Antenna for precise orbit determination

The ESA SWARM mission will consist of three satellites that will measure the Earth magnetic field. The system calls for metre accuracy knowledge of the measurement locations. To achieve this a GPS receiver is used. At least four GPS signals are tracked to determine the code and carrier ranges, from which the position can be derived. The accuracy improves when using more GPS satellites and by averaging over many measurements. The latter is achieved in ground processing with a model-based orbit prediction, resulting in cm accuracy. The main error contributions in the processing are often measurement errors due to satellite multi-path effects. The multipath effects are characterized by measuring the antenna on a 1.5 m mock-up, representing the 9 m long satellite. In order to verify that the mock-up is representative, extensive electromagnetic simulations were made. The simulations included the antenna and the complete satellite and were then reduced to the antenna and a section of the satellite. The actual design of the antenna was performed with several levels of software. First, a fast bodies-of-revolution simulation found a geometry with the right coverage. Then, a finite element method simulation allowed us to match the antenna at two frequencies simultaneously.     

Reduction of satellite electromagnetic scattering by carbon nanostructured multilayers

We propose a low observable satellite covered by layered materials. The dielectric properties of the composite materials are analyzed using the coaxial air-line method. Our solution could result in cost-effective applications for satellite stealthness. The particle swarm optimization algorithm is used to design and optimize layered absorbers. The obtained reflection is below −20 dB in many frequency subranges, also under an oblique incident condition, within the band 2–18 GHz. The optimized layered absorbers are used as a cloak for cube satellites. In particular, we investigated a very simple geometry by using a finite element commercial software in the band from 2 to 18. Finally, the transmission through an aperture on the satellite surface is analyzed. It is highlighted that the optimized layered absorbers attenuate the electric field within the satellite, improving its immunity against electromagnetic interferences.     

Satellite orbit perturbations in a dusty Martian atmosphere

In this paper we calculate the effect of atmospheric dust on the orbital elements of a satellite. Dust storms that originate in the Martian surface may evolve into global storms in the atmosphere that can last for months can affect low orbiter and lander missions. We model the dust as a velocity-square depended drag force acting on a satellite and we derive an appropriate disturbing function that accounts for the effect of dust on the orbit, using a Lagrangean formulation. A first-order perturbation solution of Lagrange's planetary equations of motion indicates that for a local dust storm cloud that has a possible density of 8.323×10⁻¹⁰ kg m⁻³ at an altitude of 100 km affects the orbital semimajor axis of a 1000 kg satellite up −0.142 m day⁻¹. Regional dust storms of the same density may affect the semimajor axis up to of −0.418 m day⁻¹. Other orbital elements are also affected but to a lesser extent.     

Compensation of gravitational attraction disturbance to pure gravity orbit for Inner Formation Flying System

The Inner Formation Flying System (IFFS) consisting of an outer satellite and an inner satellite which is a solid sphere proof mass freely flying in the shield cavity can construct a pure gravity orbit to precisely measure the earth gravity field. The gravitational attraction on the inner satellite due to the outer satellite is a significant disturbance source to the pure gravity orbit and is required to be limited to 10⁻¹¹ m s⁻² order. However, the gravitational disturbance force was on 10⁻⁹ m s⁻² order actually and must be reduced by dedicated compensation mass blocks. The region of relative motion of the inner satellite about its nominal position is within 1 cm in dimension, which raises the complexity of the compensation blocks design. The iterative design strategy of the compensation blocks based on reducing the gravitational attraction at the nominal position of the inner satellite is presented, aiming to guarantee the gravitational force in the relative motion region within requirements after the compensation. The compensation blocks are designed according to the current status of IFFS, and the gravitational disturbance force in the region is reduced to 10⁻¹¹ ms⁻² order with minimized adding mass.     

Economic effect of satellite communications based on the Great East Japan Earthquake

On 11 March 2011, an undersea earthquake of magnitude 9.0, the largest ever recorded in Japan, occurred off the Oshika Peninsula on the Pacific coast of the Tohoku region. The hypocentral region extended for 500 km in the north–south direction from Iwate Prefecture to Ibaraki Prefecture, and for 200 km in east–west direction. The earthquake generated a tsunami with a height of more than 10 m and a run-up height of up to 40.0 m in certain places, which inflicted devastating damage on the coastal areas of the Tohoku and Kanto regions. In addition to the tsunami, the earthquake caused shaking, liquefaction, subsidence, and the collapse of dams, causing major damage to vast areas in the Tohoku and Kanto regions and disrupting various types of infrastructure, including communication. In light of this unprecedented damage, satellite communications were important from various perspectives while terrestrial communications systems were damaged, and an objective evaluation of the role played by satellite communications is relevant to its future installation, adoption and use as a standalone or backup system. Furthermore, satellite communications can help reduce the extent of damage, particularly damage to communications systems, inflicted by strong earthquakes in the future. Accordingly, we report a preliminary quantitative evaluation of the role of satellite communications in the Great East Japan Earthquake, of the role of satellite communications if it becomes widespread, and of its expected role in future large-scale earthquakes in terms of the economic effect converted into cost.     

