Acoustic range error is the dominant error affecting the positioning accuracy of GNSS-Acoustics (GNSS-A). To reduce the impact, the sound speed profile (SSP) measurement data are typically used. However, the SSP measurement data may not be obtained when an unmanned observation platform is used for positioning GNSS-A. To address the issue of lacking SSP measurements, a new method that does not require SSP has been proposed. The method uses an extended cubic spline function to model the temporal variation of sound speed, employs cubic polynomials to model the spatial variation of sound speed, utilizes a squared function to model sound ray bending error, and uses least squares estimation to determine the coordinates of seafloor transponder. The observations of seafloor stations TOS2 and MYGI from 2011 to 2019 were used to test the method. The results were compared with the coordinates of the seafloor transponder provided by the GARPOS software. The results show that (1) the average root mean square (RMS) of the GNSS-A positioning residuals for TOS2 and MYGI are 6.84 cm and 8.59 cm, respectively, with the residuals exhibiting a normal distribution; (2) the centimeter-level positioning of the seafloor transponder can be realized by using the method of SSP-free GNSS-A positioning. (3) the approach using the extended cubic spline function improves the position accuracy by approximately 9.25% for TOS2 and 24.94% for MYGI, compared to the method using the cubic spline function; (4) the average difference between the baselines obtained from GARPOS software and those calculated by the method using the extended cubic spline function is 0.23 cm for TOS2 and 0.03 cm for MYGI; (5) the accuracy of the baseline derived from the method using the extended cubic spline function improves by about 17.05% for TOS2 and 13.53% for MYGI when compared to the GARPOS software results. Since no sound speed profile observation is required, the SSP-free GNSS-A positioning method is particularly suitable for unmanned GNSS-A observation systems where in-field SSP measurements are difficult to conduct. However, the SSP-free GNSS-A positioning method with the extended cubic spline function is only applicable for the positioning of seafloor stations equipped with multiple transponders within a limited sea area.

A Low Earth Orbit (LEO) constellation augmenting satellite navigation is important in the future development of Global Navigation Satellite System (GNSS). GNSS augmented by LEO constellations can improve not only the accuracy of Positioning, Navigation, and Timing (PNT), but also the consistency and reliability of secure PNT system. This paper mainly analyzes the diverse demands of different PNT users for LEO augmented GNSS, including the precision demand in real-time, the availability demand in special areas, the navigation signal enhancement demand in complex electromagnetic environments, and the integrity demand with high security. Correspondingly, the possible contributions of LEO constellations to PNT performance are analyzed from multiple aspects. A particular attention is paid to the special PNT user requirements that cannot be fulfilled with existing GNSS, such as the PNT service demand in the polar regions and the onboard GNSS orbit determination demand of some LEO satellites. The key technologies to be considered in the constellation design, function realization, and payload development of the LEO-augmented navigation system are summarized.

<p>BeiDou Global Navigation Satellite System (BDS-3) was formally commissioned to provide satellite navigation services worldwide in 2020. It not only has the normal positioning, navigation and timing (PNT) functions, but also provides several kinds of featured services. The paper  focuses on the featured services of BDS-3 and their applications in various geoscience fields. First, the featured services of BDS-3 and their performances are introduced. Then different application examples are described and analyzed based on the Geostationary Orbit (GEO) satellite-based featured services and Middle Earth Orbit (MEO) satellite-based featured services. Finally, some possible improvements to BDS in the future are discussed.</p>

