Georg Kirchner

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.

Ajisai Spin Parameter Determination Using Graz Kilohertz Satellite Laser Ranging Data

The spin rate and direction of the spherical satellite Ajisai and its slowdown between October 2003 and June 2005 were determined using Graz full-rate kilohertz satellite laser ranging (SLR) data. The high density of the kilohertz data results in a precise scanning of the satellites retroreflector panel orientation during the spin motion. Applying spectral analysis methods, the resulting frequencies allow the identification of the arrangement of the involved laser retroreflector panels at any instant in time during the pass. Using this method, the spin rate with a high accuracy (a root mean square of 4.03times10-4 Hz) and the slowdown of the spin rate during the investigated period with a magnitude of 0.0077497 Hz/year were calculated. These results were obtained in near real time from automatically performed analysis procedures during routine SLR tracking, i.e., day and night observations without any additional hardware

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.

Gravity Probe-B: New Methods to Determine Spin Parameters From kHz SLR Data

Using kilohertz data of the satellite laser ranging (SLR) station Graz only, the spin parameters of the Gravity Probe-B (GP-B) satellite are derived; these include the spin period over the course of the 1.5-year mission period, as well as spin direction and spin axis orientation. The results are compared to the actual data sets-as determined by the GP-B mission itself-thus allowing independent confirmation of the kilohertz SLR derived results.

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.

Spin Axis Precession of LARES Measured by Satellite Laser Ranging

Satellite laser ranging (SLR) is an efficient technique to measure spin parameters of the fully passive satellite LARES. Analysis of the laser range measurements gives information about the spin rate of the spacecraft and the orientation of its spin axis. A frequency analysis applied to the SLR data indicates an exponential increase of the satellites spin period: T = 11.7612 ·exp(0.00293327 ·D) , RMS = 0.115 s, where D is in days since launch. The initial spin period of LARES is calculated from the spin observations during the first 30 days after launch and is equal to T0 = 11.7131, RMS = 0.073 s. The spin axis of the satellite is precessing around the initial coordinates of right ascension RAinitial = 186.5°, RMSRA = 3.1°, and Declination Decinitial = - 73.0°, RMSDec = 0.7° (J2000 inertial reference frame), with a period of 211.7 days. The precession of the spin axis may be responsible for the observed oscillation of the slowing down rate: the spin half-life period (the time after which the spin period has doubled) varies between 209 and 267 days. The measured spin parameters of LARES are compared-and show good agreement-with the theoretical predictions given by the satellite spin model. Information about the spin parameters of LARES is necessary for the accurate modeling of the forces and torques that are affecting the orbital motion of the satellite.

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.

Circular streak camera application for satellite laser ranging

The application of the streak camera with the circular sweep for satellite laser ranging is described. The Modular Streak Camera system employing the circular sweep option was integrated into the conventional Satellite Laser System. The experimental satellite tracking and ranging has been performed. The first satellite laser echo streak camera records are presented.