Approach to inter-satellite time synchronization for micro-satellite cluster

Micro-satellite cluster enables a whole new class of missions for communications, remote sensing, and scientific research for both civilian and military purposes. Synchronizing the time of the satellites in a cluster is important for both cluster sensing capabilities and its autonomous operating. However, the existing time synchronization methods are not suitable for microsatellite cluster, because it requires too many human interventions and occupies too much ground control resource. Although, data post-process may realize the equivalent time synchronization, it requires processing time and powerful computing ability on the ground, which cannot be implemented by cluster itself. In order to autonomously establish and maintain the time benchmark in a cluster, we propose a compact time difference compensation system（TDCS）, which is a kind of time control loop that dynamically adjusts the satellite reference frequency according to the time difference. Consequently, the time synchronization in the cluster can be autonomously achieved on-orbit by synchronizing the clock of other satellites to a chosen one＇s. The experimental result shows that the standard deviation of time synchronization is about 102 ps when the carrier to noise ratio（CNR） is 95 d BHz, and the standard deviation of corresponding frequency difference is approximately0.36 Hz.

Approach to MAI cancellation for micro-satellite clusters

With the development of micro-satellite technology,traditional monolithic satellites can be replaced by micro-satellite clusters to achieve high flexibility and dynamic reconfiguration capability.For satellite clusters based on the frequency division-code division multiple access(FD-CDMA)communication system,the inter-satellite ranging precision is usually constrained due to the influence ofmulti-address interference(MAI).Themulti-user detection(MUD)is a solution to MAI,which can be divided into two categories:the linear detector(LD)and the non-linear detector(NLD).The general idea of the LD is aiming to make a better decision during the symbol decision process by using the information of all channels.However,it is not beneficial for the signal phase tracking precision.Instead,the principle of the NLD is to rebuild the interference signal and cancel it from the original one,which can improve the ranging performance at the expense of considerable delays.In order to enable simultaneous ranging and communication and reduce multi-node ranging performance degradation,this paper proposes an NLD scheme based on a delay locked loop(DLL),which simplifies the receiver structure and introduces no delay in the decision process.This scheme utilizes the information obtained from the interference channel to reconstruct the interference signal and then cancels it from the original delayed signal.Therefore,the DLL input signal-to-interference ratio(SIR)of the desired channel can be significantly improved.The experimental results show that with the proposed scheme,the standard deviation of the tracking steady error is decreased from 5.59 cm to 3.97 cm for SIR=5 dB,and 13.53 cm to 5.77 cm for SIR=-5 dB,respectively.

Analyses of noncommensurate sampling used in high-precision regenerative pseudo-noise ranging systems

A high-precision pseudo-noise ranging system is often required in satellite-formation missions. But in an actual PN ranging system, digital signal processing limits the ranging accuracy, only level up with meter-scale. Using non-integer chip to sample time ratio, noncommensurate sampling was seen as an effective solution to cope with the drawback of digital effects. However, researchers only paid attention to selecting specific ratios or giving a simulation model to verify the effectiveness of the noncommensurate ratios. A qualitative analysis model is proposed to characterize the relationship between the range accuracy and the noncommensurate sampling parameters. Moreover, a method is also presented which can be used to choose the noncommensurate ratio and the correlation length to get higher phase delay distinguishability and lower range jitter. The simulation results indicate the correctness of our analyses and the optimal ranging accuracy can be up to centimeter-level with the proposed approach.

A fast acquisition method of DSSS signals using differential decoding and fast Fourier transform

In low earth orbit (LEO) satellite or missile communication scenarios, signals may experience extremely large Doppler shifts and have short visual time. Thus, direct sequence spread spectrum (DSSS) systems should be able to achieve acquisition in a very short time in spite of large Doppler frequencies. However, the traditional methods cannot solve it well. This work describes a new method that uses a differential decoding technique for Doppler mitigation and a batch process of FFT (fast Fourier transform) and IFFT (invert FFT) for the purpose of parallel code phase search by frequency domain correlation. After the code phase is estimated, another FFT process is carried out to search the Doppler frequency. Since both code phase and Doppler frequency domains are searched in parallel, this architecture can provide acquisition fifty times faster than conventional FFT methods. The performance in terms of the probability of detection and false alarm are also analyzed and simulated, showing that a signal-to-noise ratio (SNR) loss of 3 dB is introduced by the differential decoding. The proposed method is an efficient way to shorten the acquisition time with slightly hardware increasing.