The ranges derived from acoustic measurements between seafloor stations are relatively more accurate compared with those derived from the sea surface vessel transducer to the seafloor transponders, because measurements through mixed water layers will be affected by complex acoustic range errors. Coordinates of seafloor stations can be improved by the direct-path acoustic ranging. Systematic errors in acoustic rangings, however, will significantly deteriorate the accuracy of vertical coordinates. In order to mitigate the effects of these systematic errors (e.g., acoustic ray bending and sound speed variation errors in acoustic measurements on the seafloor station location parameters), the observation model needs to be finely constructed. First, a new observation model with acoustic ray bending and sound speed bias parameters is established. Then, using a seafloor geodetic network with four moored stations at a depth of about 3000 m in the South China Sea, the significance of the acoustic ray bending parameter is tested. The results show that (1) the acoustic ray bending parameter is significant at the 90% confidence level, which means that the acoustic ray bending error in the seafloor geodetic network is not negligible; (2) by estimating the coefficient of acoustic ray bending, the influence of the acoustic ray bending error on the vertical coordinate components can be significantly mitigated; our model improves the accuracy of the seafloor stations' position with differences in the horizontal coordinate components less than 0.1 cm between the two-dimensional adjustment and three-dimensional adjustment, and also improves the vertical coordinate component to uncertainty less than 3.0 cm; (3) the relative movement between the moored stations is less than 50 cm, and the horizontal movement is larger than the vertical movement.

Satellite navigation systems are vulnerable. To guarantee the positioning, navigation and timing (PNT) safety of core infrastructure, it is necessary to establish a secure PNT system with hybrid physical principles. In this paper, the augmentations of the BeiDou satellite system (BDS) itself are analysed, namely augmentations through the BDS inter-satellite link, BDS geostationary orbit (GEO) and inclined geostationary orbit (IGSO) satellites, and BDS PNT services supported by low earth orbit (LEO) satellites. Then, taking BDS as the core component, the comprehensive PNT infrastructure seamlessly covering deep space and deep ocean is described, consisting of the deep space PNT constellation, the sea-floor PNT sonar beacon network, and the ground-based low frequency and very low frequency (VLF) long wave radio stations. Moreover, the key technologies of resilient PNT application matching comprehensive PNT and various autonomous perception PNT information are discussed, such as resilient PNT sensor integration, the resilient PNT functional model and the resilient stochastic model. As a future development direction, the key factors of intelligent PNT services are analysed, including the intelligent perception of PNT application scenes, the intelligent optimization of PNT functional and stochastic models and the intelligent fusion of multisource PNT information.

卫星导航系统具有脆弱性,为了确保核心基础设施定位、导航和定时（PNT）的安全,必须构建多物理原理混合的安全PNT体系.本文首先分析了现有北斗卫星导航系统自身能力增强途径,包括北斗星间链路的能力增强,北斗静止轨道（GEO）、倾斜同步轨道（IGSO）卫星的能力增强和低轨卫星配合的北斗系统PNT服务能力增强;其次,以北斗卫星导航系统为核心,描述了无缝覆盖深空、深海的综合PNT基础设施建设的主要内容,包括深空PNT星座、深海PNT声呐信标网络和地基低频/甚低频长波台网PNT体系.在此基础上,论述了与综合PNT和各类自主感知PNT信息配套的弹性PNT应用关键技术,包括弹性PNT传感器集成、弹性PNT函数模型和弹性随机模型.作为未来PNT应用的发展方向,文章最后论述了PNT应用的环境智能感知、PNT函数模型和随机模型智能优化以及多源PNT信息智能融合等智能PNT核心要素.

In aeromagnetic measurement, accurate compensation for the interference magnetic field generated by the aircraft platform due to the aircraft maneuvering in the Earth's magnetic field is an important prerequisite for the accurate identification of the magnetic anomaly signal of the detection target. The flaws of the traditional calibration flight scheme in aeromagnetic interference compensation are firstly analyzed, and then an improved scheme is proposed, featuring smaller calibration flight areas, more simplified maneuvers, shorter flight route and less influence from the magnetometer's direction and extreme weather. The improved scheme can enhance the robustness of the aeromagnetic interference compensation matrix and improve the compensation efficiency, and thus both the solved magnetic compensation coefficients and compensation effects are significantly improved. The experimental results of the aeromagnetic interference compensation in an area of Inner Mongolia of China show that the average accuracy of the repeated survey lines, which are compensated by the improved scheme, can be increased by 5 and 3 nT, respectively, compared with the uncompensated results and the results compensated by the traditional scheme. Therefore, the effectiveness and superiority of the improved calibration flight scheme is fully proved.

