Franz Koidl

Attitude and Spin Period of Space Debris Envisat Measured by Satellite Laser Ranging

The Environmental Satellite (Envisat) mission was finished on April 8, 2012, and since that time, the attitude of the satellite has undergone significant changes. During the International Laser Ranging Service campaign, the Satellite Laser Ranging (SLR) stations have performed the range measurements to the satellite that allowed determination of the attitude and the spin period of Envisat during seven months of 2013. The spin axis of the satellite is stable within the radial coordinate system (RCS; fixed with the orbit) and is pointing in the direction opposite to the normal vector of the orbital plane in such a way that the spin axis makes an angle of 61.86° with the nadir vector and 90.69° with the along-track vector. The offset between the symmetry axis of the retroreflector panel and the spin axis of the satellite is 2.52 m and causes the meter-scale oscillations of the range measurements between the ground SLR system and the satellite during a pass. Envisat rotates in the counterclockwise (CCW) direction, with an inertial period of 134.74 s (September 25, 2013), and the spin period increases by 36.7 ms/day.

The Impact of Solar Irradiance on AJISAI's Spin Period Measured by the Graz 2-kHz SLR System

The Graz kHz Satellite Laser Ranging (SLR) system is the first system operating with a 2-kHz-repetition-rate laser. Using Graz 2-kHz SLR data only, we applied a new analytical approach to determine the spin period of the passive satellite AJISAI. This method analyzes the range measurements to the single corner-cube-reflector panels of AJISAI, allowing accurate determination of an actual attitude of this satellite during day and night. Using Graz kHz SLR data of more than five years, we processed 877 passes of AJISAI (October 9, 2003-December 22, 2008) and calculated its spin period ( ~ 2 s) with an accuracy of 0.0042% (84 ¿s). This spin period (T) is increasing, following an exponential trend:T =1.9028 ·Exp (0.014859 . (Year - 2003.0)) s. This slow down is mainly caused by the gravitational and magnetic fields of the Earth. The high accuracy allows, for the first time, the detection of small perturbations of the spin period caused by nongravitational effects related to the solar energy flux to which the satellite is exposed.

Time-walk-compensated SPAD: multiple-photon versus single-photon operation

The SPAD has proven already its capability of timing single- photon events with picosecond accuracy; it does that also for multi-photon events, but introduces here a time walk effect: with received energies of 1000 photons and more, the measured epoch time is shifted 200 ps or more towards earlier times; although the specific SPAD type used shows the lowest time walk effect of all measured silicon avalanche diodes, this effect still might introduce range errors of up to 30 mm, when measuring distances to satellites. It has been shown that this time walk effect is connected with a very small change of the avalanche rise time; this effect has been successfully used to develop an electronic circuit which measures this rise time difference, and uses it to compensate automatically almost all of the time walk effect. Some prototypes have been built and tested successfully in the satellite laser ranging station Graz; improved versions of the circuit are operated or tested now successfully in other SLR stations. It has been shown that the time walk effect can be reduced to more or less zero, for a dynamical range from single photon up to more than 1000 photons. For best time walk compensation, the circuit is adjusted for a specific laser pulse length; it has been shown however, that this adjustment also gives good time walk compensation for other laser pulse lengths.

Compensation of SPAD time-walk effects

The single-photon avalanche diode (SPAD), operated above its breakdown voltage to detect single photons, is used at the Graz satellite laser ranging (SLR) station, where it has proven its potential capability of timing single-photon events with picosecond accuracy. It can also time multi-photon events, but introduces an intolerable time-walk effect. When the received energy is increased - to 1000 photons and more - the measured epoch time is shifted 200 ps or more towards earlier epochs, introducing range errors of up to 40 mm when measuring distances to satellites. We detected that this time-walk effect is correlated with a small change ( ps) of the avalanche rise time. This change is monitored with specially developed electronic circuits, and used to compensate the time-walk effect automatically. Some versions have been built and tested successfully at the Graz SLR station. Improved versions of the circuit are now being operated or tested successfully in other SLR stations (Germany, Japan, China and France). These new versions reduce the time-walk effect to, more or less, zero, for a dynamical range from a single photon up to more than 1000 photons.

SPAD active quenching circuit optimized for satellite laser ranging applications

We are presenting novel active quenching circuit for Single Photon Avalanche Diodes (SPADs). It was designed and optimized for satellite laser ranging applications, where the specific requirements are put on the gating performance. The goal of this work was to be able to detect the photons in short time after gate on with constant detection delay and sensitivity to minimize the measurement errors on one hand and background photon flux induced false count on the other hand. The detector sensitivity and especially the detection delay must be stabilized few nanoseconds after the gate activation. In the new circuit the SPAD can be pulse operated up to 5 volts above its breakdown voltage, the gate is opened by the incoming external pulse and is closed by the first photon detection. The new circuit was built and tested, the detector package for the field operation at the satellite laser ranging station was completed. The device performance: detection sensitivity, detection delay and timing resolution was measured and will be presented.