Dual transponder ranging performance in the presence of oscillator phase noise

A dual transponder carrier ranging method can be used to measure inter-satellite distance with high precision by combining the reference and the to-and-fro measurements. Based on the differential techniques, the oscillator phase noise, which is the main error source for microwave ranging systems, can be significantly attenuated. Further, since the range measurements are derived on the same satellite, the dual transponder ranging system does not need a time tagging system to synchronize the two satellites. In view of the lack of oscillator noise analysis on the dual transponder ranging model, a comprehensive analysis of oscillator noise effects on ranging accuracy is provided. First, the dual transponder ranging system is described with emphasis on the detailed analysis of oscillator noise on measurement precision. Then, a high-fidelity numerical simulation approach based on the power spectrum density of an actual ultra-stable oscillator is carried out in both frequency domain and time domain to support the presented theoretical analysis. The simulation results under different conditions are consistent with the proposed concepts, which makes the results reliable. Besides, the results demonstrate that a high level of accuracy can be achieved by using this oscillator noise cancelation-oriented ranging method.

Design, analysis and optimization of random access inter-satellite ranging system

In this paper, a random access inter-satellite ranging(RAISR) system is designed. The ranging accuracy is optimized by an algorithm to greatly improve the ranging accuracy. This paper verifies the feasibility of the RAISR system through a series of theoretical analysis, numerical simulation, hardware system design and testing. The research work brings the solution to the design and accuracy optimization problem of the RAISR system,which eliminates the main error caused by the satellite dynamic characteristics and frequency source drift of the RAISR system.The accuracy of the measurement system has been significantly improved.

Adaptive median threshold algorithm used in FDIS of DSSS receivers

For direct sequence spread spectrum （DSSS） receivers, the capability of rejecting narrow-band interference can be significantly improved by a process of frequency-domain interference suppression （FDIS）. The key issue of this process is how to determine a threshold to eliminate interference in the frequency domain, which has been extensively studied. However, these previous methods are tedious or very complex. A simple and ef- ficient algorithm based on medians is proposed. The elimination threshold is only related to the median by a scale factor, which can be obtained by the numerical analysis. Simulation results show that the algorithm provides excellent narrow-band interfer- ence suppression while only slightly degrading the signal-to-noise ratio （SNR）. A one-pass algorithm using logarithmic segmentation is further derived to estimate medians with low computational complexity. Finally, the FDIS is implemented in a field programmable gate array （FPGA） of Xilinx. Experiments are carried out by connecting the FDIS FPGA to a DSSS receiver, and the results show that the receiver has an effective countermeasure for a 60 dB interference-to-signal ratio （ISR）.

Integrated method for measuring distance and time difference between small satellites

The advancement of small satellites is promoting the development of distributed satellite systems,and for the latter,it is essential to coordinate the spatial and temporal relations between mutually visible satellites.By now,dual one-way ranging(DOWR)and two-way time transfer(TWTT)are generally integrated in the same software and hardware system to meet the limitations of small satellites in terms of size,weight and power(SWaP)consumption.However,studies show that pseudo-noise regenerative ranging(PNRR)performs better than DOWR if some advanced implementation technologies are employed.Besides,PNRR has no requirement on time synchronization.To apply PNRR to small satellites,and meanwhile,meet the demand for time difference measurement,we propose the round-way time difference measurement,which can be combined with PNRR to form a new integrated system without exceeding the limits of SWaP.The new integrated system can provide distributed small satellite systems with on-orbit high-accuracy and high-precision distance measurement and time difference measurement in real time.Experimental results show that the precision of ranging is about 1.94 cm,and that of time difference measurement is about 78.4 ps,at the signal to noise ratio of 80 dBHz.