The variation of sound speed is a major factor affecting the accuracy of the acoustic positioning underwater. A refined resilient model is constructed which includes temporal variations and horizontal variations on the basis of the simple resilient observation model which includes range bias and time bias parameters. The integrated estimation method and the stepwise estimation method were used to calculate the temporal and spatial varia-tions of the sound speed using the GNSS-Acoustic (GNSS-A) measurements. The sound speed model was used in the long baseline (LBL) underwater acoustic positioning, and the position of the transducer calculated by acoustic ranges is compared with that provided by GNSS. The results show that, there are obvious temporal and spatial variations in the sound speed in different water area. The temporal variation of the sound speed is less than 0.8 m/s and the spatial variation is less than 0.15 m/s. The variation of sound speed under water not only shows obvious periodicity, but also presents negative gradient in the horizontal direction. When the coordinates of the transducer are calculated by using the sound speed model considering temporal and spatial variations con-structed by the integrated estimation, the precision of the coordinates is improved by about 28%, compared with that calculated using the constant sound speed model. The 3D root mean square (RMS) of the LBL underwater acoustic positioning is about 0.73 m, and the RMS is less than 0.13 m in the vertical direction, when the co-ordinates of the ship bottom transducer are calculated within the area covered by five seafloor transponders. It indicates that the coordinate accuracy of the five seafloor transponders is high in the vertical direction.

Within the framework of differential augmentation, this paper introduces the basic technical framework and performance of the BeiDou Global Navigation Satellite System (BDS-3) Satellite-Based Augmentation System (BDSBAS), including orbit products, satellite clock offset products, ionosphere and its integrity performance. The basic principle of BDS-3 Precise Point Positioning (PPP-B2b) is expounded, the similarities and differences between the PPP service provided by BDS-3 and International Global Navigation Satellite System (GNSS) Service (IGS) are discussed, and the limitations of PPP-B2b are analyzed. Since both the BDSBAS and PPP-B2b utilize a ground monitoring station network to determine the satellite orbits and clock offset corrections, and broadcast differential corrections through the three Geostationary Orbit (GEO) satellites of BDS-3, the feasibility of the co-construction of BDSBAS and PPP-B2b is analyzed, strategies for the infrastructure sharing and correction broadcasting are presented, and the influences of BDSBAS correction broadcasting strategy adjustment are evaluated. In addition, it assesses the possibility of broadcasting differential corrections through the Inclined Geosynchronous Orbit (IGSO) satellites of BDS-3, and the feasibility of augmenting satellite navigation with Low Earth Orbit (LEO) satellites.

BeiDou Global Navigation Satellite System (BDS-3) not only performs the normal positioning, navigation and timing (PNT) functions, but also provides featured services, which are divided into geostationary orbit (GEO) and medium earth orbit (MEO) satellite-based featured services in this paper. The former refers to regional services consisting of the regional short message communication service (RSMCS), the radio determination satellite service (RDSS), the BDS satellite-based augmented service (BDSBAS) and the satellite-based precise point positioning service via B2b signal (B2b-PPP). The latter refers to global services consisting of the global short message communication service (GSMCS) and the MEO satellite-based search and rescue (MEOSAR) service. The focus of this paper is to describe these featured services and evaluate their performances. The results show that the inter-satellite link (ISL) contributes a lot to the accuracy improvement of orbit determination and time synchronization for the whole constellation. Compared with some other final products, the root mean squares (RMS) of the BDS-3 precise orbits and broadcast clock are 25.1 cm and 2.01 ns, respectively. The positioning accuracy of single frequency is better than 6 m, and that of the generalized RDSS is usually better than 12 m. For featured services, the success rates of RSMCS and GSMCS are better than 99.9% and 95.6%, respectively; the positioning accuracies of single and dual frequency BDSBAS are better than 3 and 2 m, respectively; the positioning accuracy of B2b-PPP is better than 0.6 m, and the convergence time is usually smaller than 30 min; the single station test shows that the success rate of MEOSAR is better than 99%. Due to the ISL realization in the BDS-3 constellation, the performance and capacities of the global featured services are improved significantly.

