StarFL: Hybrid Federated Learning Architecture for Smart Urban Computing

From facial recognition to autonomous driving, Artificial Intelligence (AI) will transform the way we live and work over the next couple of decades. Existing AI approaches for urban computing suffer from various challenges, including dealing with synchronization and processing of vast amount of data generated from the edge devices, as well as the privacy and security of individual users, including their bio-metrics, locations, and itineraries. Traditional centralized-based approaches require data in each organization be uploaded to the central database, which may be prohibited by data protection acts, such as GDPR and CCPA. To decouple model training from the need to store the data in the cloud, a new training paradigm called Federated Learning (FL) is proposed. FL enables multiple devices to collaboratively learn a shared model while keeping the training data on devices locally, which can significantly mitigate privacy leakage risk. However, under urban computing scenarios, data are often communication-heavy, high-frequent, and asynchronized, posing new challenges to FL implementation. To handle these challenges, we propose a new hybrid federated learning architecture called StarFL. By combining with Trusted Execution Environment (TEE), Secure Multi-Party Computation (MPC), and (Beidou) satellites, StarFL enables safe key distribution, encryption, and decryption, and provides a verification mechanism for each participant to ensure the security of the local data. In addition, StarFL can provide accurate timestamp matching to facilitate synchronization of multiple clients. All these improvements make StarFL more applicable to the security-sensitive scenarios for the next generation of urban computing.

BeiDou satellites cross-regional communication path assignment model and resource management

Time division inter-satellite communication and ranging link assignment of BeiDou satellites have made important progress; however, there is still the unsolved issue of integrated communication between ground gateway, aircraft, or even ship, and the BeiDou satellites. Therefore, in this study, we develop a path assignment model based on the idea of clustering and Markov chain. The optimal path is determined by the objective function based on the maximum transition probability. The transition probability takes into account the communication environment, congestion status, aircraft mobility, and reduces the complexity of path assignment by hiding the topology in the region. At the same time, due to the limited resources of onboard computing, storage and bandwidth, we also design a resource management strategy based on task urgency, aimed at minimizing the unreasonable allocation index, to enable the readjustment of link application resources. Finally, the performance of the model and the strategy in average link handover times, link reliability, resource allocation fairness, and network quality of service is determined by simulation.

Geopotential Determination Between Ultra-Stable Distant Clocks Based on Two-Way Space Laser Time Transfer Links

<p>The Earth’s gravity potential (geopotential) field plays an important role in geodesy, for instance, it is the basis for defining the geoid and the International Height Reference System (IHRS). In chronometric geodesy, the main challenge for directly measuring geopotential differences between two stations lies in that a reliable link for time comparison is needed. Currently, most satellite links for time comparison are dealt with in the microwave domain, for which the ionospheric and tropospheric effects are major error sources that greatly influence the signal propagation compared to optical space links. Recently, accurate laser time transfer links between satellite and ground stations have already been planned and confirmed, such as Laser Time Transfer (LTT, China) on BeiDou satellites and Tiangong II / China's space station (CSS), Time Transfer by Laser Link (T2L2, French) on Jason-2 mission and European Laser Timing (ELT, Europe) on Atomic Clock Ensemble in Space (ACES). Therefore, in this study, we propose an approach for determining the geopotential difference between two ground atomic clocks based on the Two-way Laser Time Transfer (TWLTT) technique via a space station as a bridge, which could have potential applications in geoscience. This study is supported by the National Natural Science Foundations of China (NSFC) under Grants 42030105, 41721003, 41804012, 41631072, 41874023, Space Station Project (2020)228, and the Natural Science Foundation of Hubei Province of China under Grant 2019CFB611.</p>

Prediction of BeiDou Satellite Orbit Maneuvers to Improve the Reliability of Real-Time Navigation Products

