Work at Graz on satellite signatures

The size and shape of the satellites retroreflector arrays have a major impact on the distribution and scatter of the return signal; this can be seen clearly when reaching sub-cm ranging accuracies and when using Single-Photon Detectors with single- or multi-photon returns; for other receiver systems (using MCP's), it should be checked also. As a consequence, the necessary center-of-mass correction for some satellites will differ, depending on the receiver systems. While this effect is not yet visible on small satellites or small retro-reflector arrays (like STARLETTE, ERS1), it can be in the order of centimeters on AJISAI or ETALON

Sub-cm ranging and other improvements in Graz

A lot of tests and experiments have been made in Graz during the last 2 years to increase performance and accuracy; using the SPAD from the Prag group as receiver, we have reached now about 5 mm RMS from the calibration target, and about 8 mm RMS from ERS1 and STARLETTE. In addition, by routinely using the semitrain, the number of returns has been increased significantly for most satellites

Laser Ranging to Nano-SatellitesG. Kirchner (1), Ludwig Grunwaldt (2), Reinhard Neubert (2), Franz Koidl (1), Merlin Barschke (3), Zizung Yoon (3), Hauke Fiedler (4), Christine Hollenstein (5)

Several small satellites in the class of pico- and nano-satellites will be equipped with multiple small corner cubes: OPS-SAT (ESA), S-Net and TechnoSat (8 kg resp. 15 kg; Technical University Berlin), and CubETH (ETH Zuerich). The size of these satellites is in the range from 10x10x30 cm up to about 40x40x30 cm; the planned circular orbits are in the 450 – 620 km range. Commercially available 10 mm and 0.5” corner cubes will be used for SLR; a single corner cube of this size will be sufficient for SLR to the planned LEO orbits. Placing several of these corner cubes on each side of the satellites will not only allow for standard SLR and POD, but also for an independent attitude determination with < 1° accuracy, even after the end of the satellites lifetime, or in case of problems or satellite failure. For multiple satellites flying in close formation, it will be possible to distinguish the sequence of single satellites within the formation

Laser measurements to space debris from Graz SLR station

In order to test laser ranging possibilities to space debris objects, the Satellite Laser Ranging (SLR) Station Graz installed a frequency doubled Nd:YAG pulse laser with a 1 kHz repetition rate, a pulse width of 10 ns, and a pulse energy of 25 mJ at 532 nm (on loan from German Aerospace Center Stuttgart – DLR). We developed and built low-noise single-photon detection units to enable laser ranging to targets with inaccurate orbit predictions, and adapted our standard SLR software to include a few hundred space debris targets. With this configuration, we successfully tracked – within 13 early-evening sessions of each about 1.5 h – 85 passes of 43 different space debris targets, in distances between 600 km and up to more than 2500 km, with radar cross sections from >15 m² down to <0.3 m², and measured their distances with an average precision of about 0.7 m RMS

Daylight space debris laser ranging

Space debris laser ranging is a technique to measure distances to defunct satellites or rocket bodies in orbits around Earth which was only possible within a few hours around twilight. Here, the authors show the first space debris laser ranging results during daylight while correcting inaccurate predictions using a real-time target detection software

Ubc9 sumoylation controls SUMO chain formation and meiotic synapsis in Saccharomyces cerevisiae

Posttranslational modification with the small ubiquitin-related modifier SUMO depends on the sequential activities of E1, E2, and E3 enzymes. While regulation by E3 ligases and SUMO proteases is well understood, current knowledge of E2 regulation is very limited. Here, we describe modification of the budding yeast E2 enzyme Ubc9 by sumoylation (Ubc9*SUMO). Although less than 1% of Ubc9 is sumoylated at Lys153 at steady state, a sumoylation-deficient mutant showed significantly reduced meiotic SUMO conjugates and abrogates synaptonemal complex formation. Biochemical analysis revealed that Ubc9*SUMO is severely impaired in its classical activity but promoted SUMO chain assembly in the presence of Ubc9. Ubc9*SUMO cooperates with charged Ubc9 (Ubc9∼SUMO) by noncovalent backside SUMO binding and by positioning the donor SUMO for optimal transfer. Thus, sumoylation of Ubc9 converts an active enzyme into a cofactor and reveals a mechanism for E2 regulation that orchestrates catalytic (Ubc9∼SUMO) and noncatalytic (Ubc9*SUMO) functions of Ubc9