The impact on tsunami detection from space using GNSS-reflectometry when combining GPS with GLONASS and Galileo

The devastating Sumatra tsunami in 2004 demonstrated the need for a tsunami early warning system in the Indian Ocean. Such a system has been installed within the German-Indonesian Tsunami Early Warning System (GITEWS) project. Tsunamis are a global phenomenon and for global observations satellites are predestined. Within the GITEWS project a feasibility study on a future tsunami detection system from space has therefore been carried out. The Global Navigation Satellite System Reflectometry (GNSS-R) is an innovative way of using GNSS signals for remote sensing. It uses ocean reflected GNSS signals for sea surface altimetry. With a dedicated Low Earth Orbit (LEO) constellation of satellites equipped with GNSS-R receivers, densely spaced sea surface height measurements could be established to detect tsunamis. Some general considerations on the geometry between LEO and GNSS are made in this simulation study. It exemplary analyzes the detection performance of a GNSS-R constellation at 900 km altitude and 60° inclination angle when applied to the Sumatra tsunami as it occurred in 2004. GPS is assumed as signal source and the combination with GLONASS and Galileo signals is investigated. It can be demonstrated, that the combination of GPS and Galileo is advantageous for constellations with few satellites while the combination with GLONASS is preferable for constellations with many satellites. If all three GNSS are combined, the best detection performance can be expected for all scenarios considered. In this case an 18 satellite constellation will detect the Sumatra tsunami within 17 min with certainty, while it takes 53 min if only GPS is considered.     

基于DDMR辅助的GNSS-R载波相位差测高方法

提出了一种基于时延-多普勒映射接收机（DDMR,Delay-Doppler Map Receiver）辅助的载波相位差提取方法,给出了系统结构及信号处理方法.该方法将DDMR中所观测到的码相位差作为直射信号与反射信号的码相位延时量,将完成跟踪的直射信号扩频码进行对应的延时用于完成对反射信号的开环码跟踪.该方法省去了码相位延时搜索的过程,且可以准确地对反射信号扩频码进行同步.为了验证系统的可行性及实际性能,进行了针对水面高度测量的岸基试验并给出了试验结果.岸基试验证明采用该方法的GNSS-R（Global Navigation Satellite System Reflection）接收机可以稳定地对反射信号进行跟踪并提取直射与反射信号的载波相位差,测高精度约为2.5 cm,经过0.5 s的数据平均后精度可达0.6 cm.      

基于相关功率修正的地基GNSS-R土壤湿度反演

利用全球导航卫星系统反射信号测量技术(GNSS-R)进行土壤湿度反演过程中，实际接收天线方向性会造成GNSS直反信号相关功率测量偏差。针对地基观测场景下天线方向性造成的相关功率的类余弦振荡问题，提出了基于多项式拟合的信号相关功率修正方法。为了验证所提方法的有效性，开展了地基GNSS-R土壤湿度观测实验，结果表明:基于多项式拟合的相关功率修正可以消除信号相关功率的类余弦振荡，提升GNSS-R土壤湿度反演中的观测数据有效性和反演结果准确性。      

Improved specular point prediction precision using gradient descent algorithm

Global Navigation Satellite Systems Reflectometry (GNSS-R) utilizes GNSS signals reflected off the Earth surface for remote sensing applications. Due to weak power of reflected signals, GNSS-R receiver needs to track reflected signals by open loop. The first step is to calculate the position of specular point. The specular point position error of the existing algorithm—Quasi-Spherical Earth (QSE) Approach—is about 3 km which may cause troubles in data post-processing. In this paper, gradient descent algorithm is applied to calculate position of specular point and the calculation is based on World Geodetic System 1984 (WGS 84) ellipsoid in geodetic coordinate. The benefit of this coordinate is that it is easy to investigate the effect of real surface’s altitude. Learning rate—the key parameter of the algorithm—is adaptively adjusted according to initial error, latitude and gradient descent rate. With self-adaptive learning rate strategy, the algorithm converges fast. Through simulation and test on Global Navigation Satellite System Occultation Sounder II (GNOS II), the performances of the algorithm are validated. The specular point position error of the proposed algorithm is about 10 m. The speed of the proposed algorithm is competitive compared with the existing algorithm. The test on GNOS II shows that the proposed algorithm has good real-time performance.     

