Nach oben pdf Satellite Laser Ranging with a fibre-based transmitter

Satellite Laser Ranging with a fibre-based transmitter

Satellite Laser Ranging with a fibre-based transmitter

The experimental satellite laser ranging station in Stuttgart has commenced operations in January 2016. Its modular, flexible and cost-efficient design uses only readily available components and is therefore well suited for an upgrade of existing astronomical observatories to SLR stations. One of its key features is the laser light transmission via an optical fibre, thus avoiding the need for a coudé path mount. Currently, the transmitter achieves an output pulse energy of about 25 µJ at 5 kHz (125 mW) and is operated at the fundamental Nd:YAG wavelength of 1064 nm. The complete system, including IT hardware and observer workplaces, is fitted into a 12 feet dome. With the current configuration, many cooperative targets in LEO and beyond (up to LAGEOS) have successfully been observed, with usual return rates of several hundred counts per second. Since the tracking relies on visual guiding, no accurate CPF predictions are needed, and out-of- service SLR targets like GEOS 3 can be observed as well using public TLE data.
Mehr anzeigen

6 Mehr lesen

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.
Mehr anzeigen

9 Mehr lesen

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.
Mehr anzeigen

85 Mehr lesen

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.
Mehr anzeigen

12 Mehr lesen

Satellite Laser Ranging at 100 kHz Pulse Repetition Rate

Satellite Laser Ranging at 100 kHz Pulse Repetition Rate

With these possible improvements in mind, it seems likely that SLR at very high repetition rates will become more common in the future. For existing systems, it may offer a gain in sensitivity, precision and / or data yield at a mod- erate cost. Since it enables the use of laser at wavelengths above 1.4 µm, it may also help in the development of in- herently eye-safe systems. For new systems, it may super- sede the traditional coud´e path approach, as on-mount laser systems or fibre-coupling become competitive alternatives. The German Aerospace Center will further pursue this ap- proach not only with the Uhlandsh¨ohe SLR system, but also with the new miniSLR system currently under construction, which will incorporate a whole SLR system with a multi- kHz on-mount laser into a small box [13].
Mehr anzeigen

8 Mehr lesen

Design and qualification of a recessed satellite cornercube
retroreflector for ground‑based attitude verification via satellite laser ranging

Design and qualification of a recessed satellite cornercube retroreflector for ground‑based attitude verification via satellite laser ranging

A major technological challenge in optical downlink laser communication is that the data transmitting laser on the satellite needs to be pointing accurately towards the receiving ground station during data transfer. The required precision depends on the divergence of the laser used for data transfer, which is in the order of 200 µrad with OSIRISv2. To achieve this goal, OSIRIS4CubeSat will use a combination of body pointing and active beam steering. The goal of the satellites attitude control system (ACS) is to provide a pointing accuracy of ± 1°, and the remaining accuracy comes from a fine pointing unit integrated
Mehr anzeigen

13 Mehr lesen

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.
Mehr anzeigen

5 Mehr lesen

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

The main object tracking is carried out by the receiver telescope, still a transmitter point- ing model is implemented to encounter systematic misalignment between the transmitter’s and receiver’s optical axes. Also a closed loop tracking is possible: The off-set in the im- age of the object under observation to the target point (near image center) is used to correct the pre-programmed trajectory. Moving the object to the beam’s target point increases the return rate of photons while ranging. Additionally, the beam’s direction should be controllable to iteratively adapt the beam’s target and maximize the return rate. Therefore, a beam steering possibility is built in. This is realized whilst mounting the last mirror into a kinematic mirror mount (Thorlabs - item no. KM200). The tip/tilt screws can be controlled with motorized DC Actuators (Thorlabs - item no. Z806), which are driven by a matching controller connected to a computer. The actuators have a travel length of 6 mm and a minimum achievable incremental movement of 0.05 µm. Thus, the beam can be deflected by about 54 mrad with an accuracy of 0.4 µm. A subsequent shortpass dichroic mirror (Thorlabs - item no. DMSP805L) with a cut-off wavelength of 805 nm reflects the IR-beam into the transmitter telescope.
Mehr anzeigen

56 Mehr lesen

GNSS Satellite Orbit Validation Using Satellite Laser Ranging

GNSS Satellite Orbit Validation Using Satellite Laser Ranging

BeiDou) to 20 cm (QZSS). Mean biases between GNSS-based orbits and SLR observations are presently at the 5 cm level for the aforementioned constellations. SLR tracking will also help to assess future improvements from refined orbit dynamics models, improved GNSS tracking coverage and refined processing concepts (e.g., ambiguity resolution). The diversity of orbits and spacecraft models within even a single GNSS constellation suggests a need for comprehensive SLR tracking of “all” new GNSS satellites until the GNSS only orbit determination accuracy is compatible with that of existing systems (GPS, GLONASS). Beyond this immediate goal, SLR tracking of GNSS satellites will contribute to the harmonization of GNSS- and SLR-based reference frames. The development of a consolidated SLR tracking concept for GNSS satellites within the ILRS is therefore encouraged. As part of this, special consideration should be given to the potential increase in overall tracking capacity provided by high-rate kHz laser systems. Build-up of such systems appears of particular interest for the Asia-Pacifica region, which hosts a large number of geosynchronous GNSS satellites that are less well covered than the more common GNSS satellites in medium Earth orbit.
Mehr anzeigen

