Super Low Altitude Research Articles

Evaluation of atomic oxygen fluences by analyzing transmitted light images of polyimide obtained at SLATS/MDM

Expectations for high-resolution observations by very low Earth orbit (VLEO, below 300 km) satellites are growing. However, the density of atomic oxygen (AO) is much higher in VLEO than in LEO (300 km or higher), so severe degradation of materials is a concern due to high-flux AO collisions. To clarify the degradation, we developed and deployed the material degradation monitor (MDM) mission onboard the super-low altitude test satellite (SLATS). In the MDM, several materials were exposed to the environment at 160–560 km from 23 December 2017 to 1 October 2019 and observed optically by a CCD camera. In general, AO fluence is evaluated by measuring the volume or mass loss of a polyimide, but this was difficult to do because the samples could not be returned to the ground in this mission. This study evaluated AO fluences using the CCD images of a polyimide, Vespel. The sample had nine non-penetrating holes with different bottom-plate thicknesses and was monitored by the CCD while being illuminated by LED light from the back. The CCD images reveal the intensity of the light transmitted through the holes in the bottom plate. The erosion depths of the polyimide, corresponding to the AO fluences, can be evaluated from the pixel values at the hole positions on the images. Here, the correlation between the pixel values and thickness was investigated for pristine and AO-exposed Vespel samples using the MDM engineering models. The total green pixel values followed the Beer-Lambert law for the bottom-plate thicknesses, regardless of AO exposure. Applying the correlation to the MDM flight images, we found that the AO fluence increased significantly after the end of March 2019, finally reaching (6.5–9.8) × 1021 atoms/cm2. This optical imaging method would be useful for evaluating the AO effects on the other MDM samples and high AO fluences in future VLEO missions.

초저고도 소형 지구관측위성 구조모델의 형상설계 및 발사환경시험

일반적인 지구관측 위성은 고도가 낮을수록 더 높은 영상 품질을 획득할 수 있기 때문에 주로 저궤도에서 운용된다. 그러나 300km 이하의 초저고도에서는 대기에 의해 큰 항력이 발생하며, 이는 위성의 고도유지 및 목표 수명에 치명적이다. 본 연구는 300km의 초저고도 운용에 적합한 100kg급 소형 지구관측위성 본체 구조 개발에 초점을 둔다. 설계 개념 및 구체적인 형상 설계를 제시하며, 지상검증용 구조모델을 제작하고 가상의 후보 발사체의 하중조건을 고려한 시험규격을 바탕으로 발사환경시험을 수행함으로써 개발된 위성 구조 모델이 구조적으로 안전함을 검증하였다.

Analysis of upper atmospheric effects on material per onboard atomic oxygen monitor system of SLATS

JAXA has proposed an innovative idea for satellites in Low Earth Orbit (LEO). The Super-Low Altitude Test Satellite (SLATS), also known as TSUBAME, is the first Earth observation satellite to occupy a Super-Low Orbit (S-LEO) or Very Low Earth Orbit (VLEO), below 300 km. The purposes of SLATS are 1) testing the maintenance of the satellite’s altitude with its ion engine against high atmospheric drag at a super-low altitude, 2) acquiring data on atmospheric density and atomic oxygen (AO), and 3) testing optical Earth observation. SLATS was successfully launched on 23 December 2017. SLATS was then altitude-controlled for 636 days to 271.7 km using chemical thrusters, aerodynamic drag, and ion engine propulsion. SLATS finally maintained its orbit of 167.4 km for 7 days and finished its operation on 1 October 2019. All the SLATS and Atomic oxygen MOnitor (AMO) data was acquired during these operations. The AMO is one of the mission sensors that monitor AO and its effects on spacecraft materials. The data from the AMO contributes to the choice of materials in future S-LEO satellite design. The data obtained by the AMO are valuable in that they provide considerable knowledge on AO fluence and its effects on space materials. A precise atmospheric density model and atmospheric composition model are indispensable for predicting the trajectory or re-entry of debris in orbit. Atmospheric models such as NRLMSISE-00, JB 2008, and DTM2013 have been developed, but few studies compare these models and the actual atmospheric environment in LEO. The average atmospheric density obtained from SLATS is lower than the value predicted by the atmospheric models (NRLMSISE-00, JB 2008, and DTM 2013). Understanding the model’s accuracy will contribute to the orbit control of future S-LEO satellites and the orbit prediction and control of debris in LEO.