Method of adaptive PLL bandwidth adjustment without phase slipping

In a system based on the phase lock loop（PLL）, a trade-off must be made between the tracking precision and the dynamic performance if constant parameters are adopted. To overcome this drawback, a new method called no phase slipping adaptive bandwidth（NPS-AB） is proposed, which can adjust the loop bandwidth adaptively for different working conditions. As a result, both the tracking precision and the dynamic performance can be achieved concurrently. NPS-AB has two features to keep the loop stable： one is the capability of quick response to dynamics; the other is a series of additional constraints when the bandwidth is switched. Compared with other methods, there is no phase slipping during the adjustment process for NPS-AB. The phase integer ambiguity can be avoided and the phase value is kept valid. It is meaningful for carrier ranging systems. Simulation results show that NPS-AB can deal with sudden dynamics and keep the pseudo-range value stable in the entire dynamic process.

Application and improvement of wavelet packet de-noising in satellite transponder

The satellite transponder is a widely used module in satellite missions, and the most concerned issue is to reduce the noise of the transferred signal. Otherwise, the telemetry signal will be polluted by the noise contained in the transferred signal, and the additional power will be consumed. Therefore, a method based on wavelet packet de-noising （WPD） is introduced. Compared with other techniques, there are two features making WPD more suit- able to be applied to satellite transponders： one is the capability to deal with time-varying signals without any priori information of the input signals; the other is the capability to reduce the noise in band, even if the noise overlaps with signals in the frequency domain, which provides a great de-noising performance especially for wideband signals. Besides, an oscillation detector and an av- eraging filter are added to decrease the partial oscillation caused by the thresholding process of WPD. Simulation results show that the proposed algorithm can reduce more noises and make less distortions of the signals than other techniques. In addition, up to 12 dB additional power consumption can be reduced at -10 dB signal-to-noise ratio （SNR）.

Noise behaviors of a closed-loop micro-electromechanical system capacitive accelerometer

The noise of closed loop micro-electromechanical systems(MEMS) capacitive accelerometer is treated as one of the significant performance specifications.Traditional optimization of noise performance often focuses on designing large capacitive sensitivity accelerometer and applying closed loop structure to shape total noise,but different noise sources in closed loop and their behaviors at low frequencies are seldom carefully studied,especially their behaviors with different electronic parameters.In this work,a thorough noise analysis is established focusing on the four noise sources transfer functions near 0 Hz with simplified electronic parameters in closed loop,and it is found that the total electronic noise equivalent acceleration varies differently at different frequency points,such that the noise spectrum shape at low frequencies can be altered from 1/f noise-like shape to flat spectrum shape.The bias instability changes as a consequence.With appropriate parameters settings,the 670 Hz resonant frequency accelerometer can reach resolution of 2.6 μg/(Hz)1/2 at 2 Hz and 6 μg bias instability,and 1300 Hz accelerometer can achieve 5μg/(Hz)1/2 at 2 Hz and 31 μg bias instability.Both accelerometers have flat spectrum profile from 2 Hz to 15 Hz.

An analysis method for ISL of multilayer constellation

The multilayer satellite network has high spatial spectrum utilization, flexible networking, strong survivability, and diversified functions. The inter-satellite links(ISLs) and crosslayer ISLs(CLISLs) enable direct communication paths between satellites, which improves the spatial autonomy of the constellation. Due to the existence of perturbation, ISLs are affected for a long time, which impacts reliable inter-satellite transmission. The stability and complexity of ISL establishment are related to the static and dynamic characteristics of range and azimuth. This paper presents a model of ISLs in a perturbed multilayer constellation. Series of theoretical derivation, simulation, and numerical calculation are carried out. A more comprehensive multilayer constellation ISL model is obtained. The work of this paper provides some theoretical foundations for constellation networking research.

Satellite Rotation Modulation Measurement System Based on Ultrasonic Motor

Rotation modulation technique is generally used to improve the performance of aviation and marine inertial navigation system.Considering of the performance requirements from current aerospace to MEMS micro-nano inertial device,this paper proposedrotary inertial device modulation technology application methods on the satellite.Firstly,taking the advantage ofultrasonic motor,like high resolution,fast response,electromagnetic compatibility,a lowmagnetic,high-precision and appropriate for use on satellite ultrasonic motor modulation turntable was developed.Then,through theoretical modeling and simulations,the rotation modulation technology was verified to improvethe precise of satellite attitude measurementeffectively,equivalent toimprove the accuracy of MEMS gyro over an order of magnitude.This work helpsachieve the application of rotation modulation technology in aerospace and acceleratethe promotion of the MEMS gyro in satellite attitude measurement.

