Terrain Undulation Research Articles

The use of civilian-type GPS receivers by the military and their vulnerability to jamming

We considered the impact of external influences on a GPS receiver and how these influences affect the capabilities of civilian-type GPS receivers. A standard commercial radio frequency signal generator and passive GPS antenna were used to test the sensitivity of GPS to intentional jamming; the possible effects of the terrain on the propagation of the jamming signal were also tested. It was found that the high sensitivity of GPS receivers and the low strength level of GPS satellite signals combine to make GPS receivers very vulnerable to intentional jamming or unintentional radio frequency interference. Terrain undulation was used to shield GPS antennas from the direct line-of-sight of the jamming antenna and this indicated that terrain characteristics can be used to mitigate the effects of jamming. These results illuminate the vulnerability of civilian-type GPS receivers to the possibility and the ease of disablement and establish the foundation for future work.

地形对植被生物量遥感反演的影响——以广州市为例

Terrain correction is typically used to adjust surface reflectivity for the influence of slope and aspect when plant biomass is estimated by remote sensing.There are very few studies on the differences between the pixel area of a remote sensing image and its real surface area.However,this difference inevitably influences the precision of quantitative remote sensing when the terrain is undulating.This paper addresses the effects of the difference between the pixel area of a remote sensing image and the real surface area of plant biomass obtained with remote sensing,when biomass-terrain correction model was calibrated according to surface-area calculation model and the law of the conservation of mass.The results show that the pixel area,influenced by undulating topography,was different from the actual surface area for Guangzhou city,China,and this accounted for 33.4% of the variability.Statistical values for the areas in different vegetation types were much higher after the terrain correction than before.The biggest proportions were for the coniferous forest and broad-leaved forest areas,at 6.18% and 3.70%,respectively.The grassland,shrubland,and orchard areas also increased by 2.86%,1.92%,and 1.29%,respectively.Based on the uncertainty analysis for forest biomass estimation with remote sensing,remote sensing models with high accuracy were built for different vegetation types,and their correlation coefficients were close to or more than 0.9.Therefore,these models can be applied to estimating of plant biomass.Biomass of vegetation in the eastern and northern regions was much greater than that in the western and southern regions of Guangzhou.The average biomass of woodland(broad-leaved forest and coniferous forest) was 61.86 t/hm2.This value was more than the average biomass in Pear River Delta but much lower than the biomass of the climax community of southern subtropical forest in Dinghushan.This suggests that the studied woodland biomass has a large potential for growth.The difference in biomass distribution due to terrain undulation was given by the biomass model obtained from remote sensing and the biomass-terrain correction model.The biomass of different vegetation types increased with terrain correction.The broad-leaved forest biomass increased by 7.74% which was the highest rate of change.Biomass of coniferous forest increased by 4.76%,that of grassland by 3.34%,that of shrubland by 2.50%,and that of orchard by 1.58%.Total vegetation biomass of Guangzhou increased by 5.82%.We conclude that the impact of terrain cannot be ignored because it was an important factor affecting the precision of biomass conversions.Compared with other models of surface area calculation,the pixel surface area model could be used with high accuracy to correct for the terrain impact of plant biomass estimation with remote sensing.

Building a three dimensional sealed geological model to use in numerical stress analysis software: A case study for a dam site

The dam area of the SUOXI hydropower project shows high terrain undulation and complex geological conditions, containing 6 faults and 7 weak inter-beds. A geometric model developed to represent the geology and engineering structures should incorporate the geological realities and should allow suitable mesh generation to perform numerical stress analysis. This is an important precondition to perform rock mass stability analysis of a dam foundation based on a numerical stress analysis software such as FLAC 3D. Using the modeling tools available in FLAC 3D, it is difficult to construct a complex geological model even after performing a large amount of plotting and data analyses. The 3-D geological modeling technique suggested in this paper, named as Sealed Geological Modeling (SGM), is a powerful tool for constructing complex geological models for rock engineering projects that require numerical stress analysis. Applying this technique, first, the geological interfaces are constructed for the dam area of SUOXI hydropower project using various interpolation procedures including geostatistical techniques. Then a unitary wire frame is constructed and the interfaces are connected seamlessly. As the next step, a block tracing technique is used to build a geological model that consists of 130 seamlessly connected blocks. Finally, based on the Advancing Front Technique (AFT), each block is discretized into tetrahedrons and a mesh is generated including 57,661 nodes and 215,471 tetrahedrons which is suitable to perform numerical stress analysis using FLAC 3D.

