Low-altitude Satellite Research Articles

Low Altitude Unmanned Aerial Vehicles (UAVs) and Satellite Remote Sensing Are Used to Calculated River Discharge Attenuation Coefficients of Ungauged Catchments in Arid Desert

The arid desert ecosystem is very fragile, and the change of its river discharge has a direct impact on irrigation and natural environment. River discharge attenuation coefficients is a key index to reveal the stability of desert river ecosystem. However, due to the harsh conditions in desert areas, it is difficult to establish a hydrological station to obtain data and calculate the attenuation coefficients, so it is urgent to develop new methods to master the attenuation coefficients of rivers. In this study, Taklamakan desert river was selected as the research area, and the river discharge of the desert river were estimated by combining low-altitude UAV and satellite remote sensing technology, so as to calculate the attenuation status of the river in its natural state. Combined with satellite remote sensing, the surface runoff in the desert reaches of the Hotan River from 1993 to 2017 were estimated. The results showed that the base of runoff attenuation in the lower reaches of the Hotan River is 40%. Coupled UAV and satellite remote sensing technology can provide technical support for the study of surface runoff in desert rivers within ungauged basins. Using UAV and satellite remote sensing can monitor surface runoff effectively providing important reference for river discharge monitoring in ungauged catchments.

Airglow Patches in the Polar Cap Region: A Review

Polar cap airglow patches have been known as regions of enhanced 630.0 nm airglow detected by ground-based all-sky imagers at the polar cap latitudes well inside the main auroral oval. Although they were already recognized almost four decades ago as counterparts of polar cap (plasma density) patches, such airglow observations had not been utilized extensively for the studies of ionospheric structures and/or magnetosphere-ionosphere coupling processes in the polar cap. In the last two decades, following the development of highly-sensitive airglow imagers equipped with cooled CCD (Charge Coupled Device) cameras, it has become possible to visualize the dynamical temporal evolution and complicated spatial structure of airglow patches with improved signal-to-noise ratio. Such a progress has enabled us not only to use airglow patches as tracers for plasma convection in the polar cap but also to understand the generation of small-scale plasma irregularities in the ionospheric F region. In addition, recent observations demonstrated a case in which an airglow patch was accompanied by an intense flow channel and corresponding field-aligned current structure along its edges. This implies that airglow patches can signify magnetosphere-ionosphere coupling process in the region of open field lines at the polar cap latitudes, serving as a remote sensing tool just like auroras do. Further studies showed an association of airglow patches with the intensification of aurora on the nightside (Poleward Boundary Intensification: PBI and/or streamer) leading to the expansion phase onset of substorms. This paper reviews such recent progresses in the researches of airglow patches obtained by combining data from all-sky airglow imagers, radars and low-altitude satellite observations in the polar cap.

Feasibility of quasi-frozen, near-polar and extremely low-altitude lunar orbits

Designing long-duration lunar orbiter missions is challenging due to the Moon's highly nonlinear gravity field and the third-body perturbations induced by the Earth, Sun and other large bodies. The absence of a Lunar atmosphere has offered the possibility for mission designers to search for extremely low-altitude, quasi-stable lunar orbits. In addition to the reduced amount of propellant required for station-keeping maneuvers, these orbits present great opportunities for unique scientific studies such as high resolution imaging and characterization of the polar ice deposits in deep craters. Prior to the GRAIL mission, mission planning for Lunar orbiters had suffered from inaccuracies, mainly due to the lack of an accurate Lunar gravity model, which resulted in severe deviations with respect to the spacecraft's nominal orbit.We study station-keeping feasibility for spacecraft in near-polar and extremely low-altitude, quasi-frozen orbits around the Moon, that are perturbed by a high-fidelity lunar gravity model and third-body effects from the Earth and Sun. For several candidate orbits, we compare the trade-space between mission duration and ΔV budget, considering impulsive maneuvers applied once every ‘N∈{2,6,10,14,18}’ orbits at periselene or aposelene. Additionally, we investigate the propulsive cost for different orbit insertion dates, the location of impulsive corrections for arresting argument of periselene (ω) drift, and controlling periselene altitude.

