Ionosphere Threat Space Model Assessment for GBAS
Ionosphere Threat Space Model Assessment for GBAS
复制标题
GBAS 电离层威胁空间模型评估
DOI：
--
复制
发表时间：
2009
期刊：
影响因子：
--
通讯作者：
W. Dunkel
中科院分区：
文献类型：
--
作者：
C. Mayer;B. Belabbas;N. Jakowski;M. Meurer;W. Dunkel
地基增强系统（GBAS）可以校正机场附近的飞行器所经历的大部分GNSS伪距误差。地面和机载子系统之间的未校正（空间不相关）误差必须超限，并保持尽可能小，以达到ICAO规定的要求的完整性水平。电离层梯度一般保持非常小（在美国本土区域可为4毫米/公里）。电离层的这种"正常"行为对位置误差的影响非常有限。用户位置的置信区间对于精确逼近是完全可接受的。不幸的是，电离层介质有时会受到扰动，由于电离层等离子体的强烈的时间和空间变化。当由飞行器接收的GNSS信号以与由GBAS地面设施（GGF）接收的GNSS信号不同的方式延迟时，由GGF提供的校正可能导致在飞行器水平上不可接受的大位置误差。
虽然大多数时候这种环境影响都以正常方式表现，但人们发现了一种异常的电离层行为，这种行为很少发生（10年内很少发生），但可能对GBAS的完整性构成严重威胁。CAT I GBAS架构原则上不能通过监控完全减轻这些影响。根据GBAS CAT I社区商定的方法，剩余的风险因此被处理如下：在每个时期，假设最坏情况的电离层威胁发生在100%的时间内。威胁得到缓解，例如，防止飞机使用不安全的GNSS卫星组合。
为了进行分析，必须首先界定电离层威胁空间。由于异常电离层威胁由移动的电离层锋组成，电离层威胁空间由这种电离层锋的斜率、速度和宽度所跨越。异常电离层威胁模型通过在三维威胁空间内指定一个域来定义。对于CONUS（美国接壤）地区，这一领域的威胁空间已被确定使用的经验数据收集在最后一个太阳活动周期。
为了在不同的地理区域中使用针对电离层威胁的拟议缓解算法，还必须为该区域建立异常电离层威胁模型。为了认证德国的一个GBAS地面设施，确定了包括德国在内的一个区域的异常电离层威胁空间和标称电离层去相关。这项工作是在ITMA（电离层威胁模型评估）项目中完成的，该项目是德国空中导航服务提供商DFS和德国航空航天中心DLR之间的联合项目，由DFS资助。
在该项目的第一阶段，利用本研究所涉区域上一个太阳活动周期的所有公开双频RINEX数据进行了自动数据筛选。第一阶段的目标是确定最极端的电离层事件发生的日期。当双频数据可用时，有许多方法可以提取电离层延迟。虽然使用GPS L1和L2上的相位测量值之间的差异进行数据筛选，但为了验证极端事件，我们还考虑了码减载波（CMC）和码差观测值。
在数据筛选中，我们计算了由相位差导出的电离层延迟的时间差导出的超过一定限度的（正式）空间梯度的数量。我们称这些梯度为形式的，因为它们没有校正电离层锋的运动和独立的时间变化。然而，可以利用这些正式梯度作为指标来确定电离层极端活动的时期。通过分析1998 - 2008年11年太阳活动周期的所有可用数据，使用自动筛选过程，我们确定了16个时间段的相关电离层活动。所有这些事件都经过人工检查，以排除由以下原因引起的假警报：周跳或数据损坏。
在该项目的第二阶段，开发了估算异常电离层威胁模型中参数的算法，即电离层锋的斜率、速度和宽度。为了估计关键的威胁模型参数，必须确定电离层锋的斜率、电离层锋的速度和方向。为了确定波前速度，我们使用了基于最小二乘的估计技术和一种更直接的方法，即直接从（校准的）电离层延迟计算速度和斜率。
对于电离层异常活动的每个时期，我们都手动确定了极端事件的时间和位置。已利用德国大地测量参考网（SAPOS）提供的关于所确定地点的其他数据来提高空间分辨率。然后，针对每个事件，估计了威胁模型参数。此外，为了验证目的，除了相位差导出的电离层延迟之外，还使用了CMC和代码差可观测量。结果，得出了一个对中欧特别是德国有效的威胁空间。
除了极端的电离层行为，这是捕获的异常电离层威胁模型，我们还确定了标称电离层梯度在所考虑的区域使用校准的垂直电离层延迟从安静到中等电离层活动的时期。
本文将详细介绍电离层威胁空间确定的派生方法以及在地理区域"德国"取得的成果。这些结果为欧洲GBAS系统的进一步参数化和认证提供了必要的投入。
Ground Based Augmentation Systems (GBAS) can correct the majority of the GNSS pseudo range errors experienced by an aircraft in the vicinity of an airport. Not corrected (spatially uncorrelated) errors between ground and airborne subsystems must be overbounded and kept as small as possible in order to reach the required level of integrity defined by ICAO. Ionosphere gradients remain in general very small (can be bounded by 4mm/km in the CONUS region). This "normal" behavior of the ionosphere has a very limited impact on the position error. The confidence interval of the user position is fully acceptable for precision approach. Unfortunately, the ionosphere medium is sometimes subject to perturbations due to the strong temporal and spatial variability of the ionospheric plasma. When GNSS signals received by the aircraft are delayed in a different way than the GNSS signals received by the GBAS ground facility (GGF), the corrections provided by the GGF can cause unacceptably large position errors at aircraft level.
