Differential GPS/inertial navigation approach/landing flight testresults

In November 1990, a differential GPS/inertial flight test was conducted to acquire a system performance database and demonstrate automatic landing using an integrated differential GPS/INS with barometric and radar altimeters. Flight test results obtained from postflight data analysis are presented. These results include characteristics of DGPS/inertial error, using a laser tracker as a reference. In addition, data are provided on the magnitude of the differential correlations and vertical channel performance with and without radar altimeter augmentation. Flight test results show one sigma DGPS/inertial horizontal errors of 9 ft and one sigma DGPS inertial vertical errors of 15 ft. Without selective availability effects, the differential corrections are less than 10 ft and are dominated by receiver unique errors over the time period of an approach. Therefore, the one sigma performance of the autonomous GPS (8-ft horizontal and 20-ft vertical) is very similar to the DGPS/inertial performance. Postprocessed results also demonstrate significant improvements in vertical channel performance when GPS/inertial is aided with radar altimeter along with a low-resolution terrain map     

Forward error correction for radiobeacon broadcast of differentialGPS data

Marine radiobeacon networks are being used to broadcast differential Global Positioning System (DGPS) corrections to marine users. The correction data digitally modulate signals from some of the existing marine radiobeacons, which operate in the 285 to 325 kHz band, creating DGPS/radiobeacons. The corrections improve the accuracy of the GPS fix from 100 m to 5-10 m, and provide position fixing service for many marine applications which are too demanding for the normal GPS service. Forward error correction can be used to improve the reliability or range of the DGPS/radiobeacon signal. The improvements made possible by channel coding are analyzed, and a code for DGPS/radiobeacons is recommended     

Simulation of the Canadian GOES Signal Processor

An international experiment is being conducted to assess the use of geostationary spacecraft to relay 406 MHz distress beacon signals. The development is described of a signal processor that will search a nominal 10 kHz portion of the 406 MHz band, detect anddemodulate a small number of beacons, and produce an alertmessage with low probability of message error. It is shown that thenominal composite link C/No is about 30 dB Hz. The proposedprocessor is described and simulation results are presented. Theresults indicate that the processor has a threshold of about26 dB · Hz, a message error rate of 2 percent or less, and a messagetransfer time of less than 10 min.     

Position error bound for UWB localization in dense cluttered environments

For most outdoor applications, systems such as global positioning system (GPS) provide users with accurate location estimates. However, similar range-only localization techniques in dense cluttered environments typically lack accuracy and reliability due, notably, to dense multipath, line-of-sight (LOS) blockage and excess propagation delays through materials. In particular, range measurements between a receiver and a transmitter are often positively biased. Furthermore, the quality of the range measurement degrades with distance, and the geometric configuration of the beacons also affects the localization accuracy. In this paper we derive a fundamental limit of localization accuracy for an ultrawide bandwidth (UWB) system operating in such environments, which we call the position error bound (PEB). The impact of different ranging estimation errors due to beacons distance and biases on the best positioning accuracy is investigated. The statistical characterization of biases coming from measurement campaigns can easily be incorporated into this analysis. We show that the relative importance of information coming from different beacons varies depending on the propagation conditions, such as whether the beacon is LOS or non-line-of-sight (NLOS). We show, in particular, that any a priori information knowledge on NLOS beacons can significantly improve the localization accuracy, especially in dense cluttered environments. Finally we put forth the concept of localization outage probability and epsi-localization accuracy outage, and use them to characterize the quality of localization throughout the area.     