Performance Evaluation of Homing Guidance Methods

A guided missile must be endowed with sufficient intelligence to acquire information for keeping it on course to the target, whether it was stationary or mobile. The guidance commands are generated to control the missile motion relative to target in accordance to the observed and estimated parameters for target motion. The necessity for guiding the missile motion is given by (1) the need to compensate for the non-standard conditions of the medium, technology, propulsion for the achievement of the desired accuracy, especially at greater ranges, (2) the need of reacting on the target maneuver during its flight, and (3) the need for conducting fire behind terrain undulations and artificial obstacles. According to the guidance and control philosophy utilized to control the missile motion and the location of the guidance computer, there are five types of guidance: Homing guidance, Autonomous guidance, Beam rider guidance, Command guidance and Combined guidance systems. The Homing Guidance may be active using onboard transceiver to provide the guidance signals, semi-active which combines an onboard receiver with external illuminator and passive which home on energy generated by the target. The performance of the guidance system is dependent upon the utilized guidance method which constraints the vehicles attitude during its flight. Therefore, this paper is devoted to evaluate the performance of homing guidance methods via the solution of the derived equations of c.g. motion in conjunction with ideal bond equations either numerically or analytically. The guidance performance is analyzed via flight path parameters obtained from the equations' solution. Note that the analytical solution can be very complex unless some simplifying assumptions are considered such as planer motion, constant speeds, etc. Then, the obtained equations are programmed as functions within the MATLAB environment (or any high level language) and the simulation is conducted using different engagements’ scenarios. The results justify the capabilities of each guidance method according to the engagement scenario and it will be of great value to young researchers and designers.

Computer-assisted DEM-based aero photography design

In aerial photography, the primary factor is terrain undulation. However, most of the external aerial photography software used for aerial photography design do not take terrain undulation influence into consideration. Therefore, the design result has comparative randomicity and “gaps” are expected. An aerial photography design system is developed by analyzing the terrain undulation influence to the design result with DEM data so that the forward overlap and side overlap can be justified according to the block terrain undulation to meet specifications or standards. The data designed by this system is compared with the real flying data. The results show that making use of DEM to assist in aerial photography design can ensure that the designed result fits the real terrain better.

Generation of geometrically and radiometrically terrain corrected SAR image products

Terrain undulations affect the geometric and radiometric quality of synthetic aperture radar images. The correction of these effects becomes indispensable when quantitative image analysis is performed with respect to the derivation of geo- and biophysical parameters. The paper presents a rigorous approach for geometric and radiometric correction of SAR images. Using a digital elevation model, the imaging geometry is reconstructed and is used to perform geometric and radiometric correction of terrain induced distortions. The importance of a stringent radiometric correction based on the integration of the image brightness is emphasized. The approach guarantees that the energy contained in the image data is preserved throughout the geocoding process. The resulting backscattering images are fully terrain corrected and can be used for further quantitative investigations and may also improve qualitative studies as e.g. land cover classifications. The technique is applicable for different sensor types and image products, including already geocoded SAR images. The effect of different resolutions of digital elevation models used for the correction of the backscattering coefficient is investigated.

