Testing the Galileo High Accuracy Service (HAS) at a High Latitude and under Ionospheric Scintillation

In this article, the results obtained assessing the Galileo HAS performance at high latitudes in a period of intense ionospheric activity during the ongoing 25th solar cycle are presented

Melania Susi Ph.D. in Engineering Surveying and Space Geodesy from the University of Nottingham, UK, where she was also a Marie Curie fellow. After being a scientific technical officer at the Joint Research Centre (JRC) of the European Commission she is now a GNSS senior researcher at Topcon in Concordia, Italy

Daniele Borio Currently a scientific technical officer in the JRC Food Security Unit where he is supporting the European Common Agricultural Policy (CAP) through the European satellite programs, Galileo and Copernicus

Althaf Azeez Postdoctoral researcher in the Geomatics group in Oregon State University. His research interests are highaccuracy PPP algorithms, integrating GNSS with other sensors like IMU, cellular, etc., to increase resilience of the positioning solution, and GNSS remote sensing

Jihye Park Associate Professor of Geomatics at Oregon State University. Dr. Park holds a PhD in Geodetic Science and Surveying from The Ohio State University

Kinga Wezka Research and teaching assistant at the Faculty of Geodesy and Cartography of Warsaw University of Technology. Her research interests are focused on satellite navigation, precise positioning and GNSS-based ionospheric monitoring

Ignacio Fernandez Hernandez Works at the European Commission, where he has led the design and development of Galileo high accuracy and authentication services over the last years. He has a PhD degree in Electronic Systems from Aalborg University. He also is a visiting professor at KU Leuven, and the recipient of ION’s 2021 Thurlow Award

Introduction

From the start of 2023, the Galileo High Accuracy Service (HAS) is officially transmitting free Galileo and GPS Precise Point Positioning (PPP) corrections worldwide through the Galileo E6B signal. In addition, HAS corrections are also disseminated via the internet for receivers which do not support E6B.

Already in 2022 valid corrections were transmitted through the Galileo E6B signal for testing purposes. Therefore, since last year an intense research activity has been conducted to assess HAS performance using the live signals/corrections showcasing the effectiveness of the service in terms of dissemination capabilities [1] quality of the corrections [2] and position performance [3-5]. The majority of the work focused on the assessment of the service under nominal conditions and at mid-latitudes. In this work, we analyse the HAS performance at high latitudes during a day characterised by intense ionospheric activity. The article is based on the results presented during the European Navigation Conference (ENC) 2023 [6].

High latitudes can represent a harsh environment for high-accuracy applications due to the reduced visibility of satellites at a high elevation and the occurrence of ionospheric scintillation that can affect the signal quality. Indeed, when a radio wave travels through the ionosphere, the presence of irregularities can lead to signal refraction and/or diffraction. These effects can produce phase and amplitude scintillation, namely random fluctuations of the signal phase and/or amplitude. This phenomenon occurs mainly at high latitudes and in the equatorial regions. At high latitudes, the occurrence of scintillation is mainly driven by geomagnetic activity and generally amplitude scintillation is almost absent while phase scintillation can be significant. Phase scintillation can strongly degrade the quality of the signal carrier phase, leading to cycle slips, tracking loss of lock and deteriorating the carrier-based position solutions, such as the ones obtained through PPP [7].

We are currently in the 25th solar cycle, which started in 2019, and going forward the solar cycle maximum, foreseen for 2025/2026. We are also assisting to an enhancement of the ionospheric activity which is also above the forecasts [8]. Therefore, considering the solar activity increase, it is of particular interest to assess the HAS performance under scintillation.

For this purpose, data collected at the Polish Polar Station in Hornsund at the Svalbard are used. The station is equipped with an Ionospheric Scintillation Monitoring Receiver (ISMR) that allows monitoring the occurrence of scintillation events. Moreover, the receiver has been set to record the Galileo E6B navigation message in order to be able to retrieve HAS corrections from the same receiver providing the measurements for the positioning assessment. Therefore, data demodulation is performed in the same challenging conditions. While a first analysis of the HAS demodulation performance at high latitudes was performed in [9], only quiet ionospheric conditions were considered.

