Nach oben pdf First successful satellite laser ranging with a fibre-based transmitter

First successful satellite laser ranging with a fibre-based transmitter

First successful satellite laser ranging with a fibre-based transmitter

Satellite Laser Ranging (SLR) is an established technology used for geodesy, fundamental science and precise orbit determination. This paper reports on the first successful SLR measurement from the German Aerospace Center research observatory in Stuttgart. While many SLR stations are in opera- tion, the experiment described here is unique in several ways: The modular system has been assembled completely from commercial off-the-shelf components, which increases flexibility and significantly re- duces hardware costs. To our knowledge it has been the first time that an SLR measurement has been conducted using an optical fibre rather than a coud´ e path to direct the light from the laser source onto the telescope. The transmitter operates at an output power of about 75 mW and a repetition rate of 3 kHz, and at a wavelength of 1064 nm. Due to its rather small diameter of only 80 µm, the receiver detector features a low noise rate of less than 2 kHz and can be operated without gating in many cases. With this set-up, clear return signals have been received from several orbital objects equipped with retroreflectors. In its current configuration, the system does not yet achieve the same performance as other SLR systems in terms of precision, maximum distance and the capability of daylight ranging; however, plans to overcome these limitations are outlined.
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Design and Construction of an Infrared Laser Transmitter for a Compact Satellite Laser Ranging System

Design and Construction of an Infrared Laser Transmitter for a Compact Satellite Laser Ranging System

In general, a Laser Ranging System includes three parts [11]. The first one is the laser transmitter, which includes a laser pulse source. Moreover, beam shaping optics, beam monitoring sensors and beam steering devices are reasonable to emit feasible laser pulses. After passing all optics as well as measuring and controlling units the beam is led out into space [11]. The photons hit the retroreflector on a satellite and are reflected back to the laser ranging ground station. An optical system collects the reflected photons and leads them onto a detector (e.g. a single-photon detector) in order to capture them. Hence, their ToF is recorded. This forms the second part of a SLR system [11]. The ratio of transmitted and received photons is called the return rate. The third part are controlling and measuring electronics, of which the event timer is the most important. It records the time stamps of emitted pulses and reflected signals and synchronizes with GPS to UTC time (important for satellites)[5]. A computer correlates the time stamps to each other, registers the ToF and calculates the distance to the satellite. The receiver and transmitter telescope are both installed onto an accurate and fast moving mount in order to track space objects. While tracking, a continuous ToF measurement, respectively distance measurement, is done [11] (Figure 2.1).
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Drift of the Earth’s Principal Axes of Inertia from GRACE and Satellite Laser Ranging Data

Drift of the Earth’s Principal Axes of Inertia from GRACE and Satellite Laser Ranging Data

solutions of its rotation, and thus has a broad effect on many fields, including astronomy, geodesy, and satellite-based positioning and navigation systems. That location is determined by the second-degree Stokes coefficients of the geopotential. Accurate solutions for those coefficients were limited to the stationary case for many years, but the situation improved with the accomplishment of Gravity Recovery and Climate Experiment (GRACE), and nowadays several solutions for the time-varying geopotential have been derived based on gravity and satellite laser ranging data, with time resolutions reaching one month or one week. Although those solutions are already accurate enough to compute the evolution of the Earth’s axes of inertia along more than a decade, such an analysis has never been performed. In this paper, we present the first analysis of this problem, taking advantage of previous analytical derivations to simplify the computations and the estimation of the uncertainty of solutions. The results are rather striking, since the axes of inertia do not move around some mean position fixed to a given terrestrial reference frame in this period, but drift away from their initial location in a slow but clear and not negligible manner.
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MiniSLR: A fully automated miniature satellite laser ranging ground station