基于QZSS L1信号的岸基GNSS-R码延迟海面测高

为了探究GNSS L1波段中信号不同调制方式的GNSS-R码相位延迟海面的测高性能，开展了双天线岸基GNSS-R海面测高实验，收集了采样率为40 MHz的原始中频数据。利用自主开发的GNSS-R测高软件接收机和反演软件对实验数据进行了处理分析，同时获得了基于QZSS L1C/A码和L1C码的GNSS-R海面测高结果，并分别与岸基同步观测的雷达高度计的测高值进行对比，以对反演结果进行精度评定。实验结果表明:基于QZSS L1C/A码和L1C码的海面测高精度最优可达0.63 m和0.4 m，L1C码的延迟GNSS-R测高精度明显高于L1C/A码。此外，GNSS-R测高精度会随着卫星高度角的增加而有所增加。      

A real-time GNSS-R system based on software-defined radio and graphics processing units

Reflected signals of the Global Navigation Satellite System (GNSS) from the sea or land surface can be utilized to deduce and monitor physical and geophysical parameters of the reflecting area. Unlike most other remote sensing techniques, GNSS-Reflectometry (GNSS-R) operates as a passive radar that takes advantage from the increasing number of navigation satellites that broadcast their L-band signals. Thereby, most of the GNSS-R receiver architectures are based on dedicated hardware solutions. Software-defined radio (SDR) technology has advanced in the recent years and enabled signal processing in real-time, which makes it an ideal candidate for the realization of a flexible GNSS-R system. Additionally, modern commodity graphic cards, which offer massive parallel computing performances, allow to handle the whole signal processing chain without interfering with the PC’s CPU. Thus, this paper describes a GNSS-R system which has been developed on the principles of software-defined radio supported by General Purpose Graphics Processing Units (GPGPUs), and presents results from initial field tests which confirm the anticipated capability of the system.     

Evaluation and improvement of coastal GNSS reflectometry sea level variations from existing GNSS stations in Taiwan

Global sea level rise due to an increasingly warmer climate has begun to induce hazards, adversely affecting the lives and properties of people residing in low-lying coastal regions and islands. Therefore, it is important to monitor and understand variations in coastal sea level covering offshore regions. Signal-to-noise ratio (SNR) data of Global Navigation Satellite System (GNSS) have been successfully used to robustly derive sea level heights (SLHs). In Taiwan, there are a number of continuously operating GNSS stations, not originally installed for sea level monitoring. They were established in harbors or near coastal regions for monitoring land motion. This study utilizes existing SNR data from three GNSS stations (Kaohsiung, Suao, and TaiCOAST) in Taiwan to compute SLHs with two methods, namely, Lomb–Scargle Periodogram (LSP)-only, and LSP aided with tidal harmonic analysis developed in this study. The results of both methods are compared with co-located or nearby tide gauge records. Due to the poor quality of SNR data, the worst accuracy of SLHs derived from traditional LSP-only method exceeds 1?m at the TaiCOAST station. With our procedure, the standard deviations (STDs) of difference between GNSS-derived SLHs and tide gauge records in Kaohsiung and Suao stations decreased to 10?cm and the results show excellent agreement with tide gauge derived relative sea level records, with STD of differences of 7?cm and correlation coefficient of 0.96. In addition, the absolute GNSS-R sea level trend in Kaohsiung during 2006–2011 agrees well with that derived from satellite altimetry. We conclude that the coastal GNSS stations in Taiwan have the potential of monitoring absolute coastal sea level change accurately when our proposed methodology is used.     

一种基于北斗三号系统的GNSS-R海面干涉测高技术

GNSS-R干涉测高技术可用于中尺度海面高度观测，具有空间分辨率高、测量精度高等优势。与传统的GNSS-R本地码测高技术相比，GNSS-R干涉测高技术可以有效提升高度测量精度。虽然GNSS-R干涉测高技术已有一些研究，但是基于北斗三号的干涉测高应用还很少。本文根据GNSS-R干涉测高技术优势，针对北斗三号系统在干涉测高技术上的应用，研发了支持北斗三号的GNSS-R干涉测高接收机并描述了整体架构及实现。利用所研发的接收机进行水面干涉测高试验，首次获取了北斗三号B1和B2干涉测高波形，与传统GPS L1和北斗B1本地码测高波形进行对比。对两种方法计算出的水面高度进行对比，结果显示北斗三号干涉测高精度明显优于GPS L1和北斗B1传统本地码测高精度。      

Tropospheric delays from GNSS for application in coastal altimetry

In the scope of the development of an improved methodology for the computation of the wet tropospheric correction for coastal altimetry, based on the use of tropospheric delays derived from GNSS (Global Navigation Satellite Systems), various studies have been conducted aiming to improve the estimation, at global scale, of GNSS-derived tropospheric delays.     

