ZHU Xuefen, LIN Mengying, LU Zhengpeng, CHEN Xiyuan

(Key Laboratory of Micro-Inertial Instrument and Advanced Navigation Technology of Ministry of Education,School of Instrument Science and Engineering, Southeast University, Nanjing 210096, China)

Abstract: To detect the occurrence of ionospheric scintillation in the equatorial region, a coherent/non-coherent integration method is adopted on the accumulation of intermediate frequency (IF) signal and local code, in the process of signal acquisition based on software receiver. The processes of polynomial fitting and sixth-order Butterworth filtering are introduced to detrend the tracking results. Combining with ionospheric scintillation detection algorithm and preset thresholds, signal acquisition and tracking, scintillation detection, positioning solution are realized under the influence of strong ionospheric scintillation. Under the condition that the preset threshold of amplitude and carrier phase scintillation indices are set to 0.5 and 0.15, and the percentage of scintillation occurrence is 50%, respectively, PRN 12 and 31 affected by strong amplitude scintillation are detected effectively. Results show that the positioning errors in the horizontal direction are below 5m approximately. The software receiver holds performances of accurate acquisition, tracking and positioning on the strong ionospheric scintillation conditions, which can provide important basis and helpful guidance for relevant research on ionospheric scintillation, space weather and receiver design with high performance.

Keywords: Ionospheric Scintillation, Acquisition and Tracking; Positioning Error, Coherent/Non-coherent Integration, Detrending

1 Introduction

With the development and application of GPS and navigation technology, it is increasingly important to improve the security and reliability of satellite navigation systems. The observation and study of ionospheric scintillation, as a medium for signal propagation, which can produce a negative impact on the quality of the received global navigation satellite systems (GNSS) signal[1], has become one of the important contents and hotspots in space weather. In the ionosphere which is about 60 ～ 1000 kilometers from the ground, the atmosphere interacts with high-energy particles from the sun, making atmospheric molecules and atoms charged particles with uneven density distribution, and the ionospheric scintillation is a phenomenon that the amplitude and phase of signals fluctuate randomly when signals propagate through the plasma irregularly[2-3]. There are many reasons for this phenomenon, including magnetic storms, solar activity, local electric field, electrical conductivity, etc. It is also closely related to the geographical area, season, local time, geomagnetic activity[4], etc.

Ionospheric scintillation occurs mainly in the equatorial and polar regions[5]. The scintillations in equatorial region are mainly amplitude scintillation and in the polar region are mainly phase scintillation.The frequency and intensity of ionospheric scintillation will increase in solar maximum year. Strong ionospheric scintillation will decrease signal quality and even cause signal tracking failure and make the receiver fail in navigation in serious cases[6-8].Especially in the low latitudes during the solar maximum year, frequent and strong ionospheric scintillations lead to significant attenuation in intensity and carrier-to-noise ratio of signal[9-10]. Due to the suddenness of ionospheric scintillation, frequent and severe ionospheric scintillation makes detection and satellite signal acquisition and tracking more difficult,especially in the equatorial regions.

In this paper, the software receiver is used to detect and analyze the strong ionospheric scintillation in the equatorial regions. The results of analysis will provide powerful guidance for designing high-performance tracking algorithms and improving the sensitivity and robustness of satellite receivers. The South China Sea is a low-latitude and equatorial region in China. It is of great significance to design efficient ionospheric scintillation detection and high-performance receiver algorithms for the practical application of the Beidou navigation system and the territorial security of the South China Sea.

2 Ionospheric Scintillation Indices

Frequent strong ionospheric scintillations can easily affect the acquisition and tracking of satellite signals, and even lead to positioning and navigation failures in low-latitude areas in solar maximum year. In order to realize the accurate detection and acquisition of strong ionospheric scintillations in the equatorial regions, the coherent/non-coherent integration algorithm[11]is designed to improve the signal-to-noise ratio of the signal.

Considering the influence of noise signal, the satellite signal is processed by the receiver front-end and quantified by intermediate frequency sampling.Then the GNSS digital signal is correlated with the local code and the local carrier signal, and then two signals of the same phase component I and the quadrature component Q are obtained.

