Attitude and Spin Period of Space Debris Envisat ... - Semantic Scholar

IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 52, NO. 12, DECEMBER 2014

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Attitude and Spin Period of Space Debris Envisat Measured by Satellite Laser Ranging Daniel Kucharski, Georg Kirchner, Franz Koidl, Cunbo Fan, Randall Carman, Christopher Moore, Andriy Dmytrotsa, Martin Ploner, Giuseppe Bianco, Mikhailo Medvedskij, Andriy Makeyev, Graham Appleby, Michihiro Suzuki, Jean-Marie Torre, Zhang Zhongping, Ludwig Grunwaldt, and Qu Feng

Abstract—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. Index Terms—Envisat, satellite laser ranging (SLR), satellite spin, space debris.

Manuscript received December 10, 2013; revised March 11, 2014; accepted April 4, 2014. D. Kucharski is with Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea (e-mail: [email protected]; [email protected]). G. Kirchner and F. Koidl are with the Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria. C. Fan is with Changchun Observatory, National Astronomical Observatories, Chinese Academy of Sciences, Changchun 130117, China. R. Carman and C. Moore are with Electro Optic Systems Pty., Ltd., Weston Creek, A.C.T. 2611, Australia. A. Dmytrotsa is with the Scientific Research Institute, Crimean Astrophysical Observatory, 98409 Nauchny, Ukraine. M. Ploner is with the Astronomical Institute, University of Bern, 3012 Bern, Switzerland. G. Bianco is with Agenzia Spaziale Italiana, Centro di Geodesia Spaziale “G. Colombo”, 75100 Matera, Italy. M. Medvedskij is with the Main Astronomical Observatory, The National Academy of Sciences of Ukraine, 03680 Kyiv, Ukraine. A. Makeyev is with the Crimean Laser Observatory, Main Astronomical Observatory, The National Academy of Sciences of Ukraine, 98688 Yalta, Ukraine. G. Appleby is with the Natural Environment Research Council, Space Geodesy Facility, Hailsham BN27 1RN, U.K. M. Suzuki is with Shimosato Hydrographic Observatory, Japan Coast Guard, Higashimuro 649-5142, Japan. J.-M. Torre is with Geoazur, Observatoire de la Côte d’Azur, 06304 Nice, France. Z. Zhongping is with Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China. L. Grunwaldt is with GeoForschungsZentrum Potsdam, 14473 Potsdam, Germany. Q. Feng is with the Chinese Academy of Surveying and Mapping, Beijing 100830, China. Digital Object Identifier 10.1109/TGRS.2014.2316138

I. I NTRODUCTION A. Spin Determination of the Laser-Tracked Satellites

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HE satellites dedicated to the Satellite Laser Ranging (SLR) are equipped with the corner cube reflectors (CCRs) arranged in the following four configurations: — in a form of rings—common for most of the geodetic satellites (e.g., Ajisai, LAGEOS, LARES, Starlette, and Stella); — in a form of a flat panel (e.g., Galileo, GLONASS, and GPS); — in a form of a hemispherical panel (e.g., CHAMP, Envisat, and GOCE); — in a freeform in order to cover most of the satellite’s surface and increase the satellite’s radar cross section (Beacon-C and Etalon-1 and -2). The SLR measures distances to the satellites with the laser pulses. The laser range measurements are used for precise orbit determination, study of tectonic plate motion, Earth orientation and rotation parameters, and determination of the gravity field and the geocenter position [22]. The fully passive geodetic satellites (e.g., Ajisai, Etalon-1 and -2, LAGEOS-1 and -2, LARES, Starlette, and Stella) are launched with an initial spin, which helps to stabilize the attitude of the spacecraft. In this case, the spin parameters (spin axis orientation and spin rate) of the passive heavy bodies are changing under the influence of the forces and torque motors caused by the Earth’s gravitational and magnetic fields, as well as solar irradiation [1], [12]. The SLR systems that operate with the high-repetition-rate lasers [6] offer the best accuracy of the spin measurement of the geodetic satellites. During a pass of a satellite over an SLR station, the laser pulses transmitted from the ground station are reflected by CCRs back to the receiver telescope of the SLR system. The spinning array of the CCRs causes a millimeter-scale modulation of the range measurements that engraves a frequency signal on the SLR data. This frequency can be obtained by spectral analysis of the unequally spaced data (Lomb algorithm [19]) as it was demonstrated in [3], [5], [16], and [21]. The spectral analysis can be also used for determination of the spin axis orientation of the fast-spinning spherical satellites [18]. For the slow-spinning objects (long spin periods, comparable with pass duration), the spin axis orientation can be determined by analysis of the millimeter-scale modulation of the measured distances to the single CCRs of the satellite [10], [13].

