scientific Research (ISR) at Boston College (BC). As director of the Institute, she oversees the activities of staff members working on a variety of innovative research projects. As a scientist, Patricia s own research interests are centered on the ionospheric effects on Satellite-Based Augmentation Systems (SBAS) and on promoting research and education in the science of navigation in developing countries. ABSTRACT GNSS navigation accuracy in the presence of ionospheric scintillation depends critically on tracking loop performance, which can be characterized in terms of the probability of loss-oflock (LOL) and the time for signal reacquisition following LOL events. Due to the relatively recent introduction of the new GPS modernization signals L2C and L5, there have been few statistical studies comparing L1, L2C, and L5 tracking performance under real-world scintillation conditions. While the lower frequency carriers generally experience larger signal fluctuations (due to the well-known frequency dependence of scintillation), the different codes and tracking algorithms employed for the different carrier signals make it difficult to predict their vulnerabilities and potential benefits to the NextGen aviation systems that will leverage these signals. Moreover, different GNSS receiver models employ different tracking algorithms which may exhibit unique strengths and vulnerabilities, depending on the characteristics of the scintillating environment. The most direct way to assess the tracking performance for L1, L2C, and L5 during scintillation is by the statistical analysis of experimental data collected using multiple receiver models during the current solar maximum period. With funding and support from the Federal Aviation Administration (FAA), Boston College and National Institute for Space Research (INPE) have been collecting GNSS scintillation observations in Brazil since Both GPS legacy and triple frequency receivers (L1 C/A, L2C and L5) are represented. As part of our ongoing study to assess GNSS signal tracking performance and navigation accuracy during scintillation, in this paper we report on the probability of losing code lock on L1, L2C, and L5 with two widely used GNSS scintillation monitors, the NovAtel GPStation-6 and Septentrio PolaRxS Pro. The approach we have taken is to count the number of scintillation-induced gaps in the high rate (50 Hz) receiver-reported signal amplitudes. Next, we bin these data gaps as a function of the scintillation index S4. The ratio of the number of missing samples to the total number of samples for a given S4 yields the probability of interrupted code tracking as a function of S4. Only high elevation observations are included in the statistics to exclude fluctuations caused by multipath reflections from terrestrial objects. Proc ION Pacific PNT Conference 1

2 We find that both receivers the NovAtel GPStation-6 and Septentrio PolaRxS Pro generally experienced a higher probability of losing lock on the lower frequency carriers (L2C and L5), even when quantified in terms of S4 on the same carrier, despite the enhanced codes and advanced tracking techniques available for these modernization signals. INTRODUCTION Ionospheric scintillations are fluctuations in the intensity and phase of satellite signals caused by scattering from electron density irregularities in the ionosphere. The intensity of scintillations is positively correlated with the solar cycle and the associated signal fades often exceed 20 db at L-band frequencies during solar maximum. Scintillation is generally most intense in the equatorial region of the globe after sunset, but it also occurs in the northern and southern high latitude regions. The occurrence morphology of scintillation depends on season, longitude, solar cycle, magnetic activity, and exhibits a high degree of night-to-night variability (Aarons 1982; Aarons 1993). Ionospheric scintillation affects GPS receivers in multiple ways. Amplitude scintillations result in errors decoding the GPS data messages and estimating the ranges to the satellites. Phase fluctuations stress the ability of the receiver to maintain lock on the signals and can cause cycle slips or breaks in the measured phase. These cycle slips may prevent the receiver from using the phase to refine its range measurements. When the receiver is unable to maintain lock on at least four or more GPS satellite signals, a temporary loss of positioning service occurs. The duration of these outages in service depends on the duration