MiniSLR: A fully automated miniature satellite laser ranging ground station

The German Aerospace Center is currently developing a new, small and inexpensive SLR system that may be very well suited for the further expansion of the laser ranging network. The whole system is housed in a 2m x 2m x 1.5m box, which is fully sealed and weather proof. It contains not only the mount with transmitter and receiver telescope and the laser, but also all data acquisition and experiment control systems. Our own control software, which is already used in two other SLR systems, will be used to operate the system completely autonomously. Using an infrared laser at low pulse energies avoids problems with aircraft safety. Compared to current SLR stations, which often occupy a whole observatory building and are operated by on-site staff, this miniSLR system will cut both installation and operating costs significantly. This contribution will present the set-up and first tests with the miniSLR system.
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The key role of Satellite Laser Ranging towards the integrated estimation of geometry, rotation and gravitational field of the Earth

The key role of Satellite Laser Ranging towards the integrated estimation of geometry, rotation and gravitational field of the Earth

Curretnly, the main limitation for the ERFs is the datum instability. To improve the geodetic datum and the integration of the geodetic space techniques for the ERFs, different approaches are currently under investigation at DGFI. The most important one studies the effect of different LT handling strategies on the ERFs. Until now, the introduction and weighting of available LT is purely based on experiments performed for MRF computations. In addition, the benefit of so-called space ties (difference vector between two satellite instruments) should be evaluated. Furthermore, the relative weighting of the techniques can be improved by applying VCE. First results were already published by Bloßfeld and Seitz M. (2012). Another possibility to improve the ERF datum is to incorporate the multi-satellite SLR solution to the inter-technique combination with GNSS and VLBI. The improvement of the geodetic datum in the SLR-only solution was clearly shown in this thesis. This strategy is also adapted by the ILRS which plans to include observations to LARES (in addition to LAGEOS 1/2 and Etalon 1/2 observations) in its standard TRF and EOP solution. The inclusion of LARES make it necessary to enlarge the parameter spectrum to the low degree Stokes coefficients in the standard solution since this satellite is very sensitive to the Earth’s gravitational field. The benefit of the inclusion of LARES for the TRF and EOP is demonstrated in P-V.
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Satellite Laser Ranging with a fibre-based transmitter

Satellite Laser Ranging with a fibre-based transmitter

To overcome these limitations, the dynamic research and development in the field of fibre optics offers various interesting approaches. In cooperation with Stuttgart University, our group is currently testing single-mode transmission through a multi-mode fibre with a core diameter of 50 to 70 µm, which would combine the advantage of a low beam divergence with a high damage threshold (Austerschulte, et al., 2012). Novel fibre concepts, such as hollow core fibres, promise even much higher damage thresholds and could be very well suited even for picosecond laser ranging with high energy lasers. In lab experiments, pulses with up to 30 mJ (30 ns) have been transmitted successfully through such a fibre (Dumitrache, et al., 2014). Assuming the damage threshold to depend on the square root of the pulse duration (Tien, et al., 1999), this corresponds to about 1 mJ at 30 ps.
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Mitigation of odometry drift with a single ranging link in GNSS-limited environments

Mitigation of odometry drift with a single ranging link in GNSS-limited environments

In our previous work [9], a graph-based fusion method was proposed instead of filtering to fuse stereo visual odometry and a single ranging link. The data fusion is formulated as a least-squares estimation minimizing the sum of the squared re-projection and ranging errors. The proposed algorithm is evaluated using the KITTI dataset [10] with synthetic ranging measurements, showing reduced relative pose error (RPE) and absolute trajectory error (ATE) [11], compared to stereo vision-only estimation. In [12], we propose a fusion of monocular visual odometry and ranging measurements also with a graph-based approach, presenting experimental results using a real dataset obtained in a featureless environment (grass field) with a camera and low cost UWB ranging modules. First, we recover the absolute scale in monocular visual odometry measurements, only using ranging links. Then, we reduce accumulating errors in the dead-reckoning process with a graph- based ranging fusion. However, with both [9] and [12], vehicle’s poses can only be estimated in a local navigation frame. Furthermore, only post-processing results are presented without an analysis on real-time performance.
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Simulation based draping of dry carbon fibre textiles with cooperating robots

