Application of a Multiple Hypothesis Filter to near GEO high area-to-mass ratio space objects state estimation

Optical surveys have identified a class of high area-to-mass ratio (HAMR) objects in the vicinity of the Geostationary Earth Orbit (GEO) regime. The nature of these objects is not well known, though their proximity to the GEO belt implies origins from space objects (SOs) near GEO. These HAMR objects pose a collision hazard as they transit through the vicinity of active GEO satellites. Due to their high area-to-mass ratios (AMRs), ranging from 0.1 to 20 m²/kg and higher, the effective solar radiation pressure perturbs their orbits significantly. Improvements in detection sensitivity will result in large numbers of uncorrelated tracks from surveys. A Multiple Hypothesis Filter (MHF) approach to the initial state estimation and track association provides a potentially automated and efficient approach to the processing of multiple un-correlated tracks.The availability of long-term optical angles data collected for a set of near GEO HAMR objects provides the means for testing candidate estimation processes such as the MHF. A baseline orbit determination (OD) process uses an Extended Kalman Filter/Smoother to manually estimate the 6 orbital elements and the effective area-to-mass ratio (AMR) which drives the solar radiation pressure perturbations on the orbital trajectories. In addition to allowing the characterization of the long-term behavior of the AMR, this process establishes a pseudo-truth trajectory to which the MHF analysis can be compared. An Unscented Kalman Filter (UKF) is applied in the MHF estimation process to estimate the 6 orbital elements and AMR, with no a priori state assumptions, and the results are compared to the pseudo-truth results for validation.The work to be presented summarizes the UKF/MHF process and assesses state estimation performance based on selected data for selected near GEO HAMR objects having a range of AMR value and variations. The prediction accuracy is also assessed by comparing predictions derived from filter updates to segments of the pseudo-truth trajectory determined from data not included in the updates.     

Application of a Multiple Hypothesis Filter to near GEO high area-to-mass ratio space objects state estimation

Optical surveys have identified a class of high area-to-mass ratio (HAMR) objects in the vicinity of the Geostationary Earth Orbit (GEO) regime. The nature of these objects is not well known, though their proximity to the GEO belt implies origins from space objects (SOs) near GEO. These HAMR objects pose a collision hazard as they transit through the vicinity of active GEO satellites. Due to their high area-to-mass ratios (AMRs), ranging from 0.1 to 20 m²/kg and higher, the effective solar radiation pressure perturbs their orbits significantly. Improvements in detection sensitivity will result in large numbers of uncorrelated tracks from surveys. A Multiple Hypothesis Filter (MHF) approach to the initial state estimation and track association provides a potentially automated and efficient approach to the processing of multiple un-correlated tracks.The availability of long-term optical angles data collected for a set of near GEO HAMR objects provides the means for testing candidate estimation processes such as the MHF. A baseline orbit determination (OD) process uses an Extended Kalman Filter/Smoother to manually estimate the 6 orbital elements and the effective area-to-mass ratio (AMR) which drives the solar radiation pressure perturbations on the orbital trajectories. In addition to allowing the characterization of the long-term behavior of the AMR, this process establishes a pseudo-truth trajectory to which the MHF analysis can be compared. An Unscented Kalman Filter (UKF) is applied in the MHF estimation process to estimate the 6 orbital elements and AMR, with no a priori state assumptions, and the results are compared to the pseudo-truth results for validation.The work to be presented summarizes the UKF/MHF process and assesses state estimation performance based on selected data for selected near GEO HAMR objects having a range of AMR value and variations. The prediction accuracy is also assessed by comparing predictions derived from filter updates to segments of the pseudo-truth trajectory determined from data not included in the updates.     