ABSTRACT The Advanced Topographic Laser Altimeter System (ATLAS) on ICESat-2 (Ice, Cloud, and Land Elevation Satellite-2) uses a 532 nm band photon-counting LiDAR (Light Detection and Ranging), which has certain penetrability to water bodies, and the measured data show that the bathymetric ability reaches nearly one Secchi depth. ATLAS has a limited number of beams and a fixed ground track, and only collects section elevation along the track direction. The fusion of active laser point cloud and passive optical remote sensing satellite image can fill the gap of shallow water depth data in a large range. This paper takes ATLAS as the research object, with the aim of exploring the effective algorithm flow of spaceborne active laser and passive optical fusion processing, and systematically evaluating the bathymetric accuracy of the fusion algorithm. An adaptive Gauss filtering technology based on the density of point cloud was firstly improved to achieve accurate denoising under the condition of uneven surface/underwater density. Subsequently, the depth of underwater points was calculated automatically through steps of water surface modelling, refraction correction, etc. Finally, the control points and check points were randomly extracted to solve the parameters of multispectral inversion model and verify the internal accuracy of the model. In this paper, the accuracy of ATLAS bathymetry was verified by Airborne LiDAR Bathymetry (ALB) data in Oahu Island, Hawaii, and the results indicate that the vertical root mean square error (RMSE) ranges from 0.56 m to 1.11 m. In Yongle islands and Qilianyu area of the South China Sea, WorldView-2 (WV2) 4 bands multispectral images and ATLAS data were used to carry out the active-passive fusion bathymetry, and the ALB and sonar data were used to evaluate the accuracy. Experimental results show that the internal compliance accuracy of the fusion model is better than 1.25 m (RMSE), and the real bathymetry accuracy is better than 1.42 m (RMSE). The above results reveal the great potential of active-passive fusion bathymetry based on ICESat-2 and other high-resolution remote sensing satellites, which can provide strong technical support for filling the blank of shallow water depth information.

Some spherical harmonic expressions of gravitational and geomagnetic field elements will become infinite when computation point approaches polar regions, as the sine function of the geocentric co‐latitude contained in the denominator tends to be zero. Currently, this singularity problem has been solved for gravitational field case, however, it remains unsolved for geomagnetic vectors (GVs) and geomagnetic gradient tensors (GGTs). Because the latter use Schmidt semi‐normalized associated Legendre function (SNALF), which is different from fully‐normalized associated Legendre function used in the former. To overcome this singularity problem, we derive new non‐singular expressions of the first‐ and second‐order derivatives of Schmidt SNALF (order m equals to 0 or 1), and the corresponding two kinds of spherical harmonic polynomials. When the non‐singular expressions are applied to the traditional formulae of GVs and GGTs, more practical expressions of GVs and GGTs with non‐singularity are formulated by refining the cases that the order m equals to 0, 1, 2 and other values. Furthermore, to provide flexible calculation strategies for Schmidt SNALF, we derive four kinds of recursive formulae, including the standard forward row recursion, the standard forward column recursion, the cross degree and order recursion, and the Belikov recursion. The standard formula to check the calculation accuracy of various recursive formulae of SNALF are also given. Besides, we demonstrate the effectiveness and reliability of the new derived non‐singular expressions of GVs and GGTs and analyze the computation speed and stability of the four recursive formulae by extensive numerical experiments.

The BeiDou Navigation Satellite System (BDS) has completed the constellation deployment and started to provide global services. After achieving the capabilities of global coverage, global first-class accuracy for Positioning, Navigation and Timing (PNT), global Inter-Satellite Links (ISL) networking, and global featured services, BDS will promote the construction of the comprehensive PNT infrastructure in the new era and play a more active role in international cooperation with other Global Navigation Satellite System (GNSS) providers to better serve humankind and the world.