Effect of simultaneous N2 collisions on atomic oxygen-induced polyimide erosion in sub-low Earth orbit: comparison of laboratory and SLATS data

The role of N2 in the upper atmosphere on the atomic oxygen (AO)-induced erosion of polyimide in low Earth orbit (LEO) and sub-LEO is investigated through ground-based experiments and flight data. The experiment is performed by adding an Ar beam at the same collision energy as an undecomposed O2 component in the AO beam formed by laser detonation to simulate the physical effect of simultaneous N2 collision in sub-LEO. The Ar beam is added by the dual-pulsed supersonic valve-equipped laser-detonation system developed at Kobe University. The experimental results indicate that the erosion of polyimide in the laser-detonation system is promoted by the presence of O2 and Ar in the beam, corresponding to N2 in the sub-LEO. On-ground experimental results are compared with in-orbit AO measurements. Previous space shuttle, international space station-based exposure experiments, as well as the world’s first real-time sub-LEO material erosion data aboard a super low altitude test satellite (SLATS) orbiting at an altitude of 216.8 km are presented. The SLATS data suggests the presence of an acceleration effect by N2 collision on AO-induced polyimide erosion, as predicted by ground-based experiments.

Brewster effect of super‐low altitude target above terrestrial environments using modified facet‐based weighted four‐path model

A modified facet-based weighted four-path model is presented to analyze the Brewster effect of super-low altitude target above terrestrial environments. In this approach, the facet-based weighted four-path model (FWFPM) is established to simulate the mutual interactions between the super-low altitude target and terrestrial environment. Gauss antenna radiation pattern is introduced to modify the FWFPM. The hybrid PO-MEC method is applied, where the physical optics (PO) is utilized to calculate the scattering field from each facet, and the method of equivalent currents (MEC) is used to compute the scattering field from the edge structure. The validation of the proposed method is verified by the example of a super-low altitude missile model above a rough surface. The Brewster effect of a super-low altitude aircraft above diverse terrestrial environments is studied. Moreover, the influences of relative permittivity, the roughness of environment, and the target's height on Brewster angle have been analyzed in detail to provide some useful conclusions.

MDM: A flight mission to observe materials degradation in-situ on satellite in super low Earth orbit

Satellites in low Earth orbit (LEO) are subject to several environmental factors, atomic oxygen (AO) having the greatest effect on space materials, especially polymers. Recently, the possibility of utilizing orbits at an altitude lower than 300 km (super LEO or S-LEO) has been explored, but one complication in using these orbits is that AO fluence is expected to far exceed that in an ordinary LEO. The effect of S-LEO AO on materials used in space is not well understood. To investigate the influence of S-LEO AO on space materials, we developed the Material Degradation Monitor (MDM), which is installed on a JAXA spacecraft, the Super Low Altitude Test Satellite (SLATS, also known as TSUBAME). The SLATS is the first S-LEO satellite to operate at an altitude under 200 km and the MDM is the first near real-time experiment to examine the effects of AO on materials in this orbit. The aim of the MDM is to clarify degradation behaviors of the space materials according to AO fluence. We developed the instruments needed to detect AO fluence and the changes it causes to samples of various materials in S-LEO. The MDM performed as expected until the end of the SLATS operation. Some changes in materials were found from the images taken from front and back LEDs. Although there was no breakage by AO, optical properties changed that are assumed to be due to material erosion by AO.