A laser-optical system to re-enter or lower low Earth orbit space debris

Collisions among existing Low Earth Orbit (LEO) debris are now a main source of new debris, threatening future use of LEO space. Due to their greater number, small (1–10 cm) debris are the main threat, while large (>10 cm) objects are the main source of new debris. Flying up and interacting with each large object is inefficient due to the energy cost of orbit plane changes, and quite expensive per object removed. Strategically, it is imperative to remove both small and large debris. Laser-Orbital-Debris-Removal (LODR), is the only solution that can address both large and small debris. In this paper, we briefly review ground-based LODR, and discuss how a polar location can dramatically increase its effectiveness for the important class of sun-synchronous orbit (SSO) objects. With 20% clear weather, a laser-optical system at either pole could lower the 8-ton ENVISAT by 40 km in about 8 weeks, reducing the hazard it represents by a factor of four. We also discuss the advantages and disadvantages of a space-based LODR system. We estimate cost per object removed for these systems. International cooperation is essential for designing, building and operating any such system.     

Very high delta-V missions to the edge of the solar system and beyond enabled by the dual-stage 4-grid ion thruster concept

A new and innovative type of gridded ion thruster, the “Dual-Stage 4-Grid” or DS4G concept, has been proposed and its predicted high performance validated under an ESA research, development and test programme. The DS4G concept is able to operate at very high specific impulse and thrust density values well in excess of conventional 3-grid ion thrusters at the expense of a higher power-to-thrust ratio. This makes it a possible candidate for ambitious missions requiring very high delta-V capability and high power. Such missions include 100 kW-level multi-ton probes based on nuclear and solar electric propulsion (SEP) to distant Kuiper Belt Object and inner Oort cloud objects, and to the Local Interstellar medium. In this paper, the DS4G concept is introduced and its application to this mission class is investigated. Benefits of using the DS4G over conventional thrusters include reduced transfer time and increased payload mass, if suitably advanced lightweight power system technologies are developed.A mission-level optimisation is performed (launch, spacecraft system design and low-thrust trajectory combined) in order to find design solutions with minimum transfer time, maximum scientific payload mass, and to explore the influence of power system specific mass. It is found that the DS4G enables an 8-ton spacecraft with a payload mass of 400 kg, equipped with a 65 kW nuclear reactor with specific mass 25 kg/kW (e.g. Topaz-type with Brayton cycle conversion) to reach 200 AU in 23 years after an Earth escape launch by Ariane 5. In this scenario, the optimum specific impulse for the mission is over 10,000 s, which is well within the capabilities of a single 65 kW DS4G thruster. It is also found that an interstellar probe mission to 200 AU could be accomplished in 25 years using a “medium-term” SEP system with a lightweight 155 kW solar array (2 kg/kW specific mass) and thruster PPU (3.7 kg/kW) and an Earth escape launch on Ariane 5. In this case, the optimum specific impulse is lower at 3500 s which is well within conventional gridded ion thruster capability.     

Dose estimates in a lunar shelter with regolith shielding

Beyond the Earth's atmosphere, galactic cosmic radiation (GCR) and solar energetic particles (SEPs) are a significant hazard to both manned and robotic missions. For long human missions on the lunar surface (months to a year) a radiation shelter is needed for dose mitigation and emergency protection in case of solar events. This paper investigates the interaction of source protons of solar events like those of February 1956 that emitted many fewer particles with energies up to 1000 MeV and of the October 1989 event of lower protons energy but higher fluence, with the lunar regolith and aluminum shielding of a lunar shelter. The shelter is 5 m in diameter and has a footprint of 5×8 m and a 10 cm thick aluminum support structure, however, actual thickness could be much smaller (～1–2 cm) depending on the weight of the regolith shielding piled on top. The regolith is shown to be slightly more effective than aluminum. Thus, the current results are still applicable for a thinner aluminum structure and increased equivalent (or same mass) thickness of the regolith. The shielding thicknesses to reduce the dose solely due to solar protons in the lunar shelter below those recommended by NASA to astronauts for 30 day-operation in space (250 mSv) and for radiation workers (50 mSv) are determined and compared. The relative attenuation of incident solar protons with regolith shielding and the dose estimates inside the shelter are calculated for center seeking, planar, and isotropic incidence of the source protons. With the center seeking incidence, the dose estimates are the highest, followed by those with isotropic incidence, and the lowest are those with the planar incidence.     