Extended dynamical range solid state photon counter

The paper describes the new achievements in an all solid state photon counting technique with picosecond resolution. The extended dynamical range has been achieved: the dependence of the detection delay on the detected signal strength - the time walk -has been compensated within several orders of optical signal strength. The principal application of the detector is the millimeter resolution satellite laser ranging. The detector is based on silicon avalanche photodiode pulse biased above its break voltage. The external gating and avalanche active quenching electronics is used. The time walk of the avalanche photodiode is of the order of hundreds of picoseconds in the dynamical range of single to one hundred photons input signal strengths. The additional electronics circuit has been developed to compensate for the time walk: the input optical signal strength influences the avalanche current build up time,the maximum build up time difference is 20 psec within the dynamical range 1:1000. This time difference is sensed, stretched by the factor of ten. The stretched time interval is applied, with the negative sign, as a correction to the detector propagation delay. The detector ultimate timing resolution, temporal stability, dynamical range and its dependence on the input laser pulse length have been investigated in detail. The fieldable version of the detector is been used for satellite laser ranging purposes. The timing resolution of the entire detector better than 20 picoseconds r.m.s., the maximum dynamical range > 1000:1 with the item walk bellow +/- psec have ben achieved, the results are presented. The additional applications in spectroscopy, biophysics, rangefinding and fiber optics may be considered.

Fast Response, Medium Resolution Digital Event Timer and Range Gate Generator for Satellite Laser Ranging

Fast Response, Medium Resolution Digital Event Timer and Range Gate Generator for Satellite Laser Ranging For our kHz Satellite Laser Ranging (SLR) system in Graz, we developed a fast response, medium resolution Event Timer to determine laser firing times; and a digital Range Gate Generator to activate the Single Photon Avalanche Detector (C-SPAD). The Event Timer has a resolution of about 500 ps, and determines the Event Times within 20 ns; the Range Gate Generator produces a range gate pulse with about 500 ps resolution, and with an accuracy of better than 1 ns. Both devices are fully digital, and are implemented within an FPGA circuit. These devices can be used in the present 2 kHz SLR system, as well as in future higher repetition rate SLR systems.

Graz kHz SLR LIDAR: first results

The Satellite Laser Ranging (SLR) Station Graz is measuring routinely distances to satellites with a 2 kHz laser, achieving an accuracy of 2-3 mm. Using this available equipment, we developed - and added as a byproduct - a kHz SLR LIDAR for the Graz station: Photons of each transmitted laser pulse are backscattered from clouds, atmospheric layers, aircraft vapor trails etc. An additional 10 cm diameter telescope - installed on our main telescope mount - and a Single- Photon Counting Module (SPCM) detect these photons. Using an ISA-Bus based FPGA card - developed in Graz for the kHz SLR operation - these detection times are stored with 100 ns resolution (15 m slots in distance). Event times of any number of laser shots can be accumulated in up to 4096 counters (according to > 60 km distance). The LIDAR distances are stored together with epoch time and telescope pointing information; any reflection point is therefore determined with 3D coordinates, with 15 m resolution in distance, and with the angular precision of the laser telescope pointing. First test results to clouds in full daylight conditions - accumulating up to several 100 laser shots per measurement - yielded high LIDAR data rates (> 100 points per second) and excellent detection of clouds (up to 10 km distance at the moment). Our ultimate goal is to operate the LIDAR automatically and in parallel with the standard SLR measurements, during day and night, collecting LIDAR data as a byproduct, and without any additional expenses.

Space debris science at the satellite laser ranging station Graz

Recent research activities related to space debris science at SLR station Graz are presented. Multi-static experiments were performed together with several European SLR stations. Stare and chase experiments demonstrate that space debris laser ranging is possible without a-priori knowledge of orbital information. Spin period and attitude measurements are performed via satellite laser ranging and light curves.

Observing variable stars and transiting exo-planets with single photon counting

Using our Satellite Laser Ranging (SLR) facility, our experience and our available equipment for Single Photon detection, we installed a Single Photon Counting Module (SPCM) to measure the photon flux of variable stars, and of stars with transiting exoplanets; these observations are intended as a complementary application to our standard SLR activities, to contribute observations to already known - and also to candidate - variable stars and stars with transiting exoplanets. While it is relatively easy to detect the large - in some cases up to 50% - variations of some stars, it is a challenge to detect the transiting exoplanets with this method; the decrease in photon flux here is only in the order of a few percent. In this paper, we present first results.