FORMOSAT-3/COSMIC observations of the ionospheric auroral oval development

The ionospheric radiance and electron density observed by the tiny ionospheric photometer (TIP) and GPS occultation experiment (GOX) payloads on FORMOSAT-3/COSMIC satellites are applied to determine the boundaries of the auroral oval and its width in the winter nighttime ionosphere for both hemispheres. The TIP collects ionospheric emission at 135.6 nm due to electron impact excitation, while the GOX offers ionospheric electron density profiles with radio occultation (RO) technique. Comparison between them shows similar patterns of the plasma structure in the polar caps. The mean width of the auroral bands ranges between about 2 and 11° latitude in the winter nighttime and it varies with longitudes. The comparison by month suggests that the mean radius of the auroral ovals varies with the intensity of the auroral radiance.     

Electron density profiles in the equatorial ionosphere observed by the FORMOSAT-3/COSMIC and a digisonde at Jicamarca

We examine for the first time the ionospheric electron density profiles concurrently observed by the GPS occultation experiment (GOX) onboard the FORMOSAT-3/COSMIC (F3/C) and the ground-based digisonde portable sounder DPS-4 at Jicamarca (12°S, 283°W, 1°N geomagnetic) in 2007. Our results show that the F3/C generally underestimates the F2-peak electron density NmF2 and the F2-peak height hmF2. On the other hand, when the equatorial ionization anomaly (EIA) pronouncedly appears during daytime, the total electron content (TEC) derived from the radio occultation of the GPS signal recorded by the F3/C GOX is significantly enhanced. This results in the NmF2 at Jicamarca being overestimated by the Abel inversion on the enhanced TEC during the afternoon period.     

Seismo-ionospheric precursor of the 2008 Mw7.9 Wenchuan earthquake observed by FORMOSAT-3/COSMIC

The seismo-ionospheric precursor prior to the Mw7.9 earthquake near Wenchuan, China, on 12 May 2008 was observed by the FORMOSAT-3/COSMIC satellite constellation. By binning radio occultation observations, the three-dimensional ionospheric structure can be obtained to monitor the ionospheric electron density variation prior to the earthquake. It has been determined that near the epicenter the F2-peak height, hmF2, descends about 25 km and the F2-peak electron density, NmF2, decreases about 2 × 10⁵ el/cm³ around noon within 5 days prior to the earthquake. The integrated electron content decreases more than 2 TECU between 250 and 300 km altitude.     

Analysis of ionospheric electron parameters versus geomagnetic index Dst from RO data of FORMOSAT-3/COSMIC

The FORMOSAT-3/COSMIC mission has provided ample ionospheric electron density profiles retrieved from the global positioning system radio occultation technique. Currently, there can be more than 2,000 electron density profiles acquired per day covering the global ionosphere from altitude 90 to 800 km. Utilizing the advantage of such a complete coverage, we statistically analyze how the ionospheric electron parameters NmF2, hmF2, and TEC respond to the geomagnetic index Dst for different magnetic latitudes and magnetic local time (MLT) and on quiet and storm times. A data set of 24 months is used for this study, in which most of the results focus on the low-latitude dayside regions. The results indicate that, in general, NmF2, hmF2, and TEC decrease as Dst increases at all seasons. Only during the sudden commencement phase (SSC) of storm events, NmF2 and TEC appear to increase as Dst increases.     

Global ionosphere map constructed by using total electron content from ground-based GNSS receiver and FORMOSAT-3/COSMIC GPS occultation experiment

Effects of rapidly changing ionospheric weather are critical in high accuracy positioning, navigation, and communication applications. A system used to construct the global total electron content (TEC) distribution for monitoring the ionospheric weather in near-real time is needed in the modern society. Here we build the TEC map named Taiwan Ionosphere Group for Education and Research (TIGER) Global Ionospheric Map (GIM) from observations of ground-based GNSS receivers and space-based FORMOSAT-3/COSMIC (F3/C) GPS radio occultation observations using the spherical harmonic expansion and Kalman filter update formula. The TIGER GIM (TGIM) will be published in near-real time of 4-h delay with a spatial resolution of 2.5° in latitude and 5° in longitude and a high temporal resolution of every 5 min. The F3/C TEC results in an improvement on the GIM of about 15.5%, especially over the ocean areas. The TGIM highly correlates with the GIMs published by other international organizations. Therefore, the routinely published TGIM in near-real time is not only for communication, positioning, and navigation applications but also for monitoring and scientific study of ionospheric weathers, such as magnetic storms and seismo-ionospheric anomalies.     