Probabilistic assessment of radiation risk for astronauts in space missions

Accurate estimations of the health risks to astronauts due to space radiation exposure are necessary for future lunar and Mars missions. Space radiation consists of solar particle events (SPEs), comprised largely of medium energy protons (less than several hundred MeV); and galactic cosmic rays (GCR), which include high-energy protons and heavy ions. While the frequency distribution of SPEs depends strongly upon the phase within the solar activity cycle, the individual SPE occurrences themselves are random in nature. A solar modulation model has been developed for the temporal characterization of the GCR environment, which is represented by the deceleration potential, ?. The risk of radiation exposure to astronauts as well as to hardware from SPEs during extra-vehicular activities (EVAs) or in lightly shielded vehicles is a major concern for radiation protection. To support the probabilistic risk assessment for EVAs, which could be up to 15% of crew time² on lunar missions, we estimated the probability of SPE occurrence as a function of solar cycle phase using a non-homogeneous Poisson model [1] to fit the historical database of measurements of protons with energy>30 MeV, Φ₃₀. The resultant organ doses and dose equivalents, as well as effective whole body doses, for acute and cancer risk estimations are analyzed for a conceptual habitat module and for a lunar rover during space missions of defined durations. This probabilistic approach to radiation risk assessment from SPE and GCR is in support of mission design and operational planning for future manned space exploration missions.     

Radiation engineering analysis of shielding materials to assess their ability to protect astronauts in deep space from energetic particle radiation

An analysis is performed on four typical materials (aluminum, liquid hydrogen, polyethylene, and water) to assess their impact on the length of time an astronaut can stay in deep space and not exceed a design basis radiation exposure of 150 mSv. A large number of heavy lift launches of pure shielding mass are needed to enable long duration, deep space missions to keep astronauts at or below the exposure value with shielding provided by the vehicle. Therefore, vehicle mass using the assumptions in the paper cannot be the sole shielding mechanism for long duration, deep space missions. As an example, to enable the Mars Design Reference Mission 5.0 with a 400 day transit to and from Mars, not including the 500 day stay on the surface, a minimum of 24 heavy lift launches of polyethylene at 89,375 lbm (40.54 tonnes) each are needed for the 1977 galactic cosmic ray environment. With the assumptions used in this paper, a single heavy lift launch of water or polyethylene can protect astronauts for a 130 day mission before exceeding the exposure value. Liquid hydrogen can only protect the astronauts for 160 days. Even a single launch of pure shielding material cannot protect an astronaut in deep space for more than 180 days using the assumptions adopted in the analysis. It is shown that liquid hydrogen is not the best shielding material for the same mass as polyethylene for missions that last longer than 225 days.     

Modeling the sodium potassium droplet interactions with the low earth orbit space debris environment

The NASA/JSC sodium potassium (NaK) RORSAT coolant source and propagation model has been extended to 1 mm in diameter via a size distribution, which is an inverse power law fit that has been modified to damp out in the large size regime. This function matches the observed Haystack NaK population down to diameters of about 6 mm. The extrapolated function takes the population to arbitrarily small sizes all the while retaining the mass dominance of the 1–3 cm droplets that is observed in the Haystack data. This result is physically satisfying since the mechanism of NaK ejection appears to be a nonviolent release at low relative velocities. We propose that any NaK particles smaller than about 1 mm that exist would not be due to that mechanism. Instead, we show that such a population could be the result of subsequent collisions of NaK droplets with larger resident space objects and the micrometeoroid population. Our preliminary analysis shows that collisions between these populations are likely in the time period of 1980 through present-day. Though the result of such collisions is generally unknown it is probable that some ejecta of NaK enter the low Earth orbit (LEO) environment as a result. It is these secondary NaK droplets/particles that we contend are the likely impactors noted on returned surfaces.     

