Further observations of GPS satellite oscillator anomalies mimicking ionospheric phase scintillation

An incident has previously been reported where the signal from the Navstar 43 Global Positioning System (GPS) satellite contained phase anomalies in such a way as to mimic ionospheric scintillation. We have observed another 25 events from the same satellite, plus events from three more satellites. Our data includes simultaneous observations from widely spaced receivers (up to 6,590 km apart), from different manufacturers, further ruling out the possibility of local effects. Two of the events involved a satellite (GPS IIF SV-2) broadcasting the L2C signal. This signal contained phase deviations matching those of the L1 signal, but with a 120/154 multiplicative factor. This rules out the possibility of a genuine ionospheric scintillation event, as it does not match the plasma dispersion relation. It does, however, agree with what can be expected from an anomaly in the satellite’s oscillator. While the previously reported event could be dismissed as a freak occurrence, it is now apparent that these events are a persistent phenomenon. They have the potential to corrupt geophysical research with false data and to generate false alarms in systems to forewarn of GPS outages due to scintillation.     

On the accuracy of the GPS L2 observable for ionospheric monitoring

The introduction of the unencrypted global positioning system (GPS) L2 civil (L2C) signal has the potential to improve measurements made with the L2 frequency, an important observable in GPS-based ionospheric research and monitoring. Recent work has shown significant differences between the legacy L2P(Y) and L2C-derived total electron content rate of change index (ROTI). This difference is observed between L2P(Y) and L2C-derived ROTI with certain receiver models and between zero-baseline receiver pairs. We discuss the likely cause for these differences: L1-aided tracking used to track both the L2P(Y) and L2C signals. We also present L2C data that are confirmed to be from tracking independent of L1. Using the ionospheric-free linear combination, we show that the independently tracked carrier phase dynamics are significantly more accurate than the L1-aided observables. This result is confirmed by comparing the behavior of the L2C and L2P(Y) carrier phase observables upon a sudden antenna rotation.     

Performance of various predicted GNSS global ionospheric maps relative to GPS and JASON TEC data

When using predicted total electron content (TEC) products to generate preliminary real-time global ionospheric maps (GIMs), validation of these ionospheric predicted products is essential. In this study, we evaluate the accuracy of five predicted GIMs, provided by the international GNSS service (IGS), over continental and oceanic regions during the period from September 2009 to September 2015. Over continental regions, the GPS TEC data collected from 41 IGS continuous tracking stations are used as a reference data set. Over oceanic regions, the TEC data from the JASON altimeter are used for comparison. An initial performance comparison between the IGS combined final GIM product and the predicted GIMs is also included in this study. The evaluation results show that the predicted GIMs produced by CODE outperform the other predicted GIMs for all three validation results. The accuracy of the 1-day predicted GIMs, produced by the IGS associate analysis centers (IAACs), is higher than that of the 2-day predicted GIMs. Compared to the 2-day UPC predicted GIMs, the 2-day ESA predicted GIMs are observed to have slightly worse performances over ocean regions and better positioning performances over continental regions.     

A first look at the effects of ionospheric signal bending on a globally processed GPS network

This study provides a first attempt at quantifying potential signal bending effects on the GPS reference frame, coordinates and zenith tropospheric delays (ZTDs). To do this, we homogeneously reanalysed data from a global network of GPS sites spanning 14 years (1995.0–2009.0). Satellite, Earth orientation, tropospheric and ground station coordinate parameters were all estimated. We tested the effect of geometric bending and dTEC bending corrections, which were modelled at the observation level based, in part, on parameters from the International Reference Ionosphere 2007 model. Combined, the two bending corrections appear to have a minimal effect on site coordinates and ZTDs except for low latitude sites. Considering five days (DOY 301–305, 28 October–1 November 2001) near ionospheric maximum in detail, they affect mean ZTDs by up to ~1.7 mm at low latitudes, reducing to negligible levels at high latitudes. Examining the effect on coordinates in terms of power-spectra revealed the difference to be almost entirely white noise, with noise amplitude ranging from 0.3 mm (high latitudes) to 2.4 mm (low latitudes). The limited effect on station coordinates is probably due to the similarity in the elevation dependence of the bending term with that of tropospheric mapping functions. The smoothed z-translation from the GPS reference frame to ITRF2005 changes by less than 2 mm, though the effect combines positively with that from the second order ionospheric refractive index term. We conclude that, at the present time, and for most practical purposes, the geometric and dTEC bending corrections are probably negligible at current GPS/reference frame precisions.     

