ILS glidescope evaluation of imperfect terrain

Instrument landing systems (ILS) are normally designed assuming the site around them to be flat. Uneven terrain results in undulations in the glidescope. In recent years, models have been evolved for predicting such aberrations as a simpler alternative to experimental methods. Such modeling normally assumes the ground to be fully conducting. A method is presented for considering imperfect terrain conductivity within the framework of the uniform theory of diffraction (UTD). A single impedance wedge formulation is developed to a form that resembles the standard form of UTD, with only one extra term in the diffraction coefficient. This extends the applicability of the standard UTD formulation and software packages to the case of the imperfectly conducting terrain. The method has been applied to a real airport site in India and improved agreement with measured glidescope parameters is demonstrated     

GTD Terrain Reflection Model Applied to ILS Glide Scope

The capability of calculating the reflection of electromagnetic signals from uneven terrain has many applications. One of these is the determination of instrument landing system (ILS) glide slope performance. For this application the wavelength is approximately 1 m, incidence angles are usually near grazing, and the fields are horizontally polarized, so that gross irregularities such as dropoffs and hills are more important than surface roughness. Past approaches used to calculate the ground reflections for this application have been three-dimensional physical optics models which were very cumbersome and time consuming and which neglected important diffraction and shadowing phenomenon; a two-dimensional physical optics model which was faster than the three-dimensional models but ignored many shadowing and transverse terrain variation effects; and a half-plane diffraction model which is applicable only to a specified type of terrain geometry. In this paper a terrain reflection model based on the geometrical theory of diffraction (GTD) is described which can accommodate any piecewise linear terrain profile, requires less computer time than the physical optics models, is capable of including transverse terrain effects, and determines the reflected fields with all important diffraction and blockage effects included.     

Novel minimum time trajectory planning in terrain following flights

A new methodology has been proposed to enhance inverse dynamics applications in the process of trajectory planning and optimization in terrain following flights (TFFs). The new approach uses a least square scheme to solve a general two-dimensional (2-D) TFF in a vertical plane. In the mathematical process, Chebyshev polynomials are used to model the geographical data of the terrain in a given route in a manner suitable for the aircraft at hand. The aircraft then follows the modeled terrain with sufficient clearance. In this approach the terrain following (TF) problem is effectively converted to an optimal tracking problem. Results show that this method provides a flexible approach to solve the TFF problem especially in conditions where the existing terrain to be flown over is not mathematically well behaved     

A ground and obstacle collision avoidance technique (GOCAT)

The need to provide aircrew with a timely warning of a dangerously close approach to the terrain is discussed, along with the benefits this capability is expected to provide and specific requirements that it must fulfil. The ground proximity warning techniques that are available are reviewed, and their benefits and limitations are assessed. The GEC Avionics Ground and Obstacle Collision Avoidance Technique (GOCAT) is described. The particular advantages of the GOCAT approach are identified. Because GOCAT uses a database of the terrain and obstacles around the aircraft it has a very low nuisance alarm rate but will always generate a warning when the aircraft is in a dangerous situation. Issues of parameter selection, search area definition and system limitations are examined. The results of a GOCAT simulation are presented. The system performance parameters are discussed, and database requirements are reviewed. The characteristics of a practical implementation for both civil and military applications are briefly stated     

Implementing digital terrain data in knowledge-aided space-time adaptive processing

Many practical problems arise when implementing digital terrain data in airborne knowledge-aided (KA) space-time adaptive processing (STAP). This paper addresses these issues and presents solutions with numerical implementations. In particular, using digital land classification data and digital elevation data, techniques are developed for registering these data with radar return signals, correcting for Doppler and spatial misalignments, adjusting for antenna gain, characterizing clutter patches for secondary data selection, and ensuring independent secondary data samples. These techniques are applied to select secondary data for a single-bin post-Doppler STAP algorithm using multi-channel airborne radar measurement (MCARM) program data. Results with the KA approach are compared with those obtained using the standard sliding window method for choosing secondary data. These results illustrate the benefits of using terrain information, a priori data about the radar, and the importance of statistical independence when selecting secondary data for improving STAP performance     

Multipath interferometry technique for airborne array calibration

A new technique is developed to compensate multiple-wavelength distortion in airborne antenna arrays. This approach exploits the phase information in microwave reflections from arbitrary terrain. To handle reflections incident over a broad angle, a range-Doppler preprocessor is used in each element channel to resolve wavefronts incident simultaneously from different directions. The phase information for each direction of arrival is compared between elements and processed by optimal estimators to determine the phase corrections needed to compensate the distortion. To develop the estimators, a statistical model of the complex baseband terrain reflections is developed. This is in turn used to generate conditional probability densities involving the range Doppler observations and the parameters to be estimated. These densities are subsequently used to develop minimum variance and maximum likelihood estimators. The new estimators use additional information that has not been exploited by previous techniques and therefore provide enhanced performance     