The positioning, navigation, and timing (PNT) service of the Global Navigation Satellite System (GNSS) is developing in the direction of real time and high precision. However, there are some problems that restrict the development of real-time and high-precision PNT technology. Satellite orbit maneuvering is one of the factors that reduce the reliability of real-time navigation products, especially the high-frequency orbit maneuvering of geostationary earth orbit (GEO) and inclined geosynchronous orbit (IGSO) satellites. The BeiDou Navigation Satellite System (BDS) constellation is designed to contain GEO, IGSO, and medium earth orbit (MEO). These orbit maneuvers bring certain difficulties for data processing, especially for BeiDou satellites, such as decreased real-time service performance, which results in real-time navigation products including unusable maneuvered satellites. Additionally, the performance of real-time navigation products will decrease because the orbit maneuvers could not be known in advance, which diminishes the real-time PNT service performance of BDS for users. Common users cannot obtain maneuvering times and strategies owing to confidentiality, which can lead to a decline in the BDS real-time service performance. Thus, we propose a method to predict orbit maneuvers. BDS data from the broadcast ephemeris were analyzed to verify the availability of the proposed method. In addition, the results of real-time positioning were analyzed by using ultra-rapid orbit products, demonstrating that their reliability is improved by removing maneuvered satellites in advance. This is vital to improve the reliability of real-time navigation products and BDS service performance.

An Improved Multi-Satellite Method for Evaluating Real-Time BDS Satellite Clock Offset Products

Two methods are widely used for evaluating the precision of satellite clock products, namely the single-satellite method (SSM) and the multi-satellite method (MSM). In the satellite clock product evaluation, an important issue is how to eliminate the timescale difference. The SSM selects a reference satellite to eliminate the timescale difference by between-satellite differencing, but its evaluation results are susceptible to the gross errors in the referenced satellite clock offsets. In the MSM, the timescale difference is first estimated and then removed. Unlike the GPS, the BeiDou Navigation Satellite System (BDS) consists of three types of satellites, namely geosynchronous earth orbit (GEO), inclined geosynchronous orbit (IGSO), and medium earth orbit (MEO) satellites. The three types of satellites have uneven orbital accuracy. In the generation of satellite clock products, the orbital errors are partly assimilated into the clock offsets. If neglecting the orbital accuracy difference of the three types of BeiDou satellites, the MSM will obtain biased estimates of the timescale difference and finally affect the clock product evaluation. In this study, an improved multi-satellite method (IMSM) is proposed for evaluating the real-time BDS clock products by removing the assimilated orbital errors of the three types of BDS satellites when estimating the timescale difference. Three real-time BDS clock products disseminated by three different International GNSS Service (IGS) analysis centers, namely CLK16, CLK20, and CLK93, over a period of two months are used to validate this method. The results indicate that the assimilated orbital errors have a significant impact on the estimation of the timescale difference. Subsequently, the IMSM is compared with the SSM in which the referenced satellite is rigorously chosen, and their RMS difference is only 0.08 ns, which suggests that the evaluation results obtained by the IMSM are accurate. Compared with the traditional MSM, the IMSM improves the RMS by 0.16, 0.11, and 0.07 ns for CLK16, CLK20, and CLK93, respectively. Finally, three real-time BDS clock products are evaluated using the proposed method, and results reveal a significant precision difference among them.

Penggunaan Mobile Base Station South Galaxy G1 untuk Pengukuran Batas Bidang Tanah di Kawasan Padat Bangunan

