Low-altitude Measurement Research Articles

Stability Derivatives of Various Lighter-than-Air Vehicles: A CFD-Based Comparative Study

An aerostat with a single tether is proposed for the application of wind measurements at low altitudes. In the current study, the aerodynamic model parameters (stability derivatives) of the aerostat are investigated based on a CFD-based approach. The static, as well as the dynamic stability derivatives of the aerostats are presented. The calculation of the dynamic stability derivatives involves the simulation of the oscillations of the aerostats in their axial direction (surge), the vertical direction (heave) and angular motions with respect to the lateral direction (pitch). A forced sinusoidal oscillation is used for the simulation of the aerostat, and one stable period of oscillation is taken for the derivatives’ extraction. Four different aerostats are considered for the current study with four different angles of attack. The Zhiyuan aerostat, HAA aerostat, NPL aerostat and GNVR aerostat are the aerostats considered for this study. The stability derivative results obtained for the four aerostats are analyzed and compared with respect to their geometrical features. From the static aerodynamic characteristics, the Zhiyuan aerostat shows better performance than the other aerostats in terms of the lift–drag ratio. The dynamic stability derivatives of the Zhiyuan aerostat suggest its application as the proposed low-altitude wind measurement system.

Low elevation target altitude measurement for ubiquitous radar based on known transmitted waveform and sparse representation

Low elevation target altitude measurement for ubiquitous radar based on known transmitted waveform and sparse representation

The Lunar Polar Hydrogen Mapper CubeSat Mission

The Lunar Polar Hydrogen Mapper (LunaH-Map) mission will map the distribution of hydrogen around the lunar South Pole using a miniature neutron spectrometer. The mission builds upon a decade of lunar science, which has revealed both regional and more localized enrichments of water ice near the lunar poles. Localized enrichments are primarily within permanently shadowed regions (PSRs) and craters throughout the South Pole. The spatial extent of these regions is often below the resolution of previous neutron instruments that have flown on lunar missions. The neutron leakage spectrum from planetary surfaces is primarily sensitive to hydrogen abundance in the top meter of regolith, however, for neutron spectrometers with omnidirectional sensitivity, the spatial resolution is limited by the spacecraft orbital altitude above the surface. A low altitude measurement from a distance on the same scale of the PSRs could spatially isolate and constrain the hydrogen enrichments both within and around within those regions. A small spacecraft mission is ideally suited to acquire the low-altitude measurements required to localize hydrogen enrichments using neutron spectroscopy at the lunar South Pole. LunaH-Map will use a solid iodine ion propulsion system, X-Band radio communications through the NASA Deep Space Network, star tracker, Command & Data Handling System, and EPS systems from Blue Canyon Technologies, solar arrays from MMA Designs, LLC, mission design and navigation by KinetX. Spacecraft systems design, integration, qualification, test, and mission operations are performed by Arizona State University, AZ Space Technologies and Qwaltec.

An Aircraft Wetland Inundation Experiment Using GNSS Reflectometry

In early May of 2017, a flight campaign was conducted over Caddo Lake, Texas, to test the ability of Global Navigation Satellite System-Reflectometry (GNSS-R) to detect water underlying vegetation canopies. This paper presents data from that campaign and compares them to Sentinel-1 data collected during the same week. The low-altitude measurement allows for a more detailed assessment of the forward-scattering GNSS-R technique, and at a much higher spatial resolution, than is possible using currently available space-based GNSS-R data. Assumptions about the scattering model are verified, as is the assumption that the surface spot size is approximately the Fresnel zone. The results of this experiment indicate GNSS signals reflected from inundated short, thick vegetation, such as the giant Salvinia observed here, results in only a 2.15 dB loss compared to an open water reflection. GNSS reflections off inundated cypress forests show a 9.4 dB loss, but still 4.25 dB above that observed over dry regions. Sentinel-1 data show a 6-dB loss over the inundated giant Salvinia, relative to open water, and are insensitive to standing water beneath the cypress forests, as there is no difference between the signal over inundated cypress forests and that over dry land. These results indicate that, at aircraft altitudes, forward-scattered GNSS signals are able to map inundated regions even in the presence of dense overlying vegetation, whether that vegetation consists of short plants or tall trees.