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Fig. 1. (Left) Envisat and (right) its RRA. Courtesy of ESA.

SLR can measure spin parameters of the satellites during day and night, regardless to the sun–satellite–station geometry, and without any additional equipment. Measurement and analysis of the spin dynamics of the passive satellites can improve knowledge about the perturbing forces and torque motors, which are of magnetic, gravitational, and non-gravitational nature. This in turn can increase the accuracy of the scientific information obtained from the analysis of the orbital motion of the artificial satellites. In the case of the active missions designed for the Earth remote sensing (Envisat) or measurement of the Earth’s gravity field (GOCE), it is necessary to stabilize the orientation of the spacecraft in such a way that the satellite is always pointing to the Earth, i.e., in the nadir direction. The active missions use SLR as an independent, absolute, and alternative to GPS technology providing position of the spacecraft. The active, low Earth orbit, and laser-tracked satellites are equipped with small hemispherical panels that consist of four to nine CCRs. The high-repetition-rate SLR measurements to such panels can be successfully used to determine attitude and spin rate of a spacecraft as it was demonstrated with Gravity Probe B mission [7]. B. Envisat Envisat [20] (see Fig. 1) is the successor to the European Space Agency (ESA) Remote Sensing Satellites ERS-1 and ERS-2. The satellite was launched on March 1, 2002, and the expected lifetime of the mission was five years. The satellite was placed in a circular sun-synchronous polar orbit at a perigee of 796 km (inclination 98.54◦ and eccentricity of 0.001165). The dimensions of the launch configuration were 10.5-m length, 4.57-m envelope diameter, and 26 m × 10 m × 5 m in-orbit configuration. The total weight of the spacecraft was 8211 kg. The objectives of the Envisat mission were intended to support monitoring and studying of the Earth’s environment and climate changes; the management and monitoring of the Earth’s resources, both renewable and nonrenewable; and the development of a better understanding of the structure and dynamics

of the Earth’s crust and interior. The mission provided longterm data sets that are essential for both operational applications of spaceborne remote sensing systems and climatological and environmental research. As such, it was a major contributor to the global study and monitoring of the Earth and its environment as expressed by programmes such as the International Geosphere and Biosphere Programme (TGBP) and the World Climate Research Programme. Envisat is equipped with a retroreflector array (RRA) panel for SLR. The panel has a hemispherical shape (20 cm in diameter) and consists of nine CCRs (see Fig. 1), i.e., one nadir looking and eight symmetrically distributed around a hemispherical housing (oriented 50◦ off the symmetry axis of the panel). The mass of the RRA is 2 kg. The Envisat mission ended on April 8, 2012, following the unexpected loss of contact with the satellite. After this time, the satellite became a passive space debris and lost ability to maintain its attitude in the orbit. The satellite’s immense size makes Envisat a major space junk risk for many decades. The collision with an uncataloged space debris was probably the reason for the loss of the experimental geodetic satellite BLITS on January 22, 2013 [2], [4]. The nanosatellite BLITS has been designed as the zero-signature spherical lens for SLR [15]. The pioneer nonconductive glassy body did not allow the magnetic field of the Earth to decrease the rotational rate of the spacecraft during its mission [14]. Since Envisat occupies a crowded region of the low Earth orbiting satellites, it is of high importance to monitor its orbital motion and analyze possibilities of avoiding a potential future collision [2]. During the International Technical Laser Workshop 2012 in Frascati, Italy, it has been suggested that the SLR stations shall make an attempt of tracking the inactive Envisat in order to better predict the trajectory of the satellite and measure evolution of its spin parameters [8], [9]. On May 30, 2013 (day of year: 150), the International Laser Ranging Service (ILRS), i.e., basing on the request from ESA, asked the global network of the SLR stations [23] to resume tracking of Envisat. Since that

KUCHARSKI et al.: ATTITUDE AND SPIN PERIOD OF SPACE DEBRIS ENVISAT MEASURED BY SLR

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Fig. 2. RCS: Orientation angles (longitude and latitude) of the laser vector L. Vectors: AT (along-track), N (normal to the orbital plane), R (radial vector), and n (nadir vector).