Terrain database integrity monitoring for synthetic vision systems

A real-time terrain database integrity monitor for synthetic vision systems (SVS) that are to be used in civil aviation is presented. SVS provides pilots with advanced display technology including terrain information as well as other information about the external environment such as obstacles and traffic. The use of SVS to support strategic and tactical decision-making and the compelling nature of the terrain depiction may require terrain database server certification at the essential and flight-critical levels. SVS and terrain database characteristics are discussed and a failure model is identified. Real-time integrity monitors are proposed that check the consistency between terrain profiles described by the database and terrain profiles that are sensed in flight by either a downward-looking (DWL) sensor or a forward-looking (FWL) season A DWL sensor scheme is discussed in detail and it is shown that this scheme can provide the necessary integrity required for an essential certification of a terrain database server.     

Multi-sensor terrain classification for safe spacecraft landing

A novel multi-sensor information fusion methodology for intelligent terrain classification is presented. The focus of this research is to analyze safety characteristics of the terrain using imagery data obtained by on-board sensors during spacecraft descent. This information can be used to enable the spacecraft to land safely on a planetary surface. The focus of our approach is on robust terrain analysis and information fusion in which the terrain is analyzed using multiple sensors and the extracted terrain characteristics are combined to select safe landing sites for touchdown. The novelty of this method is the incorporation of the T-Hazard Map, a multi-valued map representing the risk associated with landing on a planetary surface. The fusion method is explained in detail in this paper and computer simulation results are presented to validate the approach.     

Layover solution in multibaseline SAR interferometry

In this work, spectral estimation techniques are used to exploit baseline diversity of a multichannel interferometric synthetic aperture radar (SAR) system and overcome the layover problem. This problem arises when different height contributions collapse in the same range-azimuth resolution cell, due to the presence of strong terrain slopes or discontinuities in the sensed scene. We propose a multilook approach to counteract the presence of multiplicative noise, which is due to the extended nature of natural targets; to this purpose we extend the RELAX algorithm to the multilook data scenario (M-RELAX). A thorough performance analysis of nonparametric (beamforming and Capon) and parametric (root MUSIC and M-RELAX) techniques is carried out based on Monte Carlo simulations and Cramer-Rao lower bounds (CRLB) calculation. The results suggest the superiority of parametric methods over nonparametric ones.     

基于数据操作的航天测控软件测试复用模型

测试复用技术是提高软件测试效率和质量的重要技术手段,在分析航天测控软件行为模式的基础上提出了基于数据操作的航天测控软件测试复用模型,给出了测试用例搜索和匹配算法;以航天测控外测数据处理软件的可靠性测试为例给出了模型的具体应用。结果表明,基于给定模型的测试复用方法可提供该软件约65%的测试用例,有效提高了测试效率,所提出的模型和方法对专业领域软件测试效率和质量的提高具有一定的借鉴意义。     

Autonomous airborne navigation in unknown terrain environments

We address the issue of autonomous navigation, that is, the ability for a navigation system to provide information about the states of a vehicle without the need for a priori infrastructure such as GPS, beacons, or a map. The algorithm is known as simultaneous localisation and mapping (SLAM) and it is a terrain aided navigation system (TANS) which has the capability for online map building, and simultaneously utilising the generated map to bound the errors in the navigation solution. Since the algorithm does not require any a priori terrain information or initial knowledge of the vehicle location, it presents a powerful navigation augmentation system or more importantly, it can be implemented as an independent navigation system. Results are first provided using computer simulation which analyses the effect of the spatial density of landmarks as well as the quality of observation and inertial navigation data, and then finally the real time implementation of the algorithm on an unmanned aerial vehicle (UAV).     

基于位图的三维地形景观生成

地形模型的三维实时显示技术是虚拟现实的一项关键技术。地形模型的建立是地形三维仿真研究的重点之一。在现存的各种地形模型的生成方法中,利用地形位图建立地形模型的方法一经提出,便因其算法简单,形象直观,易于实现等特点日益得到重视并且广泛的应用于游戏制作和虚拟场景的构建等领域。基于地形位图图像,成功的建立了较大规模的地形模型,进行了纹理映射,结果真实感较强,并实现了实时交互的场景漫游。     