A small spacecraft to probe the interior of the Jovian moon Europa: Europa Tomography Probe (ETP) system design

We propose the system design of a small orbiter of the Jovian moon Europa (Europa Tomography Probe, ETP in short) aimed at unveiling its interior structure. ETP is conceived as a piggyback probe of a larger spacecraft to the Jovian system (such as Europa Clipper). Its payload comprises only a magnetometer and transponder. The former will be used to measure the time-varying, induction magnetic field of the moon at different orbital and rotational frequencies, a measurement inaccessible to a flyby spacecraft. An Inter-Satellite Link (ISL) between ETP and the main spacecraft, enabled by the on-board transponder, will be used to accurately determine Europa's gravity field, rotational state and tidal deformation. By combining magnetic and gravity field measurements, ETP could characterize the interior structure of Europa with an accuracy only attainable by a low-altitude orbiter, thus constraining the thickness and conductivity of the subsurface ocean. Following the announcement that a 250-300-kg mass allowance was available on the upcoming NASA Europa Clipper spacecraft, we propose ETP's mission and spacecraft design assuming Clipper's nominal trajectory as a baseline. The concept can be adapted to a class of missions to the Jovian and Saturnian systems based upon a mother-daughter spacecraft system. The spacecraft design has been pursued under the philosophy of determining the minimum required total mass and volume that allows to meet the scientific requirements, rather than finding out what science return could be obtained with pre-assigned system constraints. Since ETP shall autonomously reach its final orbit, the propulsion system has been one of the primary focuses. The radiation analysis has also been essential because the shielding structure affects both the mission duration and the mass budget. We show that the ETP concept could be, indeed, technically feasible with an overall system mass budget just above 250 kg, thus providing a valuable yet affordable augmentation to a larger flyby mission to Europa.

Ionospheric parameters in the European sector during the magnetic storm of August 25–26, 2018

Variations of ionospheric parameters Total Electron Content (TEC) by GNSS, critical frequency (foF2) by vertical sounding and electron density (Ne) by low-altitude satellite were studied at high, mid and low latitudes of the European sector during the magnetic storm of August 25–26, 2018. During the main phase of the storm the ionospheric F2-layer was under the positive disturbance at mid and low latitudes. Then the transition from the positive to negative ΔfoF2 values occurred at all latitudes. The recovery phase was characterized by negative ionospheric disturbance at all latitudes. This is due to the decrease of thermospheric O/N2 ratio during the recovery phase of the storm. The intense Es layers screened the reflections from the F2-layer on August 26th at high and at low latitudes but at different times. Some blackouts occurred due to the high absorption level at high latitudes. In general, foF2 and TEC data were highly correlated. The major Ne changes were at the low latitudes. In general, Ne data confirmed the ionospheric dynamics revealed with foF2 and TEC.

The Gamma-Ray Background Model for Detectors Onboard Low-Altitude and High-Inclination Satellites in the Energy Band up to Several MeV

Results are presented from an analysis of the temporal behavior of the background count rate in an energy band of up to several MeV for the γ-ray detectors onboard low-altitude satellites, using data of the AVS-F instrument as an example. The instrument was installed onboard the CORONAS-F satellite with orbital parameters of an altitude of ~500 km and an inclination of 82.5°. The temporal profiles of the background count rate in the equatorial orbit regions are approximated by fourth- or fifth-degree polynomials. The applicability of the approximating polynomials is shown for the instruments installed onboard satellites with orbital inclinations of up to 38°, allowing for the evolution of the Kp index in the preceding time interval of 12–24 h before passing the geomagnetic equator. In modeling the RHESSI data of October 27, 2003, at an initial orbital altitude of ~600 km and an inclination of 38°, a mean count rate value of 1017 ± 8 s−1 is obtained in the geomagnetic latitude interval ±5° in the energy range Е > 0.1 MeV (analysis of the data yields a value of 1094 ± 153 s−1).

