一种精确估计区域北斗接收机硬件延迟的方法

提出了一种精确估计区域北斗接收机硬件延迟(DCB)的方法。该方法不需要传统复杂的电离层模型,在已知一个参考站接收机硬件延迟的条件下,利用正常情况下电离层延迟量和卫星-接收机几何距离强相关这一特点,采用站间单差法来精确估计区域内BDS接收机的硬件延迟。试验结果表明,该方法单站估计的单站北斗接收机连续30d的硬件延迟RMS在0.3ns左右。通过GEO卫星双频观测值扣除已知卫星DCB和本文方法估计的接收机DCB,计算对应穿刺点一天的VTEC并和GIM格网内插结果并进行比对分析,二者大小和变化趋势均符合较好,进一步验证了本文提出的方法具有可靠性。     

基于河北省CORS的区域电离层模型精度分析

为提高区域电离层模型和导航定位服务的精度,利用河北省连续运行参考站系统（CORS） 6个基准站的GPS卫星观测数据进行区域电离层建模和接收机差分码偏差（DCB）估计,并引入中国科学院（CAS）发布的电离层产品内插得到的垂直总电子含量（VTEC）进行区域电离层模型精度验证。实验结果表明,估计的单日GPS卫星DCB与产品值精度相当,偏差控制在0.5 ns以内;河北省CORS站GPS系统接收机DCB稳定性较好,5 d的标准偏差均小于0.1 ns;利用河北省CORS建立的区域电离层TEC在地磁平静期与磁暴期均与CAS产品值具有较高的一致性,TEC偏差控制在2 TECU以内。河北省区域电离层模型能有效监测电离层TEC在不同地磁状态下的时空变化,提高区域导航定位服务水平。     

Swarm低轨卫星星座的GPS接收机差分码偏差估计

差分码偏差(differential code bias,DCB)是指由全球导航卫星系统(global navigation satellite system, GNSS)信号接收和发射硬件导致的频率相关的偏差项,对电离层估计有显著的影响,在利用GNSS观测数据提取电离层总电子含量时需要被精确修正,研究利用低轨卫星的星载GNSS观测数据估计DCB尤为重要。使用Swarm星座3颗卫星GPS接收机2016年1月的双频观测值,设计了独立估计和联合估计两种估计方案,采用附加限制条件的间接平差方法对GPS卫星以及星载接收机的DCB进行估计。以中国科学院和德国宇航中心的DCB产品作为参考,分析了两种估计方案的精度和稳定性,相较于独立估计方案,联合估计方案得到的GPS卫星DCB的稳定性较独立估计方案提高了16.6%,且与参考DCB具有更好的一致性。     

利用非组合精密单点定位技术确定斜向电离层总电子含量和站星差分码偏差

联合双频GPS数据,利用相位平滑伪距算法,可得到包含斜向电离层总电子含量（slant total electron content,sTEC）、测站和卫星差分码偏差（differential code bias,DCB）的电离层观测值（称之为＂平滑伪距电离层观测值＂）,常应用于与电离层有关的研究。然而,平滑伪距电离层观测值易受平滑弧段长度和与测站有关的误差影响。提出一种新算法：利用非组合精密单点定位技术（precise point positioning,PPP）计算电离层观测值（称之为＂PPP电离层观测值＂）,进而估计sTEC和站星DCB。基于短基线试验,先用一台接收机按上述两种方法估计sTEC,用于改正另一接收机观测值的电离层延迟以实施单频PPP,结果表明,利用PPP电离层观测值得到的sTEC精度较高,定位结果的可靠性较强。随后,选取全球分布的8个IGS（internationalGNSS service）连续跟踪站2009年1月内某四天的观测数据,利用上述两种电离层观测值计算所有卫星的DCB,并将计算结果与CODE发布的月平均值进行比较,其中,平滑伪距电离层观测值的卫星DCB估值与CODE（Centre for Orbit Deter mination in Europe）发布值的差别较大,部分卫星甚至可达0.2～0.3 ns,而PPP电离层观测值而言,绝大多数卫星对应的差异均在0.1 ns以内。     