Camera calibration method for an infrared horizon sensor with a large field of view

Inadequate geometric accuracy of cameras is the main constraint to improving the precision of infrared horizon sensors with a large field of view(FOV).An enormous FOV with a blind area in the center greatly limits the accuracy and feasibility of traditional geometric calibration methods.A novel camera calibration method for infrared horizon sensors is presented and validated in this paper.Three infrared targets are used as control points.The camera is mounted on a rotary table.As the table rotates,these control points will be evenly distributed in the entire FOV.Compared with traditional methods that combine a collimator and a rotary table which cannot effectively cover a large FOV and require harsh experimental equipment,this method is easier to implement at a low cost.A corresponding three-step parameter estimation algorithm is proposed to avoid precisely measuring the positions of the camera and the control points.Experiments are implemented with 10 infrared horizon sensors to verify the effectiveness of the calibration method.The results show that the proposed method is highly stable,and that the calibration accuracy is at least 30%higher than those of existing methods.

High precision attitude dynamic tracking control of a moving space target

On-orbit spacecraft face many threats,such as collisions with debris or other spacecraft.Therefore,perception of the surrounding space environment is vitally important for on-orbit spacecraft.Spacecraft require a dynamic attitude tracking ability with high precision for such missions.This paper aims to address the above problem using an improved backstepping controller.The tracking mission is divided into two phases:coarse alignment and fine alignment.In the first phase,a traditional saturation controller is utilized to limit the maximum attitude angular velocity according to the actuator’s ability.For the second phase,the proposed backstepping controller with different virtual control inputs is applied to track the moving target.To fulfill the high precision attitude tracking requirements,a hybrid attitude control actuator consisting of a Control Moment Gyro(CMG)and Reaction Wheel(RW)is constructed,which can simultaneously avoid the CMG singularity and RW saturation through the use of an angular momentum optimal management strategy,such as null motion.Finally,five simulation scenarios were carried out to demonstrate the effectiveness of the proposed control strategy and hybrid actuator.

Residual intensity modulation in resonator fiber optic gyros with sinusoidal wave phase modulation

We present how residual intensity modulation(RIM) affects the performance of a resonator fiber optic gyro(R-FOG) through a sinusoidal wave phase modulation technique. The expression for the R-FOG system's demodulation curve under RIM is obtained. Through numerical simulation with different RIM coefficients and modulation frequencies, we find that a zero deviation is induced by the RIM effect on the demodulation curve, and this zero deviation varies with the RIM coefficient and modulation frequency. The expression for the system error due to this zero deviation is derived. Simulation results show that the RIM-induced error varies with the RIM coefficient and modulation frequency. There also exists optimum values for the RIM coefficient and modulation frequency to totally eliminate the RIM-induced error, and the error increases as the RIM coefficient or modulation frequency deviates from its optimum value; however, in practical situations, these two parameters would not be exactly fixed but fluctuate from their respective optimum values, and a large system error is induced even if there exists a very small deviation of these two critical parameters from their optimum values. Simulation results indicate that the RIM-induced error should be considered when designing and evaluating an R-FOG system.

Bipartite graph-based control flow checking for COTS-based small satellites

Abstract Single event upset （SEU） effect, caused by highly energized particles in aerospace, threatens the reliability and security of small satellites composed of commercialofftheshelves （COTS）. SEU induced control flow errors （CFEs） may cause unpredictable behavior or crashes of COTSbased small satellites. This paper proposes a generic softwarebased control flow checking technique （CFC） and bipartite graphbased control flow checking （BGCFC）. To simplify the types of illegal branches, it transforms the conventional control flow graph into the equivalent bipartite graph. It checks the legal ity of control flow at runtime by comparing a global signature with the expected value and introduces consecutive IDs and bitmaps to reduce the time and memory overhead. Theoretical analysis shows that BGCFC can detect all types of internode CFEs with constant time and memory overhead. Practical tests verify the result of theoretical analysis. Compared with previous techniques, BGCFC achieves the highest error detection rate, lower time and memory overhead; the composite result in evaluation fac tor shows that BGCFC is the most effective one among all these techniques. The results in both theory and practice verify the applicability of BGCFC for COTSbased small satellites.