Sliding mode and SDRE control laws on a tethered satellite system to de-orbit space debris

Since space debris is a problem that has been continuously increasing, removal missions should be considered. Tethered space system (TSS) has wide application prospects in the future on-orbit missions such as debris removal. However, it is rather complex and difficult for TSS to realize stabilization of tumbling combinations after connecting to the debris. In this paper, the stabilization problem of this combination is studied.An adaptive sliding mode and State-Dependent Riccati Equation control methods are applied on a TSS to stabilize the system and de-orbit the space debris. The tether tension and stability of the in-plane and out-of-plane libration angles of the system are taken into account. The tether can only resist axial stretching. The thrusters, which are the sources of the system inputs are equipped on the satellite. The controllers regulate the tether to remain fully stretched and to decrease the altitude of the orbit continuously. The numerical simulation validates the proposed control schemes for de-orbiting the debris and put it in lower altitude orbit. This makes the debris retrieve to the atmosphere in less time than the actual orbit lifetime. The comparison between two control schemes is discussed.

Exploiting dynamical perturbations for the end-of-life disposal of spacecraft in LEO

As part of the dynamical analysis carried out within the Horizon 2020 ReDSHIFT project, this work analyzes the possible strategies to guide low altitude satellites towards an atmospheric reentry through an impulsive maneuver. We consider a fine grid of initial conditions in semi-major axis, eccentricity and inclination and we identify the orbits that can be compliant with the 25-year rule as the target of a single-burn strategy. Besides the atmospheric drag, we look for the aid provided by other dynamical perturbations - mainly solar radiation pressure - to facilitate a reentry. Indeed, in the case of typical area-to-mass ratios for objects in LEO, we observed that dynamical resonances can be considered only in combination with the atmospheric drag and for a very limited set of initial orbits. Instead, if an area augmentation device, as a solar sail, is available on-board the spacecraft, we verified that a wider range of disposal solutions become available. This information is exploited to design an improved mitigation scheme, that can be applied to any satellite in LEO.

Analysis of lifetime extension capabilities for CubeSats equipped with a low-thrust propulsion system for Moon missions

In this paper, we analyze the mission lifetime extension capability for a CubeSat smaller than 3U in a circular lunar orbit at a 100-km altitude, assuming the utilization of a state-of-the-art low-thrust electric propulsion system such as pulsed plasma thrusters with an impulse bit (Ibit) and velocity change (ΔV) below 60 μNs and 120 m/s, respectively. Because of the non-spherical gravity field of the Moon and its strong influence on low-altitude lunar orbits, a long orbital lifetime is achievable only within a set of stable orbits which mainly depends on initial inclination and right ascension of the ascending node (RAAN); moreover, as a piggyback on a main mission, the deployment of CubeSats in those stable orbits is not guaranteed. For this reason, we propose an orbit correction strategy whose performance is constrained to the initial orbital parameters of the CubeSat (i.e., inclination and RAAN), its solar power generation capacity, its attitude control strategy, and its propulsion subsystem features, such as the thruster Ibit and budgeted ΔV. By analyzing the required time to perform the orbit correction maneuvers to extend the orbital lifetime and the minimum altitude achieved throughout the mission lifetime of the spacecraft, we demonstrated that a one-year mission can be achieved within initial orbital inclination values greater than 65°. For unstable orbits bounded by initial orbital inclination values smaller than 65°, the orbit lifetime can also be extended from a few days to up to one year. Better performance with our proposed orbit correction strategy can be achieved by using an electric propulsion system featuring Ibit and ΔV values greater than 40 μNs and 80 m/s, respectively. Our results show the feasibility of performing any orbit correction maneuver for the enhancement of the mission lifetime of a CubeSat, expanding the performance capabilities of CubeSats to any mission in a lunar orbit by reducing the limitation of deploying them in unstable orbits.