In order to eliminate noise, we use the sixth-order Butterworth high-pass filter[12]with a cutoff frequency of 0.1 Hz to detrend the carrier phase φ and finally obtain φtrend. The six-order filter is composed of 3 cascaded second-order filters to improve the stability of the filter.

Fig.1 shows the time-varying information of the carrier phase, which is collected from GPS satellite No.25 in the equatorial regions of Brazil on November 27,2013. The right side of the longitudinal axis represents the original carrier phase value, and the left side of the longitudinal axis represents the corresponding detrended result of carrier phase by the sixth-order Butterworth at the same period.

Fig.1 Comparison of Butterworth Filtering before and after

In the shadow region, we can clearly observe that the detrended carrier phase fluctuates over time, indicating that the signal is affected by ionospheric scintillation.

3 Detection System of Strong Ionospheric Scintillation

Ionospheric scintillation is a natural phenomenon that it is difficult to predict the scintillation. In order to save data storage space, we combine the full-band receiving antenna with the commercial Septentrio receiver to monitor ionospheric scintillation. The acquisition system will be triggered to store data when a strong ionospheric scintillation is detected.

The software receiver receives the real-time satellite signal. After successfully tracking the signal,the software receiver collects the output and carrier phase data of I and Q channels, and then calculates the scintillation indices and judgment index of scintillation event in real time. The judgment index is compared with the set threshold.

If the trigger of the scintillation event is reached,the acquisition system will collect the multi-channel RF front-end data which is defined by the software and that data is the original IF data affected by ionospheric scintillation. The specific acquisition system is shown in Fig.2.

Fig.2 Data Collection System Driven by Ionospheric Scintillation

Because of the strong ionospheric scintillation in the equatorial regions, we adopt the coherent/noncoherent integration acquisition algorithm to improve the signal-to-noise ratio and achieve the purpose for accurate acquisition.

Compared with the serial search algorithm and the cyclic correlation algorithm[13], we adopt the coherent integration to improve the signal-to-noise ratio effectively, and then adopt the non-coherent integration to remove the limitation that the coherent integration time is easily affected by navigation data bit-flipping.Therefore, we can accurately capture satellite signals in a weak signal-to-noise ratio environment, which provides basic data for ionospheric scintillation detection.

For amplitude scintillation and phase scintillation,two different detrending methods, polynomial fitting and sixth-order Butterworth filtering, are taken to eliminate the influence of satellite motion, clock error and Doppler. By setting the scintillation threshold and recording the number of simultaneous occurrences of amplitude and phase scintillations, we successfully determine the occurrence of scintillations. The schematic diagram of ionospheric scintillation detection and analysis in software receiver is shown in Fig.3.

Fig.3 Principle of Ionospheric Scintillation Detection

4 The Analysis of Ionospheric Scintillation Detection Data

Solar maximum year takes 11 years a cycle.Compared with other years, there is a frequent and strong ionospheric scintillation phenomenon in the Solar maximum year, and the latest year is 2013.Therefore, in order to test the feasibility of the designed algorithm under strong ionospheric scintillation mentioned in this paper, the data was collected by the data acquisition system deployed in the equatorial regions of Brazil on November 27, 2013.The specific location is shown in Fig.4:

Fig.4 Location of the Data Collection System on the Map

In MATLAB, we select the intermediate frequency signal of GPS L1 band recorded on November 27. We use the raw algorithm in GPS software receiver and acquisition algorithm improved by coherent/non-coherent integration to simulate the recorded data. The acquisition results corresponding to the two methods are shown in Fig.5.

Fig.5 Acquisition under the Influence of Strong Ionospheric Scintillation

Firstly, we set the acquisition threshold as 1.75.Satellite signals are seriously disturbed under the strong ionospheric scintillation, and the GPS software receiver with raw algorithm fail in capturing Satellite signal. As shown in Fig.5 (a).

The GPS receiver improved by the coherent/non-coherent integration algorithm captured 7 satellites, which were satellites 5, 12, 18, 21, 25, 29 and 31. As shown in Fig.5 (b).

In the case of successful acquisition, we track the signal. Fig.6 shows the tracking results of the instantaneous code phase and quadrature code phase of No. 25 satellite.