Impact of Terrain Undulation on Precision of Radar Scene Matching

Impact of Terrain Undulation on Precision of Radar Scene Matching

On the influence of urbanization on the water budget in Nairobi city: A numerical study

The impact of the growth and development of the City of Nairobi on the water budget is simulated using a high-resolution limited-area numerical model. The water substance fields are modelled with full physics in a control experiment. Five sensitivity experiments are then performed by altering the land-use/cover over the domain of study to assess the influence of the city, forests and terrain undulations on the water substance fields. Results showed that the highest evaporation occurred in areas of the study domain with open grasslands/scattered bush-land's vegetation types and the least at the city centre. Deforestation would lead to a substantial increase in the loss of water effected through evaporation despite a reduction in transpiration. The observed rainfall amount and frequency were highest in the high ground portions to the northwest of the study domain. Numerical analyses showed that the urban heat island had a destabilizing effect on the flow, which enhanced convection that resulted in increased rainfall downwind of the urban area. Further growth and expansion of the city of Nairobi would increase the area and amount of rainfall received. Deforestation would decrease rainfall amounts. Massive reforestation would increase the observed rainfall. There has been a decrease in soil moisture at the current location of the city centre; the decrease is bound to increase with the expansion of the city. The City of Nairobi has resulted in a large decrease in the soil moisture through converting the natural fabric to concrete/asphalt material. Deforestation would result in a marginal decrease in the soil moisture. Further growth and development of Nairobi City would modify the water substance budget appreciably.

Adaptive dynamic walking of a quadruped robot using a neural system model

We are trying to induce a quadruped robot to walk dynamically on irregular terrain by using a neural system model. In this paper, we integrate several reflexes, such as a stretch reflex, a vestibulospinal reflex and extensor/flexor reflexes, into a central pattern generator (CPG). We try to realize adaptive walking up and down a slope of 12°, walking over an obstacle 3 cm in height, and walking on terrain undulation consisting of bumps 3 cm in height with fixed parameters of CPGs and reflexes. The success in walking on such irregular terrain in spite of stumbling and landing on obstacles shows that the control method using a neural system model proposed in this study has the ability for autonomous adaptation to unknown irregular terrain. In order to clarify the role of a CPG, we investigate the relation between parameters of a CPG and the mechanical system by simulations and experiments. CPGs can generate stable walking suitable for the mechanical system by receiving inhibitory input as sensory feedback and generate adaptive walking on irregular terrain by receiving excitatory input as sensory feedback. MPEG footage of these experiments can be seen at: http://www.kimura.is.uec.ac.jp.

Quantitative roughness characterization of geological surfaces and implications for radar signature analysis

Stochastic surface models are useful for analyzing in situ roughness profiles and synthetic aperture radar (SAR) images of geological terrain. In this paper, two different surface models are discussed: surfaces with a stationary random roughness (conventional model) and surfaces with a power-law roughness spectrum (fractal model). In the former case, it must be considered that for short profiles (L<200l/sub 0/), the measured values of rms-height s and correlation length l may be significantly smaller than the intrinsic values s/sub 0/ and l/sub 0/. In the latter case, rms-height and correlation length depend on the profile length L, and the surface is better characterized by slope and offset of the roughness spectrum (which are independent of L), The sensitivity of the SAR signature to variations in surface roughness parameters is evaluated by means of theoretical scattering models. For smoother geological surfaces such as most arid terrain types, single scattering is dominant, which means that the roughness parameters can be determined from SAR data using comparatively simple algorithms. Multiple scattering processes on rough surfaces such as a'a lava and variations of the local incidence angle due to large-scale terrain undulations make the retrieval of roughness parameters by means of inverse modeling much more complex. Field data of surface roughness indicate that rougher geological surfaces may be in the diffractal regime at higher radar frequencies, in which the scattering characteristics deviate significantly from the patterns observed for stationary surfaces. On the basis of surface and scattering models, recently published observations of roughness data and radar signatures from volcanic, alluvial, and arid surfaces are examined.

Terrain influences in SAR backscatter and attempts to their correction

Synthetic aperture radar (SAR) images reveal radiometric image distortions that are caused by terrain undulations. The authors present the results of a study extracting and investigating the various components of these terrain influences. An imaging model, is set up for the geometric rectification of the SAR image and for a reconstruction of the imaging geometry. A prerequisite for the setup of this model is the use of a digital elevation model. Eight different geometric parameters are derived and investigated for their influence on grey-value variations in the geocoded SAR image. Image grey-value variations of three major land-use classes-forest, agricultural land, and urban/suburban areas-are examined. Empirical models of the SAR-backscatter variations are used to describe the relations between image grey values and various geometric parameters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>