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.     

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.     

YES2 optimal trajectories in presence of eccentricity and aerodynamic drag

YES2 (launching 2007) aims to demonstrate a tether-assisted re-entry concept, whereby payload will be returned to Earth using momentum provided from a swinging tether. Deployment takes place in two phases: (1) deployment of 3.5 km of tether to the local vertical and hold, and (2) deployment to 30 km for a swinging cut. Optimal trajectories are determined for both phases after comparing the effect of different cost functions on the deployment dynamics. Closed-loop control is provided by linearizing the dynamics around the optimal trajectories and solving a receding horizon control problem for a set of linear feedback gains. The controllers are tested in a flexible tether model with large disturbances to the hardware model and environmental variables. Closed-loop simulations show that the system can be controlled quite well using only feedback of length and length rate.     

Design of a 40/50 GHz satellite ground station for fade mitigation experiments

Due to the increasing demand in satellite capacity, driven by applications such as high-definition television (HDTV), 3D-TV and interactive broadband services, higher frequency bands will have to be exploited. The capacity on Ku-band is already becoming scarce and Ka-band systems are more commonly used. It can be expected that 40 and 50 GHz (Q and V band) will have to be used in the future. At these frequencies the wave propagation effects have a significant impact on the performance. The traditional approach of implementing large fade margins in the system design is not suitable as it leads to expensive ground terminals. Fade mitigation by adaptive coding and modulation (ACM) is a cost-efficient method. To investigate the Q/V-band for future commercial exploitation, ESA's ALPHASAT satellite will provide experimental payloads for communications and wave propagation experiments. In Graz a Q/V-band ground station is currently under development. It will be equipped with a 3 m tracking antenna, a 50 W Klystron amplifier and a 290 K LNA. Fade mitigation experiments will be conducted, initially using DVB-S2 modems which allow to vary the modulation scheme, the Forward Error Correction code and the symbol rate under control of the ACM computer. In addition, uplink power control can be combined with the ACM methods. A specially developed signal analyser provides precise measurement of the signal/noise ratio. In addition, propagation data will be available from a beacon receiver, also developed by Joanneum Research. Important goals of the experiment are to investigate the reliability of links under realistic operating conditions using ACM and to develop efficient ACM and signal/noise ratio measurement algorithms which can be later implemented in optimised modems for Q/V-band.The paper describes the ground station design and addresses the planned fade mitigation experiments.     

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.     

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.     

Modelling of the optical observations of a geosynchronous fragmentation event

A study of the evolution and optical detectability of a fragmentation debris cloud in geosynchronous orbit has been carried out. The 1998 NASA breakup model has been used to generate orbit data for 95 fragments larger than 10 cm size from a 1000 kg satellite. The orbital evolution of these fragments is studied using a precision numerical propagator, employing a high-fidelity force model. Although the fragments rapidly disperse throughout the geostationary arc, they remain localised in right ascension of ascending node and inclination, and are driven along a narrow inertial corridor by luni-solar perturbations. The ESA PROOF software is used to study the detectability of the fragments using a 1- and 0.5-m telescope design. The 1-m telescope can detect 82% of the fragments (down to 13 cm in size) whilst the 0.5-m telescope can detect 39% of the fragments (down to 30 cm size). Due to the large along-track spread of the fragments, a time limit of 1-month post-breakup can be established for a space surveillance system to catalogue the breakup fragments. After this time the angular separation is such that the fragments disperse into the background population, and are no longer distinguishable as originating from a common breakup event.     

Parabolic maneuvers of the Swiss Air Force fighter jet F-5E as a research platform for cell culture experiments in microgravity