A combined modulated feedback and temperature compensation approach to improve bias drift of a closed-loop MEMS capacitive accelerometer

目的:根据Allan方差定义,分析引起闭环MEMS电容式加速度计中长期漂移的因素,研究相应的消除和补偿方法,实现降低漂移、提高闭环MEMS加速度计稳定性的目的。创新点:首先设计利用载波调制反馈电压信号的方法,避开并滤除反馈通道上的低频噪声,降低Allan方差的偏置不稳定性;然后在反馈通道上加入参考信号,利用该参考信号经过模拟部分后被解调的相位来表征温度,进行温度补偿,进一步降低Allan方差的长期漂移。方法:首先,根据Allan方差的偏置不稳定性定义,其影响因素来自于闭环系统的低频噪声。根据文献资料和作者之前的实验测试情况,大部分低频噪声来自于反馈通道,因反馈信号是低频电压信号。为降低反馈通道低频噪声,设计载波调制反馈信号及通过高通滤波器滤除低频噪声的方案MFA(图4)。经测试,采用MFA的输出噪声和1小时的稳定性明显优于反馈信号直接反馈的方案DFA(图10-12)。在MFA基础上,继续增加一个参考信号,该信号通过整个系统的模拟部分后被数字系统解调,其相位信息携带了外界温度变化信息,从而可利用该相位信息进行实时温度补偿(图5、6)。对温度补偿方案进行静态温度范围测试(图16),补偿后的度系数是补偿前的1/46;在快速动态温度变化下(图17),实时补偿结果是未补偿的1/8,显示了较好的补偿特性。温度补偿后的Allan方差长期漂移获得明显降低,而偏置不稳定性略微升高(图18、19)。总体上看,利用MFA和温度补偿,闭环MEMS电容式加速度计的整体漂移特性得到较好抑制。结论:针对闭环MEMS电容式加速度计漂移问题,提出载波调制反馈和新型温度补偿方案。两种方案能够在同一闭环系统中同时使用,分别降低了偏置不稳定性和长期温度漂移,1 h的Allan方差显示,偏置不稳定性约为13μg,100 s积分时间的漂移为17μg,温度系数降到0.1 mg/°C,显示了较好的稳定性。

An online error calibration method for spaceflight TT&C systems based on LEO-ground DDGPS

To overcome the shortcomings of the traditional measurement error calibration methods for spaceflight telemetry, tracking and command(TT&C) systems, an online error calibration method based on low Earth orbit satellite-to-ground doubledifferential GPS(LEO-ground DDGPS) is proposed in this study. A fixed-interval smoother combined with a pair of forward and backward adaptive robust Kalman filters(ARKFs) is adopted to solve the LEO-ground baseline, and the ant colony optimization(ACO) algorithm is used to deal with the ambiguity resolution problem. The precise baseline solution of DDGPS is then used as a comparative reference to calibrate the systematic errors in the TT&C measurements, in which the parameters of the range error model are solved by a batch least squares algorithm. To validate the performance of the new online error calibration method, a hardware-in-the-loop simulation platform is constructed with independently developed spaceborne dual-frequency GPS receivers and a Spirent GPS signal generator. The simulation results show that with the fixed-interval smoother, a baseline estimation accuracy(RMS, single axis) of better than 10 cm is achieved. Using this DDGPS solution as the reference, the systematic error of the TT&C ranging system is effectively calibrated, and the residual systematic error is less than 5 cm.

A spaceborne advanced storage system for remote sensing microsatellites

With the development of satellite miniaturization and remote sensing,the establishment of microsatellite constellations is an inevitable trend.Due to their limited size,weight,and power,spaceborne storage systems with excellent scalability,performance,and reliability are still one of the technical bottlenecks of remote sensing microsatellites.Based on the commercial off-the-shelf field-programmable gate array and memory devices,a spaceborne advanced storage system(SASS)is proposed in this paper.This work provides a dynamic programming,queue scheduling multiple-input multiple-output cache technique and a high-speed,high-reliability NAND flash controller for multiple microsatellite payload data.Experimental results show that SASS has outstanding scalability with a maximum write rate of 2429 Mb/s and preserves at least 78.53%of the performance when a single NAND flash fails.The scheduling technique effectively shortens the data scheduling time,and the data remapping method of the NAND flash controller can reduce the retention error by at least 50.73%and the program disturbance error by at least 37.80%.