GNSS CORS as one of land and area measuring instrument has the weakness of limited range of base station and can only be used in open area. Nowadays, this weakness can be overcome by mobile base station technology. One GNSS tool that uses mobile base station technology is Galaxy G1, South type. Another advantage is that, it can capture the Beidou satellite signal so that the satellite configuration becomes better. Thus, the researcher conducted a study with the aims (1) to know the difference between accuracy of measurement MBS South Galaxy G1 with and without Beidou satellites; (2) to test the accuracy of the results of measurements using Mobile Base Station South Galaxy G1 type towards the results of measurements with Electronic Total Station (ETS) in the densely building area. This research used a comparison experimental research method with a quantitative approach. The results of the analysis show that (1) the coordinates of the observation with Beidou have an average horizontal accuracy of 0.025 m, while the results of the calculation of coordinates without Beidou have an average horizontal accuracy of 0.421 m. (2) The difference between the MBS South Galaxy G1 coordinate value and the terrestrial coordinate value is 0.132 m on average. The results of the t-test with a significance level of 5% found that the MBS South Galaxy G1 coordinate value has a significant difference to the terrestrial coordinate value. Keywords: mbs, south galaxy g1 Intisari: CORS sebagai salah satu alat ukur bidang tanah mempunyai kelemahan terbatasnya jangkauan base station dan hanya dapat digunakan di daerah terbuka. Kelemahan tersebut kini dapat diatasi dengan adanya teknologi MBS. Salah satu alat GNSS yang menggunakan teknologi MBS adalah South tipe Galaxy G1. Kelebihan lain adalah dapat menangkap sinyal satelit Beidou sehingga konstalasi satelitnya lebih baik. Berdasarkan hal tersebut peneliti ini bertujuan (1) mengetahui perbedaan ketelitian hasil pengukuran MBS South Galaxy G1 dengan dan tanpa satelit Beidou; (2) menguji ketelitian hasil pengukuran menggunakan MBS South Tipe Galaxy G1 terhadap hasil pengukuran dengan Electronik Total Station (ETS) pada kawasan padat bangunan. Metode yang digunakan adalah metode penelitian eksperimen perbandingan dengan pendekatan kuantitatif. Hasil analisis diketahui (1) Koordinat pengamatan dengan Beidou memiliki ketelitian horisontal rata-rata sebesar 0.025 m, sedangkan hasil perhitungan koordinat tanpa Beidou memiliki ketelitian horisontal rata-rata sebesar 0.421 m. (2) Perbedaan nilai koordinat MBS South Galaxy G1 terhadap nilai koordinat terestris rata-rata sebesar 0.132 m. Hasil uji t dengan taraf signifikansi 5% diperoleh bahwa nilai koordinat MBS South Galaxy G1 memiliki perbedaan yang signifikan terhadap nilai koordinat terestris. Kata Kunci: mbs, south galaxy g1.

Performance Analysis of Ground Target Detection Utilizing Beidou Satellite Reflected Signals

This paper presents a method of ground target detection using reflected signals of BeiDou satellites. The phase difference information, which is the output of the phase-lock loop (PLL) in the tracking process, is an important observation in this technique. The geometric relationships between the specular point of different BeiDou satellites and the target are established. In addition, the detection and false alarm probability are also analyzed. In order to verify the reliability of the method, an experiment in the suburb area of Beijing was completed. The target was placed in the coverage area of the left-handed circular polarization (LHCP) antenna for two time periods (10–20 s and 40–55 s). By observing the phase difference in BeiDou reflected signals in the presence of a target, it was found that the changing trend was in good agreement with the target placement time periods. In the second experiment, the target moved east and west at a speed of 0.5 m/s, and the range of motion was 6 m. During the acquisition of the BeiDou reflection signal, the target passed through the antenna 14 times. The performance of target detection with different parameters was observed by extracting in-phase (I) branch component data, phase difference information, and the carrier-to-noise ratio (CNR) of five BeiDou reflected signals. The experimental results allowed three conclusions to be drawn as follows: (1) The target detection performance of the three parameters has a certain relationship with the altitude angle and the azimuth angle of the satellite; (2) target motion direction information can be reflected in the change of the satellite I branch component data; (3) The CNR information of different satellite reflected signals varies greatly when the target moves, which is quite different from that of the first experimental target when it is stationary. Thus, the feasibility of target detection using BeiDou reflection signal was demonstrated through these two experiments.