Attitude Stabilization of Spacecraft in Very Low Earth Orbit by Center-Of-Mass Shifting

At very low orbital altitudes (≲450 km) the aerodynamic forces can become major attitude disturbances. Certain missions that would benefit from a very low operational altitude require stable attitudes. The use of internal shifting masses, actively shifting the location of the spacecraft center-of-mass, thus modulating, in direction and magnitude, the aerodynamic torques, is here proposed as a method to reject these aerodynamic disturbances. A reduced one degree-of-freedom model is first used to evaluate the disturbance rejection capabilities of the method with respect to multiple system parameters (shifting mass, shifting range, vehicle size, and altitude). This analysis shows that small shifting masses and limited shifting ranges suffice if the nominal center-of-mass is relatively close to the estimated center-of-pressure. These results are confirmed when the analysis is extended to a full three rotational degrees-of-freedom model. The use of a quaternion feedback controller to detumble a spacecraft operating at very low altitudes is also explored. The analysis and numerical simulations are conducted using a nonlinear dynamic model that includes the full effects of the shifting masses, a realistic atmospheric model, and uncertain spacecraft aerodynamic properties. Finally, a practical implementation on a 3U CubeSat using commercial-off-the-shelf components is briefly presented, demonstrating the implementation feasibility of the proposed method.

Closed-Loop Control of Earth Observation Satellites

Low-altitude satellite orbits, while enabling high-resolution imagery of the Earth surface, have limited coverage properties. This disadvantage of the remote sensing satellites can be offset by the low-thrust electric propulsion capability, which permits to combine high-resolution with short revisit times. The purpose of this work is to present a simple closed-loop control function that allows the direct control of the orbit, making the spacecraft to pass over prescribed sites on the terrestrial surface without the need of precomputed reference trajectories. The simplicity of application contributes to the autonomy of the remote sensing satellites. The performance of the new control algorithm is illustrated by representative numerical simulation results.

Auroral Hiss Emissions During Cassini's Grand Finale: Diverse Electrodynamic Interactions Between Saturn and Its Rings

AbstractThe Cassini Grand Finale orbits offered a new view of Saturn and its environment owing to multiple highly inclined orbits with unprecedented proximity to the planet during closest approach. The Radio and Plasma Wave Science instrument detected striking signatures of plasma waves in the southern hemisphere. These all propagate in the whistler mode and are classified as (1) a filled funnel‐shaped emission, commonly known as auroral hiss. Here however, our analysis indicates that they are likely associated with currents connected to the rings. (2) First observations of very low frequency saucers directly linked to the planet on field lines also connected to the rings. The latter observations are unique to low altitude orbits, and their presence at the Earth and Saturn alike shows that they are fundamental plasma waves in planetary ionospheres. Our results give an insight, from a unique perspective, into the dynamic and diverse nature of Saturn's environment.

Variations of the main nighttime ionospheric density anomalies observed by DEMETER during the descending phase of solar cycle 23

This paper is related to analysis of the Weddell Sea Anomaly (WSA) and the Mid-latitude Summer Nighttime Anomaly (MSNA) observed by the low altitude satellite DEMETER during nighttime between 2004 and 2010. This time interval corresponds to the decrease of the solar cycle 23 which was unusually long. It appears that, if these two anomalies have a peak in local summer (December in the Southern hemisphere for the WSA, June in the Northern hemisphere for the MSNA), the anomalies are also observed during the months around December and June with a decreased intensity. But at the end of the solar cycle 23 the summer peaks dramatically decrease and even relatively more quickly than the solar index F10.7. This phenomenon is much more significant for the WSA. It is shown that the mechanism producing the two anomalies (thermospheric neutral winds and magnetic declination effects) is strengthened by the solar ionization which is active during the night above the WSA and the MSNA areas. But at solar minimum, this mechanism is weakened. These results are valid at the satellite altitude (660 km) and may vary at lower altitudes.

Lunar orbit dynamics and maneuvers for Lunisat missions

Lunisat represents a next-generation microsatellite aimed at orbiting the Moon, and equipped with dispensers for the release of nanosatellites. This research is focused on the orbital dynamics of both the main microsatellite and the nanosatellites after release. Due to the irregular concentrations of mass in the Moon, low altitude, near-circular lunar orbits are affected by a considerable number of harmonics of the Moon gravitational field. Nonsingular equinoctial orbit elements are employed for orbit propagations, in conjunction with numerical averaging, which is a numerical technique that allows substantial computational improvements. The dynamical model considers a large number of harmonics of the lunar gravitational field, as well as the Earth and Sun perturbing influence as third bodies. Low-altitude lunar satellites turn out to impact the lunar surface after a few weeks or months. In case of an unsatisfactory lifetime, two simple orbit maintenance strategies are evaluated, together with the related propellant budget. Nanosatellites will be released from Lunisat by means of springs. The orbit of each nanosatellite depends on the release conditions, i.e. the relative velocity magnitude and direction. This work investigates the nanosatellite lifetimes as functions of these conditions. Lastly, minimum-time low-thrust transfers for reducing the orbit altitude are investigated, both for nanosatellite release and for the conclusive phase of the Lunisat mission, which finally terminates with the impact on the lunar surface.