In this study, we present an assessment in terms of orbit/clock corrections quality and position performance. For the analysis, the corrections were extracted using an in-house developed correction parser and then applied to the broadcast data, properly formatted and sent as input to a PPP algorithm to assess the positioning performance. The article is structured as follows. At first, we describe the methodology adopted to demodulate and apply HAS corrections to broadcast data in order to carry out an HAS-based PPP processing. Then the experimental setup is introduced. Experimental results of the PPP analysis are then described. Finally, the conclusions are presented.

Decoding and applying HAS corrections

The HAS corrections, which are transmitted through the live E6B signal, are encoded through a high parity vertical Reed Solomon (RS) code. The HAS message pages are vertically stacked and multiplied by the Reed-Solomon encoding matrix in order to generate a redundant set of pages. Then, each Galileo satellite broadcasts a subset of such pages, which are currently stored in binary format by receivers supporting E6B processing. In order to retrieve the HAS corrections from the received binary pages, a decoder has been developed in Python [10]. The decoder is now provided as open-source tool and available on github (https:// github.com/borioda/HAS-decoding). The decoder, denoted as Galileo HAS Parser (GHASP), is able to process binary files from different receiver manufacturers, such as Septentrio, Novatel and Javad. The output of GHASP is a set of Comma-Separated Values (CSV) files containing HAS orbit and clock corrections, code and phase biases. The tool can be used through a user-friendly Graphical User Interface (GUI), which offers plotting options as well. In Figure 1, the full processing chain adopted in this work is illustrated: from the reception of the Galileo E6B signal to the demodulation and application of HAS corrections.

The corrections have been applied to the broadcast data as indicated in [11]. For this purpose, a dedicated script was developed to apply the HAS corrections to the broadcast data and generate Standard Product 3 (SP3) files that can be easily processed by any commercial and open-source PPP software. In this study, the Oregon State University PPP (ORPPP) engine, developed by the Oregon State University, was used.

The ORPPP software exploits ionosphere- free linear combinations of GNSS measurements and it estimates the unknown vector composed of the position, clock error, zenith wet delay, float ambiguity parameter, differential code biases, and intersystem bias. An adaptive robust Kalman Filter (KF) is used to estimate the unknown parameters [12]. Tidal corrections, phase wind-up, and relativistic corrections are applied as well.

A schematic representation of the processing adopted by ORPPP engine is provided in Figure 2, which also shows the retrieval and integration of the corrections extracted by the GHASP decoder.

Experimental set-up

The data were collected by the WUTH GNSS reference station located in the Polish Polar Station Hornsund (77.00°N, 15.54°E) on Spitsbergen in the Norwegian Svalbard archipelago. Figure 3 shows the the antenna location while an aerial view of the reference station location is provided in Figure 4. Figure 5 shows the map of the Svalbard archipelago indicating the location of the station with the related coordinates.

The WUTH GNSS station is operated by the Warsaw University of Technology and plays an important role in interdisciplinary research like analysis of GNSS signals at high latitudes, monitoring of geodynamic movements (isostatic movements of the Earth’s crust), Earth atmosphere monitoring and indirect climate change monitoring (glacier movement, glacier mass balance). Furthermore, the WUTH station is included in international GNSS networks such as the International GNSS Service (IGS), the European Reference Frame (EUREF) Permanent GNSS Network (EPN) and European Plate Observing System (EUPOS).

The station is equipped with a high-precision reference Septentrio receiver, a PolaRx5S, connected to a SEPCHOKE_B3E6– SPKE choke-ring antenna. The receiver is set to track the full spectrum of current GNSS signals, and it is logging highfrequency (50 Hz) measurements for ionospheric analysis. Indeed, the receiver is an ISMR embedding an ultra-low phase noise Oven Controlled Crystal Oscillator (OCXO). The use of an OCXO ensures that the oscillator phase noise does not exceed the effects of low-level phase scintillation.

The high rate of data sampling and the low noise clock allows for monitoring of the amplitude and phase fluctuations due to ionospheric scintillation. Moreover, for this research, the receiver has been set to record the E6B Galileo navigation messages, in order to be able to demodulate the HAS corrections from the same receiver providing the observations and, consequently, under the same harsh conditions.