6 Mehr lesen

Optical Performance Measurements of the BELA EQM and FM Transmitter Laser during AIV

Optical Performance Measurements of the BELA EQM and FM Transmitter Laser during AIV

The Transmitter – consisting of Laser Head Box (LHB), Laser Electronics Unit (LEU), Electronics Unit (ELU) and Analog Electronics Unit dummy (AEU dummy) are integrated in very clean (plasma cleaned) TV chamber (see Fig. 1). All the integration tasks were performed under a laminar flow box in ISO 5 environment. In the EQM campaign the laser was firing directly horizontally through a window onto an optical bench where the beam was analyzed before, during and after thermal cycling. Due to problems in accurately measuring the beam pointing and beam pointing stability in this configuration, the FM LHB was mounted in a vertical position with a 90° tilt mirror for its measurement campaign.
Mehr anzeigen

4 Mehr lesen

A low-power SiGe impulse radio-UWB transmitter with biphase modulation function

A low-power SiGe impulse radio-UWB transmitter with biphase modulation function

Abstract—In this paper a compact and low-power IR-UWB transmitter is presented. The transmitter is based on a cross- coupled LC oscillator core which is transiently turned on and off by current spikes generated on-chip. A simple phase control circuit enables biphase modulation by controlling the start-up phase condition of the oscillator. The UWB transmitter has a low power consumption of 9.8 mW when biphase modulated with a 200 MHz impulse sequence. The transmitter IC occupies an area of 0.3 mm 2 including bond pads.
Mehr anzeigen

4 Mehr lesen

Satellite-based DNI nowcasting based on a sectoral atmospheric motion approach

Satellite-based DNI nowcasting based on a sectoral atmospheric motion approach

The nowcast is being initialized with the situation being cloudy or not cloudy at the pixel at the start of the nowcast. Clouds but also cloud gaps moving towards the power plant are tracked within each sector over 3 time slots of satellite imagery. The surroundings of the location of interest are separated into 32 sectors with equal angular distribution. All pixels having a polar angle within the sector are mapped onto the bisecting line inside the sector at the distance to the central pixel. By that a vector of cloud mask values (cloudy, cloud free) is created along each sector’s bisecting line and for both cloud masks separately. The more detailed structure inside the two- dimensional sector is reduced to a one-dimensional vector by this approach. This reduces the spatial resolution, but also increases the clearness of the signal itself. Within this one-dimensional vector again a low pass filter is performed. Cloud gaps up to 7 km along each bisecting line are set to cloudy as well to avoid small gaps being created by the mapping to the bisecting line process. Clouds being only 1 pixel long are set to cloud free on the other hand.
Mehr anzeigen

8 Mehr lesen

Laser-Based High-Voltage Metrology with ppm Accuracy

Laser-Based High-Voltage Metrology with ppm Accuracy

For performing pump-and-probe experiments in the Λ-system of Ca + two different laser systems are re- quired. The optical pumping will take place at 393 nm whereas an 854-nm laser has to be employed for probing. As discussed before, the pump laser can be fixed to one frequency whereas the probe laser has to be flexible due to the different Doppler shifts caused by the different post-acceleration voltages. For optical pumping a diode laser system has been chosen. An external cavity diode laser (ECDL) with a laser diode producing light between 780 nm and 787 nm was already available [82] and hence, the D2-transition was selected for optical pumping (see section 2.3.1). As shown in Fig. 3.12, the laser frequency can be measured and controlled with a WSU10 wavelength meter (wavemeter) from High Finesse. To generate laser light at 393 nm, the light from the ECDL is amplified and frequency doubled. The amplification is realized with a tapered amplifier TEC-400-780-2500 from Sacher Lasertechnik and the second harmonic generation (SHG) with a commercial Wavetrain system from Spectra Physics based on a lithium triborate (LBO) crystal. The SHG light possesses several side fringes due to the production process in the crystal [83], which would increase the laser-induced stray light. Therefore, the 393 nm light is sent through a mode cleaner (spatial filter) consisting of two lenses and a pinhole to guarantee a perfect Gaussian beam before guiding it directly to the beamline. For background-free laser spectro- scopic experiments, an acousto-optical modulator (AOM) is placed before the beamline which offers the opportunity to block the laser for optical pumping while the ions are probed with the infrared laser [84]. But due to the already low background rates, its benefits are compensated by the longer measurement cycle and hence, the measurements described in chapter 4 were done with continuous beams. The total setup is explained in more detail in [85].
Mehr anzeigen

124 Mehr lesen

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

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

Two routing policies are considered within this paper. The first one (policy 1) tries to minimize the number of handovers at the up and downlink (UDL) segment by selecting the satellite which shows the maximum remaining visibility time[2], whereas the second policy (policy 2) tries to minimize the number of hops at the ISL segment, which can lead to many handovers at the UDL segment. Both policies cannot be applied at the same time. In order to find a suitable sequence of satellites for establishing a path between the ingress point (the origin) and the egress point of the satellite system (the destination), the Dijkstra algorithm [3], [4] is used for computing the shortest path i.e. minimizing the delay.
Mehr anzeigen

5 Mehr lesen

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?