Identification of iron-bearing minerals based on HySpex hyperspectral remote sensing data

Our research group built a super-low altitude detection platform with power delta wings and mounted it with a HySpex hyperspectral sensor. This platform has great potential for extracting alteration information as it can provide high spectral and spatial resolution images. The aim of this study is to verify the reliability and accuracy of alteration information extraction using this platform. The seven-scene hyperspectral images of the Karatagh region in Hami of Xinjiang obtained by the platform in September 2017 were used to identify iron-bearing minerals. First, these images were preprocessed by splicing, atmospheric correction, and geometric correction. Second, the images were compared with the United States Geological Survey (USGS) standard spectra of similarities between the end-member spectra of goethite-, limonite-, and jarosite-altered minerals. Third, the three altered minerals were mapped in the test area based on the end-member spectrum. The results show that the correlations between the end-member spectra of three altered minerals, including goethite, limonite, and jarosite, obtained from the HySpex image and corresponding spectra in USGS reach 0.92873, 0.95098, and 0.8875, respectively. The end-member spectral reflectance values were higher than those in the spectral library. The original spectral data after the continuum-removal and the local quantitative display show that the spectrum at the feature absorption location was similar to the reference spectrum. On this basis, the distribution maps of goethite, limonite, and jarosite iron-stained altered minerals in the Hami Gobi area were produced and field verified. The data show that the information of identified minerals from the HySpex hyperspectral data were in accordance with the actual situation. Both methods can recognize jarosite well, the spectral angular mapping method is better than the back propagation (BP) neural network for goethite recognition, and the BP neural network of limonite recognition is better than the spectral angular mapping method. The above results prove that the alteration information can be effectively identified using the HySpex hyperspectral data integrated on a super-low altitude detection platform, which is of great significance to the rapid delineation of minerogenic prospects area.

A Modified Four Path Method for Calculating Coupling Field of Super-Low Altitude Aircraft Target

A novel modified four path method (FPM) is presented for calculating coupling field of super-low altitude aircraft target. Based on the hybrid method PO + MEC (Physical Optics and Method of Equivalent Currents), the antenna radiation pattern is introduced to consider the multipath interference from side lobe of seeker. The modified FPM is used to calculate the coupling field from super-low altitude aircraft target above different terrestrial environments. The curves of scattering coefficient are analyzed. The influences of height of target, root mean square (RMS), and incident angle on coupling field characteristics are discussed. The simulation results can be used for reference in detection for super-low altitude target and optimization for radar system.

Study on Characteristics of Coupling Field from Super-low Altitude Target under Diverse Terrestrial Environments in Semi-active Radar System

A novel electromagnetic scattering model is presented for studying coupling field of super-low altitude target under diverse terrestrial environments in semi-active radar system. The PO+MEC hybrid method is applied to obtain the electromagnetic scattering of target. The Physical Optics (PO) is introduced to calculate the scattering from facets of target. The Method of Equivalent Currents (MEC) is introduced to calculate the scattering from edges of target. Antenna radiation pattern of seeker is introduced to modify traditional Four Path Method (FPM) in semi-active radar system. Based on the modified FPM, the characteristics of coupling field from super-low altitude target under diverse terrestrial environments in semi-active radar system are studied. Simulation results can use for reference in the fields of remote sensing and detection for super-low altitude target.

Aerodynamic performance optimization for the rotor design of a hovering agricultural unmanned helicopter

The importance of using Agriculture unmanned helicopters (AUHs), especially for spraying pesticides and fertilizers on any terrain type to ensure crop yields, has been recently acknowledged. Apart from flying these helicopters at a super-low altitude and low speed, using an efficient and optimum rotor blade ensures a uniform and deep penetration of pesticide and fertilizers over a specified area. Accordingly, this work attempts to optimize the rotor blade of an AUH by using coupling statistics and several numerical techniques, including design of experiments, response surface method, and computational fluid dynamics. The experiments are designed using the central composite design method and by selecting the geometric variables that affect the aerodynamic performance of the rotor blade, including the root chord, tip chord, and angle of attack. The angle at the root and tip is optimized in order for the resulting twist to produce a uniform blade loading, achieve maximum lift, and minimize the required hover power. The required aerodynamic forces and limited availability of engine power are identified as constraints. The blade is optimized only when the helicopter is hovering at a persistent rotational speed, and the hover efficiency of the rotor blade with an optimal twist distribution is significantly higher than the baseline.