Long-term sensitivity of human cells to reduced gravity has been supposed since the first Apollo missions and was demonstrated during several space missions in the past. However, little information is available on primary and rapid gravi-responsive elements in mammalian cells. In search of rapid-responsive molecular alterations in mammalian cells, short-term microgravity provided by parabolic flight maneuvers is an ideal way to elucidate such initial and primary effects. Modern biomedical research at the cellular and molecular level requires frequent repetition of experiments that are usually performed in sequences of experiments and analyses. Therefore, a research platform on Earth providing frequent, easy and repeated access to real microgravity for cell culture experiments is strongly desired. For this reason, we developed a research platform onboard the military fighter jet aircraft Northrop F-5E “Tiger II”. The experimental system consists of a programmable and automatically operated system composed of six individual experiment modules, placed in the front compartment, which work completely independent of the aircraft systems. Signal transduction pathways in cultured human cells can be investigated after the addition of an activator solution at the onset of microgravity and a fixative or lysis buffer after termination of microgravity. Before the beginning of a regular military training flight, a parabolic maneuver was executed. After a 1 g control phase, the parabolic maneuver starts at 13,000 ft and at Mach 0.99 airspeed, where a 22 s climb with an acceleration of 2.5g is initiated, following a free-fall ballistic Keplerian trajectory lasting 45 s with an apogee of 27,000 ft at Mach 0.4 airspeed. Temperature, pressure and acceleration are monitored constantly during the entire flight. Cells and activator solutions are kept at 37 °C during the entire experiment until the fixative has been added. The parabolic flight profile provides up to 45 s of microgravity at a quality of 0.05g in all axes. Access time is 30 min before take-off; retrieval time is 30 min after landing. We conclude that using military fighter jets for microgravity research is a valuable tool for frequent and repeated cell culture experiments and therefore for state-of-the art method of biomedical research.     

Antenna for precise orbit determination

The ESA SWARM mission will consist of three satellites that will measure the Earth magnetic field. The system calls for metre accuracy knowledge of the measurement locations. To achieve this a GPS receiver is used. At least four GPS signals are tracked to determine the code and carrier ranges, from which the position can be derived. The accuracy improves when using more GPS satellites and by averaging over many measurements. The latter is achieved in ground processing with a model-based orbit prediction, resulting in cm accuracy. The main error contributions in the processing are often measurement errors due to satellite multi-path effects. The multipath effects are characterized by measuring the antenna on a 1.5 m mock-up, representing the 9 m long satellite. In order to verify that the mock-up is representative, extensive electromagnetic simulations were made. The simulations included the antenna and the complete satellite and were then reduced to the antenna and a section of the satellite. The actual design of the antenna was performed with several levels of software. First, a fast bodies-of-revolution simulation found a geometry with the right coverage. Then, a finite element method simulation allowed us to match the antenna at two frequencies simultaneously.     

Electromagnetic shielding of thermal protection system for hypersonic vehicles

The numerical simulation and the measurement of electromagnetic shielding at microwave frequencies of thermal protection system for hypersonic vehicles is presented using nested reverberation chamber. An example of a possible thermal protection system for a re-entry vehicle is presented. This system based on carbon material is electromagnetically characterized. The characterization takes into account not only the materials but also the final assembly configuration of the thermal protection system. The frequency range is 2–8 GHz. The results of measurements and simulations show that the microwave shielding effectiveness of carbon materials is above 60 dB for a single tile and that the tile inter-distance is able to downgrade the shielding effectiveness on the average to about 40 dB.     

Detection of volcanic gases and particles by satellite

In this paper the detection of components of volcanic eruption has been carried out investigating, in appropriate bands of the electromagnetic spectrum (6.25, 8.7, 10.8, 12 μm), the values of the brightness temperature. The analysis has been performed in the Thermal Infrared Region (TIR) studying both the absorption–emission and scattering phenomena related to the interactions between electromagnetic radiation and volcanic emissions. The results have been achieved by means of a combined use of numerical simulations, devoted to examining the behaviour of the atmosphere gases and volcanic components, and remotely sensed satellite images. The proposed methodologies allow an estimate of the amount of gaseous and solid components, of the size of the emitted particles, of the height of the volcanic plume and of the distance of the volcanic components from the crater. The processed images come from the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) sensor on board the geo-stationary satellite Meteosat Second Generation (MSG) and take into consideration an eruption of the Etna volcano as a study case (1st of April 2012, 04:30 and 05:30 UTC). The procedures are general and may therefore be extended to any other similar case.     

Super-capacitor energy storage for micro-satellites: Feasibility and potential mission applications

Small satellites, weighting between 100 and 200 kg, have witnessed increasing use for a variety of space applications including remote sensing constellations and technology demonstrations. The energy storage/stored power demands of most spacecraft, including small satellites, are currently accommodated by rechargeable batteries—typically nickel–cadmium cells (specific energy of 50 Wh kg⁻¹), or more recently lithium-ion cells (150 Wh kg⁻¹). High energy density is a primary concern for spacecraft energy storage design, and these batteries have been sufficient for most applications. However, constraints on the allowable on-board battery size have limited peak power performance such that the maximum power supply capability of small satellites currently ranges between only 70 and 200 W. This relatively low maximum power limits the capabilities of small satellites in terms of payload design and selection. In order to enhance these satellites' power performance, the research reported in this paper focused on the implementation of super-capacitors as practical rechargeable energy storage medium, and as an alternative to chemical batteries. Compared to batteries, some super-capacitors are able to supply high power at high energy-efficiency, but unfortunately they still have a very low energy density (5–30 Wh kg⁻¹). However, the provision of this high power capability would considerably widen the range of small satellite applications.     