While most of the time this environmental effect is behaving in a normal way, there has been found an anomalous ionospheric behavior which occurs rarely (few occurences in 10 years) but can be a serious threat to GBAS integrity. The CAT I GBAS architecture can principally not fully mitigate these effects by monitoring. According to the agreed approach in the GBAS CAT I community, the remaining risk is therefore treated as follows: At each epoch the worst-case ionospheric threat is assumed to occur in 100% of the time. The threat is mitigated, e.g., by preventing the aircraft from using unsafe combinations of GNSS satellites.
To permit the analysis, it is essential to first define the ionosphere threat space. Since the anomalous ionosphere threat consists of moving ionospheric fronts, the ionosphere threat space is spanned by the slope, velocity and width of such an ionospheric front. The anomalous ionosphere threat model is defined by specifying a domain inside the three-dimensional threat space. For the CONUS (conterminous US) region this domain in threat space has been determined by using empirical data collected during the last solar cycle.
In order to use the proposed mitigation algorithms for the ionosphere threat in a different geographical region, the anomalous ionosphere threat model has to be established for that region as well. For the certification of a GBAS ground facility in Germany, both the anomalous ionosphere threat space and the nominal ionospheric de-correlation for a region including Germany were determined. This work has been done within the ITMA (ionosphere threat model assessment) project which is a joint project between the German Air Navigation Service Provider DFS and the German Aerospace Center DLR, funded by DFS.
In the first phase of the project, an automated data-screening has been performed using all publicly available dual-frequency RINEX data of the entire last solar cycle in the region considered in this study. This first phase aims to identify the days of most extreme ionospheric events. When dual-frequency data is available, there are a number of ways of extracting the ionospheric delays. While for the data-screening the difference between the phase measurements on GPS L1 and L2 were used, we also consider code minus carrier (CMC) and code-difference observable in order to validate extreme events.
In the data-screening we computed the number of (formal) spatial gradients above certain limits derived from time-differences of phase-difference-derived ionospheric delays. We call these gradients formal, since they are not corrected for the movement of the ionospheric front and independent temporal changes. Nevertheless periods of extreme ionospheric activity can be identified using these formal gradients as an indicator. By analyzing all available data from a complete 11 years solar activity cycle period (1998-2008) using an automatic screening process, we have determined 16 time periods of relevant ionospheric activity. All these events have been examined manually in order to exclude false alarms caused, e.g., by cycle slips or corrupted data.
In the second phase of the project, algorithms have been developed for estimating the parameters in the anomalous ionosphere threat model, i.e. the slope, speed and width of a ionospheric front. In order to estimate the key threat model parameters, the slope of the ionospheric front, the velocity and direction of the ionospheric front have to be determined. For determining the front velocity, we used both a least-squares-based estimation technique and a more direct way of computing the velocity and slope directly from (calibrated) ionospheric delays.
For each period of anomalous ionospheric activity we have manually identified the time and location of extreme events. Additional data from the German geodetic reference network (SAPOS) for the determined locations has been used to increase the spatial resolution. Then, for each event threat model parameters have been estimated. Again, CMC and code-difference observables were used in addition to phase-difference-derived ionosphere delays for validation purposes. As a result, a threat space valid for the mid Europe especially Germany was derived.
In addition to the extreme ionosphere behavior, which is captured by the anomalous ionosphere threat model, we have also determined the nominal ionospheric gradients in the considered region using calibrated vertical ionospheric delays from periods of quiet to moderate ionospheric activity.
The paper will give a detailed insight in the derived methods for ionospheric threat space determination and the achieved results for the geographical region "Germany". The results act as an essential input for further parameterization and certification of GBAS systems in Europe.