Precise Measurements of Ionospheric Delay Gradient at Short Baselines Associated with Low Latitude Ionospheric Disturbances

S. Saito, S. Fujita, T. Yoshihara

Abstract:	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.
Published in:	Proceedings of the 2012 International Technical Meeting of The Institute of Navigation January 30 - 1, 2012 Marriott Newport Beach Hotel & Spa Newport Beach, CA
Pages:	1445 - 1450
Cite this article:	Saito, S., Fujita, S., Yoshihara, T., "Precise Measurements of Ionospheric Delay Gradient at Short Baselines Associated with Low Latitude Ionospheric Disturbances," Proceedings of the 2012 International Technical Meeting of The Institute of Navigation, Newport Beach, CA, January 2012, pp. 1445-1450.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In