Statistics of GPS ionospheric scintillation and irregularities over polar regions at solar minimum

A statistical study of the occurrence characteristic of GPS ionospheric scintillation and irregularity in the polar latitude is presented. These measurements were made at Ny-Alesund, Svalbard [78.9°N, 11.9°E; 75.8°N corrected geomagnetic latitude (CGMLat)] and Larsemann Hills, East Antarctica (69.4°S, 76.4°E; 74.6°S CGMLat) during 2007–2008. It is found that the GPS phase scintillation and irregularity activity mainly takes place in the months 10, 11 and 12 at Ny-Alesund, and in the months 5, 6 at Larsemann Hills. The seasonal pattern of phase scintillation with respect to the station indicates that the GPS phase scintillation occurrence is a local winter phenomenon, which shows consistent results with past studies of 250 MHz satellite beacon measurements. The occurrence rates of GPS amplitude scintillation at the two stations are below 1%. A comparison with the interplanetary magnetic field (IMF) B _y and B _z components shows that the phase scintillation occurrence level is higher during the period from later afternoon to sunset (16–19 h) at Ny-Alesund, and from sunset to pre-midnight (18–23 h) at Larsemann Hills for negative IMF components. The findings seem to indicate that the dependence of scintillation and irregularity occurrence on geomagnetic activity appears to be associated with the magnetic local time (MLT).     

Investigating the inconsistency of ionospheric ROTI indices derived from GPS modernized L2C and legacy L2 P(Y) signals at low-latitude regions

GPS Solutions - The rate of change of total electron content (TEC) index (ROTI), an important parameter to characterize ionospheric irregularities and associated scintillation activities, can be...     

Use of the L2C signal for inversions of GPS radio occultation data in the neutral atmosphere

Results from processing FORMOSAT-3/COSMIC radio occultations (RO) with the new GPS L2C signal acquired both in phase locked loop (PLL) and open loop (OL) modes are presented. Analysis of L2P, L2C, and L1CA signals acquired in PLL mode shows that in the presence of strong ionospheric scintillation not only L2P tracking, but also L1CA tracking often fails, while L2C tracking is most stable. The use of L2C improves current RO processing in the neutral atmosphere mainly by increasing the number of processed occultations (due to significant reduction in the number of L2 tracking failures) and marginally by a reduction in noise in statistics. The latter is due to the combination of reduced L2C noise (compared to L2P) and increased L1CA noise in those occultations where L2P would have failed. This result suggests application of OL tracking for L1CA and L2C signals throughout an entire occultation to optimally acquire RO data. Two methods of concurrent processing of L1CA and L2C RO signals are considered. Based on testing of individual occultations, these methods allow: (1) reduction in uncertainty of bending angles retrieved by wave optics in the lower troposphere and (2) reduction in small-scale residual errors of the ionospheric correction in the stratosphere.     

GPS L1C信号多径抑制性能分析

MBOC（multiplexed binary offset carrier）是GPS—GALILEO互用和兼容性工作组推荐的信号调制体制,其实现方式分为TMBOC和CBOC两种,其中TMBOC（6,1,4／33）已被GPSL1C信号所采用。多径是众多卫星导航系统应用的主要误差源之一,由于不具备时间和空间的相关性,无法通过差分技术消除。本文基于窄相关技术和Double—Delta技术对TMBOC（6,1,4／33）的抗多径性能进行了分析,并与BPSK（1）及BOC（1,1）进行了对比。在同等条件下,TMBOC（6,1,4／33）的抗多径性能优于或等同于BPSK（1）及BOC（1,1）。     

L5载波的信号质量分析

增加第三民用频率L5载波是GPS现代化措施中的一项,目前美国已经有4颗卫星具有L5观测值。从L5的信号结构上进行分析,应用L5载波的实际观测数据,与L1载波和L2载波相比较,L5载波不但在信噪比上得到提高,而且其多路径效应的影响也比较小,具有较好的数据观测质量。     