基于球谐函数模型的GPS差分码延迟估计

电离层延迟是GNSS观测值中最大的误差源,因此如何利用GNSS观测值确定高精度电离层模型逐渐成为实时导航、定位及大气相关研究的重要内容。在通常采用组合观测值建立模型的方法中,精确估计电离层总电子含量（TEC）的重要误差之一是差分码硬件延迟（DCBs）。为了实时得到P1、P2、C2相互间硬件差分码延迟偏差,本文采用IGS跟踪站的观测数据并利用载波平滑后的差分伪距建立观测方程,对卫星和接收机硬件差分码延迟偏差进行实时解算。经比较模型解算DCB值与IGS最大差异不超过0.8 ns,C1、P1码延迟偏差72%差异值小于0.3 ns,P1、P2的74%差异值小于0.3 ns。     

利用北斗观测实验网解算北斗卫星差分码偏差

差分码偏差(DCB)是电离层总电子含量(TEC)监测和建模的主要系统误差,卫星DCB也是卫星导航系统导航电文的重要参数。研究了卫星DCB的估计算法,推导了不同基准下DCB的转换公式,利用北斗观测实验网解算了2013年北斗卫星的DCB。在同一基准下分析了北斗卫星DCB的稳定性,并与MGEX发布的DCB产品进行了比较分析。实验结果表明,该方法解算的北斗卫星B1-B2DCB在-9~17ns之间,北斗卫星DCB的稳定性优于0.4ns;北斗倾斜地球同步轨道卫星(IGSO)卫星稳定性优于地球静止轨道卫星(GEO)和中圆地球轨道卫星(MEO);利用北斗观测实验网解算的北斗卫星DCB与MGEX解算结果存在最大约1.7ns的系统偏差,可能由于测距码的不一致性所致;接收机硬件材质的不同是导致接收机DCB差异的主要影响因素。     

基于BDS/GPS观测的北斗硬件延迟解算

研究了联合BDS/GPS观测数据基于球冠谐函数的中国区域电离层建模,并精确估计了北斗卫星和接收机DCB。联合解算得到的GPS卫星DCB相对CODE精度优于0.2 ns,GPS接收机DCB相对CODE精度优于1 ns;联合解算得到的中国区域上空VTEC相对CODE事后产品的精度可达2~3 TECU。     

GNSS接收机DCB短期时变特征分析和估计方法

提出一种使用非差非组合精密单点定位(PPP)估计和分析接收机DCB短时时变特征的方法。首先利用非差非组合PPP得到包含接收机DCB的重构电离层参数估值;然后通过IGS电离层GIMs格网模型内插剥离各历元站星斜向电离层距离延迟;最后通过最小二乘约束得到各历元接收机DCB解。由于格网本身精度(2～8 TECU)和插值精度限制,解算出来的接收机DCB并不能真实反映其短期时变特征。为此,提出利用站间单差或者历元间差分的方法还原其真实的变化态势。实验结果表明,所提出的方法能够正确估计接收机DCB,并能真实还原其短期时变特征,具有良好的适用性。     