Low Altitude Measurement Accuracy Improvement of the Airborne FMCW Radio Altimeters

This manuscript focuses on the analysis of a critical height of radio altimeters that can help for the development of new types of aeronautical radio altimeters with increased accuracy in measuring low altitudes. Altitude measurement accuracy is connected with a form of processing the difference signal of a radio altimeter, which carries information on the measured altitude. The definition of the altitude measurement accuracy is closely linked to the value of a critical height. Modern radio altimeters with digital processing of a difference signal could shift the limit of accuracy towards better values when the basics of the determination of critical height are thoroughly known. The theory results from the analysis and simulation of dynamic formation and the dissolution of the so-called stable and unstable height pulses, which define the range of the critical height and are presented in the paper. The theory is supported by a new method of derivation of the basic equation of a radio altimeter based on a critical height. The article supports the new theory of radio altimeters with the ultra-wide frequency deviation that lead to the increase the accuracy of a low altitude measurement. Complex mathematical analysis of the dynamic formation of critical height and a computer simulation of its course supported by the new form of the derivation of the basic equation of radio altimeter guarantee the correctness of the new findings of the systematic creation of unstable height pulses and the influence of their number on the altitude measurement accuracy. Application of the presented findings to the aviation practice will contribute to increasing the accuracy of the low altitude measurement from an aircraft during its landing and to increasing air traffic safety.

Possibilities of Increasing the Low Altitude Measurement Precision of Airborne Radio Altimeters

The paper focuses on the new trend of increasing the accuracy of low altitudes measurement by frequency-modulated continuous-wave (FMCW) radio altimeters. The method of increasing the altitude measurement accuracy has been realized in a form of a frequency deviation increase with the help of the carrier frequency increase. In this way, the height measurement precision has been established at the value of ±0.75 m. Modern digital processing of a differential frequency cannot increase the accuracy limitation considerably. It can be seen that further increase of the height measurement precision is possible through the method of innovatory processing of so-called height pulses. This paper thoroughly analyzes the laws of height pulse shaping from the differential frequency presented by the number that represents the information about the measured altitude for this purpose. This paper presents the results of the laboratory experimental altitude measurement with the use of a so-called double-channel method. The application of obtained results could contribute to the increase of air traffic safety, mainly in the phase of the aircraft approaching for landing and landing itself.

Correction method of the manned spacecraft low altitude ranging based on γ ray

γ photons altimeter, which is a low altitude measurement of the spacecraft based on Compton effect at the moment when it lands on earth, is an important equipment for spacecraft landing control. Manned spacecraft landing control needs γ-ray altitude control system to revise ignition signal for retrograde landing rocket at different landing speed. To make the spacecraft speed zero at the moment landing on the surface, a method that makes ignition altitude correction of the spacecraft at different landing speeds is figured out, which is based on the number of γ-ray particles reflected field gradient. According to the established theoretical model, the method feasibility is proved by a mathematical derivation and verified by an experiment, and the adaptability of the method under different parameters is also described. The results indicate that the error of the altitude signal error is less than 0.05 m at the height of 0.5 m to 1.5 m when the landing speed of spacecraft is in the range of 6 m/s to 10 m/s. The method provided has great significance for the manned spacecraft engineering need.

Impact of Multipath Effect on Low-Altitude Elevation Measurement of Radar

The main impact of multipath effect on low-altitude radar measurement is elevated in this paper. The influence of multipath effect on monopulse radar measurement of elevation is analyzed. Examples suggest that the multipath error caused by the signals from pattern main lobe of radar antenna should be mainly considered in the actual project. According to the three elevation range of reflected signal entering the radar beam area, the multipath error model of elevation is established. The problem of anti-multipath interference for low-altitude target tracking with radar is solved by analyzing the error source which causes multipath effect.