Effects of stand-off bursts on rubble-pile targets: Evaluation of a hazardous asteroid mitigation strategy

We explore the aftereffects of stand-off burst mitigation on kilometer-scale rubble pile asteroids. We use a simple model of X-ray energy deposition to calculate the impulse transferred to the target, in particular to burst-facing blocks on the target surface. The impulse allows us to estimate an initial velocity field for the blocks on the outer side of the target facing the burst. We model the dynamics using an N-body polyhedron program built on the Open Dynamics Engine, a “physics engine” that integrates the dynamical equations for objects of general shapes and includes collision detection, friction, and dissipation.We tested several different models for target objects: rubble piles with different mass distributions, a “brick-pile” made of closely fitting blocks and zero void space, and a non-spherical “contact binary” rubble pile. Objects were bound together by self-gravity and friction/inelastic restitution with no other cohesive forces. Our fiducial cases involved objects of m=3.5×10¹² kg (corresponding to a radius of 0.7 km for the bulk object), an X-ray yield of 1 megaton, and stand-off burst distances of R=0.8–2.5 km from the target center of mass.Kilometer-scale rubble piles are robust to stand-off bursts of a yield ($Y\sim 1$ megaton) that would be sufficient to provide an effective velocity change ($\mathrm{\Delta }v\sim 0.05\mathrm{m}{\mathrm{s}}^{-1}$). Disaggregation involving some tens of percent of the target mass happens immediately after the impulse; the bulk of the object re-accretes on a few gravitational timescales, and the final deflected target contains over 95% (typically, 98–99%) of the original mass. Off-center components of the mitigation impulse and the target mass distribution cause a small amount of induced spin and off-axis components of velocity change. The off-axis velocity component amounts to an angular deviation of $\sim$ 0.05–0.1 radians from the nominal impulse vector, which may be important for mitigation planning.     

Right ventricular tissue Doppler assessment in space during circulating volume modification using the Braslet device

Introduction: This joint US–Russian work aims to establish a methodology for assessing cardiac function in microgravity in association with manipulation of central circulating volume. Russian Braslet-M (Braslet) occlusion cuffs were used to temporarily increase the volume of blood in the lower extremities, effectively reducing the volume in central circulation. The methodology was tested at the International Space Station (ISS) to assess the volume status of crewmembers by evaluating the responses to application and release of the cuffs, as well as to modified Valsalva and Mueller maneuvers. This case study examines the use of tissue Doppler (TD) of the right ventricular (RV) free wall. Results: Baseline TD of the RV free wall without Braslet showed early diastolic E′ (16 cm/s), late diastolic A′ (14 cm/s), and systolic S′ (12 cm/s) velocities comparable with those in normal subjects on Earth. Braslet application caused 50% decrease of E′ (8 cm/s), 45% increase of A′, and no change to S′. Approximately 8 beats after the Braslet release, TD showed E′ of 8 cm/s, A′ of 12 cm/s, and S′ of 13 cm/s. At this point after release, E′ did not recover to baseline values while l A′ and S′ did recover. The pre-systolic cross-sectional area of the internal jugular vein without Braslet was 1.07 cm², and 1.13 cm² 10 min after the Braslet was applied. The respective cross-sectional areas of the femoral vein were 0.50 and 0.54 cm². The RV myocardial performance Tei index was calculated by dividing the sum of the isovolumic contraction time and isovolumic relaxation time by the ejection time ((IVCT+IVRT)/ET); baseline and Braslet-on values for Tei index were 0.25 and 0.22, respectively. Braslet Tei indices are within normal ranges found in healthy terrestrial subjects and temporarily become greater than 0.4 during the dynamic Braslet release portion of the study. Conclusions: TD modality was successfully implemented in space flight for the first time. TD of RV revealed that the Braslet influenced cardiac preload and that fluid was sequestered in the lower extremity interstitial and vascular space after only 10 min of application. This report demonstrates that Braslet application has an effect on RV physiology in long-duration space flight based on TD, and that this effect is in part due to venous hemodynamics.     