附加闭合差约束的BDS频间偏差估计模型

中国的北斗卫星导航系统(BeiDou navigation satellite system,BDS)是全球导航卫星系统(global navigation satellite system,GNSS)中唯一全星座提供3频信号的卫星导航系统,其信号频率间共存在3种频间偏差(differential code bias,DCB),分别是DCB_(C2I-C7I)、DCB_(C2I-C6I)、DCB_(C7I-C6I)。理论上,这3种DCB之间存在代数和为零的关系。基于3种频间偏差闭合差约束,加入DCB观测方程,以北斗中轨道(medium earth orbit,MEO)和倾斜地球同步轨道(inclined geosynchronous satellite orbit,IGSO)卫星作为参考卫星,采用附加限制条件的间接平差方法同步估计BDS的3种DCB。选取2018年1月1日—30日多模GNSS实验(multi-GNSS experiment,MGEX)基准站的BDS 3频数据,分别采用附加闭合差约束估计和独立求解两种方法计算北斗二代卫星的3种频间偏差。以中国科学院(Chinese Academy of Sciences,CAS)和德国宇航中心(Deutsches Zentrum für Luft-und Raumfahrt,DLR)的DCB产品作为参考,分析了所提方法估计的DCB精度、稳定性及部分典型卫星的DCB时间序列,验证了所提方法对北斗3频DCB估计的适用性和科学性,并通过BDS单频标准单点定位(standard point positioning,SPP)实验验证了DCB对单点定位精度的影响效果。     

基于站间单差相位绕转数据估计测站天线旋转速率的方法研究

相位绕转在GNSS定位中是一种误差源,但包含接收机天线旋转的有用信息。提出了一种基于站间单差相位绕转观测数据估计测站天线旋转速率的方法。首先由无几何距离观测值的变化判断天线旋转的开始与结束时间,然后利用单颗卫星站间差分的无几何距离观测值求出测站的天线旋转角度,以及单颗卫星的测站天线旋转速率,最后将所有卫星计算的测站天线旋转速率按照高度角加权平均得到最终的天线旋转速率。通过精心设计实验方案,经实测数据验证,该方法可以精确地估计测站天线的旋转速率,在本实验中,天线旋转平均速率估计精度约为0.5°/s。     

M_DCB: Matlab code for estimating GNSS satellite and receiver differential code biases

Global navigation satellite systems (GNSS) have been widely used to monitor variations in the earth’s ionosphere by estimating total electron content (TEC) using dual-frequency observations. Differential code biases (DCBs) are one of the important error sources in estimating precise TEC from GNSS data. The International GNSS Service (IGS) Analysis Centers have routinely provided DCB estimates for GNSS satellites and IGS ground receivers, but the DCBs for regional and local network receivers are not provided. Furthermore, the DCB values of GNSS satellites or receivers are assumed to be constant over 1?day or 1?month, which is not always the case. We describe Matlab code to estimate GNSS satellite and receiver DCBs for time intervals from hours to days; the software is called M_DCB. The DCBs of GNSS satellites and ground receivers are tested and evaluated using data from the IGS GNSS network. The estimates from M_DCB show good agreement with the IGS Analysis Centers with a mean difference of less than 0.7?ns and an RMS of less than 0.4?ns, even for a single station DCB estimate.     

Determining receiver biases in GPS-derived total electron content in the auroral oval and polar cap region using ionosonde measurements

Global Positioning System (GPS) total electron content (TEC) measurements, although highly precise, are often rendered inaccurate due to satellite and receiver differential code biases (DCBs). Calculated satellite DCB values are now available from a variety of sources, but receiver DCBs generally remain an undertaking of receiver operators and processing centers. A procedure for removing these receiver DCBs from GPS-derived ionospheric TEC at high latitudes, using Canadian Advanced Digital Ionosonde (CADI) measurements, is presented. Here, we will test the applicability of common numerical methods for estimating receiver DCBs in high-latitude regions and compare our CADI-calibrated GPS vertical TEC (vTEC) measurements to corresponding International GNSS Service IONEX-interpolated vTEC map data. We demonstrate that the bias values determined using the CADI method are largely independent of the topside model (exponential, Epstein, and α-Chapman) used. We further confirm our results via comparing bias-calibrated GPS vTEC with those derived from incoherent scatter radar (ISR) measurements. These CADI method results are found to be within 1.0 TEC units (TECU) of ISR measurements. The numerical methods tested demonstrate agreement varying from within 1.6 TECU to in excess of 6.0 TECU when compared to ISR measurements.     