레이저 고도계와 초음파 거리계를 이용한 무인항공기 지면고도측정 알고리즘 설계

본 논문은 레이저 고도계와 초음파 거리계를 이용한 무인항공기의 지면고도측정에 대한 알고리즘에 대하여 제시하였다. 무인항공기의 저고도 거리 측정시에 사용하는 레이저 고도계와 초음파 거리계를 이용하여 간단한 지상시험을 하였고, 각 센서들의 특징을 확인할 수 있었다. 특히, 레이저 고도계의 단점을 확인하여 초음파 거리계로 보완하는 알고리즘을 설계하였고, 실험을 진행하였다. 레이저 고도계와 초음파 거리계는 칼만필터를 이용하여 약결합 방식으로 융합하였으며, 실험결과 레이저 고도계의 단점을 초음파 거리계로 보완하여 안정적인 고도값이 측정될 수 있음을 확인하였다. This paper presents an algorithm concerning the ground altitude measurement using a laser altimeter and an ultrasonic rangefinder for UAV(Unmanned Aerial Vehicle). A simple ground test conducted using the laser altimeter and ultrasonic rangefinder that are used for conducting the low altitude measurement of UAV and identify the characteristics of each sensor. Especially, the disadvantages of the laser altimeter were checked through the ground test. After that who those are participated in this paper planned the algorithm which is complemented by the ultrasonic rangefinder and the experiment was conducted. The laser altimeter and the ultrasonic rangefinder were fused by a loosely coupled method by Kalman filter. The paper shows that stable value of altitude complemented by the ultrasonic rangefinder that covers the laser altimeter's drawbacks can be measured through the ground test.

다중표적 비행시험을 위한 비행 자료처리 시스템 설계

본 논문에서는 다중표적 비행시험을 위한 비행 자료처리 시스템이 설계되었다. 비행자료 처리를 위해 표적그룹 처리, 자료융합 처리 및 자료연동 처리가 수행 되었으며, 자료융합 필터로서 집중형 칼만필터와 연합형 칼만필터를 설계하였다. 특히 집중형 필터에 레이더의 SNR과 추정기법을 적용하여 비행체의 저고도 계측을 개선하였다. 개발된 시스템을 다중표적 비행시험에 적용한 결과, 저고도 및 초기구간에서 개선된 비행궤적을 확인할 수 있었다. In this paper, The flight data processing system was designed for multiple target flight test. For flight data processing, multiple target grouping, data fusion processing, and data slaving processing were performed and, as a data fusion filter, centralized, and federated Kalman filters were designed. A centralized kalman filter was modified in order to improve the vehicle's low altitude measurement using radar's SNR and estimation process. From the testing of multiple target missile, it confirmed flight trajectory measurement was improved in low altitude area and the beginning stage of vehicle.

In-line type micropulse lidar with an annular beam: experiment

An in-line type compact micropulse lidar (MPL) with an annular beam was developed for low-altitude cloud measurement. An optical circulator and a couple of axicon prisms for an annular beam were installed on the lidar optics. The advantage of using the in-line MPL is its ability to obtain a near-range measurement with a narrow field of view of 0.1 mrad and to obtain a depolarization measurement of the orthogonally polarized echoes caused by ice crystals of a low-altitude cloud. The total insertion loss of the lidar optics was 3 dB. Detectors such as avalance photodiode detectors can be operated in an analog mode near the breakdown voltage because of the high isolation of the optical circulator. The ideal lidar echo variation from the nearest distance was verified by measuring the mountain echoes at various distances. The depolarization measurement of a low-altitude ice cloud was also demonstrated.

Determination of Path Radiance for Downward Path of Sight from Ground-Based or Low-Altitude Measurement*

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text Almerian R. Boileau, "Determination of Path Radiance for Downward Path of Sight from Ground-Based or Low-Altitude Measurement*," J. Opt. Soc. Am. 58, 586-587 (1968) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article