Computational mission analysis and conceptual system design for super low altitude satellite

Computational mission analysis and conceptual system design for super low altitude satellite

Image Positioning Accuracy Analysis for Super Low Altitude Remote Sensing Satellites

Super low altitude remote sensing satellites maintain lower flight altitudes by means of ion propulsion in order to improve image resolution and positioning accuracy. The use of engineering data in design for achieving image positioning accuracy is discussed in this paper based on the principles of the photogrammetry theory. The exact line-of-sight rebuilding of each detection element and this direction precisely intersecting with the Earth's elliptical when the camera on the satellite is imaging are both ensured by the combined design of key parameters. These parameters include: orbit determination accuracy, attitude determination accuracy, camera exposure time, accurately synchronizing the reception of ephemeris with attitude data, geometric calibration and precise orbit verification. Precise simulation calculations show that image positioning accuracy of super low altitude remote sensing satellites is not obviously improved. The attitude determination error of a satellite still restricts its positioning accuracy.

New Structures and Algorithms of Altimeters for Controlling Vehicles Motion at Low and Superlow Altitudes

The methods and devices for measuring of a low and superlow altitude for the wide class of the moving vehicles (ekranoplanes, helicopters, aircrafts, hovercrafts and etc) are considered. Special attention devotes to the altimeters setting on the board of the vehicle moving over water's surface, especially for landing vehicles and for vehicles, which have sensors for NOE (Nap-of-the Earth- is a type of very low-altitude flight, some times it is known as “flying under the radar”). Special attention devotes the questions of the earth's surface reflection ability. Diffuse reflection and mirror reflection and main reflection characteristics are considers. The values of errors for different methods of measuring altitudes are analised, that may help to develop new technique for new vehicle. Recommendations concerning with the choice of the altitude measuring method are done. Results of the experimental researches are discussed.

Research and Development of a New Power Processing Control Unit of Ion Engine System for the Super Low Altitude Test Satellite

JAXA has researched the Super Low Altitude Test Satellite (SLATS), which orbits the Earth at the altitude of nearly 200 km, for use in next-generation Earth observation satellites. An electric propulsion system is very useful to compensate for air drag of the SLATS since it has low thrust and long lifetime. Therefore, based on the Kiku-8 ion engine system, we started research and development of a new ion engine system to apply to the SLATS program. The SLATS ion engine system must be small and light. So, we decided to develop a power processing control unit (PPCU) which combines power supplies and a controller. According to the SLATS requirements, the performance requirements for the PPCU were determined. Then, we manufactured and tested the breadboard model of the high-voltage converter in the PPCU. The test results showed that the power efficiency is over 90 percent, which meets the performance requirements for the PPCU.

The Study of a Super Low Altitude Satellite

This paper reports the result of a study for super low altitude satellite. The altitude of this satellite's orbit is lower than ever. The altitude of a conventional earth observing satellite is generally around from 600km to 900km. The lowest altitude of earth observing satellite launched in Japan was 350km; the Tropical Rainfall Measuring Mission (TRMM). By comparison, the satellite reported in this paper is much lower than that and it is planned to orbit below 200km. Furthermore, the duration of the flight planned is more than two years. Any satellite in the world has not achieved to keep such a low altitude that long term. The satellite in such a low orbit drops quickly because of the strong air drag. Our satellite will cancel the air drag effect by ion engine thrust. To realize this idea, a drag-free system will be applied. This usually leads a complicated and expensive satellite system. We, however, succeeded in finding a robust control law for a simple system even under the unpredictable change of air drag. When the altitude of the satellite is lowered successfully, the spatial resolution of an optical sensor can be highly improved. If a SAR is equipped with the satellite, it enables the drastic reduction of electric power consumption and the fabulous spatial resolution improvement at the same time.