Fig.6 Tracking Results of GPS 25

Taking the tracking results of the captured satellite signal in GPS L1 band as an example, we detect the ionospheric scintillation within 1 hour, and discard the unstable data in the first 10 s, and the output frequency of scintillation indices is 1 Hz. The results are shown in Fig.7. Fig.7 (a) is the time-varying amplitude scintillation of GPS satellites 12, 18, 25 and 31, and Fig.7 (b)is the corresponding phase scintillation. When the amplitude scintillation indices S4 satisfies S4 < 0.2, it can be defined that there is no ionospheric scintillation;the amplitude scintillation indices satisfying 0.2< S4 <0.5 is defined as weak scintillation; the amplitude scintillation indices satisfying S4 > 0.5 is defined as strong scintillation.

Fig.7 Ionospheric Scintillation Detection of GPS L1 Frequency

The red dotted line in Fig.7 (a) represents S4 = 0.5,and the moment that exceeds the red line indicates strong ionospheric scintillation occurs. Moreover,there is a small period of time when S4 is greater than 1,indicating that the scintillation reaches saturation, and that shows the signal is affected by strong ionospheric scintillation during this period.

In addition, since the detected signal comes from the Brazilian region near the equator, there should be relatively strong amplitude scintillation and relatively weak phase scintillation in signals. As is shown in Fig.7, strong amplitude scintillation is basically accompanied by phase scintillation, and the phase scintillation indices σφsatisfy σφ> 0.15 at some moments. The specific number and ratio of scintillation are shown in Table 1.

Table 1 Time and Proportion of Scintillation

According to the ratio, amplitude scintillation mainly occurs in this region with lower intensity of phase scintillation. This result is consistent with the characteristics that there are mainly strong amplitude scintillations in equatorial and mainly phase scintillations in polar regions. The amplitude scintillation ratios of satellite No.12 and No.31 are 0.7339 and 0.6951, respectively. Therefore, in this period, the satellite signals are affected by strong ionospheric scintillation for more than 30 minutes.Meanwhile, the results show that satellites No. 18 and No. 25 are affected by strong ionospheric amplitude scintillation for nearly 30 minutes. Considering the factors such as the false acquisition of satellite signals and the enhancement of scintillation indices caused by other Interferences, we set the threshold of scintillation ratio to 50%. With this threshold, we successfully detected that satellites No. 12 and No. 31 were affected by strong ionospheric scintillation.

Taking actual acquisition point of satellite signal(Lat: –23°12′27.5772" Lng: –45°51′35.1414" Hgt:+693.4m) as reference, we test the positioning performance of the improved software receiver. The positioning error in the local cartesian coordinates coordinate system is shown in Fig. 8.

Fig.8 UTM Coordinates Variation

As shown in Fig.8, the horizontal direction error of the information calculated by the improved software receiver is controlled within 5 m, and height direction error is controlled within 10 m.

The satellite constellation after the data acquisition begins for 6 hours is shown in Fig.9.

Fig.9 Satellites Sky Plot

(From http://www.gnssplanning.com)

5 Conclusion

Due to the complex space electromagnetic environment in signal transmission, the satellite signal quality is affected by ionospheric scintillation, which leads to the decrease of carrier-to-noise ratio of receiver and the increase of tracking error and positioning error. Especially in low latitudes and equatorial regions, severe ionospheric scintillation will lead to hours of navigation interruption. The software receiver improved in this paper realizes the detection of strong ionospheric scintillation in the equatorial regions and also realizes the successful acquisition,tracking, positioning and demodulation of satellite signals. The test results show that the horizontal positioning error can be controlled within 5 m,indicating that the software receiver is still reliable under the weak signal conditions caused by strong ionospheric scintillation. The improved software receiver can modify the scintillation detection threshold and detect ionospheric scintillation with different intensities, which provides important basis and useful guidance for analyzing ionospheric scintillation and space weather. This paper will be of significance for the future design of high-performance Beidou navigation receiver and also the territorial security of the South China Sea.

Acknowledgment

We would like to thank Professor Jade Morton and Professor Dennis M Akos in the University of Colorado at Boulder for providing the original measurement data of São José dos Campos, Brazil, and also thank their kind supports.