Simulation based draping of dry carbon fibre textiles with cooperating robots

The usage of cooperating robots with reshapeable, stripe-shaped grippers for the draping of large cut pieces of dry carbon fibre textiles showed high potential both in the preliminary and in the field tests. In order to obtain the desired draping, simulation results were successfully used to determine grip- and droppoints and the gripper’s module orientations. The use of a hardware abstraction execution layer for handling the grip- and droppoints as well as the transfer was demonstrated. It proved reliable and far more flexible than conventional approaches, although speed optimization is definitely necessary. The material supply proved to be of crucial influence for the quality due to unwanted material deformation. Further tests concerning reliability, accuracy and speed are to be conducted in the future. Concerning the cell layout we believe that the collision control must be incorporated far earlier in the process allowing a optimized cell layout where all cut pieces can be processed at shortest time. This way constellations with unprocessable cut pieces can be avoided in advance and a flexible, automated production directly from the CAD data becomes possible.
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Repeat-Pass Spaceborne Transmitter-Stationary Receiver Bistatic Sar Interferometry - First Results

Repeat-Pass Spaceborne Transmitter-Stationary Receiver Bistatic Sar Interferometry - First Results

tion). The interferometric phase is extracted from the highest peak of the west side. Fig. 6 displays the estimated displace- ments of the Cathedral plaza building among with the temper- atures at the acquisition dates. The curves corresponding to the same orbit and different sub-swath are highly correlated, while two curves from different orbits are a bit less correlated, but the general trend is the same. This aspect can be linked to at least two facts: the acquisitions on one orbit are one day shifted from the acquisitions on the other, and the phase cen- ters may not be the same for the two orbits. All the curves show an average evolution in keeping with the temperature variation, which can be explained by the increase of the build- ing’s height due to dilatation. The mentioned correlations can be viewed as a qualitative validation of the proposed repeat- pass bistatic interferograms generation procedure.
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Multipath Assisted Positioning with Transmitter Visibility Information

Multipath Assisted Positioning with Transmitter Visibility Information

On its third propagation path drawn teal, the transmit signal is first scattered at the point scatterer and afterward reflected at the wall. In this case, the location of the resulting virtual transmitter vTx3 is obtained by applying the corresponding two cases iteratively. The location of Tx3 is thus the scatterer location mirrored at the reflecting wall. Accordingly, the time offset between the physical transmitter and vTx3 is again τ 0 . The third case can be generalized to any number of con- secutive reflections and/or scattering. If the transmit signal undergoes only specular reflections at plane surfaces, the cor- responding virtual transmitter is time synchronized with the physical transmitter, and accordingly τ 0 = 0 s. If the transmit signal is scattered at least one time, the traveled distance of the signal between the physical transmitter and the last involved scatterer is the additional propagation distance and determines the time offset.
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A delay model for satellite constellation networks with inter-satellite links

A delay model for satellite constellation networks with inter-satellite links

• In total each satellite has four ISLs plus one uplink (UL) and one downlink (DL). Moreover the traffic on the forward link has to be differentiated from the traffic on the return link, so each link is bi-directional. This means that in reality each satellite has 12 links (4× inter orbit, 4× intra orbit, 2× UL, 2× DL). Although satellites with this functionality appear to be rather complex we believe that optical inter-satellite communication will be the key because optical transmitters are very compact with low power consumption. With technology available today the onboard-switching should not be a major obstacle. For complexity reasons it is not possible to simulate three queues for each link because this would mean that each satellite has more than 30 different queues. Consequently the queues were setup just for the outgoing ISLs as Fig. 3 shows. Queues for the DL are not considered in the simulations done here; omitting the incoming queues does not have significant influence on the simulation results since these queues are no bottlenecks for the system. In order to further reduce the complexity of the problem, the model only considers the queue waiting time in the first satellite. This means that it is implicitly assumed that sufficient bandwidth in the ISL segment is available so no further queuing delay is introduced along the way.
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Epoxy-based fibre reinforced nanocomposites