电离层二阶项模型的构建及其变化规律分析研究

在高精度大地测量GPS数据处理研究和应用领域,电离层二阶项延迟影响不容忽视。采用带有三角级数的曲面拟合模型构建电离层延迟二阶项模型,并结合西安市高精度GPS地面沉降监测数据,分析近五年来该区域电离层总电子含量、一阶项、二阶项延迟的半年与年变化规律及其对定位结果的影响,在电离层二阶项的研究方面获得了一些有意义的结论。     

Ionospheric climatology derived from gps occultation observations made by the ionospheric occultation experiment

The ionospheric occultation experiment (IOX) is a GPS occultation sensor with an ionospheric mission focus. IOX measurements of GPS L1 and L2 carrier phase during Earth limb views of setting GPS satellites are used together with the Abel transform to determine vertical profiles of electron density from which F-region peak parameters are determined. Data from a four and a half month period beginning in November 2001 are statistically binned and compared with a climatological model. To account for potential errors in interpretation that could arise from violation of the Abel transform assertion of spherical symmetry, the data are compared to both the climatology and to statistics of simulated ionospheric inversions using the climatological model. General characteristics of the climatology are reproduced by the occultation data. However, several significant discrepancies between the model and the data are observed during this near-solar maximum time period. In particular, average mid-latitude daytime densities are shown to be higher than the climatological prediction and the height of F2 layer in the post-sunset equatorial region is underestimated by up to 150 km.

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LEO星座参数对GPS掩星数量和时空分布影响的模拟研究

利用低地球轨道LEO(Low Earth Orbit)卫星上的星载接收机对GPS卫星进行掩星观测反演地球的大气性质,对于气象学和大气研究具有重要意义.与单颗LEO卫星相比,由多个卫星组成的LEO星座观测到的掩星事件在时空分辨率和数量上都有很大的提高.考虑特定的区域,采用数值模拟方法,对不同卫星数和不同星座参数k的玫瑰型星座在一天内所观测的掩星事件的数量和分布进行比较分析,得到相应结论.研究成果对实际发射LEO星座时相关参数的选取具有参考价值.     

Improving the Abel transform inversion using bending angles from FORMOSAT-3/COSMIC

The FORMOSAT-3/COSMIC satellite constellation has become an important tool toward providing global remote sensing data for sounding of the atmosphere of the earth and the ionosphere in particular. In this study, the electron density profiles are derived using the Abel transform inversion. Some drawbacks of this transform in LEO GPS sounding can be overcome by considering the separability concept: horizontal gradients of vertical total electron content (VTEC) information are incorporated by the inversion method, providing more accurate electron density determinations. The novelty presented in this paper with respect to previous works is the use of the phase change between the GPS transmitter and the LEO receiver as the main observable instead of the ionospheric combination of carrier phase observables for the implementation of separability in the inversion process. Some of the characteristics of the method when applied to the excess phase are discussed. The results obtained show the equivalence of both approaches but the method exposed in this work has the potentiality to be applied to the neutral atmosphere. Recent FORMOSAT-3/COSMIC data have been processed with both the classical Abel inversion and the separability approach and evaluated versus colocated ionosonde data.     

Ionospheric electron density observed by FORMOSAT-3/COSMIC over the European region and validated by ionosonde data

This research is motivated by the recent IGS Ionosphere Working Group recommendation issued at the IGS 2010 Workshop held in Newcastle, UK. This recommendation encourages studies on the evaluation of the application of COSMIC radio occultation profiles for additional IGS global ionosphere map (GIM) validation. This is because the reliability of GIMs is crucial to many geodetic applications. On the other hand, radio occultation using GPS signals has been proven to be a promising technique to retrieve accurate profiles of the ionospheric electron density with high vertical resolution on a global scale. However, systematic validation work is still needed before using this powerful technique for sounding the ionosphere on a routine basis. In this paper, we analyze the properties of the ionospheric electron density profiling retrieved from COSMIC radio occultation measurements. A comparison of radio occultation data with ground-based measurements indicates that COSMIC profiles are usually in good agreement with ionosonde profiles, both in the F2 layer peak electron density and the bottom side of the profiles. For this comparison, ionograms recorded by European ionospheric stations (DIAS network) in 2008 were used.     