Carrier phase prediction method for GNSS precise positioning in challenging environment

Since the signals of global navigation satellite system (GNSS) are blocked frequently in challenging environments, the discontinuous carrier phases seriously affect the application of GNSS precise positioning. To improve the carrier phase continuity, this paper proposes a carrier phase prediction method based on carrier open-loop tracking. In the open-loop tracking mode, the carrier numerically controlled oscillator (NCO) is controlled by the predicted Doppler, but not by the loop filter output. To improve the phase prediction effective time, accurate receiver clock drift estimation is studied in the prediction method. The phase prediction performance is tested on GNSS software receiver. In the phase prediction effective time tests, open-loop processes were set for the tested channel. The test results show that, when some satellite signals are blocked in 15?s, the probability of carrier phase error less than quarter cycles is more than 94%. In the real time kinematic (RTK) positioning tests, some satellite signals are blocked in 10–15?s repeatedly. The test results show that, the carrier phase continuity is basically not affected by the signal interruption, and the RTK can almost keep continuous centimeter-level positioning accuracy without re-fixing the integer ambiguity.     

Deformation monitoring using GNSS-R technology

GNSS reflectometry (GNSS-R) has been widely studied in recent years for various applications, such as soil moisture monitoring, biomass analysis, and sea state monitoring. This paper presents the concept of a novel application of using GNSS-R technology for deformation monitoring. Instead of installing GNSS on the deformation body to sense the movement, GNSS-R deformation monitoring system estimates the deformation from receiving GNSS signal reflected by the deformation body remotely. A prototype of GNSS-R deformation monitoring system has been developed based on GNSS software receiver technology. A 3D geometrical model of GNSS signal reflection has been used to reveal the relationship between the change of carrier phase difference and deformation. After compensating the propagation path delay changes caused by satellite movement, the changes in the remaining carrier phase difference are linked to the deformation. Field tests have been carried using the GNSS-R system developed and the results show sub-centimeter level deformation can be observed with the new technology. Unlike other GNSS deformation monitoring methods, GNSS-R receivers are not installed on the slope which makes this new technology more attractive.     

GNSS reflectometry and remote sensing: New objectives and results

The Global Navigation Satellite System (GNSS) has been a very powerful and important contributor to all scientific questions related to precise positioning on Earth’s surface, particularly as a mature technique in geodesy and geosciences. With the development of GNSS as a satellite microwave (L-band) technique, more and wider applications and new potentials are explored and utilized. The versatile and available GNSS signals can image the Earth’s surface environments as a new, highly precise, continuous, all-weather and near-real-time remote sensing tool. The refracted signals from GNSS radio occultation satellites together with ground GNSS observations can provide the high-resolution tropospheric water vapor, temperature and pressure, tropopause parameters and ionospheric total electron content (TEC) and electron density profile as well. The GNSS reflected signals from the ocean and land surface could determine the ocean height, wind speed and wind direction of ocean surface, soil moisture, ice and snow thickness. In this paper, GNSS remote sensing applications in the atmosphere, oceans, land and hydrology are presented as well as new objectives and results discussed.     

一种用于电离层TEC监测的GNSS信号载波跟踪算法

全球卫星导航系统（GNSS）是电离层TEC监测中应用最普遍的手段. 目前方法通常是在传统导航用途的GNSS接收机输出的原始观测量基础上,经过数据后处理得到电离层TEC信息,其GNSS信号的跟踪处理算法依然采用GNSS导航接收机的算法. 针对GNSS系统用于电离层TEC监测的特殊性,提出一种称为GNSS双频信号和差联合跟踪的新算法,与传统方法相比,该算法直接跟踪电离层TEC的变化,可以提高电离层TEC跟踪的灵敏度和TEC的观测精度,改善电离层TEC监测性能.      

一种适用于高轨空间的GNSS矢量跟踪方案设计

高轨空间中全球卫星导航系统（GNSS）信号可用性严重变差，对GNSS接收机的跟踪性能提出更高要求。利用GNSS信号传播链路模型分析了高轨空间GNSS信号特点，对比了标量跟踪和矢量跟踪这2类典型跟踪环路在高轨空间的适用性，进而设计了一种适用于高轨空间的GNSS矢量跟踪方案。该方案通过估计载噪比确定量测噪声方差阵，以对各通道量测信息进行加权处理来获得高精度的导航参数；并根据高轨航天器的动态性能确定过程噪声方差阵，利用轨道动力学模型对导航参数进行一步预测，从而实现了对各通道信号跟踪参数的准确预测及联合跟踪。仿真验证表明:所设计的跟踪方案可实现高轨空间中强信号对弱信号的辅助跟踪，从而提高了高轨空间中弱信号的跟踪性能及可用性，并对中断信号具有一定的桥接能力。      

Testing a new multivariate GNSS carrier phase attitude determination method for remote sensing platforms