date, many of the SLR systems (monument ID and name: 1824 Golosiiv, 1873 Simeiz, 1893 Katzively, 7090 Yarragadee, 7237 Changchun, 7249 Beijing, 7810 Zimmerwald, 7821 Shanghai, 7825 Mount Stromlo, 7838 Simosato, 7839 Graz, 7840 Herstmonceux, 7841 Potsdam, 7845 Grasse, and 7941 Matera) have tracked the passive Envisat and delivered the full rate data to the global data centers. II. S PIN PARAMETER D ETERMINATION A. Attitude Determination During passing of the satellite over the SLR station, the range measurements are collected. After passing, the predicted range trend is fitted to the measured values by adjusting time bias and range bias estimations. As the next step, the range measurements are converted into range residuals by subtracting the predicted range from the observed (measured) range values (O − C, observed minus calculated range). In the course of this process, fitting functions (orbital function or low-degree polynomials) are used in order to remove systematic trends from the distribution of the range residuals. Postprocessing of the range residuals allows eliminating most of the noise from the measured data, leaving the true distance observations between the ground station and the satellite. In order to determine the attitude of Envisat, we have analyzed the full rate data delivered by 15 SLR stations during the special ILRS campaign (started on May 30, 2013) and the data collected by Graz SLR station (38 passes) before the campaign started, i.e., total 347 passes measured until September 25, 2013. The laser vector of the SLR system is a vector centered at the satellite’s position and oriented toward the observing ground station. Orientation of the laser vector in the RCS can be described by longitude and latitude angles (see Fig. 2). The RCS (see Figs. 2 and 3, right) is a right-handed Cartesian coordinate system, where the R-axis (+Z) always points from the Earth’s center along the radius vector toward the satellite as it moves through the orbit; the AT -axis (+X) is the along-track vector perpendicular to the radius vector, and the N -axis (+Y ) is normal to the orbital plane. The nadir direction is opposite to the radius vector. Fig. 3 (left) presents the orientations of the laser vectors of the SLR systems at the epochs of the range measurements, expressed in the RCS. We assume the maximum incident angle between the laser beam and the optical axis of a CCR at which the reflections are possible to be 40◦ . Because of the off-nadir orientation of the eight CCRs around the perimeter of the RRA,

Fig. 3. (Left) Orientation of the laser vectors in the RCS. (Right) Position of Envisat during an evening flight over Graz: September 23, 2013, UTC 20:57, and RCS: AT : along-track (+X), N : normal to the orbital plane (+Y ), R: radial vector (+Z), and n: nadir direction (−Z). XT , YT , and ZT : axes of the terrestrial coordinate system. The ground/orbital tracks are plotted. Altitude of an orbit not to scale.

the maximum incident angle between the laser beam and the symmetry axis of the CCR panel at which the laser echoes can occur is 90◦ . Due to the configuration of the orbit, the laser beam of a ground SLR station can point at the latitude angles of the RCS (see Fig. 2) lower than −28◦ . In order to determine the position of the symmetry axis of the retroreflector panel in the RCS, we assume that the orientation of the panel is stable in this coordinate system after April 10, 2013, and fit the reflection cone (cone half-angle of 90◦ ) to the observed coordinates of the laser vectors (see Fig. 3, left). The circumference of the reflection cone determines the limit of the laser echoes, whereas its axis indicates the position of the symmetry axis of the retroreflector panel. Assuming that the symmetry axis of the panel is parallel to the satellite’s axis of rotation, we obtain the spin axis in the RCS at Lon = 269.22◦ , Lat = −28.14◦ (see Fig. 3, left). B. Retroreflector Panel Offset and the Spin Period from Meter-Scale-Range Oscillations During its mission, Envisat was stabilized in such a way that the retroreflector panel was always pointing in the nadir direction. After the satellite became inactive, its attitude changed and the body started to rotate. The spin axis of Envisat is not coincident with the symmetry axis of the retroreflector panel, and the two axes are separated by an offset H. The rotation of the retroreflector panel about the spin axis causes oscillations of the range measurements between a ground SLR system and the satellite in the meter scale. This effect is clearly visible in the distribution of the range residuals calculated as observed minus predicted range. Fig. 4 presents the range residuals of Envisat pass measured by Graz SLR station on July 12, 2013. The range residuals oscillate due to an offset between the spin axis and the retroreflector panel, and the oscillation period can be determined by frequency analysis of the given data set. This analysis uses the Lomb algorithm [19], which is a method of estimating a frequency spectrum of unequally spaced data (the case of SLR), based on the least squares fit of sinusoids to the data samples. Due to the long spin period of the satellite, we have selected 87 continuous passes measured by different

