An ionospheric spatial decorrelation is one of the most dominant error factors that affects the availability of safety-critical differential global navigation satellite systems (DGNSS). This is because systems apply significant conservatism on the error source when ensuring navigation safety due to its unpredictable error characteristic. This paper investigates a correlation between GNSS-derived ionospheric spatial decorrelation and space weather intensity. The understanding of the correlation has significant advantages when modeling residual ionospheric errors without being overly pessimistic by exploiting external sources of space weather information. An ionospheric spatial decorrelation is quantified with a parameter of spatial gradient, which is an ionosphere total electron content (TEC) difference per unit distance of ionospheric pierce point (IPP). We used all pairs of stations from dense GNSS networks in the conterminous United States (CONUS) that provide an IPP separation distance of less than 100 km to obtain spatial gradient measurements under both ionospherically quiet and active conditions. Since the correlation results would be applied to safety-critical navigation applications, special attention was paid by taking into consideration all non-Gaussian tails of a spatial gradient distribution when determining spatial gradient statistics. The statistics were compared with space weather indices which are disturbance storm time (Dst) index and interplanetary magnetic field (IMF) Bz index. As a result, the ionospheric spatial decorrelation showed a significant positive correlation with both indices, especially under active ionospheric conditions. Under quiet conditions, it showed positive correlation slightly weaker than those under active conditions, and the IMF Bz showed preceding response to the spatial gradient statistics revealing the potential applicability for predicting the spatial decorrelation conditions.

https://www.mdpi.com/1424-8220/19/9/2127/pdf

Correlation between Ionospheric Spatial Decorrelation and Space Weather Intensity for Safety-Critical Differential GNSS Systems

This paper demonstrates the use of SBAS, supported by a limited number of local-area reference receivers, as the primary source of corrections and integrity verification for the use of GNSS to replace fixed transponders in the European Rail Traffic Management System (ERTMS). The recent RHINOS project has demonstrated how a trackside network of local-area differential GNSS (LADGNSS) reference receivers can provide corrections and integrity information in areas with no SBAS coverage. However, in locations where SBAS is available and provides good coverage, it can provide corrections and integrity of similar quality when supported by a much smaller network of local reference stations, making the overall ERTMS installation more cost-effective. This SBAS-based implementation is described and compared to the existing LADGNSS-based implementation developed in RHINOS.

Use of SBAS corrections with local-area monitoring for railway guidance and control applications

The concept of Satellite‐Based Augmentation System (SBAS) has its roots in the 1980s. An SBAS is a system designed to improve global navigation satellite system (GNSS) service such that the augmented service meets the strict requirements of air navigation. This chapter uses the existing Wide Area Augmentation System L1 design to describe the different SBAS monitors. The SBAS protection level equations are based upon the observation that the error sources are approximately Gaussian and that an inflated Gaussian model can be used to conservatively describe the positioning errors. The SBAS integrity parameters sent to the users are the user differential range errors to bound the ranging errors to a specific satellite and the grid ionospheric vertical errors to bound the estimated errors in the SBAS ionospheric delay model. The chapter is intended to provide an overview to assist the reader in understanding how the different message types connect together to form a differential GPS correction.

Satellite-Based Augmentation Systems (SBASs)

As an important part of the BeiDou global navigation satellite system (BDS-3), the BeiDou satellite-based augmentation system (BDSBAS) aims to provide a high-integrity navigation service for users in China and its surrounding areas. At present, BDSBAS has completed the preliminary construction of the system and formed the single frequency (SF) augmentation service. The paper introduces the composition and working process of BDSBAS, and expounds the solution and performance evaluation methods of the BDSBAS SF service. Tests based on a general aviation aircraft were carried out at Caihu Airport in Shenyang, China, which simulated the aircraft’s take-off and landing operation during the approach phase and the transition flight operation during the en-route phase. On this basis, the BDSBAS SF service performance was analyzed and evaluated. The horizontal positioning errors of the three flight tests are 1.62, 1.27, and 1.49 m, respectively, and the vertical positioning errors are 4.38, 4.30, and 4.04 m, respectively. In the first two tests, the APV-I availability and continuity of the service reach 100%. In the last test, a receiver false alarm event occurred, resulting in an APV-I availability and continuity of 99.993% and 99.989%, respectively. The event was confirmed to be caused by some aviation environmental factors and was not related to the system service. No integrity events occurred during the tests. The test results show that the positioning accuracy, availability, and integrity of the BDSBAS SF service meet the APV-I level requirements of the ICAO standards, which preliminarily illustrates the ability of BDSBAS to be used in the field of civil aviation.

/pdf/construction-progress-and-aviation-flight-test-of-bdsbas-1tflo0xx.pdf

Construction Progress and Aviation Flight Test of BDSBAS

Satellite Based Augmentation Systems (SBASs) broadcast a series of 250-bit messages that together contain a full set of differential correction and integrity information. Each message requires one second to send and contains only a small subset of the full set of required information. Different types of data have different update periods, ranging from six seconds for the satellite integrity information, to five minutes for the ionospheric corrections. A key component of an SBAS is deciding which message needs to be sent at any given second. Each message has an associated time-out period that is approximately two times the update period. It is possible to get into a situation where several messages will time out within a very short interval and it may not be possible to update all of them within the desired timeframe. Further, if there is a change in integrity status that requires an alert to be broadcast, these alert messages will preempt and delay the broadcast of otherwise intended messages. The Wide Area Augmentation System (W AAS) currently uses a dynamic message scheduler that actively determines which messages should go out over the next several seconds. As the corrections or confidence levels change, some messages may change in priority. This scheduler has been carefully tuned for the current number of GPS satellites and ionospheric grid points, but the algorithm is not adaptable should either value change appreciably. Further, with the advent of L5 messages that have their content organized very differently (and that have no ionospheric corrections) a new message scheduling algorithm is needed.

Chris Harris

Papers

WAAS at 15

WAAS at 15