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Long-term monitoring of satellite microvibrations generates a significant amount of data streams, placing strain on satellites with limited transmission capacity. To relieve this transmission strain, a dynamic compressed sensing (CS) framework is proposed for measuring satellite microvibrations. Microvibration streams are measured block by block and then reconstructed using a dynamic recovery algorithm. The recovery solution of one block can be used as a priori knowledge for the next block, allowing for faster updates. However, existing dynamic recovery algorithms are only applicable in the real domain and cannot be applied to microvibrations projected on a Fourier basis in the complex domain. In light of this event, the dynamic homotopy algorithm is expanded to the complex domain to deal with microvibration signals that are sparse in the Fourier basis. In comparison to conventional uniform sampling methods, the experimental results show that the dynamic CS with the expanded recovery algorithm can achieve a maximum root-mean-square acceleration (Grms) deviation of 4% in power spectrum density with one-fifth of the sampling points. Compared to recovery algorithms applicable to fixed measurements, the dynamic algorithm can achieve comparable accuracy in about one-third of the computation time. The experimental findings demonstrate the feasibility of satellite microvibrations measurements using dynamic CS.

In this letter, a dynamic channel reservation (DCR) strategy based on deep Q network (DQN) is proposed for multi-service low earth orbit (LEO) satellite communication system. We develop a novel modeling method to represent DCR problem of multiple services as a reinforcement learning (RL) task. Based on this model, we calculate the influence of current channel allocation results on future environment and take it as one of the factors for channel allocation decision. Moreover, a corresponding neural network is designed as a decision evaluator to provide an end to end mapping of decision to its value, which effectively avoids the influence of artificial preconditions. Simulation results show that the proposed strategy can improve the overall quality of service (QOS) of the system.

In this letter, we simulate GNSS/LEO measurements and propose a process strategy for LEO-augmented GNSS medium length baseline RTK. Experiments show that, the performance of GNSS medium length baseline RTK can be significantly improved by introducing LEO satellites. The convergence speed of LEO-augmented GPS or BDS float solution maybe better than GPS/BDS combined under the conditions of similar satellite geometry. Besides, the RMS error of fixed solutions are improved to better than 4 cm from sub-decimeter level.

The technology of chirp spread spectrum (CSS) has been around for decades, widely used in the fields of radar and sonar. In recent years, CSS has been used in the IEEE 802.15.4a and Long Range (LoRa) Internet of Things (IoT), called LoRa modulation. On the other hand, CSS has never been used in low-Earth-orbit (LEO) satellite communication systems for low-data-rate transmission. CSS applied in LoRa as a kind of successful commercial IoT technology will promote research on its application in satellite communication for IoT. Recently, new types of chirp signals with the same chirp rate have become available to realize multiple accesses. This letter examines those types of chirp signal and the revised SCS called Asymmetry Chirp Signal (ACS) is proposed for better performance in LEO satellite IoT.

With the development of Internet of Things (IoT), the world gradually moves toward the interconnection of all things. In terrestrial networks, narrowband-IoT and Long Rage (LoRa) are used widely in the IoT industry. For low-earth-orbit (LEO) satellites, they have a lot of conspicuous advantages, such as broader coverage and lower construction cost. In this letter, we place emphasis on the application of LoRa-related technologies to LEO satellite communication systems, and the modulation is the prior research. Chirp spread spectrum (CSS) modulation is the most important technology in LoRa. Therefore, the research of CSS modulation used in the satellite IoT is imperative.

In this letter, we consider blind estimation of channel parameters over a frequency-selective channel. We use a blocked Rhee–Glynn smoothing estimator to derive E-step in the expectation-maximization (EM) algorithm. The proposed algorithm copes with the curse of dimensionality of a forward–backward algorithm; meanwhile, it is easy to parallelize, which is amenable to a modern computing hardware and speeds up the estimation of channel parameters. The experiment results show that the proposed algorithm is close to the Baum–Welch algorithm in terms of convergence of channel coefficients and outperforms the EM algorithm coupled with a joined two-filter smoothing algorithm in terms of convergence of channel coefficients and running time.

By cognitive radio, the low Earth orbit (LEO) satellites may prefer to operate in the unlicensed spectrum which is open to all the users, and compete for the limited resources with terrestrial cognitive radio networks (CRNs). The competition can be regarded as a game and analyzed with game theory. This particular unlicensed spectrum sharing problem is modeled here, and the special properties of “spatially-distinguished-interference” and the short period of the interactions between satellites and terrestrial CRNs are explored. Then, the problem is formulated as a “partially-blind” finitely repeated prisoner's dilemma by game theory. Finally, we begin with two promising spectrum sharing schemes, which can be used to enforce the frequency reuse among the remotely located terrestrial CRN players as well as to overcome the observation noise. By analysis and comparison, it is proposed that the novel refreshing-contrite-tit-for-tat (R-CTFT) is the optimal spectrum sharing scheme. Simulation results verify that it can be used to utilize the spectrum most efficiently.

With the development of the LEO satellite communication technology, highly dependable wireless communication and sensor data collection using LEO satellites have been getting much attention for emergency, marine research, and forest fire disaster in the remote region. The satellite system is expected to have the following features: rapid production, low cost, and fast construction of the satellite network. In this paper, a QS-CDMA uplink access technique in the LEO satellite is presented and discussed, which is focused on the local clocks using GPS 1PPS timing signals and the Doppler compensation for terminal uplink. The spreading code with length of 1023, which is used for the uplink preamble, selects the shift-m-sequence that can greatly reduce the MAI and increase the number of simultaneous access users. A novel analysis method for the accuracy of clock synchronization and a novel method for the estimation of Doppler shift and propagation delay are presented. These methods are used to guide the specific hardware implementation of the QS-CDMA LEO satellite sensor data collection system. Through simulations and experiments, it results in that this system structure can drastically reduce the complexity in implementing the acquisition in the satellite and increase the adaptability of the satellite system in different environments.

Pyrolysis utilizes biomass to produce a liquid product bio-oil which is widely used as a potential energy source and a low-cost feedstock for chemical production. Considerable efforts have been made to convert biomass to bio-oil since the oil crisis in mid-1970s. This review focuses on the recent developments in catalysts used in pyrlysis of biomass for bio-oil production. In this review, 5 types of catalysts such as metal oxide, molecular sieve, mineral, transition metal and others, have been given. Molecular sieves, such as HZSM-5, ZSM-5, MCM-41, SBA-15, HUSY, REY and their modified catalysts are most widely used in biomass catalytic pyrolysis process. The effects of catalysts on bio-oil yields, quality, composition, stability etc. are also discussed. Although catalysts in pyrolysis of biomass produce markedly influences on bio-oil products, the variety of catalyst types and properties make generalizations difficult to define, in regard to trying to critically analyze the literature.