Road-map assisted ground moving target tracking

Tracking ground targets with airborne GMTI (ground moving target indicator) sensor measurements proves to be a challenging task due to high target density, high clutter, and low visibility. The exploitation of nonstandard background information such as road maps and terrain information is therefore highly desirable for the enhancement of track quality and track continuity. The present paper presents a Bayesian approach to incorporate such information consistently. It is particularly suited to deal with winding roads and networks of roads. The target dynamics is modeled in quasi one-dimensional road coordinates and mapped onto ground coordinates using linear road segments taking road map errors into account. The case of several intersecting roads with different characteristics, such as mean curvature, slope, or visibility, is treated within an interacting multiple model (IMM) scheme. Targets can be masked both by the clutter notch of the sensor and by terrain obstacles. Both effects are modeled using a sensor-target state dependent detection probability. The iterative filter equations are formulated within a framework of Gaussian sum approximations on the one hand and a particle filter approach on the other hand. Simulation results for single targets taken from a realistic ground scenario show strongly reduced target location errors compared with the case of neglecting road-map information. By modeling the clutter notch of the GMTI sensor, early detection of stopping targets is demonstrated     

三维地形不变性特征描述及其在地形匹配中的应用

 针对实时地形图与基准地形图之间存在旋转、缩放和平移等相似变换的地形匹配问题,提出一种基于地形不变性特征描述的匹配方法。分析了地形的凹凸特征,根据三维地形表面的局部极值点相对位置分布稳定的特点,采用极值点相对位置归一化表示的方法构造了地形的不变性特征向量,基于二维地形特征向量间的距离比较,实现了适应相似变换的地形匹配。该地形描述与匹配方法不依赖于地形的绝对尺度值,仿真实验表明了方法具有较好的实用性,是解决导航定位中存在缩放变换的地形匹配的有效方法。     

Simple Generation of One-Parameter Pseudoterrain Surfaces

A simple method for generating digital terrain surfaces as a function of only the terrain standard deviation is presented. This is accomplished by fitting a postulated autocorrelation model to actual terrain data. Observed relations between model parameters are used to obtain a one-parameter model.     

A Rader Clutter Model: Average Scattering Coefficients of Land, Snow, and Ice

A model has been developed for average radar backscatter from terrain based on recent carefully controlled wide-bandwidth measurements of vegetation, snow-covered ground, and sea ice and on a comparison with measurements over North America by the Skylab S-193 scatterometer. The models for the thiree cases take the form ?° dB = A + B? + Cf+ Df?, 20° ? angle of incidence ? 70°, where the constants vary depending on polarization and terrain class. They also differ above and below a critical frequency (6 GHz for general terrain, 8 GHz for sea ice, and between 8 and 12 GHz for snow). For angles of incidence of 0° (vertical) and 10°, the model is of the form ?° dB = M(?) + N(?)f over the range 1 to 18 GHz. Hundreds of thousands of measurements contributed to the general (vegetated terrain) model, and smaller numbers contributed to the snow and sea ice models. Since 1974 all measurements have been made with University of Kansas microwave spectrometers. A brief discussion of fading shows that insufficient data are available to describe the ranges adequately.     

Bayesian gamma mixture model approach to radar target recognition

This paper develops a Bayesian gamma mixture model approach to automatic target recognition (ATR). The specific problem considered is the classification of radar range profiles (RRPs) of military ships. However, the approach developed is relevant to the generic discrimination problem. We model the radar returns (data measurements) from each target as a gamma mixture distribution. Several different motivations for the use of mixture models are put forward, with gamma components being chosen through a physical consideration of radar returns. Bayesian formalism is adopted and we obtain posterior distributions for the parameters of our mixture models. The distributions obtained are too complicated for direct analytical use in a classifier, so Markov chain Monte Carlo (MCMC) techniques are used to provide samples from the distributions. The classification results on the ship data compare favorably with those obtained from two previously published techniques, namely a self-organizing map and a maximum likelihood gamma mixture model classifier.     

一种基于Hopfield神经网络的TF／TA航迹规划算法

提出一种基于Hopfield神经网络的无人机地形跟随(TF)/地形回避(TA)迹规划方法.该方法将地研信息反映到算法参数连接权中,利用扩展的Hopfield模型结合无人机约束实现航迹安全、合理的规划.仿真实验证明该方法能获得满意的TF/TA航迹.     

轮地力学模型参数灵敏度分析与主参数估计

在星球探测过程中,星球车需要估计地面力学参数以及时调整控制策略,快速适应地形变化。针对形式复杂,参数耦合的轮地相互作用模型,采用Sobol灵敏度分析方法,分别对模型中的地面承压特性参数和剪切特性参数的灵敏度进行定量分析,筛选出沉陷指数与内摩擦角作为模型的主导参数,用于反映地面承压特性和剪切特性的变化。基于轮地相互作用力学平衡方程,通过简化应力分布公式,进一步推导出主导参数的解析表达式。通过以典型值固定非主导参数,利用系统状态参数和滤波处理完成地面力学特性主导参数的估计。结果表明,所提出的地面力学主导参数解析式及相应的估计方法能够快速反映地形的变化,沉陷指数估计的平均相对误差为2.8%,内摩擦角估计的平均相对误差小于3%。估计结果可准确预测车轮牵引力,为实现实时控制提供必要信息。