Epoxy-based fibre reinforced nanocomposites

Gilbert et al. [81, 82] and Timmerman et al. [83] demonstrated that fracture toughness and mechanical properties are increased by incorporation of metal and inorganic particles. In these studies, they have been developing the concept of La PolynanoGrESS (Layered Polynanomeric Graphite Epoxy Scaled System) which utilizes the nanoparticle effect in an epoxy matrix and scales to a continuous carbon fibre reinforced composites systems. Typically, Timmerman et al. [83] modified the matrices of carbon fibre/epoxy composites with layered inorganic clays and a traditional filler to determine the effects of particle reinforcement, both micro and nano scale, on the response of these materials to cryogenic cycling. The mechanical properties of the laminates studied were not significantly altered through nanoclay modification of the matrix. The incorporation of nanoclay reinforcement in the proper concentration resulted in laminates with microcrack densities lower than those seen in the unmodified or macro-reinforced materials as a response to cryogenic cycling. Lower nanoclay concentrations resulted in a relatively insignificant reduction in microcracking and higher concentrations displayed a traditional filler effect. In a recent development, Brunner et al. [84] exploited Timmerman et al. [83] work on use epoxy with a relatively small amount of nano-size filler as matrix in fibre-reinforced laminates. They [84] focused on investigating whether a nano-modified epoxy matrix yields improved delamination resistance in a fibre- reinforced laminate compared to a laminate with neat epoxy as matrix material. To start with, neat and nano- modified epoxy specimens without fibre reinforcement were prepared for a comparison of the fracture toughness of the matrix material itself. Additional properties of the neat and nano-modified epoxy were also determined (partly taken from Timmerman et al. [83]) and compared. The study reported fracture toughness improvement up to about 50% and energy release rates increased by about 20% were observed for addition of 10 wt.% of organosilicate clay.
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Optical fibre Fizeau-based OCT

Optical fibre Fizeau-based OCT

Both the reference and the backscattered light pass through the coupler to the receiving interferometer (in figure 2 a Michelson receiving interferometer is shown, but this could be a Fa[r]

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Laser-Based High-Voltage Metrology with ppm Accuracy

Laser-Based High-Voltage Metrology with ppm Accuracy

The loss of efficiency due to the delayed detection can be observed in Fig. 5.4a where Doppler tuning is performed in the drift tube and a continuous, anticollinear laser beam is used. The optical detection region was fixed at a potential far off the resonance condition whereof only the first chamber is used since the signal in the second chamber has diminished. Since the measurement parameters like ion current and laser power are similar to the conditions where the interactions took place in the optical detection region, the signal-to-background ratio of 0.35:1 can be used to estimate the relative amount of detected fluorescence photons. Assuming the background rate to be constant, the fluorescence rate dropped to 14 %. Even with this loss, the procedure has the advantage that the background from the scattered light can be completely suppressed by turning off the laser during the detection time which leads to a considerably increased signal-to-noise ratio. This has been realized for the collinear case, which has a dramatically higher background rate, with an acousto-optical modulator placed behind the quadrupling stage as described in section 3.7.2. A resonance spectrum taken under these conditions in pulsed mode and in a similar measurement time (8 min) is shown in Fig. 5.4b. The beam-on/beam-off periods were chosen as 10 µ s/2 µ s leading to another efficiency loss of a factor six. Hence, the total intensity in the collinear pulsed mode is much smaller but since there is very little background, a similar signal-to-noise ratio of 25:1 compared to the anticollinear continuous mode (22:1) is achieved. It turned out after the measurement that most of the remaining background light is caused by the vacuum gauges. In both cases the spectra have a width of Γ FWHM ≈ 500 MHz and their resonance frequency can be determined with an uncertainty of approximately 4 MHz. The Voigt profile fitted to the experimental data is dominated by the Gaussian contribution which confirms the expectations on the ion production process regarding the initial velocity distribution.
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Support of satellite-based data analyses with CCM simulations

Support of satellite-based data analyses with CCM simulations

Discussions of difference between Boulder measure- ments and satellite observa- tions are still going on since the beginning of the early 2000s (e.g. Randel et al., 2004; Hegglin et al., 2014). Is the Boulder location (FPH measurements) representa- tive for the detection of global water vapor trends in the LS?