A global database and statistical analyses of (4) Vesta craters

In this paper, we present a global database of craters larger than 0.7 km in diameter on the surface of (4) Vesta, which we created based on a global mosaic of the Dawn spacecraft's high-altitude mapping orbit (HAMO) images at a resolution of ∼60 m/pixel and cross-checked using the images from the low-altitude mapping orbit (LAMO) mission phase. The method to produce the crater database consists of visual crater identification and mathematical shape determination. To give precise measurements, topographic corrections have also been considered by combining the digital elevation model (DEM) data in this global survey. We calculated the morphometric parameters of the craters, which consist of diameter, depth, depth-to-diameter ratio, ellipticity, and azimuth of the major axis. Our results show that the craters on (4) Vesta are very shallow, with a mean depth-to-diameter ratio of ∼0.065, which is similar to that of the craters on (1) Ceres and (25143) Itokawa but lower than for craters on other solid bodies. The results also show that the fractions of craters with an ellipticity greater than 1.1 or 1.2 are smaller than those of craters on the Martian surface, which is probably because few secondaries exist on the surface of (4) Vesta. In addition, the azimuths of the major axis are almost uniformly distributed in different directions. Our globally consistent crater database has many potential applications for future studies such as geologic mapping and surface age dating.

System analysis and test-bed for an atmosphere-breathing electric propulsion system using an inductive plasma thruster

Challenging space mission scenarios include those in low altitude orbits, where the atmosphere creates significant drag to the S/C and forces their orbit to an early decay. For drag compensation, propulsion systems are needed, requiring propellant to be carried on-board. An atmosphere-breathing electric propulsion system (ABEP) ingests the residual atmosphere particles through an intake and uses them as propellant for an electric thruster. Theoretically applicable to any planet with atmosphere, the system might allow to orbit for unlimited time without carrying propellant. A new range of altitudes for continuous operation would become accessible, enabling new scientific missions while reducing costs. Preliminary studies have shown that the collectible propellant flow for an ion thruster (in LEO) might not be enough, and that electrode erosion due to aggressive gases, such as atomic oxygen, will limit the thruster lifetime. In this paper an inductive plasma thruster (IPT) is considered for the ABEP system. The starting point is a small scale inductively heated plasma generator IPG6-S. These devices are electrodeless and have already shown high electric-to-thermal coupling efficiencies using O2 and CO2. The system analysis is integrated with IPG6-S tests to assess mean mass-specific energies of the plasma plume and estimate exhaust velocities.

Statistical Study on Variations of the Ionospheric Ion Density Observed by DEMETER and Related to Seismic Activities

AbstractIn this paper, we present the result of a statistical study performed on the ionospheric ion density variations above areas of seismic activity. The ion density was observed by the low altitude satellite DEMETER between 2004 and 2010. In the statistical analysis a superposed epoch method is used where the observed ionospheric ion density close to the epicenters both in space and in time is compared to background values recorded at the same location and in the same conditions. Data associated with aftershocks have been carefully removed from the database to prevent spurious effects on the statistics. It is shown that, during nighttime, anomalous ionospheric perturbations related to earthquakes with magnitudes larger than 5 are evidenced. At the time of these perturbations the background ion fluctuation departs from a normal distribution. They occur up to 200 km from the epicenters and mainly 5 days before the earthquakes. As expected, an ion density perturbation occurring just after the earthquakes and close to the epicenters is also evidenced.

Medical Implications of Space Radiation Exposure Due to Low-Altitude Polar Orbits.