A global geographical survey of received signal strength in the VHF band

Sumbandila (SO-67) is an 81 kg LEO satellite launched on 17 September 2009. Its primary payload is a multi-spectral imager with a ground sampling distance of 6.25 m at an orbit altitude of 500 km. Its two command transceivers operate in the commercial and amateur radio VHF and UHF bands and one of them provides a VHF to UHF repeater service to radio amateurs. This paper presents initial results of a global three week monitoring period of two VHF frequency ranges. The data is obtained by executing on-board flight control procedures to select the frequencies to measure. The existing on-board telemetry gathering system is employed to record the data, most notably the received signal strength for the selected frequency. The data is downloaded using an adaptation of the imagery data download path. We determine regions of high signal levels by distributing individual measurements over cells in the satellite footprint before averaging over the cells. The data is then plotted on a geographical signal strength heatmap. We compare our results with that of a similar study of the late 1990s and point out changes since then. The data provides useful information for selecting future ground station locations for minimum interference. It further gives an indication of frequencies to use for command and telemetry communication at existing ground stations. We propose that including a receiver capable of measuring frequency interference across a desired frequency range is very useful to future missions for selecting communication frequencies from this range for ground station locations.     

MOA2—an R&D paradigm buster enabling space propulsion by commercial applications

More than 60 years after the late Nobel laureate Hannes Alfvén had published a letter stating that oscillating magnetic fields can accelerate ionised matter via magneto–hydrodynamic interactions in a wave like fashion, the technical implementation of Alfvén waves for propulsive purposes has been proposed, patented and examined for the first time by a group of inventors.Consequently improved since then, the name of the latest concept, relying on magneto-acoustic waves to accelerate electric conductive matter, is MOA²—Magnetic field Oscillating Amplified Accelerator. Based on computer simulations, which were undertaken to get a first estimate on the performance of the system, MOA² is a corrosion free and highly flexible propulsion system, whose performance parameters might easily be adapted in operation, by changing the mass flow and/or the power level. As such the system is capable of delivering a maximum specific impulse of 13116 s (12.87 mN) at a power level of 11.16 kW, using Xe as propellant, but can also be attuned to provide a thrust of 236.5 mN (2411 s) at 6.15 kW of power. First tests—that are further described in this paper—have been conducted successfully with a 400 W prototype system at an ambient pressure of 0.20 Pa, delivered 9.24 mN of thrust at 1472 s I_SP, thereby underlining the feasibility of the concept.Based on these results, space propulsion is expected to be a prime application for MOA²—a claim that is supported by numerous applications such as Solar and/or Nuclear Electric Propulsion or even as an ‘afterburner system’ for Nuclear Thermal Propulsion. However, MOA² has so far seen most of its R&D impetus from terrestrial applications, like coating, semiconductor implantation and manufacturing as well as steel cutting. Based on this observation, MOA² resembles an R&D paradigm buster, as it is the first space propulsion system, whose R&D is driven primarily by its terrestrial applications. Different terrestrial applications exist, but the most successful scenarios so far revolve around MOA²'s unique features with respect to high throughput/low target temperature coatings on sensitive materials. In combination with its intrinsic high flexibility, MOA² is highly suited for a common space-terrestrial application research and utilisation strategy.This paper presents the recent developments of the MOA² R&D activities at Q₂ Technologie(s), the company in Vienna, Austria, which has been set up to further develop and test the magneto-acoustic wave technology and its applications.     

Field emission performance of multiwalled carbon nanotubes for a low-power spacecraft neutraliser

Field electron emission from aligned multiwalled carbon nanotubes has been assessed to determine if the performance, defined by power consumption, lifetime and emission current, is suitable for use in spacecraft charge neutralisation for field emission electric propulsion (FEEP). Carbon nanotubes grown by chemical vapour deposition (CVD) were mounted on a dual in line chip with a macroscopic (nickel mesh) extractor electrode mounted ～1 mm above the tubes. The nanotubes’ field emission characteristics (emission currents, electron losses and operating voltage) were measured at ～10^?4 Pa. An endurance test of one sample, running at a software-controlled constant emission current lasted >1400 h, approaching the longest known FEEP thruster lifetime. The emission corresponds to a current density of ～10 mA/cm² at a voltage of 150 V. These results, implementing mature extractor-electrode geometry, indicate that carbon nanotubes have considerable potential for development as robust, low-power, long-lived electron emitters for use in space.