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Fig. 4. Range residuals calculated for Envisat pass measured by Graz SLR station on July 12, 2013, UTC 20:40. (a) Distribution of the range residuals (0 level indicates the mean). (b) Incident angle between the laser vector and the symmetry axis of the retroreflector panel. (c) Representation of the amplitude function (see gray lines on graph c) A = 2.52 · sin(incident angle) [m].

Fig. 5. SLR data of Envisat measured by Graz on July 14, 2013. (a) Range residuals. Zero is the mean level. (b) Zoom of a peak given by a single CCR. The polynomial function is fitted to the accepted points (black dots). The closest approach (CA) is marked. (c) Spin phase of the satellite is changing over time, i.e., the linear trend represents an apparent spin rate of the body ω = 3.1187◦ /s. The timescale is in seconds from the second of day = 76180. The vertical lines indicate epochs of the CA of the CCRs.

SLR stations with duration longer than 4 min (average duration of 5.7 min). For every pass, the range residuals have been calculated and spectrally analyzed giving the spin period of the satellite. The frequency analysis of the range residuals is a wellknown process [3], [5], [16], [21] and allows the determination of the apparent spin rate of the moving satellite, as shown from the observing ground site. The apparent spin periods calculated from the analysis of the meter-scale-range oscillations (see Fig. 6) can be approximated by the linear trend function (unweighted least squares fit): Tapparent = 0.0290416 · D + 109.352 [s] and RMS = 2.23 s, where D is in days of year 2013. Assuming that the spin axis of Envisat and the symmetry axis of the retroreflector panel are parallel allows finding the offset H between the axes. All collected amplitudes A [see example pass: Fig. 4(a)] of the range oscillations expressed in the domain of the incident angle IA between the laser vector and the symmetry axis of the retroreflector panel can be approximated by a function A = H · sin(IA) [see Fig. 4(c)], and the best fit occurs for H = 2.52 m (RMS = 0.12 m). The H parameter is larger than 1.70-m offset between the panel and the center of mass of the satellite (as on April 5, 2012).

C. Spin Period From Millimeter-Scale-Range Oscillations During postprocessing of the SLR data, the range residuals (observed minus predicted range) are calculated [see Fig. 4(a)]. During the next step of postprocessing, the sinusoidal variation over time of the range residuals (in the meter scale) is removed by the polynomial functions. This process allows obtaining the millimeter-scale-range variations caused by the spinning single CCRs of the panel. Among the analyzed Envisat passes, only the highly accurate data delivered by Graz SLR station allow distinguishing between the range measurements given by the single CCRs. The Graz SLR system measures distances to the satellites with a 2-kHz repetition rate laser (10-ps pulsewidth) since October 9, 2003 [6]. The system uses a “Compensated Single Photoelectron Avalanche Detector” (C-SPAD), which detects the arrival time of the first photons reflected by the satellite and thus measures the distance to the nearest CCR of the panel observed during a pass. Fig. 5(a) shows the range residuals after the polynomial fitting of a 130-s part of an Envisat pass measured by Graz on July 14, 2013.