and severity of the disturbances, the geometry of the satellites in view, and the signal reacquisition time of the equipment (Carrano et al., 2005, Carrano et al., 2010). The S4 index is the standard deviation of normalized signal intensity fluctuations and is directly related to the probability that signal fades will reach a particular level (Basu et al., 1987). Loss of lock is more likely to occur when the GPS signal level drops below the fade margin of the receiver s internal tracking loops. Therefore, it is not surprising that the probability of losing lock varies as a function of the S4 index (Carrano et al, 2010). In this paper we explore the statistical relationship between intensity fluctuations due to ionospheric scintillation and loss of lock occurrence on the GPS L1, L2C, and L5 for two widely used GNSS scintillation monitors. We expect this information will be useful in future modeling and simulation studies concerning the impacts of scintillation on GNSS navigation accuracy. THE DATA COLLECTION SITE The data considered in this study was collected at São José dos Campos, Brazil, a location near the crest of the Appleton anomaly where global scintillation levels tend to be strongest (coordinates: S ' / W '16.9 degrees). Figure 1 shows the location of the site. We initially selected scintillation observations between November 2012 and January 2013 for this analysis but, for reasons we shall discuss, we also considered data during the period September to October Figure 1. The asterisk indicates the location of São José dos Campos, Brazil. The dashed line shows the location of the magnetic dip equator. GNSS RECEIVER HARDWARE Two different GNSS receiver models were used to collect the scintillation observations in this project, namely the NovAtel GPStation-6 and the Septentrio PolaRxS Pro. Both receivers were co-located at the site but operated with separate L1/L2/L5 antennas made by their respective manufacturers. Below, we provide a brief description of the equipment. Additional information is available from the receiver manufacturers websites. NovAtel GPStation-6 This GNSS scintillation monitor can track the GPS L1/L2/L2C/L5, SBAS L1/L5, GLONASS L1/L2, Galileo E1/E5a/E5b/Alt-BOC and BeiDou signals. Signal power and phase measurements are provided at a sampling rate of 50 Hz. A total of 120 independent channels are available for tracking signals. Signal intensity measurements are provided as the difference between narrow band and wide band Proc ION Pacific PNT Conference 2

3 power measured over 20 millisecond periods (Van Dierendonck, et al., 1993; Falletti et al., 2010). An Oven Controlled Crystal Oscillator (OCXO) is employed for low phase noise. This receiver has no direct access to the GPS P(Y) code. It monitors the open codes on L2C and L5 to produce observations on those carrier frequencies suitable for scintillation analysis (Shanmugan et al., 2012). Septentrio PolaRxS Pro This Global Navigation Satellite System (GNSS) receiver can monitor satellites from the following constellations: GPS, GLONASS, Galileo, and SBAS. Using 136 channels, it can track the L1, L2, L2C, L5, and E5ab / AltBOC satellite signals. While this receiver is capable of providing power and phase measurements at 100 Hz, we operated it at 50 Hz. We computed signal intensity from the post-correlator Inphase (I) and Quadrature (Q) samples acquired during 20 millisecond intervals. This receiver uses a high-quality OCXO for low phase noise. This receiver has no direct access to the GPS P(Y) code. It monitors the open codes on L2C and L5 to produce observations on those carrier frequencies suitable for scintillation analysis (Spoglia et al., 2013). METHODOLOGY The approach we have taken is to count the number of scintillation-induced gaps in the high rate (50 Hz) receiver-reported signal amplitudes, and then compute the probability of scintillation-induced loss of lock as a function of S4. The steps we take to accomplish this are as follows. First we attempt to exclude data gaps which may be caused by processes other than ionospheric scintillation. These include multipath and receiver noise. To avoid the former, we discard data from low elevation satellites (<30 ) and also data gaps that last longer than 5 minutes (setting satellites). To avoid the latter, we consider only data for which S The remaining data gaps are considered to be the result of scintillation-induced loss of lock. We count the total number of missing samples in these gaps. Next, we interpolate the S4 