On the short-term temporal variations of GNSS receiver differential phase biases

As a first step towards studying the ionosphere with the global navigation satellite system (GNSS), leveling the phase to the code geometry-free observations on an arc-by-arc basis yields the ionospheric observables, interpreted as a combination of slant total electron content along with satellite and receiver differential code biases (DCB). The leveling errors in the ionospheric observables may arise during this procedure, which, according to previous studies by other researchers, are due to the combined effects of the code multipath and the intra-day variability in the receiver DCB. In this paper we further identify the short-term temporal variations of receiver differential phase biases (DPB) as another possible cause of leveling errors. Our investigation starts by the development of a method to epoch-wise estimate between-receiver DPB (BR-DPB) employing (inter-receiver) single-differenced, phase-only GNSS observations collected from a pair of receivers creating a zero or short baseline. The key issue for this method is to get rid of the possible discontinuities in the epoch-wise BR-DPB estimates, occurring when satellite assigned as pivot changes. Our numerical tests, carried out using Global Positioning System (GPS, US GNSS) and BeiDou Navigation Satellite System (BDS, Chinese GNSS) observations sampled every 30 s by a dedicatedly selected set of zero and short baselines, suggest two major findings. First, epoch-wise BR-DPB estimates can exhibit remarkable variability over a rather short period of time (e.g. 6 cm over 3 h), thus significant from a statistical point of view. Second, a dominant factor driving this variability is the changes of ambient temperature, instead of the un-modelled phase multipath.     

Estimation and analysis of Galileo differential code biases

When sensing the Earth’s ionosphere using dual-frequency pseudorange observations of global navigation satellite systems (GNSS), the satellite and receiver differential code biases (DCBs) account for one of the main sources of error. For the Galileo system, limited knowledge is available about the determination and characteristic analysis of the satellite and receiver DCBs. To better understand the characteristics of satellite and receiver DCBs of Galileo, the IGGDCB (IGG, Institute of Geodesy and Geophysics, Wuhan, China) method is extended to estimate the satellite and receiver DCBs of Galileo, with the combined use of GPS and Galileo observations. The experimental data were collected from the Multi-GNSS Experiment network, covering the period of 2013–2015. The stability of both Galileo satellite and receiver DCBs over a time period of 36 months was thereby analyzed for the current state of the Galileo system. Good agreement of Galileo satellite DCBs is found between the IGGDCB-based DCB estimates and those from the German Aerospace Center (DLR), at the level of 0.22 ns. Moreover, high-level stability of the Galileo satellite DCB estimates is obtained over the selected time span (less than 0.25 ns in terms of standard deviation) by both IGGDCB and DLR algorithms. The Galileo receiver DCB estimates are also relatively stable for the case in which the receiver hardware device stays unchanged. It can also be concluded that the receiver DCB estimates are rather sensitive to the change of the firmware version and that the receiver antenna type has no great impact on receiver DCBs.     

Phase center modeling for LEO GPS receiver antennas and its impact on precise orbit determination

Most satellites in a low-Earth orbit (LEO) with demanding requirements on precise orbit determination (POD) are equipped with on-board receivers to collect the observations from Global Navigation Satellite systems (GNSS), such as the Global Positioning System (GPS). Limiting factors for LEO POD are nowadays mainly encountered with the modeling of the carrier phase observations, where a precise knowledge of the phase center location of the GNSS antennas is a prerequisite for high-precision orbit analyses. Since 5 November 2006 (GPS week 1400), absolute instead of relative values for the phase center location of GNSS receiver and transmitter antennas are adopted in the processing standards of the International GNSS Service (IGS). The absolute phase center modeling is based on robot calibrations for a number of terrestrial receiver antennas, whereas compatible antenna models were subsequently derived for the remaining terrestrial receiver antennas by conversion (from relative corrections), and for the GNSS transmitter antennas by estimation. However, consistent receiver antenna models for space missions such as GRACE and TerraSAR-X, which are equipped with non-geodetic receiver antennas, are only available since a short time from robot calibrations. We use GPS data of the aforementioned LEOs of the year 2007 together with the absolute antenna modeling to assess the presently achieved accuracy from state-of-the-art reduced-dynamic LEO POD strategies for absolute and relative navigation. Near-field multipath and cross-talk with active GPS occultation antennas turn out to be important and significant sources for systematic carrier phase measurement errors that are encountered in the actual spacecraft environments. We assess different methodologies for the in-flight determination of empirical phase pattern corrections for LEO receiver antennas and discuss their impact on POD. By means of independent K-band measurements, we show that zero-difference GRACE orbits can be significantly improved from about 10 to 6 mm K-band standard deviation when taking empirical phase corrections into account, and assess the impact of the corrections on precise baseline estimates and further applications such as gravity field recovery from kinematic LEO positions.     