GNSS (Global Navigation Satellite Systems)-based attitude determination is an important field of study, since it is a valuable technique for the orientation estimation of remote sensing platforms. To achieve highly accurate angular estimates, the precise GNSS carrier phase observables must be employed. However, in order to take full advantage of the high precision, the unknown integer ambiguities of the carrier phase observables need to be resolved. This contribution presents a GNSS carrier phase-based attitude determination method that determines the integer ambiguities and attitude in an integral manner, thereby fully exploiting the known body geometry of the multi-antennae configuration. It is shown that this integral approach aids the ambiguity resolution process tremendously and strongly improves the capacity of fixing the correct set of integer ambiguities. In this contribution, the challenging scenario of single-epoch, single-frequency attitude determination is addressed. This guarantees a total independence from carrier phase slips and losses of lock, and it also does not require any a priori motion model for the platform. The method presented is a multivariate constrained version of the popular LAMBDA method and it is tested on data collected during an airborne remote sensing campaign.     

Prospects of Global Navigation Satellite System (GNSS) reflectometry for geodynamic studies

Innovative processing of satellite radar altimetry over solid Earth has been successfully applied for observing geodynamic process of glacial isostatic adjustment over the former Laurentide Ice Sheet in the present-day Hudson Bay land region. In this contribution, a simulation is conducted to study the prospects of the applications of space-/airborne and land-based Global Navigation Satellite System (GNSS) reflectometry to synoptically observe global-scale geodynamic processes with a vertical accuracy of ∼2 mm/yr.     

Seasonal variations of storm-time TEC at European middle latitudes

Global Navigation Satellite System (GNSS) measurements of the Total Electron Content (TEC) from local (Dourbes, 50.1°N, 04.6°E) and European IGS (International GNSS Service) stations were used to obtain the TEC changes during the geomagnetic storms of the latest solar activity cycle. A common epoch analysis, with respect to geomagnetic storm intensity, season, and latitude, was performed on data representing nearly 300 storm events. In general, the storm-time behaviour of TEC shows clear positive and negative phases, relative to the non-storm (median) behaviour, with amplitudes that tend to increase during more intense storms. The most pronounced positive phase is observed during winter, while the strongest and yet shortest negative phase is detected during equinox. Average storm-time patterns in the TEC behaviour are deduced for potential use in ionosphere prediction services.     

CYGNSS海面风速固有误差与时空分布特征

全球导航卫星系统反射测量（GNSS-R）是一种新兴的海面风速遥感技术,对GNSS-R反演风速进行详细定量分析是该技术从科学研究走向业务应用的必要条件。 以气旋全球导航卫星系统（CYGNSS）的风速数据为例,利用时空匹配的浮标风速和欧洲中期天气预报中心（ECMWF）的预报风速数据,详细分析了CYGNSS遥感风速的气候态特征和时空分布特征。基于三配对数据分析方法,阐明了CYGNSS遥感风速的固有误差,并提出了相应的风速标定系数。研究表明:GYGNSS的中低风速（w <10 m·s–1）精度较好,但高风速的误差显著增大;风速误差具有良好的时间一致性,但呈现明显的空间分布不均匀现象;总体而言,CYGNSS风速的固有误差约为1.79 m·s–1。研究结果一方面可为CYGNSS风速数据的业务应用提供参考,另一方面也为进一步标定CYGNSS的反射测量信号提供依据。      

机载GNSS海洋反射信号的建模与仿真

由于全球导航卫星系统反射(GNSS-R)机载试验耗费大和重复性差,需研制GNSS-R信号模拟器,但没有相应的反射信号模型。提出了一种基于数据拟合的机载GNSS海洋反射信号建模方法。首先,对复杂的GNSS海面反射信号进行近似简化。然后利用ZV模型生成的时延相关功率曲线,通过最小二乘拟合和非线性拟合,建立了多条等时延间隔的海洋反射信号功率衰减模型,从而得到机载GNSS海洋反射信号的时延、功率、多普勒频率参数。最后,对多条反射信号的合路信号进行相关的仿真验证。验证的结果表明模拟的14条反射信号的相关功率曲线与ZV模型理论曲线的相关系数优于0.99,能够有效地用于GNSS海洋反射信号的生成。该方法可根据海面风场、浪高、波高等海面信息,模拟不同海况的海洋反射信号,为GNSS-R信号模拟器的研制奠定基础。     

Using the Global Navigation Satellite System and satellite altimetry for combined Global Ionosphere Maps

For deriving global maps of the Total Electron Content (TEC) from space geodetic techniques usually observations from the Global Navigation Satellite System (GNSS) are taken. However, the GNSS stations are inhomogeneously distributed, with large gaps particularly over the sea surface.