25 Mehr lesen

Multipath Assisted Positioning with Transmitter Visibility Information

Multipath Assisted Positioning with Transmitter Visibility Information

The evaluations above consider an approximation of a converged prior map of transmitter visibilities that is known to the user to illustrate solely the effect of mapping of and using information about transmitter visibilities. In reality, such a prior map is created by users with crowdsourcing. A first user maps the transmitter states and visibilities with Channel-SLAM and hands the map over to a second user. Since the second user does in general not know their starting location, they need to estimate the translation and rotation relating the coordinate system of the two users in order to be able to use the map. This estimation is referred to as map matching and can be done based on transmitter states estimated by the second user and in the map as in [43]. Once a map match has been found, the second user exploits the information in the map, updates it with own observations. The map is then handed over to a third user, who again performs map matching, uses and updates the information in the map, hands the map on to the fourth user, and so on. We refer to this approach as collaborative Channel-SLAM.
Mehr anzeigen

14 Mehr lesen

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.
Mehr anzeigen

8 Mehr lesen

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).
Mehr anzeigen

13 Mehr lesen

SOFOS - A new Satellite-based Operational Fog Observation Scheme

SOFOS - A new Satellite-based Operational Fog Observation Scheme

Table 5.4 summarises the results of the statistical appraisal of very low cloud detection based on the validation methodology discussed above. Figures for both the single-pixel and the 3 by 3 pixel environment approaches are pre- sented. The accuracy figures reveal that 56% of situations were correctly classified in the former and 71% in the latter case. At 15% the differences be- tween both approaches are therefore evident. Both methods however agree in that by tendency the number of very low cloud situations is underestimated by the algorithm, as shown by bias scores of 0.67 and 0.77. Underestimation is marked for the single-pixel approach (BS = 0.67) and smaller, but still considerable for 3 by 3 pixels. The combination of hit rate and false alarm rate corroborates this conclusion: 52 and 68% of situations are properly de- tected. Therefore, even in the 3 by 3 pixel comparison, slightly more than 30% of very low cloud situations go undetected. At the same time, the num- ber of very low cloud situations falsely reported by the algorithm for this approach is low at 12% (22% for single-pixel comparison). The probability of false detection is somewhat higher than the FAR, resulting in Hanssen– Kuipers figures of 0.17 and 0.46 for the approaches respectively. At 0.62 the threat score computed for the 3 by 3 pixel case shows that overall, good skill can be expected from the algorithm. In this as in all other figures a marked difference between the single-pixel and more suitable 3 by 3 pixel approaches is evident.
Mehr anzeigen

165 Mehr lesen

Process observation in fiber laser-based selective laser melting

Process observation in fiber laser-based selective laser melting

c Federal Institute of Santa Catharina Catarina, Av Mauro Ramos, 950 Centro, CEP: 88020-300, Florianópolis, Santa Catarina, Brazil Abstract. The process observation in selective laser melting (SLM) focuses on observing the interaction point where the powder is processed. To provide process relevant information, signals have to be acquired that are resolved in both time and space. Especially in high-power SLM, where more than 1 kW of laser power is used, processing speeds of several meters per second are required for a high-quality processing results. Therefore, an implementation of a suitable process observation system has to acquire a large amount of spatially resolved data at low sampling speeds or it has to restrict the acquisition to a predefined area at a high sampling speed. In any case, it is vitally important to synchronously record the laser beam position and the acquired signal. This is a prerequisite that allows the recorded data become information. Today, most SLM systems employ f -theta lenses to focus the processing laser beam onto the powder bed. This report describes the drawbacks that result for process observation and suggests a variable retro-focus system which solves these issues. The beam quality of fiber lasers delivers the processing laser beam to the powder bed at relevant focus diameters, which is a key prerequisite for this solution to be viable. The optical train we present here couples the processing laser beam and the process observation coaxially, ensuring consistent alignment of interaction zone and observed area. With respect to signal processing, we have developed a solution that synchronously acquires signals from a pyrometer and the position of the laser beam by sampling the data with a field programmable gate array. The relevance of the acquired signals has been validated by the scanning of a sample filament. Experiments with grooved samples show a correlation between different powder thicknesses and the acquired signals at relevant processing parameters. This basic work takes a first step toward self-optimization of the manufacturing process in SLM. It enables the addition of cognitive functions to the manufacturing system to the extent that the system could track its own process. The results are based on analyzing and redesigning the optical train, in combination with a real-time signal acquisition system which provides a solution to certain technological barriers. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or repro- duction of this work in whole or in part requires full attribution of the original publication, including its DOI. [DOI: 10.1117/1.OE.54.1.011008 ]
Mehr anzeigen

8 Mehr lesen

Show all 10000 documents...