Deployment history and design considerations for space reactor power systems

The history of the deployment of nuclear reactors in Earth orbits is reviewed with emphases on lessons learned and the operation and safety experiences. The former Soviet Union's “BUK” power systems, with SiGe thermoelectric conversion and fast neutron energy spectrum reactors, powered a total of 31 Radar Ocean Reconnaissance Satellites (RORSATs) from 1970 to 1988 in 260 km orbit. Two of the former Soviet Union's TOPAZ reactors, with in-core thermionic conversion and epithermal neutron energy spectrum, powered two Cosmos missions launched in 1987 in ～800 km orbit. The US’ SNAP-10A system, with SiGe energy conversion and a thermal neutron energy spectrum reactor, was launched in 1965 in 1300 km orbit. The three reactor systems used liquid NaK-78 coolant, stainless steel structure and highly enriched uranium fuel (90–96 wt%) and operated at a reactor exit temperature of 833–973 K. The BUK reactors used U-Mo fuel rods, TOPAZ used UO₂ fuel rods and four ZrH moderator disks, and the SNAP-10A used moderated U-ZrH fuel rods. These low power space reactor systems were designed for short missions (～0.5 kW_e and ～1 year for SNAP-10A, <3.0 kW_e and <6 months for BUK, and ～5.5 kW_e and up to 1 year for TOPAZ). The deactivated BUK reactors at the end of mission, which varied in duration from a few hours to ～4.5 months, were boosted into ～800 km storage orbit with a decay life of more than 600 year. The ejection of the last 16 BUK reactor fuel cores caused significant contamination of Earth orbits with NaK droplets that varied in sizes from a few microns to 5 cm. Power systems to enhance or enable future interplanetary exploration, in-situ resources utilization on Mars and the Moon, and civilian missions in 1000–3000 km orbits would generate significantly more power of 10's to 100's kW_e for 5–10 years, or even longer. A number of design options to enhance the operation reliability and safety of these high power space reactor power systems are presented and discussed.     

Suborbital and low-thermospheric experiments using sounding rockets in Taiwan

The suborbital flight is a kind of flight, which reaches the space and then comes back to ground without completing one orbital revolution. The atmospheric thermosphere extends from 85 km to 600 km in altitude. Therefore, the suborbital and low-thermospheric experiments to be performed at altitude below 300 km can be combined using the sounding rocket. These experiments include rocket staging, fairing separation, ultrasonic flight, reentry, aerobrake and recovery test, ultraviolet and ionization observations, ozone measurement, etc. The advent of Taiwan's sub-orbital and thermospheric experiments project can be traced back to 1997. This is the year Taiwan's National Space Organization (NSPO) was assigned to be responsible for procuring the sounding rocket for applications in science experiments and space technology research effort. From 1997 to 2010, 8 launches have been completed including one experimental hybrid rocket. All onboard instruments and sensors for sub-orbital and low-thermospheric experiments are developed and integrated by the domestic universities. More launches have been planned in the future. Opportunities for international cooperation in developing new instruments and payloads for future experiments will be possible.     

Interaction of mental and orthostatic stressors

We assessed hemodynamic responses induced by orthostatic and mental stressors, using passive head up tilt (HUT) and mental arithmetic (MA), respectively. The 15 healthy males underwent three protocols: (1) HUT alone, (2) MA in supine position and (3) MA+HUT, with sessions randomized and ≥2 weeks apart. In relation to baseline, HUT increased heart rate (HR) (+20.4±7.1 bpm; p<0.001), mean blood pressure (MBP) (+4.7±11.3 mmHg; p<0.05), diastolic blood pressure (DBP) (+6.1±11.6 mmHg; p<0.05) and total peripheral resistance (TPR) (+155±232 dyne*s/cm⁵; p<0.001) but decreased stroke volume (SV) (?33.1±13.4 ml; p<0.001) and cardiac output (CO) (?0.6±1.0 l/min; p<0.01). MA increased HR (+8.0±6.0 bpm; p<0.001), systolic blood pressure (SBP) (+9.0±7.7 mmHg; p<0.001), MBP (+10.0±6.5 mmHg; p<0.001), DBP (+9.5±7.2 mmHg; p<0.001) and CO (+0.6±0.8 l/min; p<0.01). MA+HUT increased HR (+28.8±8.4 bpm; p<0.001), SBP (+4.6±14.3 mmHg; p<0.05), MBP (+11.2±11.6 mmHg; p<0.001), DBP (+13.5±10.1 mmHg; p<0.001) and TPR (+160±199 dyne*s/cm⁵; p<0.001) but SV (?34.5±14.6 ml; p<0.001) decreased. Mental challenge during orthostatic challenge elicited greater increases in heart rate, despite similar reductions in stroke volume such as those during orthostatic stress alone. Overall, cardiac output decreases were less with combinations of mental and orthostatic challenges in comparison to orthostasis alone. This would suggest that carefully chosen mental stressors might affect orthostatic responses of people on standing up. Therefore, additional mental loading could be a useful countermeasure to alleviate the orthostatic responses of persons, particularly in those with histories of dizziness on standing up or on return to earth from the spaceflight environment of microgravity.     