The Ladybug5 is an integrated, multi-camera system that features a near-spherical field of view. It is commonly deployed on mobile mapping systems to collect imagery for 3D reality capture. This paper describes an approach for the geometric modelling and self-calibration of this system. The collinearity equations of the pinhole camera model are augmented with five radial lens distortion terms to correct the severe barrel distortion. Weighted relative orientation stability constraints are added to the self-calibrating bundle adjustment solution to enforce the angular and positional stability between the Ladybug5's six cameras. Centimetre-level 3D reconstruction accuracy can be achieved, with image-space precision and object-space accuracy improved by 92% and 93%, respectively, relative to a two-term lens distortion model. Sub-pixel interior orientation stability and millimetre-level relative orientation stability were also demonstrated over a 10-month period.

In November 2016, China launched its first X-ray pulsar navigation satellite (XPNAV-1), in order to study the feasibility of using the regularly emitted X-ray signals from pulsars for spacecraft navigation. Over the past two years, a large amount of observations have been obtained by the XPNAV-1 satellites. The Crab pulsar (i.e. PSR0531+21) is its main observational source in which 3506627 s observations are made in 1251 orbits during 451 days. In order to study the X-ray pulsar observation performance of XPNAV-1 satellite in the past two years, those observations of Crab pulsar are processed fully. Each recovered pulse profile from daily observation have a mean similarity of 90.67

In order to improve the precision of BeiDou orbit determination under the conditions of regional ground monitoring station and make good use of increasingly rich on-board data and upcoming ISL technology, a method of BeiDou precision orbit determination is proposed which combines the use of ground monitoring stations data, low earth orbit satellite(LEOs) data and Inter-Satellite Link(ISL) data.The effects of assisting data of LEOs and ISL on the precision orbit determination of navigation satellite are discussed.Simulation analysis is carried out mainly from the number of LEOs, orbit slot configuration and ISL.The results show that the orbit precision of BeiDou will greatly improve about 73% with a small number of LEOs, while improvement of clock bias is not remarkable;the uniform orbit slot configuration of the same number of LEOs has a modest effect on the accuracy of combined orbit determination;compared with LEOs, the increase of ISL will significantly improve the accuracy of orbit determination with a higher efficiency.

The precision of orbit determination for navigation satellites with regional distributed stations may be improved by introducing onboard GNSS observation data. The method of orbit determination combining the onboard data of LEO with the observation data of regional monitor stations is discussed. In order to validate this proposed approach and its performance, the observation data of seven GPS stations in China and onboard data of GRACE-A and GRACE-B during March 16–31, 2011 were collected and orbit determination experiments were carried out. The result shows that, compared with the result of seven stations, the visibility of GPS satellites is increased by about 14 %; meanwhile, the precision of GPS orbit expressed in RTN components is improved by about 35, 44, and 45 %, respectively; when introducing only GRACE-B, improvements in RTN components of 51, 60, and 62 % are achieved when both GRACE-A and GRACE-B are introduced and the visibility of GPS satellites is increased by 18 % in this case. The proposed approach will provide a new idea to improve the orbit determination accuracy of navigation satellite under the restriction of regional monitoring stations.

面对市场经济条件下科技期刊的改革与创新,科技期刊编辑必须具备5大意识,即创新意识、特色意识、质量意识、责任意识、服务意识,才能保证期刊良性发展.

撰写科技论文英文摘要,不仅是学术交流的基本要求,而且是论文被引用和收录的必要条件.而英语摘要的文题又是论文的总纲,它必须准确表达论文的中心内容,向读者提供直接的信息.文题表述清晰,简明扼要,能使读者一目了然,同时便于读者进行文献检索.因此,撰写科技论文英语摘要文题应遵循ABC原则,即:Accuracy(准确)、Brevity(简洁)、Clarity(清晰).