Recent progress in satellite radio beacon studies with particular emphasis on the ATS-6 radio beacon experiment

In May 1974 a new era in satellite radio beacon studies of the ionosphere opened with the ATS-6 Radio Beacon Experiment. The history of radio beacon studies up to that time is reviewed briefly and the particular features of the ATS-6 beacon are discussed together with the basic theory required to interpret the measurements. The main emphasis is on the ATS-6 beacon experiment but other beacon data are discussed which provide the necessary background. The diurnal and seasonal variations of the total electron content and the plasmaspheric content are presented for the U.S.A. and Europe. In winter the plasmaspheric content over the Western Hemisphere maximizes at night while in Europe and the Pacific it appears to peak near noon. This is thought to be caused by flow of plasma from the local and conjugate ionospheres. Night maxima of total electron content are found showing that they do not arise from depletions of the plasmaspheric content. The plasmaspheric content is highly sensitive to solarterrestrial disturbance, it reaches a minimum on the third day of a storm and may take between 10 and 20 days of partial filling and emptying to recover. Traveling disturbances in U.S.A., Europe, and India show similarities of speeds but not of direction. Beacon observations of micropulsations in total content, tropospheric fluctuations and Fresnel diffraction by intense ionospheric irregularities are discussed together with radio wave scintillations and some applications of beacon radio data to communications and navigation.     

Detection of interference/jamming and spoofing in a DGPS-aidedinertial system

Previous research at the Air Force Institute of Technology (AFIT) has resulted in the design of a differential Global Positioning System (DGPS) aided INS-based (inertial navigation system) precision landing system (PLS) capable of meeting the FAA precision requirements for instrument landings. The susceptibility of DGPS transmissions to both intentional and nonintentional interference/jamming and spoofing must be addressed before DGPS may be safely used as a major component of such a critical navigational device. This research applies multiple model adaptive estimation (MMAE) techniques to the problem of detecting and identifying interference/jamming and spoofing in the DGPS signal. Such an MMAE is composed of a bank of parallel filters, each hypothesizing a different failure status, along with an evaluation of the current probability of each hypothesis being correct, to form a probability-weighted average state estimate as an output. For interference/jamming degradation represented as increased measurement noise variance, simulation results show that, because of the good failure detection and isolation (FDI) performance using MMAE, the blended navigation performance is essentially that of a single extended Kalman filter (EKF) artificially informed of the actual interference noise variance. However, a standard MMAE is completely unable to detect spoofing failures (modeled as a bias or ramp offset signal directly added to the measurement). This work describes a moving-bank pseudoresidual MMAE (PRMMAE) to detect and identify such spoofing. Using the PRMMAE algorithm, spoofing is very effectively detected and isolated; the resulting navigation performance is equivalent to that of an EKF operating in an environment without spoofing     

频率选择性衰落信道DS-CDMA无人机中继通信系统航迹规划

无人机中继通信是实现远距离无线通信的一种重要技术手段，无人机的飞行航迹对无人机中继通信系统的链路传输可靠性存在显著的影响，在频率选择性衰落信道环境下研究了基于直序列码分多址（DS-CDMA）的无人机中继通信系统的航迹优化的问题。首先，给出了基于DS-CDMA的译码转发无人机中继通信系统的模型，并理论分析给出无人机中继通信系统的链路中断概率及平均误码率计算公式，以此为基础，基于链路中断概率最小化准则提出了中继无人机的航迹规划方法，最后通过仿真验证了所提出方法的正确性与有效性。研究表明：最大比值合并DS-CDMA无人机中继通信系统可充分获取频率选择性衰落信道提供的分集增益，显著改善链路传输的可靠性。     

用于SAR运动补偿的DGPS/SINS组合系统研究

使用考虑位置误差相关项的伪距率观测模型 ,研究了用于合成孔径雷达运动补偿的差分 GPS/ SINS伪距率组合系统。结果表明 ,组合系统的长期位置精度能达到 1 m左右。 GPS数据更新率低于 INS,在 GPS测量时间间隔内 ,组合系统的性能仅由 INS决定。虽然 INS误差随时间积累 ,在 GPS数据更新率为 1 s的情况下 ,即使采用中等精度的惯性仪表 ,其相对位置精度为厘米级 (这里相对位置精度指组合系统在 GPS测量时间间隔内位置误差的变化范围)。     