The range residuals oscillate with an amplitude of about 6 mm that is caused by the rotation of the retroreflector panel. The single downpeak [see Fig. 5(b)] represents change of the incident angle between the optical axis of the given CCR and the laser beam of the SLR system. The minimum incident angle occurs when the CCR points toward the ground SLR station and indicates the CA between the CCR and the SLR system (minimum incident angle between the optical axis of the reflector and the laser beam). The epoch of the CA can be determined by finding a minimum of a low degree (third degree) polynomial function applied to the selected range residuals of the single peak [see Fig. 5(b), black points]. In the case of the symmetrical distribution of the CCRs around the panel (equal angular separation between the CCRs), it is possible to apply the spectral filter [17] to the data set and eliminate the noise points [see Fig. 5(b), gray points]; this process improves accuracy of the CA determination. The RRA of Envisat consists of the central CCR mounted on the top of the panel and eight CCRs distributed symmetrically around the ring with an azimuthal separation angle of 45◦ . Supposing that the spin parameters of the satellite are stable during a single pass, it is possible to calculate the spin rate of Envisat by analyzing time between the CA epochs of the consecutive peaks; it takes peak-to-peak time for the satellite to rotate by an angle of 45◦ about its spin axis. The observed change of the spin phase of the satellite [see Fig. 5(c)] can be approximated by a linear trend function, which indicates the apparent spin rate ω (spin) of the body. During a satellite pass over the SLR station, the laser beam vector changes its orientation about the spin axis of the spacecraft. This motion of the laser vector around the satellite’s body causes the ground system to observe an apparent spin of the passing-by spacecraft. The apparent spin is composed of the inertial rotation of the satellite and the apparent effect, which depends on the geometry of the pass and on the orientation of the observed body [11]. The apparent effect can be eliminated from the observation by the simulation model, which expresses rotation of the satellite in the J2000 inertial reference frame. The spin of the satellite is realized by three right-handed Cartesian coordinate systems of the same origin in the optical center of the RRA. The body coordinate system (BCS) is fixed with the array in such a way that its axis +ZBCS coincides with the symmetry axis of the array and is oriented toward the central CCR. The BCS rotates within the spin coordinate system (SCS), and the spin axis +ZSCS coincides with +ZBCS . The spin phase is calculated as an angle from axis +XSCS to +XBCS . Orientation of the SCS is expressed by longitude and latitude of the spin axis (+ZSCS ) within the RCS (see Fig. 3, right) which is fixed with the orbit and moves with the satellite. This spin model allows for determination of the inertial spin phase of the satellite at the CA epochs [see Fig. 5(c)]. For an observed epoch of the CA, the model calculates the position of the satellite and adjusts the spin phase in such a way that a single CCR of the ring is facing the ground SLR station. The process of the spin phase determination continues for the consecutive CA epochs of a given pass. The determined spin phase values are approximated by a liner trend function, and the slope of the

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Fig. 6. Spin period of Envisat during year 2013. (Black circles) Inertial spin period and (gray points) apparent spin period. The linear functions indicate the slowing down trends after 100th day of the year.

function represents the inertial spin rate of the spacecraft about the axis of rotation. The accuracy of the spin determination can be quantified by RMS of the spin phase residuals calculated to the linear trend function. In the spin model, we assume that the orientation of Envisat’s spin axis is stable and equal to the orientation of the retroreflector panel (determined in Section II-A). In this configuration, the RMS of the spin phase residuals depends mainly on the spin direction of the satellite: clockwise (CW) or CCW. The RMS (4.26◦ ) of the phase residuals for the CCW rotation is about 40% smaller than for the opposite case; thus, we conclude that the satellite rotates CCW about its spin axis (the spin phase increases during a pass). In order to determine the inertial spin rate of Envisat, we have analyzed 46 passes measured by Graz SLR station between April 13 and September 24, 2013. The average pass has a duration period of 6.8 min, return rate of 38%, and presents 17 peaks of the range residuals given by the single CCRs [see Fig. 5(a)]. The inertial spin periods determined by the analysis of the satellite’s spin phase can be approximated by the linear trend function (unweighted least squares fit): Tinertial = 0.0367320 · D + 124.883 [s], RMS = 0.91 s, where D is in days of year 2013. D. Spin Determination Results During its mission, Envisat was stabilized in such a way that the retroreflector panel was always pointing in the nadir direction. After the end of the mission (April 8, 2012), the orientation of the spacecraft started to deviate in the opposite direction to the normal vector of the orbital plane. The passive spacecraft could change its attitude under the influence of the Earth’s magnetic and gravity fields. The magnetic field induces eddy currents in the metallic parts of the satellite while the gravity field acts on the offset between the center of mass and the center of gravity of the body. The obtained results indicate that the incident angle between the symmetry axis of the retroreflector panel (spin axis) and the along-track vector remains close to the initial value of 90◦ and is equal to 90.69◦ (see Fig. 3). The apparent and inertial spin period values are presented in Fig. 6. The apparent spin periods are calculated from the analysis of the range oscillations in the meter scale (see Section II-B), gray circles, preceded by four values (gray triangles) estimated from the time spans between the consecutive peaks of the range residuals visible in the passes measured

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Fig. 7. Orientation of Envisat. (Left) During the mission, i.e., nadir stabilized. (Right) After May 2013, the satellite spins in the CCW direction about the spin axis S. Vectors: radial R, normal to the orbital plane N , along-track AT , and nadir n; a ground track of the polar orbit is marked.