GNSS receiver implementations to mitigate the effects of commensurate sampling frequencies on DLL code tracking

The sampling frequency of a digitized intermediate frequency signal has a strong effect on the measurement accuracy of Global Navigation Satellite System (GNSS) receivers. The delay-locked loop tracking error is significant when the sampling frequency is an integer multiple of the code chipping rate, the so-called commensurate sampling frequency, and the number of distinct instantaneous residual code phases is low. This results in distortions of the correlation shape and discriminator functions that lead to a significant accuracy degradation. These effects are most pronounced when the sampling frequency is low. Notwithstanding, it is generally good for receivers to keep the sampling frequency to a minimum owing to the processing load and power consumption. It creates a challenge for existing GNSS signal processing techniques. Random, sine and sawtooth jitters have been found to mitigate these distortions considerably. A software algorithm and two hardware receiver implementations of these solutions are proposed. A register-based architecture can be directly applied to the conventional receiver architecture, while the increase in resource and power consumption is insignificant. A RAM-based design cannot only considerably minimize utilized resources but also slightly reduce the power consumption compared to the conventional architecture.     

Calibration errors on experimental slant total electron content (TEC) determined with GPS

The Global Positioning System (GPS) has become a powerful tool for ionospheric studies. In addition, ionospheric corrections are necessary for the augmentation systems required for Global Navigation Satellite Systems (GNSS) use. Dual-frequency carrier-phase and code-delay GPS observations are combined to obtain ionospheric observables related to the slant total electron content (sTEC) along the satellite-receiver line-of-sight (LoS). This observable is affected by inter-frequency biases [IFB; often called differential code biases (DCB)] due to the transmitting and the receiving hardware. These biases must be estimated and eliminated from the data in order to calibrate the experimental sTEC obtained from GPS observations. Based on the analysis of single differences of the ionospheric observations obtained from pairs of co-located dual-frequency GPS receivers, this research addresses two major issues: (1) assessing the errors translated from the code-delay to the carrier-phase ionospheric observable by the so-called levelling process, applied to reduce carrier-phase ambiguities from the data; and (2) assessing the short-term stability of receiver IFB. The conclusions achieved are: (1) the levelled carrier-phase ionospheric observable is affected by a systematic error, produced by code-delay multi-path through the levelling procedure; and (2) receiver IFB may experience significant changes during 1 day. The magnitude of both effects depends on the receiver/antenna configuration. Levelling errors found in this research vary from 1.4 total electron content units (TECU) to 5.3 TECU. In addition, intra-day vaiations of code-delay receiver IFB ranging from 1.4 to 8.8 TECU were detected.     

The role of Global Navigation Satellite System (GNSS) software radios in embedded systems

This paper outlines the motivation for Global Navigation Satellite System (GNSS) software receivers. Features of traditional and software-based GNSS receiver architectures are highlighted and compared, focusing on the advantages of the software design. The choice of which architecture is advantageous, particular in the case of embedded systems, is present along with design criteria—both for the current environment as well as what can be expected in the future. Electronic Publication