Analysis of airborne GPS multipath effects using high-fidelity EM models

Airborne GPS systems are being upgraded to provide sufficient positioning accuracy to support automatic landing operations in low visibility conditions. This is made possible by differential GPS (DGPS), in which the errors common to the airborne receiver and ground station are removed by knowledge of the latter's precise location. However, errors specific to the airborne system remain, of which the dominant components are receiver noise and multipath. To support the assessment of the integrity of the signal in space, these residual errors are incorporated in a statistically based error model, designated as the "standard model." The standard model is defined as the standard deviation of a Gaussian distribution that overbounds the residual pseudo-range (PR) error. It relates the standard deviation of the overbounding distribution to the elevation angle of the satellite relative to the local level coordinate system. The international community is currently developing improved standards to enable DGPS systems to support landings in the worst visibility conditions (i.e., CAT III). As a part of this development, the standard model for multipath is being re-evaluated and an improved model is sought. In order to better characterize the residual multipath errors, tools for accurate calculation of the airframe scattering effects are needed. Development of such tools is the subject of this paper. A new method for accurately computing pseudo-range error, based on the use of high-fidelity EM models, is described. This approach provides new insight into the mechanisms causing multipath error.     

应用信标模式测速的实现方法及论证

根据未来固体火箭型号试验越来越多,首区无线电设备受火箭尾焰干扰的情况以及测控体制的发展要求,探讨了利用信标模式[1]进行高精度测量时所面临的工程问题,包括信标模式下测量值的误差项分析,减小测量误差的方法等内容。     

面向室内场景的惯性磁感应定位方法

针对卫星拒止的室内导航问题,在现有磁信标定位技术的基础上,通过分析低频旋转磁场的特点,建立了一种不受传感器姿态和磁信标磁矩信息影响的磁感应矢量夹角观测模型,并结合MEMS惯性导航的误差模型提出了一种以磁感应矢量夹角的正余弦值为量测信息的惯性磁感应动态定位方法,避免了磁传感器姿态误差和磁矩误差对定位结果的影响。利用实验室的双轴磁信标实验系统进行静态测试,验证了磁感应定位模型具有不受传感器姿态、环境中遮挡物以及磁矩影响的特点;通过数值仿真的方式对基于无迹卡尔曼滤波(UKF)的惯性磁感应定位方法进行验证,结果表明该方法能有效地抑制惯性导航的发散,且位置最大误差小于0.75 m,满足大多数室内场景下的高精度导航定位需求。     

Adaptive Forward Error Control for Digital Satellite Systems

Two hybrid schemes of time-frequency resource sharing to increase the rain margin of Ku-and Ka-band satellite systems are proposed. Scheme 1 requires sharing a small pool of bandwidth for adaptive forward error control coding, sharing a small pool of time frame for rate reduction, and sharing a portion of low frequency time-division multiple access (TDMA) back-up frame for downlink transmission to the rain affected stations. Scheme 2 utilizes variable rate modulation and forward error correction, shares a small pool of time frame for rate reduction, and shares a portion of low frequency TDMA back-up frame. Effective usable capacities of the system using these schemes are calculated. Distribution of resources in order to maximize the effective usable capacity is also analyzed. The results obtained are compared with other adaptive schemes. Preliminary analysis shows that the utilized capacity of scheme 1 exceeds 99 percent of the effective usable capacity possible if it never rains for an outage of 0.05 percent and fade margin of 2.5 dB. For scheme 2 similar performance is achievable at a fade margin of 1.5 dB. For higher outage objectives the loss of effective utilized capacity is higher for scheme 2.     

用太阳源法测量遥测系统的品质因数G/T

阐述利用太阳源法测量遥测系统品质因数G/T的原理,提出可用于工程实际的实现方法,重点对太阳流量密度、K1因子、K2因子等参数进行分析,给出工程应用计算方法。最后,以某遥测系统为例,给出了采用太阳源法和信标塔法的测量值,并对测试结果进行比对分析。实测结果分析表明,太阳源法测得的G/T重复测试精度小于0.4 dB,与信标塔法测试结果相差小于0.6 dB。因此,太阳源法合理可行,可广泛应用于遥测系统指标测试。     