by Graz SLR station between March 3 and April 10, 2013. Polynomial approximation of the apparent spin periods (see Fig. 6, dashed line) indicates that the rotational period of Envisat was decreasing until April 2013; after that, the trend changed and the spin of the satellite started to slow down. The apparent spin trend (see Fig. 6, gray points) indicates an increase in Envisat spin period of 29.0 ms/day, whereas the inertial trend (true spin; see Fig. 6, black points) gives 36.7 ms/day. Due to the long spin period in relation to the pass duration (about 3 rotations/pass), the apparent effect has a significant impact on the spin determination, i.e., the observed (apparent) spin period is about 15% shorter than the true (inertial) spin of the spacecraft. The inertial spin estimates do not depend on the geographical position of the observing SLR system; thus, the RMS of the obtained values is lower (0.91 s) compared with the case of the apparent spin (2.23 s). The analysis presented here relies on the assumption that the spin axis of Envisat is stable from April 10 until September 25, 2013. It possible that the orientation of the spacecraft wobbles within a limit of a few degrees; however, the amount of the available SLR passes does not allow detecting the small attitude changes of the satellite. The change of the spin axis orientation in the range of 5◦ could increase the RMS of the inertial spin period values not more than 15%. The obtained spin period trends indicated that the change of the satellite’s attitude (after the end of the mission) was accompanied by an increase in the rotational rate of the body about its spin axis until April 2013. After this stage, the satellite started to lose its rotational energy what might be related to the stabilization of the spacecraft’s attitude with respect to the orbital plane (see Fig. 7). The rotation of Envisat might have been caused by the influence of the forces and torque motors generated by the Earth’s gravity field as it acts on the offset between the center of mass and the center of gravity of the satellite. The magnetic field of the Earth could also have an impact on the spin of Envisat due to the change of the satellite’s orientation in the rotating magnetic field during an orbital cycle. III. C ONCLUSION AND P OTENTIAL A PPLICATION Envisat mission was dedicated to the remote sensing of the Earth’s land, atmosphere, oceans, and ice caps. The satellite has been operated for ten years, doubling its planned five-year lifetime. The unexpected loss of the contact with the satellite on April 8, 2012, led to an end of the mission leaving the satellite in an unknown condition.

During the ILRS campaign, the SLR stations successfully tracked Envisat and delivered the range measurements that allowed to determine the attitude and the spin period of the satellite. During the mission, the orientation of Envisat was fixed with the orbit in such a way that the retroreflector panel was always pointing in the nadir direction. This situation changed after the spacecraft became inactive, and the recent SLR measurements indicate stable orientation of the body within the RCS, i.e., the retroreflector panel points in the direction opposite to the normal vector of the orbital plane. The satellite spins about the axis parallel to the symmetry axis of the retroreflector panel with a period of 134.74 s (September 25, 2013) and is slowly losing its rotational energy; the spin period increases by 36.7 ms/day. The satellite spins in the CCW direction in such a way that the solar array approaches the alongtrack and the radial vectors consecutively. The passive Envisat is a massive space debris on the crowded low Earth orbit. Its orbital motion should be carefully analyzed and the possibility of the conjunctions with another object should be monitored. Knowledge of the Envisat’s spin dynamics, as well as its evolution, can help model perturbations and determine the orbit more accurately. The spin information can be also used for planning an automated mission for the removal of Envisat from the orbit. SLR is a very efficient source of information about the spin dynamics of the passive Envisat; thus, we recommend the ILRS community to continue tracking this satellite.