index (computed every 60 sec) onto the high rate (50 Hz) data epochs, and bin the scintillation-induced data gaps according to S4. The ratio of the number of missing samples to the total number of samples for each S4 yields the probability of a scintillation-induced data gap (interrupted code tracking) as a function of S4. Both the NovAtel GPStation-6 and the Septentrio PolaRxS Pro have been specifically designed to provide robust signal tracking during ionospheric scintillation. Nevertheless, loss of lock does occur when the scintillations are sufficiently intense, leading to gaps in the measured data. Figures 2 and 3 compare 50 Hz L1 signal power fluctuations measured by the Novatel and Septentrio receivers, respectively, between 0-4 UT on 16 November, Only data for PRNs 21, 22, 24, 25, 26, 29, 30, and 31 are shown (the data for other tracked satellites are omitted for clarity). Missing data samples (data gaps) associated with loss of lock events are colored red, whereas green indicates uninterrupted signal tracking. Despite the label C/No used for the vertical axes in Figures 3 and 4, the data plotted are actually signal intensities, plus arbitrary offsets (determined by us). Hence one should not compare the absolute magnitudes of the C/No values shown between the two receivers. Our LOL statistics will be reported in terms of S4, which is computed from the normalized signal intensity, so that the absolute magnitude of signal intensity is unimportant. A number of specific events have been identified in Figure 2 for discussion. The events circled in yellow were recorded while the transmitting satellites were high in the sky (>30 elevation). Hence the data gaps (red) that occurred during these events were likely caused by the signal fluctuations associated with ionospheric scintillation. Other events circled in red were recorded while the transmitting satellites were at low elevation angles. Signal fluctuations associated with these events are due to multipath, ionospheric scintillation, or some combination of both. Data gaps associated with these low elevation events have been excluded from our statistics. When we first examined the statistics of interrupted signal tracking for the PolaRxS Pro receiver, we were surprised to find many gaps in the data from high elevation satellites, even in the absence of any scintillation. This problem is evident in Figure 3, which shows that there are multiple instances of simultaneously interrupted tracking events for all satellites tracked. Clearly this is not an environmental effect but instead due to some problem with the equipment. Since the PolaRxS Pro and GPStation-6 receivers were co-located, the problem is unlikely due to the source of local RF interference. We conjecture that either the receiver was unable to track the requested satellite signals due to an overburdened CPU, or it was unable to send the data to the computer quickly enough over the manufacturersupplied USB to high-speed serial adaptor cable. In any case, it became clear that we could not use the data from this particular PolaRx Pro receiver for our analysis. Proc ION Pacific PNT Conference 3

4 Figure 2. L1 signal power for the NovAtel GPStation-6 (not all tracked satellites are shown). Missing data samples (data gaps) due to loss of lock are shown in red, whereas green indicates uninterrupted signal tracking. Figure 3. L1 signal power for the Septentrio PolaRxS Pro (not all tracked satellites are shown). Missing data samples (data gaps) due to loss of lock are shown in red, whereas green indicates uninterrupted signal tracking. Proc ION Pacific PNT Conference 4

5 We should note that when the Septentrio receiver was tracking normally, it tended to experience fewer scintillation-induced data gaps than the NovAtel receiver when tracking L1 signals. For example, compare the scintillation event circled in yellow toward the upper-right hand corner of Figure 2 with the corresponding scintillation event in Figure 3. Thankfully, there is another Septentrio PolaRxS Pro operating at São José dos Campos (about 10 km distant from our site), as part of the CIGALA network (Bougard et al., 2011). João Francisco Galera Monico kindly provided data from the CIGALA PolaRxS Pro receiver for the period September-October We are very grateful to Dr. Galera Monico and his team for supplying this data for use in our study. From this point in the paper onward, the Septentrio data we will discuss is from the CIGALA receiver. When we examined the data from the CIGALA PolaRxS-Pro receiver, we noted elevated S4 values throughout the day (as large as 0.5), even at high elevation angles. A closer examination of the raw signal intensities revealed evidence of simultaneous fading (but not loss of lock) for all satellites tracked. We suspect a source of local RF interference may be responsible for the elevated S4 values, but thankfully our LOL statistics should be unaffected, since (as we shall see) the PolaRxS Pro maintained satellite tracking on L1 when S4 was less than 0.6. Figure 4. A comparison of L1 tracking performance between the GPStation-6 (left) and PolaRxS Pro (right). Proc ION Pacific PNT Conference 5

