Long Baseline Precise Ionospheric Gradient Measurements and its Application to GBAS

Takayuki Yoshihara, Susumu Saito, Shinji Saitoh

Peer Reviewed

Abstract: Ionospheric delay spatial gradient is one of the most important integrity risk in Ground-based Augmentation System (GBAS) because it could produce significant positioning errors. To accurately measure ionospheric delay spatial gradients is desired in GBAS safety system design from two viewpoints, which are construction of a threat model for GBAS and development of ionospheric delay spatial gradient monitor for implementing category-III (CAT-III) GBAS ground subsystem. Regarding the gradient monitor, a typical prototype uses its GNSS reference stations with a separation of several hundred meters. Because ionospheric delay in common-satellite single difference (ISD) is larger in proportional to baseline length, it is expected that estimation accuracy of ISD is improved if random noise is the same. To expand a baseline length up to 10 km, we propose a new setup to simplify error models in a method based on single-frequency carrier-based and code-aided technique (SF-CBCA) by reduction of receiver clock contribution to single difference measurements. Namely, we used a multi-port GNSS receiver with a single receiver clock and Radio-over-Fiber (RoF) equipment to input two GNSS antenna signals received at the both ends of a baseline to the receiver. We utilized the existing facilities of optical fiber cables for proof of our concepts. From examination of fundamental characteristics, we confirmed that RoF equipment enable us to directly identify ionospheric rapid changes. Next, we examined estimation performance of ISD from a data set with a common clock in comparison with another data set without a common clock. The both data sets were collected by the same antennas with a baseline length of 2.3 km. To evaluate performance improvement with RoF equipment, we examined reliability of ISD solutions using a parameter of fix rate in ambiguity resolution process. As result, there are not so large differences between the both data sets under quiet ionosphere conditions. The same analysis was applied to the both data sets under disturbed condition, when plasma bubble events were observed. As a result, fix rate was worse in the case with a common clock against our initial expectation. Reliability analysis of ISD result without a common clock suggests there were some inconsistency solutions in comparison with the other ISD results. The results also showed further work was needed to check data quality and preform data screening for comparison under an equivalent condition.
Published in: Proceedings of the ION 2017 Pacific PNT Meeting
May 1 - 4, 2017
Marriott Waikiki Beach Resort & Spa
Honolulu, Hawaii
Pages: 885 - 896
Cite this article: Yoshihara, Takayuki, Saito, Susumu, Saitoh, Shinji, "Long Baseline Precise Ionospheric Gradient Measurements and its Application to GBAS," Proceedings of the ION 2017 Pacific PNT Meeting, Honolulu, Hawaii, May 2017, pp. 885-896. https://doi.org/10.33012/2017.15066
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