# Ionospheric Remote Sensing with GNSS
## 4 Ionospheric Remote Sensing
According to [24], ionospheric irregularities are \"small-scale structures in the ionospheric plasma density generally oriented so that the plasma density variations occur rapidly across the geomagnetic field but slowly (or not at all) along the geomagnetic field\". Two ionospheric scintillation indices are used to quantify the scintillation severity:  
Figure 2: A global VTEC map from MIT Madrigal world-wide GPS receiver network [23]. (a) S4 (amplitude scintillation index): the ratio of the standard deviation of the signal power to the average signal power computed over a period of time (typically 1 min) as defined by [25]:  
\[S4=\sqrt{\frac{\left\langle A^{2}\right\rangle-\left\langle A\right\rangle^{2}}{ \left\langle A\right\rangle^{2}}} \tag{10}\]  
where \(A\) is the signal intensity or power, and \(\left\langle\right\rangle\) denotes ensemble averaging (\(\approx\)time averaging). (b) sigma phi or \(\sigma_{\phi}\) (phase scintillation index): the standard deviation of \(\sigma\) in radians, where \(\sigma\) is the refractive component of the GNSS signal phase as defined by [25] and \(\phi_{det}\) is the detrended phase. \[\sigma_{\phi}=\sqrt{\left\langle\phi_{detr}^{2}\right\rangle-\left\langle\phi _{detr}\right\rangle^{2}} \tag{11}\]  
Ionospheric irregularities may simultaneously lead to GNSS ranging errors (TEC delay) and GNSS signal phase scintillations. The relationship between the GPS phase scintillations and TEC variation can be expressed as (described in detail in [26]):  
\[\Delta\text{TEC}=\frac{0.75f_{\text{L1}}\Delta\phi_{\text{L112}}}{f_{\text{L1 }}^{2}/f_{\text{L2}}^{2}-1} \tag{12}\]  
where the differential carrier phase between L1 and L2 frequency bands (\(\Delta\phi_{\text{L112}}\)) is proportional to TEC variation (\(\Delta\)TEC). As shown in Figure 3, an example GPS scintillation event observed at the Antarctic McMurdo scintillation Station from MIT Madrigal contains both high amplitude and phase scintillation measurements. The background color represents VTEC level in a similar way in Figure 2, and the red circles represent S4 in Figure 3a and sigma phi (\(\sigma_{\phi}\)) in Figure 3b. On 7 January 2014, the polar ionospheric irregularities and density structures in the southern polar region induced by an incoming solar storm caused an observation of this scintillation event (with relatively high S4 and \(\sigma_{\phi}\)) using ground-based GPS receivers. Figure 3: _Cont._  
GNSS is widely used to measure S4 and \(\sigma_{\phi}\) in order to observe and study the associated ionospheric irregularities. GNSS phase scintillations can cause cycle slips in carrier-phase and put pressure on the tracking loops of GNSS receivers. Severe GNSS scintillations can even lead to GNSS receiver loss-of-track and thus reduce positioning accuracy and availability. A great number of ground-based receivers are deployed in different regions around the world to detect and measure ionospheric space weather including the plasma irregularities that disturb GNSS signals. For instance, the chain of autonomous adaptive low-power instrument platforms (AAL-PIP) [28] on the East Antarctic Plateau has been used to observe ionospheric activity in the South Polar region. Together with six ground-based magnetometers, four dual frequency GPS receivers of the AAL-PIP project have been used to capture ionospheric irregularities and ultra-low frequency (ULF) waves associated with geomagnetic storms by analyzing the GPS TEC and scintillation data collected in Antarctica [29]. Furthermore, the ESA Space Weather Service Network is hosting several ionospheric scintillation monitoring systems developed by the German Aerospace Center (DLR), Norwegian Mapping Authority (NMA), and Collecte Localisation Satellites (CLS) [30]. Figure 4 gives a high-level illustration of two ionospheric impacts on GNSS--ranging errors and scintillation. Figure 4: An illustration of ionospheric impacts on GNSS. Figure 3: An example GPS scintillation event observed at the Antarctic McMurdo scintillation Station from MIT Madrigal.