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[14] D. Kucharski, G. Kirchner, and F. Koidl, “Spin parameters of nanosatellite BLITS determined from Graz 2 kHz SLR data,” Adv. Space Res., vol. 48, no. 2, pp. 343–348, Jul. 2011. [15] D. Kucharski, G. Kirchner, H.-C. Lim, and F. Koidl, “Optical response of nanosatellite BLITS measured by the Graz 2 kHz SLR system,” Adv. Space Res., vol. 48, no. 8, pp. 1335–1340, Oct. 2011. [16] D. Kucharski, T. Otsubo, G. Kirchner, and G. Bianco, “Spin rate and spin axis orientation of LARES spectrally determined from satellite laser ranging data,” Adv. Space Res., vol. 50, no. 11, pp. 1473–1477, Dec. 2012. [17] D. Kucharski, T. Otsubo, G. Kirchner, and H.-C. Lim, “Spectral filter for signal identification in the kHz SLR measurements of the fast spinning satellite AJISAI,” Adv. Space Res., vol. 52, no. 5, pp. 930–935, Sep. 2013. [18] D. Kucharski, H.-C. Lim, G. Kirchner, and J.-Y. Hwang, “Spin parameters of LAGEOS-1 and LAGEOS-2 spectrally determined from Satellite Laser Ranging data,” Adv. Space Res., vol. 52, no. 7, pp. 1332–1338, Oct. 2013. [19] N. R. Lomb, “Least-squares frequency analysis of unequally spaced data,” Astrophys. Space Sci., vol. 39, no. 2, pp. 447–462, Feb. 1976. [20] J. Louet and S. Bruzzi, “ENVISAT mission and system,” in Proc. IEEE IGARSS, Jun. 1999, vol. 3, pp. 1680–1682. [21] T. Otsubo, J. Amagai, H. Kunimori, and M. Elphick, “Spin motion of the AJISAI satellite derived from spectral analysis of laser ranging data,” IEEE Trans. Geosci. Remote Sens., vol. 38, no. 3, pp. 1417–1424, May 2000. [22] E. C. Pavlis, “Geophysical parameters from laser ranging to the Lageos and Etalon satellites,” in Proc. 34th COSPAR Sci. Assembly, 2nd World Space Congr., Houston, TX, USA, Oct. 2002. [23] M. R. Pearlman, J. J. Degnan, and J. M. Bosworth, “The international laser ranging service,” Adv. Space Res., vol. 30, no. 2, pp. 135–143, Jul. 2002.

Daniel Kucharski was born in Pulawy, Poland, in 1978. He received the M.Sc. degree in mechatronics from Warsaw University of Technology, Warsaw, Poland, in 2003 and the Ph.D. degree in geodesy and cartography from the Polish Academy of Sciences, Warsaw, in 2008. From 2006 to 2011, he was with the Space Research Institute, Austrian Academy of Sciences, Graz, Austria, where he was working with the analysis of the kilohertz Satellite Laser Ranging (SLR) data. He was a Guest Researcher (2009–2010) with Hitotsubashi University, Tokyo, Japan (under the scholarship of the National Institute of Information and Communications Technology, Tokyo), where he was investigating the spin parameters of AJISAI. Since 2011, he has been with Korea Astronomy and Space Science Institute (KASI), Daejeon, Korea. Since 2013, he has been an Appointed Senior Researcher with KASI, where he is investigating the spin dynamics of artificial satellites with SLR data. Dr. Kucharski is a member of the International Laser Ranging Service.

Georg Kirchner was born in Bruck (near Salzburg), Austria, in 1950. He received the M.Sc. degree in electrical engineering/electronics from Graz University of Technology, Graz, Austria, in 1978. In 1979, he joined the Space Research Institute, Austrian Academy of Sciences, Graz, and started to establish the Satellite Laser Ranging station at GrazLustbuehel Observatory. He was responsible for the planning, construction, and operation of the laser station. During the last decades, he has developed the system to millimeter-ranging capability and added the capability to range to space debris. Dr. Kirchner is an Appointed Member of the International Laser Ranging Service Governing Board. He was the recipient of the Christian Doppler Prize from the Provincial Government of Salzburg in 1988.

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Franz Koidl, photograph and biography not available at the time of publication.

Cunbo Fan, photograph and biography not available at the time of publication.

Randall Carman, photograph and biography not available at the time of publication.

Christopher Moore, photograph and biography not available at the time of publication.

Andriy Dmytrotsa, photograph and biography not available at the time of publication.

Martin Ploner, photograph and biography not available at the time of publication.

Giuseppe Bianco, photograph and biography not available at the time of publication.

Mikhailo Medvedskij, photograph and biography not available at the time of publication.

Andriy Makeyev, photograph and biography not available at the time of publication.

Graham Appleby, photograph and biography not available at the time of publication.

Michihiro Suzuki, photograph and biography not available at the time of publication.

Jean-Marie Torre, photograph and biography not available at the time of publication.

Zhang Zhongping, photograph and biography not available at the time of publication.

Ludwig Grunwaldt, photograph and biography not available at the time of publication.

Qu Feng, photograph and biography not available at the time of publication.