无人机下滑着陆DGPS/INS导航及引导系统的仿真与试验研究

介绍了一种采用输出校正方式DGPS/INS组合导航系统引导无人机下滑着陆的方案。给出了该方案的数学模型,对组合导航系统进行了数字仿真,并在数字仿真的基础上进行了动态跑车试验,验证了所设计的输出校正方式DGPS/INS组合导航系统能够满足引导无人机自动下滑着陆的精度要求。     

Carrier phase navigation architecture for shipboard relative GPS

Carrier phase differential GPS (DGPS) navigation architectures and algorithms for automatic shipboard landing of aircraft are described. Processing methodologies are defined to provide high integrity carrier phase cycle estimation and positioning by optimally exploiting the complementary benefits of measurement filtering and satellite geometric redundancy for the terminal navigation problem. Navigation performance sensitivity to the standard deviations of raw carrier and code phase measurement errors, measurement error correlation times, and the filtering duration is quantified. Necessary conditions to ensure acceptable terminal navigation availability are specifically defined.     

差分GPS在飞行试验中的应用

ＧＰＳ是美国正在大力发展的高精度卫星导航系统。主要叙述差分ＧＰＳ在飞行试验中的应用，首先介绍ＧＰＳ的伪距测量定位原理和测速原理，ＤＧＰＳ提高定位精度的原理。     

Polar ionospheric disturbance effects on NNSS satellite positioning

From the analysis of the United States Navy navigation satellite system (NNSS) positioning data of about 10000 passes, it is clearly found that polar ionospheric disturbance affects the positioning. The positioning error increases with increasing geomagnetic disturbance level (local K-index), and the pass number of position fixes decreases by one or two per day when the K-index is large. These effects may arise from the spatial gradients of electron density and/or the radio wave scattering due to well-developed ionospheric irregularities inherent to the disturbed auroral ionosphere     

基于低频磁信标的全天时自主定位方法

由于以GPS、北斗为代表的GNSS信号容易受到天气和位置的影响,单纯使用GNSS在某些条件下会有较大的定位误差,甚至无法提供定位服务。针对这种情况研究了基于低频磁信标的自主定位方法,分析了电磁学中的毕奥-萨伐尔定律等原理,建立了基于低频磁信标定位的系统模型,给出了分离式双信标定位方法。通过实验分析了信标安装的误差特性,最后通过仿真和实际实验验证了该定位方法的有效性,对于室内机器人导航、地下生物探索、水下跟踪定位等单独采用GNSS定位影响较大的场景下的自主定位与导航具有重要的工程应用价值。     

Relay selection based on MAP estimation for cooperative communication with outdated channel state information

In this paper, we consider an amplify-and-forward (AF) cooperative communication system when the channel state information (CSI) used in relay selection differs from that during data transmission, i.e., the CSI used in relay selection is outdated. The selected relay may not be actually the best for data transmission and the outage performance of the cooperative system will deteriorate. To improve its performance, we propose a relay selection strategy based on maximum a posteriori (MAP) estimation, where relay is selected based on predicted signal-to-noise ratio (SNR). To reduce the computation complexity, we approximate the a posteriori probability density of SNR and obtain a closed-form predicted SNR, and a relay selection strategy based on the approximate MAP estimation (RS-AMAP) is proposed. The simulation results show that this approximation leads to trivial performance loss from the perspective of outage probability. Compared with relay selection strategies given in the literature, the outage probability is reduced largely through RS-AMAP for medium-to-large transmitting powers and medium-to-high channel correlation coefficients.     

Phase modulated DGPS transmitter implemented with a clamped-mode resonant converter

A clamped-mode resonant converter (CMRC) is proposed to be used as a transmitter which broadcasts correction signals in a differential Global Positioning System (DGPS). The digital information of the DGPS modulates the carrier with different modulation methods minimum shift keying (MSK) (recommended for the marine radiobeacon band), stepped binary phase shift keying (S-BPSK) and BPSK. The first two methods allow keeping the converter's operation in soft switching mode. Extensive trials under impedance mismatch and modulation conditions confirm that the soft switching mode is preserved.