Post-disposal orbital evolution of satellites and upper stages used by the GPS and GLONASS navigation constellations: The long-term impact on the Medium Earth Orbit environment

The long-term evolution and environmental impact in MEO of all the abandoned spacecraft and upper stages associated with the GPS and GLONASS navigation constellations were analyzed. The orbits of the disposed objects, as of 1 May 2011, were propagated for 200 years and snapshots of their evolving distribution were obtained, together with an estimation of the changing collision probability with the spacecraft of the operational navigation systems existing or planned in MEO, i.e., GLONASS, GPS, Beidou and Galileo. The probability that the abandoned objects considered will collide with the operational spacecraft of the navigation constellations is very low, even taking into account the intrinsic eccentricity instability of the disposal orbits. Assuming the present or envisaged configuration of the constellations in MEO, the probability of collision, integrated over 200 years, would be <1/300 with a GLONASS spacecraft, <1/15,000 with a GPS or Beidou spacecraft, and <1/250,000 with a Galileo spacecraft. The worst disposal strategy consists in abandoning satellites and upper stages close to the altitude of the operational constellation (GLONASS), while a re-orbiting a few hundred km away (GPS) is able to guarantee an effective long-term dilution of the collision risk, irrespective of the eccentricity instability due to geopotential and luni-solar perturbations. The disposal strategies applied so far to the GPS satellites should be able to guarantee for at least a few centuries a sustainable MEO environment free of collisions among intact objects. Consequently, there would be no need to adopt disposal schemes targeting also the optimal value of the eccentricity vector. However, it should be pointed out that the GPS disposal strategy was devised well in advance of the Beidou constellation announcement, so most of the abandoned satellites were re-orbited fairly close to the altitude of the new Chinese system. A new re-orbiting approach will be therefore needed in the future.     

Thrust measurement of dimethyl ether arcjet thruster

The present paper describes thrust measurement results for an arcjet thruster using Dimethyl ether (DME) as the propellant. DME is an ether compound and can be stored as a liquid due to its relatively low freezing point and preferable vapor pressure. The thruster successfully produced high-voltage mode at DME mass flow rates above 30 mg/s, whereas it yielded low-voltage mode below 30 mg/s. Thrust measurements yielded a thrust of 0.15 N and a specific impulse of 270 s at a mass flow rate of 60 mg/s with a discharge power of 1300 W. The DME arcjet thruster was comparable to a conventional one for thrust and discharge power.     

Flight performance analysis of GRACE K-band ranging instrument with simulation data

A dual one-way ranging (DOWR) system provides very high accuracy range measurements between two satellites. The GRACE satellite mission implements the DOWR, called KBR (K-band ranging), to measure very small inter-satellite range change in order to map the Earth gravity field. The flight performance of the KBR is analyzed by using a hybrid software simulator that incorporates actual satellite orbit data into a comprehensive KBR simulator, which was earlier used for computing the GRACE baseline accuracy. Three types of experiments were performed. First is the comparison of the flight data with the simulated data in spectral domain. Second is the comparison of double differenced noise level. Third is the comparison of the range-rate difference with GPS clock estimates. The analysis shows a good agreement with the simulation model except some excessive high frequency noise, e.g. 10⁻⁴ m/√Hz at 0.1 Hz. The range-rate difference shows 0.003 cyc/s discrepancy with the clock estimates. These analyses are helpful to refine the DOWR simulation model and can be benefit to future DOWR instrument development.     