6 STATISTICS OF SCINTILLATION-INDUCED TRACKING INTERRUPTIONS Next we present the results of our statistical analysis. Figure 4 compares the results for the GPStation-6 (left) and PolaRxS Pro (right) while tracking the GPS L1 signal. The format of this figure, and the ones to follow, is as follows. The top panel is a histogram showing the total number of 50 Hz data samples collected as a function of S4. Data for satellites below 30 elevation and data gaps lasting longer than 5 minutes are excluded from this number. The middle panel shows the number of missing 50 Hz data samples as a function of S4. The lower panel shows the ratio of the latter to the former, which gives the probability of a scintillation-induced data gap as a function of S4. The statistics summarized in Figure 4 include 17 days for the GPStation-6 (collected between Nov 2012 and Jan 2013) and 46 days for the PolaRxS Pro (collected between Sep and Oct 2014). For both receivers the probability of interrupted tracking on L1 increases as a function of S4, but this probability was significantly larger for the NovAtel than for the Septentrio. Figure 6 summarizes the statistics of L2C tracking for the PolaRxS Pro. Unfortunately, the GPStation-6 did not track L2C signals (presumably due to a receiver configuration issue), so we cannot compare results between the two receivers. The plots on the left are organized in terms of the S4 index computed for the L2C signal, while the plots on the right are organized in terms of the S4 index computed for the L1 signal. We fully expected a higher probability of interrupted tracking of L2 for the same level of ionospheric disturbance, because scintillations are generally more intense on lower frequency carriers due to the well-known frequency dependence of scintillation. Comparing the plots on the right side of Figure 6 with those on the right side of Figure 5 shows this expectation is clearly met (the probability of a data gap is at least 5 times larger on L2C than L1, for a given S4 value on L1. Nevertheless, we anticipated a similar probability of interrupted tracking on L1 as L2C when parameterized in terms of their own signal fluctuations (i.e. the S4 on the same carrier). This turned out to not be the case. Comparing the left hand plots in Figure 6 with the right hand plots in Figure 5, we see that the probability of interrupted tracking is larger on the lower frequency carrier, even when viewed in terms of its own S4. Figure 7 summarizes the statistics of L5 tracking for the GPStation-6. Unfortunately, the GPStation-6 did not track L5 signals (presumably due to a receiver configuration issue), so we cannot compare results between the two receivers. The plots on the left are organized in terms of the S4 index computed for the L5 signal, while the plots on the right are organized in terms of the S4 index computed for the L1 signal. This receiver experienced a higher probability of a data gap on L5 than L1, for a given S4 value on L1. Similarly, the probability of interrupted tracking is larger on the L5 than L1, even when viewed in terms of its own S4. More precisely stated, the probability of a data gap on L5 (when viewed as a function of S4 on L5) was higher than the probability of a data gap on L1 (when viewed as a function of S4 on L1). We found the observation that the probability of interrupted tracking was larger on L2C and L5 than on L1, even when viewed in terms of their own S4 values to unexpected. This seems to suggest that L2C and L5 tracking may be less robust to scintillation effects than L1 tracking, despite the availability of new codes on these GPS modernization signals and the advanced algorithms that can be employed to track them. Proc ION Pacific PNT Conference 6

7 Figure 6. L2 tracking performance for the PolaRxS Pro as a function of S4 on L2 (left) and L1 (right). Figure 7. L5 tracking performance for the GPStation-6 as a function of S4 on L5 (left) and L1 (right). Proc ION Pacific PNT Conference 7