GPS现代化后L2C的信号质量分析

分析了L2C码的主要作用,比较了GPS现代化后L2载波信噪比的变化特点,阐述了伪距观测值的噪声水平。     

Determination of the ionospheric foF2 using a stand-alone GPS receiver

The critical frequency of ionospheric F2 layer (foF2) is a measure of the highest frequency of radio signal that may be reflected back by the F2 layer, and it is associated with ionospheric peak electron density in the F2 layer. Accurate long-term foF2 variations are usually derived from ionosonde observations. In this paper, we propose a new method to observe foF2 using a stand-alone global positioning system (GPS) receiver. The proposed method relies on the mathematical equation that relates foF2 to GPS observations. The equation is then implemented in the Kalman filter algorithm to estimate foF2 at every epoch of the observation (30-s rate). Unlike existing methods, the proposed method does not require any additional information from ionosonde observations and does not require any network of GPS receivers. It only requires as inputs the ionospheric scale height and the modeled plasmaspheric electron content, which practically can be derived from any existing ionospheric/plasmaspheric model. We applied the proposed method to estimate long-term variations of foF2 at three GPS stations located at the northern hemisphere (NICO, Cyprus), the southern hemisphere (STR1, Australia) and the south pole (SYOG, Antarctic). To assess the performance of the proposed method, we then compared the results against those derived by ionosonde observations and the International Reference Ionosphere (IRI) 2012 model. We found that, during the period of high solar activity (2011–2012), the values of absolute mean bias between foF2 derived by the proposed method and ionosonde observations are in the range of 0.2–0.5 MHz, while those during the period of low solar activity (2009–2010) are in the range of 0.05–0.15 MHz. Furthermore, the root-mean-square-error (RMSE) values during high and low solar activities are in the range of 0.8–0.9 MHz and of 0.6–0.7 MHz, respectively. We also noticed that the values of absolute mean bias and RMSE between foF2 derived by the proposed method and the IRI-2012 model are slightly larger than those between the proposed method and ionosonde observations. These results demonstrate that the proposed method can estimate foF2 with a comparable accuracy. Since the proposed method can estimate foF2 at every epoch of the observation, it therefore has promising applications for investigating various scales (from small to large) of foF2 irregularities.     

Performance evaluation of different ionospheric models in single-frequency code-based differential GPS positioning

Differential ionospheric slant delays are obtained from a quiet-time, three-dimensional ionospheric electron density model, called the TaiWan Ionosphere Model (TWIM), to be used in code-based differential GPS positioning. The code observations are acquired from nine continuously operating GPS stations around Taiwan whose baseline ranged from 19 to 340 km. Daily 24-hour epoch-per-epoch positioning obtained for 70 most geomagnetic quiet days (2008–2010) for each of the 72 baselines. The performance of TWIM has been compared with the standard operational Klobuchar model (KLB) used by typical single-frequency receivers and the IGS global ionospheric model (GIM). Generally, TWIM performed well in reducing the differential ionospheric delay especially for long baselines and different levels of low solar activity. It has a much better performance compared to the operational KLB model. TWIM also performed similarly with GIM, though GIM has the best performance overall. GIM has the best ionospheric gradient estimates among the three models whose differential ionospheric delay-to-horizontal error ratio is more than 0.25. This is followed closely by TWIM with about 0.20. KLB only has a ratio of <0.10. The similarity of the performance of TWIM and GIM demonstrates the feasibility of TWIM in correcting for differential ionospheric delays in the C/A code pseudorange that is caused by electron density gradients in the ionosphere. It can provide decimeter-to-centimeter level accuracy in differential GPS positioning for single-frequency receivers during geomagnetic quiet conditions across all seasons and different levels of low solar activities.     

Improved precise point positioning in the presence of ionospheric scintillation

Ionospheric disturbances can be detrimental to accuracy and reliability of GNSS positioning. We focus on how ionospheric scintillation induces significant degradation to Precise Point Positioning (PPP) and how to improve the performance of PPP during ionospheric scintillation periods. We briefly describe these problems and give the physical explanation of highly correlated phenomenon of degraded PPP estimates and occurrence of ionospheric scintillation. Three possible reasons can contribute to significant accuracy degradation in the presence of ionospheric scintillation: (a) unexpected loss of lock of tracked satellites which greatly reduces the available observations and considerably weakens the geometry, (b) abnormal blunders which are not properly mitigated by positioning programs, and (c) failure of cycle slip detection algorithms due to the high rate of total electronic content. The latter two reasons are confirmed as the major causes of sudden accuracy degradation by means of a comparative analysis. To reduce their adverse effect on positioning, an improved approach based on a robust iterative Kalman filter is adopted to enhance the PPP performance. Before the data enter the filter, the differential code biases are used for GNSS data quality checking. Any satellite whose C1–P1 and P1–P2 biases exceed 10 and 30 m, respectively, will be rejected. Both the Melbourne–Wubbena and geometry-free combination are used for cycle slip detection. But the thresholds are set more flexibly when ionospheric conditions become unusual. With these steps, most of the outliers and cycle slips can be effectively detected, and a first PPP estimation can be carried out. Furthermore, an iterative PPP estimator is utilized to mitigate the remaining gross errors and cycle slips which will be reflected in the posterior residuals. Further validation tests based on extensive experiments confirm our physical explanation and the new approach. The results show that the improved approach effectively avoids a large number of ambiguity resets which would otherwise be necessary. It reduces the number of re-parameterized phase ambiguities by approximately half, without scarifying the accuracy and reliability of the PPP solution.     