Impaired T-wave amplitude adaptation to heart-rate induced by cardiac deconditioning after 5-days of head-down bed-rest

The study of QT/RR relationship is important for the clinical evaluation of possible risk of acquired or congenital ventricular tachyarrhythmias. In the hypothesis that microgravity exposure could induce changes in the repolarization mechanisms, our aim was to test if a short 5-days strict 6° head-down bed-rest (HDBR) could induce alterations in the QT/RR relationship and spatial repolarization heterogeneity. Twenty-two healthy men (mean age 31±6) were enrolled as part of the European Space Agency HDBR studies. High fidelity (1000 Hz) 24 h Holter ECG (12-leads, Mortara Instrument) was acquired before (PRE), the last day of HDBR (HDT5), and four days after its conclusion (POST). The night period (23:00–06:30) was selected for analysis. X, Y, Z leads were derived and the vectorcardiogram computed. Selective beat averaging was used to obtain averages of P–QRS–T complexes preceded by the same RR (10 ms bin amplitude, in the range 900–1200 ms). For each averaged waveform (i.e., one for each bin), T-wave maximum amplitude (Tmax), T-wave area (Tarea), RTapex, RTend, ventricular gradient (VG) magnitude and spatial QRS-T angle were computed. Non-parametric Friedman test was applied. Compared to PRE, at HDT5 both RTapex and RTend resulted shortened (−4%), with a decrease in T-wave amplitude (−8%) and area (−13%). VG was diminished by 10%, and QRS-T angle increased by 14°. At POST, QT duration and area parameters, as well as QRS-T angle were restored while Tmax resulted larger than PRE (+5%) and VG was still decreased by 3%. Also, a marked loss in strength of the linear regression with RR was found at HDT5 in Tmax and Tarea, that could represent a new dynamic marker of increased risk for life-threatening arrhythmias. Despite the short-term HDBR, ventricular repolarization during the night period was affected. This should be taken into account in astronauts for risk assessment during space flight.     

Endurance testing of a pulsed plasma thruster for nanosatellites

The mission complexity of Nanosatellites has increased tremendously in recent years, but their mission range is limited due to the lack of an active orbit control or ∆v capability. Pulsed Plasma Thrusters (PPT), featuring structural simplicity and very low power consumption are a prime candidate for such applications. However, the required miniaturization of standard PPTs and the adaption to the low power consumption is not straightforward. Most investigated systems have failed to show the required lifetime. The present coaxial design has shown a lifetime of up to 1 million discharges at discharge energies of 1.8 J in previous studies. The present paper focuses on performance characterizations of this design. For this purpose direct thrust measurements with a µN thrust balance were conducted. Thrust measurements in conjunction with mass bit determination allowed a comprehensive assessment. Based on those measurements the present µPPT has a total impulses capability of approximately I≈1.7 Ns, an average mass bit of 0.37 µg s⁻¹ and an average specific impulse of I_sp≈904 s. All tests have shown very good EM compatibility of the PPT with the electronics of the flight-like printed circuit board. Consequently, a complete µPPT unit can provide a ∆v change of 5.1 m/s or 2.6 m/s to a standard 1-unit or 2-unit CubeSat respectively.     

Comparison of rainfall models with Ku-band beacon measurement

Predictions of rain rate and rain attenuation are the most vital steps when analyzing a satellite link operating at frequencies above 10 GHz. Rain attenuation at 12.594 GHz over a satellite path link was measured for the period of 3 years (i.e. January 2002 to December 2004) at Bangkok (13.7°N, 100.7°E). In this paper, a comparison between the current methodologies available to model the impact of rain in earth-space propagation and a dataset of 3 years of rain accumulation with a sampling period of 1 min is made.