Design and Implementation of Highly Integrated Electronic System for Small-Size Spacecraft

Due to the tight installation space, strict weight requirements, numerous functional units and dense wireless equipments of small-size spacecraft, the traditional electronic system design of aircraft cannot meet the requirements of miniaturization and lightweight for small-size spacecraft. In this paper, a highly integrated electronic system suitable for small-size spacecraft is designed, in which many key technologies such as miniaturization and lightweight design technology, multi frequency wireless electromagnetic compatibility design technology, small aircraft low delay cooperative networking and high-precision differential positioning are adopted. The proposed highly integrated electronic system makes timing control, attitude and orbit control, wireless telemetry, networking communication, satellite navigation, power distribution and other functions integrated. The total weight of the system is about 7.62kg, which is much lighter than existing electronic system. The experiment results that the highly integrated electronic system achieves good effect. This technology has broad application prospects in small-size aircraft with strict weight and space requirements.

Precise Linear Positioning Method Based on Transmissive Two-Stage Diffraction Grating System

This paper mainly studies the precise linear positioning method based on the transmissive type two-stage diffraction grating system. Starting from the analysis of the two-stage diffraction principle, the mathematical model of the two-stage grating diffraction is established, and the positioning characteristics of the differential positioning method and the modified positioning method are discussed. The simulation experiment of the linear positioning device is carried out to study the displacement characteristics. The experimental results show that the precise positioning based on the diffraction grating can obtain a positioning accuracy of ±0.4 μm.

High accuracy positioning with low-cost single-frequency GPS system in multipath environments

Positioning using low-cost, single-frequency GPS receivers provides an economical solution, but these receivers are subject to biases leading to degradation of the accuracy required. Factors contributing to degradation in the accuracy of low-cost systems are ionospheric delay, multipath, and measurement noise. Unless carefully addressed, these errors distort the ambiguity resolution process, and result in less accurate positioning solutions. However, with the modern hardware improvements, measurement noise is now almost neglibible. Ionospheric delay has been dramatically reduced with the availablity of global or local ionospheric maps produced by various organizations (e.g., International GNSS Service (IGS), and National Oceanic and Atmospheric Administraion (NOAA). The major remaining constraint and challenging problem is multipath. This is because mulitpath is environmentally dependant, difficult to model mathematically, and cannot be reduced through differential positioning. The research proposes a new approach to identify multipath-contaminated L1 measurements. The approach is based on wavelet analysis using Daubechies family wavelets. First, the difference between the code and carrier phase measurements was estimated, leaving essentially twice the ionospheric delay, multipath and system noise. The ionospheric delay is largely removed by using high resolution ionospheric delay maps produced by NOAA. The remaining residuals contain mainly low-frequency multipath, if existed, and high-frequency part of the residual component described above. The L1 measurements obtaines from the staellites with lowest multipath were used to compute the final positions using Trimble Total Control (TTC) and Bernese scientific processing software packages. The AC12 single-frequency GPS receiver was extensively tested in static and kinematic modes. Accuracies within 5 cm was demostrated for baselines up to 65 km under various multipath environments.

High accuracy positioning with low-cost single-frequency GPS system in multipath environments

Positioning using low-cost, single-frequency GPS receivers provides an economical solution, but these receivers are subject to biases leading to degradation of the accuracy required. Factors contributing to degradation in the accuracy of low-cost systems are ionospheric delay, multipath, and measurement noise. Unless carefully addressed, these errors distort the ambiguity resolution process, and result in less accurate positioning solutions. However, with the modern hardware improvements, measurement noise is now almost neglibible. Ionospheric delay has been dramatically reduced with the availablity of global or local ionospheric maps produced by various organizations (e.g., International GNSS Service (IGS), and National Oceanic and Atmospheric Administraion (NOAA). The major remaining constraint and challenging problem is multipath. This is because mulitpath is environmentally dependant, difficult to model mathematically, and cannot be reduced through differential positioning. The research proposes a new approach to identify multipath-contaminated L1 measurements. The approach is based on wavelet analysis using Daubechies family wavelets. First, the difference between the code and carrier phase measurements was estimated, leaving essentially twice the ionospheric delay, multipath and system noise. The ionospheric delay is largely removed by using high resolution ionospheric delay maps produced by NOAA. The remaining residuals contain mainly low-frequency multipath, if existed, and high-frequency part of the residual component described above. The L1 measurements obtaines from the staellites with lowest multipath were used to compute the final positions using Trimble Total Control (TTC) and Bernese scientific processing software packages. The AC12 single-frequency GPS receiver was extensively tested in static and kinematic modes. Accuracies within 5 cm was demostrated for baselines up to 65 km under various multipath environments.

Improved In-Flight Estimation of Inertial Biases through CDGNSS/Vision Based Cooperative Navigation

This paper discusses the exploitation of a cooperative navigation strategy for improved in-flight estimation of inertial sensors biases on board unmanned aerial vehicles. The proposed multi-vehicle technique is conceived for a “chief” Unmanned Aerial Vehicle (UAV) and relies on one or more deputy aircrafts equipped with Global Navigation Satellite System (GNSS) antennas for differential positioning which also act as features for visual tracking. Combining carrier-phase differential GNSS and visual estimates, it is possible to retrieve accurate inertial-independent attitude information, thus potentially enabling improved bias estimation. Camera and carrier-phase differential GNSS measurements are integrated within a 15 states extended Kalman filter. Exploiting an ad hoc developed numerical environment, the paper analyzes the performance of the cooperative approach for inertial biases estimation as a function of number of deputies, formation geometry and distances, and absolute and relative dynamics. It is shown that exploiting two deputies it is possible to improve biases estimation, while a single deputy can be effective if changes of relative geometry and dynamics are also considered. Experimental proofs of concept based on two multi-rotors flying in formation are presented and discussed. The proposed framework is applicable beyond the domain of small UAVs.

Connection Between Chromosomal Location and Function of CtrA Phosphorelay Genes in Alphaproteobacteria

Most bacterial chromosomes are circular, with replication starting at one origin (ori) and proceeding on both replichores toward the terminus (ter). Several studies have shown that the location of genes relative to ori and ter can have profound effects on regulatory networks and physiological processes. The CtrA phosphorelay is a gene regulatory system conserved in most alphaproteobacteria. It was first discovered in Caulobacter crescentus where it controls replication and division into a stalked and a motile cell in coordination with other factors. The locations of the ctrA gene and targets of this response regulator on the chromosome affect their expression through replication-induced DNA hemi-methylation and specific positioning along a CtrA activity gradient in the dividing cell, respectively. Here we asked to what extent the location of CtrA regulatory network genes might be conserved in the alphaproteobacteria. We determined the locations of the CtrA phosphorelay and associated genes in closed genomes with unambiguously identifiable ori from members of five alphaproteobacterial orders. The location of the phosphorelay genes was the least conserved in the Rhodospirillales followed by the Sphingomonadales. In the Rhizobiales a trend toward certain chromosomal positions could be observed. Compared to the other orders, the CtrA phosphorelay genes were conserved closer to ori in the Caulobacterales. In contrast, the genes were highly conserved closer to ter in the Rhodobacterales. Our data suggest selection pressure results in differential positioning of CtrA phosphorelay and associated genes in alphaproteobacteria, particularly in the orders Rhodobacterales, Caulobacterales and Rhizobiales that is worth deeper investigation.