Ionospheric GPS radio occultation measurements on board CHAMP

The GPS radio occultation technique is a rather simple and inexpensive tool for getting information about the global characteristics of the vertical electron density distribution. No other ionospheric sounding technique (bottomside/topside vertical sounding, incoherent scatter) unifies vertical profiling through the entire ionosphere with global coverage. The paper addresses retrieval methods and algorithms applied for the generation of operational products including their limitations in accuracy and spatial resolution. Preliminary results of ionospheric radio occultation (IRO) measurements carried out onboard the German CHAMP satellite are reported. The achieved accuracy of the retrieved electron density profiles (EDPs) is estimated in particular by comparing the IRO results with independent vertical sounding data from European stations. It is concluded that CHAMP-IRO measurements have the potential to establish global data sets of EDPs, contribute to ionospheric research, develop and improve global ionospheric models and to provide operational space weather information.     

利用GPS/MET与CHAMP掩星资料反演地球中性大气参数的比较

采用Abel积分反演算法 ,在干大气模式下 ,分别针对GPS/MET与CHAMP特定掩星事件的Level2数据进行处理 ,得到了相应的折射指数与大气参数廓线。分别将反演结果与UCAR和GFZ公布的结果进行比较 ,证明了反演算法的正确性与通用性。分析了两个温度反演结果都存在系统误差和CHAMP温度反演结果误差较大的原因 ,并提出了改进建议     

Ground- and space-based GPS data ingestion into the NeQuick model

This paper presents a technique for ingesting ground- and space-based dual-frequency GPS observations into a semi-empirical global electron density model. The NeQuick-2 model is used as the basis for describing the global electron density distribution. This model is mainly driven by the F2 ionosphere layer parameters (i.e. the electron density, N _m F2, and the height, h _m F2 of the F2 peak), which, in the absence of directly measured values, are computed from the ITU-R database (ITU-R 1997). This database was established using observations collected from 1954 to 1958 by a network of around 150 ionospheric sounders with uneven global coverage. It allows computing monthly median values of N _m F2 and h _m F2 (intra-month variations are averaged), for low and high solar activity. For intermediate solar activity a linear interpolation must be performed. Ground-based GNSS observations from a global network of ~350 receivers are pre-processed in order to retrieve slant total electron content (sTEC) information, and space-based GPS observations (radio occultation data from the FORMOSAT-3/COSMIC constellation) are pre-processed to retrieve electron density (ED) information. Both, sTEC and ED are ingested into the NeQuick-2 model in order to adapt N _m F2 and h _m F2, and reduce simultaneously both, the observed minus computed sTEC and ED differences. The first experimental results presented in this paper suggest that the data ingestion technique is self consistent and able to reduce the observed minus computed sTEC and ED differences to ~25–30% of the values computed from the ITU-R database. Although sTEC and ED are both derived from GPS observations, independent algorithm and models are used to compute their values from ground-based GPS observations and space-based FORMOSAT-3/COSMIC radio occultations. This fact encourages us to pursue this research with the aim to improve the results presented here and assess their accuracy in a reliable way.     

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.     

Ionospheric calibration of single frequency VLBI and GPS observations using dual GPS data

Summary The ionospheric effect is one of the main sources of error in Very Long Baseline Interferometry (VLBI) and Global Positioning System (GPS) high precision geodesy. Although the use of two frequencies allows the estimation of this effect, in some cases dual observations are not possible due to the available equipment or the type of observation. This paper presents the ionospheric calibration of single frequency VLBI and GPS observations based on the ionospheric electron content estimated from dual frequency GPS data. The ionospheric delays obtained with this procedure and the VLBI baseline length results have been compared with those obtained with dual frequency data. For the European geodetic VLBI baselines, both solutions agree at the 3–5 parts in 10^–9 level. The noise introduced by the GPS-based calibration is in the order of 3 cm for the VLBI observables and of 10 cm for the GPS observables.     