Data

The data analysed in this paper were collected from the WUTH station on the 4th of September 2022 (DOY 247). The day was selected for the occurrence of very strong ionospheric activity. In Figure 7 the sky plot observed from the WUTH station for the full selected day is reported. It can be observed that the maximum satellite elevation is around 60 degrees for the entire day. Despite the reduced maximum satellite elevation, good PDOP/HDOP and VDOP values are recorded for the combined GPS + Galileo case (i.e. below 1.5), as shown in Figure 7. In Figure 8 , the level of scintillation is shown through the Phi60 scintillation index reported by the ISMR during a period of the day with strong ionospheric event. As the name suggests, the Phi60 estimates the level scintillation by computing the standard deviation of the detrended carrier phase measurement, averaged over one minute of data [13].

For the analysis, we focused on the interval between 15:17 and 19:00 that was affected by very strong phase scintillation. More specifically, we can observe a very strong phase scintillation peak around 15:20. Moreover, scintillation of weak /moderate intensity is present until the end of the observation period. In the literature slightly different Phi60 thresholds are used to classify different levels of phase scintillation, for example considering different locations. Based on [14] and [15] the values of Phi60 corresponding to different levels of scintillation are reported in Table 1.

Results

The quality of the HAS orbits and clocks was assessed by comparing them with the final precise products downloaded from the Center for Orbit Determination in Europe (CODE). Summary statistics (95th percentile) of the residual errors observed are provided in Table 2 for Galileo and GPS.

The corrections, after proper formatting were then used as input for the PPP processing. PPP processing was set to start just before the very strong scintillation event discussed above. The GPS L1C/A-L2 P and Galileo E1-E5b configuration was selected for the processing. Moreover, the PPP has been configured in static mode. The horizontal and vertical position errors are shown in Figure 9. With respect to the results presented at ENC, in this article, we assess the PPP algorithm by starting the processing even closer to the maximum scintillation peak to further challenge the PPP engine. The processing started at 15:17 only almost 3 minutes before the maximum scintillation peak that occurred at about 15:20. This represents a very challenging and extreme scenario since the PPP convergence starts in the middle of the very strong scintillation period. As expected, this choice impacts the convergence time which is of about 20 minutes. Moreover, it should be underlined that no HAS phase biases have been used, since they were not available yet at the time of the data collection. The engine was set to provide a float PPP solution, so no ambiguity resolution was performed to speed up the convergence. After the convergence the positioning accuracy is well below the maximum target values of 20 and 40 centimetres for the horizontal and vertical cases, respectively.

The error is stable below the target values even if moderate/ strong scintillation is still present until the end of the observation period. For comparison, the horizontal and vertical errors are reported in Figure 10 for a period of quiet ionospheric conditions during the same testing day. In this case, the target accuracy is achieved after only 230 seconds (less than four minutes). After convergence, the positioning accuracy is also in this case well below the maximum target values.

Conclusions

Phase scintillation can strongly affect the quality of the signal carrier phase which is crucial for PPP positioning algorithms used by HAS users. In this article, we presented the results obtained assessing the Galileo HAS performance at high latitudes in a period of intense ionospheric activity during the ongoing 25th solar cycle. We analysed a particular day (4th September 2022) when very strong phase scintillation was recorded, and tested PPP convergence using Galileo HAS data applied to GPS and Galileo. The PPP processing was started in the middle of a very strong scintillation event, only few minutes before the maximum scintillation peak. This represents a very extreme event, that as expected, leaded to an increase of the converg time. Despite the very strong phase scintillation event, HAS achieved the target position performance after convergence in a stable way even if scintillation persisted until the end of the observation period. Further work may include a statistical characterization of the convergence time during scintillation (of which only one occurrence is presented here), and using more recent HAS data, particularly after service declaration in January 2023, from which a slightly higher performance is expected.

Acknowledgement

This article is based on a paper published in the ENC Proceedings and reported in the references [6]. The operation of the WUTH GNSS station is supported by the Warsaw University of Technology within the Excellence Initiative: Research University (IDUB) programme.

Reference

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