Improving the GNSS positioning stochastic model in the presence of ionospheric scintillation

Ionospheric scintillations are caused by time- varying electron density irregularities in the ionosphere, occurring more often at equatorial and high latitudes. This paper focuses exclusively on experiments undertaken in Europe, at geographic latitudes between ~50°N and ~80°N, where a network of GPS receivers capable of monitoring Total Electron Content and ionospheric scintillation parameters was deployed. The widely used ionospheric scintillation indices S4 and s_j{\sigma_{\varphi}} represent a practical measure of the intensity of amplitude and phase scintillation affecting GNSS receivers. However, they do not provide sufficient information regarding the actual tracking errors that degrade GNSS receiver performance. Suitable receiver tracking models, sensitive to ionospheric scintillation, allow the computation of the variance of the output error of the receiver PLL (Phase Locked Loop) and DLL (Delay Locked Loop), which expresses the quality of the range measurements used by the receiver to calculate user position. The ability of such models of incorporating phase and amplitude scintillation effects into the variance of these tracking errors underpins our proposed method of applying relative weights to measurements from different satellites. That gives the least squares stochastic model used for position computation a more realistic representation, vis-a-vis the otherwise ‘equal weights’ model. For pseudorange processing, relative weights were com- puted, so that a ‘scintillation-mitigated’ solution could be performed and compared to the (non-mitigated) ‘equal weights’ solution. An improvement between 17 and 38% in height accuracy was achieved when an epoch by epoch differential solution was computed over baselines ranging from 1 to 750 km. The method was then compared with alternative approaches that can be used to improve the least squares stochastic model such as weighting according to satellite elevation angle and by the inverse of the square of the standard deviation of the code/carrier divergence (sigma CCDiv). The influence of multipath effects on the proposed mitigation approach is also discussed. With the use of high rate scintillation data in addition to the scintillation indices a carrier phase based mitigated solution was also implemented and compared with the conventional solution. During a period of occurrence of high phase scintillation it was observed that problems related to ambiguity resolution can be reduced by the use of the proposed mitigated solution.     

GPS现代化后L_2载波的定位精度研究

利用实际采集的数据,从信噪比和定位残差上分析了L2载波的数据质量,指出GPS现代化后,用L2C码恢复的L2载波信噪比明显提高,与L1载波信噪比接近,两者的差值仅在5 dBHz以内;用L2C码恢复的L2载波比GPS现代化前的L2载波的信噪比更加稳定,不容易受卫星高度角的影响;同时利用L2C码恢复的L2载波的定位残差比没有L2C码的L2载波小,定位精度明显提高。     

Tuning a Kalman filter carrier tracking algorithm in the presence of ionospheric scintillation

GPS Solutions - Strong ionospheric electron content gradients may lead to fast and unpredictable fluctuations in the phase and amplitude of the signals from Global Navigation Satellite Systems...     

Different mechanisms of the ionospheric influence on GPS occultation signals

A local mechanism for strong ionospheric effects on radio occultation (RO) global positioning satellite system (GPS) signals is described. Peculiar zones centered at the critical points (the tangent points) in the ionosphere, where the gradient of the electron density is perpendicular to the RO ray trajectory, strongly influence the amplitude and phase of RO signals. It follows from the analytical model of local ionospheric effects that the positions of the critical points depend on the RO geometry and the structure of the ionospheric disturbances. Centers of strong ionospheric influence on RO signals can exist, for example, in the sporadic E-layers, which are inclined by 3–6° relative to the local horizontal direction. Also, intense F2 layer irregularities can contribute to the RO signal variations. A classification of the ionospheric influence on the GPS RO signals is introduced using the amplitude data, which indicates different mechanisms (local, diffraction, etc.) for radio waves propagation. The existence of regular mechanisms (e.g., local mechanism) indicates a potential for separating the regular and random parts in the ionospheric influence on the RO signals.     

Temporal and spatial variations of global ionospheric total electron content under various solar conditions

By utilizing the numerical technique of principal component analysis (PCA), this work analyses temporal and spatial variations of the ionosphere under various solar conditions during the period 1999–2013. Applying the PCA technique to the time series of the global ionospheric total electron content (TEC) maps provides an efficient method for analyzing the main ionospheric variability on a global scale that is able to decompose periodic variations (e.g., annual and semiannual oscillations) while retaining the asymmetry in the temporal and spatial domains (e.g., seasonal and equator anomalies). The TEC series of different local times are processed separately at two time scales: (1) the whole 15 years of the period of study and (2) the individual years. In contrast with previous studies, the analysis of the dataset of the 15 years shows that dawn (e.g., LT4–6) and late morning (LT10–12) are the more remarkable characteristic times for ionospheric variability. This study also reveals a cyclic trend of the variability with respect to local times. The first two modes, which contain 80–90% of the total variance, represent spatial distributions and temporal variations with respect to the different stages of the solar cycle and local times. Annual and semiannual variations are demodulated from the first two modes, and the results show that these variations evidently have distinct trends for daytime and nighttime. An exception is that, under active solar conditions, extremely strong solar irradiance during the daytime has a residual effect on the variability of the nighttime.