Use of radio-occultation data for ionospheric imaging during the April 2002 disturbances

In 1994, Hajj et al. (1994) proposed the use of radio occultation data in ionospheric imaging. The advantages gained by including this data source are examined in this paper. Many data sources including ground-based and satellite-based observations are available for the events of the April 2002 ionospheric disturbance. This period has been chosen to study simultaneous images of the disturbed ionosphere over the USA and Europe. A 4D tomographic imaging technique known as Multi-instrument Data Analysis System (MIDAS) (Mitchell and Spencer, 2003) is applied in this study. The primary purpose of the study is to compare images produced with and without the use of radio-occultation data. The work investigates whether GPS occultation combined with ground-based GPS data improves the determination of peak height and peak density in the images. The results indicate that the occultation data improve both the peak height and the peak density in the images. The use of ionosonde data is also examined and the results are compared between the USA and Europe.     

天文经纬度和天文方位角测定的基本原理

大地天文测量无论在基础科学,还是在国民经济和国防建设中都具有重要的意义。随着空间大地测量技术(VLB、ILLR、SLR和GPS等)的发展,虽然天文测量在应用上有所减少,但在大地测量的绝对定位和中远程武器的发射等领域中仍然是不可替代的技术。本文详细地介绍了天文经纬度和天文方位角测量的基本原理和方法,综述了天文测量目前的发展状况和存在的问题。本文的工作可为天文大地测量的研究提供一定的参考和依据。     

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.     

Performance comparison of semicodeless GPS receivers for LEO satellites

This report provides a detailed performance analysis of three semicodeless dual-frequency GPS receivers for use in low Earth orbit (LEO). The test set comprises the IGOR receiver, which represents a follow-on of the flight-proven BlackJack receiver, as well as two geodetic receivers (NovAtel OEM4-G2 and Septentrio PolaRx2), which are entirely based on commercial-off-the-shelf technology (COTS). All three receivers are considered for upcoming flight projects or experiments and have undergone at least a preliminary environmental qualification program. Using extensive signal simulator tests, the cold start signal acquisition, tracking sensitivity, differential code biases, raw measurement accuracy, and navigation accuracy of each receiver have been assessed. All tests are based on a common scenario that is representative of an actual space mission and provides a realistic simulation of the signal dynamics and quality on a scientific LEO satellite. Compared to the other receivers, the IGOR instrument exhibits a superior tracking sensitivity and is thus best suited for occultation measurements with low tangent point altitudes. The OEM4-G2 and PolaRx2 receivers are likewise shown to properly track dual-frequency GPS signals and normal signal levels and to provide accurate code and carrier phase measurements. Given their limited resource requirements, these receivers appear well suited for precise orbit determination applications and ionospheric sounding onboard of microsatellites with tight mission budgets.     

不同GPS掩星电离层剖面产品相关性分析

将在一定时空限定范围内的不同低轨卫星COSMIC、GRACE、CHAMP、FY3C的电离层掩星电子密度剖面定义为一个掩星对来对比分析不同类型掩星电离层产品。结果表明：COSMIC掩星对之间的电子密度剖面整体轮廓符合得很好,电子密度剖面主要在250 km以下和500 km以上存在较大的偏差,250~500 km的电子密度整体偏差较小,统计得到的COSMIC掩星对的电子密度参量NmF2和hmF2的相关系数能分别达到0.99和0.97,具有高度相关性,不同COSMIC卫星之间没有明显的系统误差;COSMIC、GRACE、CHAMP和FY3C不同低轨卫星间的电子密度剖面略有差异,通过统计电子密度参量NmF2和hmF2之间的相关系数,COSMIC和CHAMP的相关系数分别为0.95和0.86,COSMIC和GRACE的相关系数分别为0.98和0.94,COSMIC和FY3C的相关系数分别为0.96和0.92,不同掩星类型之间的电子密度参量之间也具有高度相关性,验证了不同卫星任务GPS掩星电离层剖面的一致性。