Space radiation research has progressed rapidly in recent years, but there remain large uncertainties in predicting and extrapolating biological responses to humans. Exposure to cosmic radiation and solar particle events (SPEs) may pose a critical health risk to future spaceflight crews and can have a serious impact on all biomedical aspects of space exploration. The relatively minimal shielding of the cancelled 1960s Manned Orbiting Laboratory (MOL) program's space vehicle and the high inclination polar orbits would have left the crew susceptible to high exposures of cosmic radiation and high dose-rate SPEs that are mostly unpredictable in frequency and intensity. In this study, we have modeled the nominal and off-nominal radiation environment that a MOL-like spacecraft vehicle would be exposed to during a 30-d mission using high performance, multicore computers. Projected doses from a historically large SPE (e.g., the August 1972 solar event) have been analyzed in the context of the MOL orbit profile, providing an opportunity to study its impact to crew health and subsequent contingencies. It is reasonable to presume that future commercial, government, and military spaceflight missions in low-Earth orbit (LEO) will have vehicles with similar shielding and orbital profiles. Studying the impact of cosmic radiation to the mission's operational integrity and the health of MOL crewmembers provides an excellent surrogate and case-study for future commercial and military spaceflight missions.Chancellor JC, Auñon-Chancellor SM, Charles J. Medical implications of space radiation exposure due to low-altitude polar orbits. Aerosp Med Hum Perform. 2018; 89(1):3-8.

Radar Sounding Through the Earth’s Ionosphere at 45 MHz

Radar sounding from aircraft or ground-coupled radars has long provided scientists with a powerful technique to sound through ice layers to retrieve local depth and layering structure. More recently, it has been used to detect shallow aquifers in warm, dry, and desert regions. At Mars, a long-wavelength radar sounding from low orbit altitudes has produced global maps that reveal the presence of ice layering at all latitudes and glacial deposits on the flanks of volcanoes. Until now, sounding from the earth orbit at wavelengths long enough to penetrate ice sheets and arid sand was thought to be infeasible, because of the electromagnetic properties of the ionosphere. In this paper, we show that a radar sounding at frequencies as low as 45 MHz is, in fact, theoretically possible under viewing conditions that occur often enough to be practical. This conclusion opens up a previously unutilized portion of the electromagnetic spectrum for large-scale, spaceborne remote sensing of subsurface features on the earth.

NOAA POES and MetOp particle observations during the 17 March 2013 storm

Five National Oceanic and Atmospheric Administration and Meteorological Operational polar orbiting low altitude satellites were in operation during the 17 March 2013 storm. They were equipped with identical instruments to measure the precipitation of energetic particles over a large energy range. Well calibrated particle data, gives the opportunity of complete ILAT/MLT plots on a time resolution of 3 h throughout the storm. During the storm sudden commencement and main phase of the storm, an isotropic proton precipitation occurred at almost all magnetic local time and the Isotropic Boundary (IB) coincided with the inner boundary of the Ring Current (RC) as determined by the Van Allen Probes. Thus the IB on the night/dusk sector is a measure of how deep the ring current develops in the inner magnetosphere. The electron and proton precipitation is closely connected with substorms and the convective electric field created by the solar wind. A loss cone totally filled with particles indicates the occurrence of particles injection into the RC, while a partly filled loss cone means that the RC particles experienced a loss.The above 1 MeV electrons disappear in the storm main phase, but appears with increased intensity in the recovery phase. They are limited to the invariant latitude region 50∘ to 62∘. The lower energy electrons penetrate deeper into the magnetosphere. During geomagnetic storms a Storm Time Equatorial Belt (STEB) of energetic neutral atoms and ions is found to exist at low altitudes around the geomagnetic equator. Their source is the RC protons existing at larger L-values. The STEB appears first in the midnight/evening sector and then later in the morning sector largely consistent with the drift of the RC ions and in accordance with results from ground based magnetic observations. During the main phase of the storm, the outer part of the RC will experience magnetopause shadowing and will be lost at the magnetopause. Maximum particle injection into the ring current is associated with the most rapid fall in Dst. The maximum STEB intensity is associated with minimum Dst and strongest RC.