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A Model-Based Approach for Performing Successful Multi-Driver Scenarios

A Model-Based Approach for Performing Successful Multi-Driver Scenarios

Whenever the predicted values of one vehicle does not comply with the equally calculated value of the vehicle supposed to be met, the scenario has to react dynamically. In general, such dynamic reactions must be performed carefully and indirectly, in line with the restrictions of the study. Possible dynamic reactions may include: (1) changed timing of traffic lights, (2) changed behaviour of surrounding vehicles, like e.g. slower leading vehicles or congestions, (3) planned detours, (4) changes in the environment like changed speed limits, weather conditions or even enlarged or shortened tracks (in case the virtual track is changeable online), (5) visual effects like showing a slightly modified speed in the speedometer or even changing the frustum values. As the dynamic reactions can be seen as scenario alternatives, they can be included in the scenario graph as nodes with probabilities. The alternatives may also be combined, e.g. by using a detour, a slow lead vehicle and changing traffic light durations. During scenario preparation, the possible
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Multi-sensor satellite-based monitoring of a volcanic eruption

Multi-sensor satellite-based monitoring of a volcanic eruption

The emission of volcanic gases due to the 2014/15 Holuhraun fissure eruption was investi- gated by SO 2 measurements derived from GOME-2 data. The data was kindly provided by Dr. Pascal Hedelt (Remote Sensing Technology Institute (IMF), DLR). The processing of GOME-2 SO 2 data is based on the differential optical absorption spectroscopy (DOAS) meth- od in the UV wavelength range around 320 nm [5-8]. GOME-2 is an ultraviolet spectrometer (290-790 nm) aboard the polar-orbiting satellites MetOp-A (launched in 2006) and MetOp-B (launched in 2012) which takes global measurements of atmospheric composition on a daily basis. GOME-2 provides nadir-view scans with a ground pixel resolution of 40 x 40 km 2 (MetOp-A) and 80 x 40 km 2 (MetOp-B).
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SOFOS - A new Satellite-based Operational Fog Observation Scheme

SOFOS - A new Satellite-based Operational Fog Observation Scheme

The 1030 UTC scene of 8 November 2005 is a good example of such a sit- uation. The central and south-eastern parts of the scene are dominated by a high pressure area centred on the Czech Republic and reaching to Spain and the Ukraine. Radiation fog patches are present in several countries, includ- ing France, Germany, Italy and Switzerland. In the north-western part of the scene, a low pressure area with its centre to the north of the British isles influences the weather. A cold-frontal cloud band stretches from northern Portugal to Finland. Figure 5.4 shows the satellite classification (simpli- fied into high and low cloud classes) in conjunction with the corresponding METAR observations (classified into low cloud, high cloud and clear cases). The thermal infrared image shown in figure 5.5 very clearly underlines the presence of the cold (high) cloud band. A great number of ’low cloud’ METAR observations are located within this band. Assuming that these measurements are correct, high-reaching clouds are obviously the source of this disagreement between satellite and reference data assessment.
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First steps to establish an Small Satellite Program in Peru

First steps to establish an Small Satellite Program in Peru

In the past years a couple of developing countries did manage to establish and develop satellite programs in their countries. Now it is the time for Perú in South America to do the same. The aim of this paper is to inform the space community on the first steps and achievements carried out by some Peruvian institutions and individuals to strengthen a national aerospace program. This challenging endeavor will be carried out with the involvement and in collaboration of governmental, research and educational institutions in Peru. Since the last two years, the efforts of Peruvian engineers and academic personal have been centralized in the development of a small satellite program with emphasis on the definition of mission planning and mission design. The Peruvian government is currently working on the further maturity of the aerospace area in the country. The National Commission of Investigation and Aerospace Development of Peru (CONIDA) and the Department of Defense support such challenging and promising plan. A first step has been achieved establishing the National Center for Satellite Imagery Operations (CNOIS). The objective and purposes of CNOIS is to promote the technological and scientific development of the country in the remote sensing area. This paper will describe the, until now, achieved tasks by Peruvian individuals and institutions by setting up a plan for accomplishing this challenging endeavour. The progress of our society and country can be ensured developing the space technology. It will give us the great opportunity to perform space science and space exploration.
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Signal analysis of the first GPS III satellite

Signal analysis of the first GPS III satellite

assessment based on high-gain antenna measurements and receiver tracking data is provided in the subsequent 68.. section.[r]

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