Precise Measurements of Ionospheric Delay Gradient at Short Baselines Associated with Low Latitude Ionospheric Disturbances
Precise Measurements of Ionospheric Delay Gradient at Short Baselines Associated with Low Latitude Ionospheric Disturbances
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发表时间：
2012-02
期刊：
影响因子：
1.6
通讯作者：
S. Saito;S. Fujita;T. Yoshihara
中科院分区：
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作者：
S. Saito;S. Fujita;T. Yoshihara
对于天基增强系统（SBAS）或地基增强系统（GBAS）等差分GNSS系统，电离层延迟的空间不均匀性（或空间梯度）是一个关键问题。在设计安全和可用的系统时，需要界定描述电离层变化可能范围的所谓电离层威胁模型。以适当的方式设计威胁模型是一个非常重要的问题。在低纬地区，电离层扰动的主要形式是等离子体泡，而在中纬地区则是暴增强密度。等离子体泡是一种局部的电离层损耗，在其边缘具有非常陡峭的梯度。然而，与等离子体泡相关的电离层梯度的行为并不为人所知，因为与等离子体泡相关的梯度的测量是不够的。因此，研究等离子体泡的电离层延迟梯度对于用真实的观测数据验证威胁模型具有重要意义。与等离子体气泡相关的梯度通常具有几十公里的尺度大小。在短基线的情况下，通常的双频电离层延迟估计方法很难准确地校正两个接收机之间的接收机频间偏差。此外，L2信号更容易受到通常与等离子体气泡相关联的闪烁的影响。为了克服这些困难，我们使用了由[Fujita et al.，2010年]。该方法是基于载波相位测量，并辅以码测量。它实现了电离层延迟差小于10 mm的测量精度。正在日本石垣岛（北纬24.3度，东经124.2度）进行观测。在石垣设置了5个接收器，间隔为0.4至1.0公里。每个台站都配备了一个双频全球导航卫星系统接收器（NovAtel Euro-3），经改装后可进行2赫兹采样。对2008年4月和2010年10月获得的数据进行了分析，以使用Fujita等人开发的新的基于单频载波相位的方法估计与等离子体泡相关的电离层延迟梯度，2010年]。我们成功地估计电离层延迟的差异与短基线从0.4到1.6公里，在L1频率的高精度为几毫米。虽然在分析期间太阳活动很低，但经常观察到与等离子体泡有关的电离层延迟梯度高达200毫米/公里，这是有把握地估计的。在分析期间发现的电离层延迟梯度在II/III类GBAS开发标准中使用的威胁空间范围内。然而，有一些事件梯度可能很大，但由于分析失败而无法验证。该方法需要改进。考虑到太阳活动的增加，甚至更大的梯度也可能被探测到。分析了观测梯度与卫星指向的关系。最大梯度和大梯度事件的数量都有很强的卫星方向依赖性。在接收器的椭圆平面内和低仰角处往往会观察到大的梯度。这与等离子体泡的三维结构的预期是一致的，与中纬度SED的特征有很大不同。也有一个非常大的梯度在一个相对较高的仰角（45？）这需要进一步调查对于即将到来的太阳活动高峰，继续进行观测和分析以验证电离层威胁模型是重要的。
For differential GNSS systems such as the space-based augmentation system (SBAS) or the ground-based augmentation system (GBAS), spatial inhomogeneity (or spatial gradient) in ionospheric delay is a critical problem. In designing safe and available systems, so-called ionospheric threat models that describe possible ranges of ionospheric variability need defined. Designing a threat model in an adequate way is a very important issue. In the low latitude region, the dominant ionospheric disturbance is the plasma bubble which is different from that in the mid-latitude region, the storm enhanced density. Plasma bubble is a local ionospheric depletion with very steep gradients at its edges. The behavior of the ionospheric gradient associated with plasma bubble is, however, not well known, because measurements of gradients associated with plasma bubble are not enough. Therefore, it is important to investigate the ionospheric delay gradient associated with plasma bubbles for validating the threat models with real observation data. The gradient associated with plasma bubbles typically have a scale size of a few 10 km. In such short base line occasions, usual dual-frequency ionospheric delay estimation is not accurate enough, because it is difficult to calibrate the receiver inter-frequency bias between two receivers with necessary accuracy. Furthermore, L2 signal is more susceptible to scintillation that is usually associated with plasma bubble. To overcome these difficulties, we used single frequency- based short baseline ionosphere delay difference estimation method developed by [Fujita et al., 2010]. The method is based on carrier-phase measurements and is aided by code measurements. It achieves a measurement accuracy of less than 10 mm in the ionospheric delay difference. Observations are being conducted on Ishigaki Island (24.3N, 124.2E), Japan. Five receivers were set up in Ishigaki with separation of 0.4 to 1.0 km. Each station is equipped with a dual-frequency GNSS receiver (NovAtel Euro-3) modified for 2 Hz sampling. The data obtained in April 2008 and October 2010 were analyzed to estimate the ionospheric delay gradient associated with plasma bubbles with the new single-frequency carrier phase-based method developed by [Fujita et al., 2010]. We succeeded in estimating the ionospheric delay differences with short baselines from 0.4 to 1.6 km with high precision of several millimeters at the L1 frequency. Slant ionospheric delay gradients as large as 200 mm/km that were estimated with confidence were frequently observed associated with plasma bubbles, although the solar activity was low during the periods of analysis. Ionospheric delay gradients found in the periods of analysis were within the range of the threat space used in the development standards for the Category-II/III GBAS. However, there were a number of events where the gradients could be large but could not be verified due to analysis failure. The method needs improvement. Considering the increasing solar activity, even larger gradients may well be detected. The relationship between the observed gradient and the satellite direction was analyzed. Both the maximum gradients and the number of large gradient events had a strong satellite direction dependence. The large gradients tend to be observed in the meridional plane of the receiver and at low elevation angles. This is consistent with the expectation from the three-dimensional structure of the plasma bubble and very different from the characteristics of the mid-latitude SED. There was also a single event of very large gradient at a relatively high elevation angle (45?) which needs more investigation. For the coming peak of the solar activity, it is important to continue observation and analysis to validate the ionospheric threat model.