8 CONCLUSIONS As part of our ongoing study to assess GNSS signal tracking performance and navigation accuracy during scintillation we evaluated the probability of scintillation-induced loss of code lock on L1, L2C, and L5 as encountered by two widely used GNSS scintillation monitors, the NovAtel GPStation-6 and Septentrio PolaRxS Pro. The approach we have taken is to count the number of scintillation-induced gaps in the high rate (50 Hz) receiver-reported signal amplitudes. Next, we bin these data gaps as a function of the scintillation index S4. The ratio of the number of missing samples to the total number of samples for a given S4 yields the probability of interrupted code tracking as a function of S4. Only high elevation observations are included in the statistics to exclude fluctuations caused by multipath reflections from terrestrial objects. We found the probability of scintillation-induced L1 tracking interruption appears to be significantly larger for the GPStation-6 than the PolaRxS Pro. We also found that both receivers experienced a higher probability of losing lock on the lower frequency carriers, as one might expect due to the well-known frequency dependence of scintillation. Perhaps less expected, however, was the observation that that the probability of interrupted tracking was larger on the lower frequency carriers than on L1, even when parameterized in terms of the S4 for their own carriers. This seems to suggest that L2C and L5 tracking may be less robust to scintillation effects than L1 tracking, despite the availability of new codes on these GPS modernization signals and the advanced algorithms that can be employed to track them. This study has several inherent limitations. Firstly, the probabilities of losing lock we report are specific to the particular GNSS receiver model tested. Due to problems with data from one of our receivers we were unable to compare the performance of the GPStation-6 than the PolaRxS Pro during the same time period. Also, there are many external factors which could affect our results. Differences in the receiver installation such as antenna types, cables, and placement are a few possible hindrances to a fair comparison of receiver tracking performance during scintillation. Furthermore, the number of samples included in this study is limited (particularly for L2C and L5). Back in 2012, 10 satellites transmitted the L2C signal while only 3 transmitted L5. Currently, 16 satellites transmit L2C and 8 transmit L5. We hope to expand this study to include more recent data in the future. AKNOWLEDGEMENTS This research was supported by Boston College Cooperative Agreement FAA 11-G-006, sponsored by Deane Bunce. The authors are indebted to Eurico de Paula and João Francisco Galera Monico for providing GNSS data from São José dos Campos. REFERENCES Aarons, J. (1982), Global morphology of ionospheric scintillations, Proc. IEEE, 70, , doi: /proc Aarons, J. (1993), The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence, Space Sci. Rev. 63, Bougard, B., Sleewaegen, J-M., Spogli, L., Veettil, Sreeja Vadakke, Monico, J.F. Galera (2011), CIGALA: Challenging the Solar Maximum in Brazil with PolaRxS, Proceedings of the 24th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS 2011), Portland, OR, September 2011, pp Carrano, C. S., and Groves, K.M. (2010), Temporal Decorrelation of GPS Satellite Signals due to Multiple Scattering from Ionospheric Irregularities, Proceedings of the 23rd International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS 2010), Portland, OR, September 2010, pp Carrano, C. S., Groves, K. M., McNeil, W. J., Doherty, P. H. (2012), Scintillation Characteristics Across the GPS Frequency Band, Proceedings of the 25th International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS 2012), Nashville, TN, September 2012, pp Carrano, C. S., K. Groves and J. Griffin (2005), Empirical Characterization and Modeling of GPS Positioning Errors Due to Ionospheric Scintillation, Proceedings of the Ionospheric Effects Symposium, Alexandria, VA, May 3-5, 2005 Carroll, M., Y. Morton, and E. Vinande (2014), Triple Frequency GPS Signal Tracking During Strong Ionospheric Scintillations over Ascension Island, Position, Location and Navigation Symposium - PLANS 2014, 2014 IEEE/ION, 5-8 May 2014, Monterey, CA. Proc ION Pacific PNT Conference 8

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