Analysis on the experiment of satellite laser ranging of defunct Tiangong-1 spacecraft

Abstract

Tiangong-1, which was launched on 29 September 2011, is the first prototype space station of China. During the past two years in its lifetime, Tiangong-1 has been visited by a series of Shenzhou spacecraft. On 21 March 2016, it had formally ended its service and now is replaced by Tiangong-2. The orbit of Tiangong-1 is about 400 km above the ground where Tiangong-2, the international space station and other satellites are located as well. The space debris is now posing great collision risk to these space assets. The satellite laser ranging (SLR) is a space tracking technique with the highest precision. Its precise data lay a solid foundation for reliable conjunction assessment. On October 2002, Greene, Mount Stromlo SLR station of Australia, announced that it was possible to track a piece of space debris of a size of 15 cm at a distance of 1250 km using the SLR technology. In 2011, both Graz SLR station of Austrian and Grasse LLR station of France succeeded in debris laser ranging data. In China, Shanghai Astronomical Observatory obtained debris laser ranging data in 2008, Yunnan Observatory in 2010 and Changchun Observatory in 2014 also obtained debris data using SLR. In 2014, Changchun Observatory observed a total of 244 space debris in altitude about 400 to 1800 km. Tiangong-1 spacecraft was equipped with corner cube reflectors (CCRs), which were designed mainly for orbital rendezvous with Shenzhou spacecraft and now provide convenience for SLR. From March to May of 2016, a coordinated laser ranging experiment on Tiangong-1 was made by Changchun Observatory (ILRS site ID: 7237) of National Astronomical Observatories, Sheshan station (ILRS site ID: 7821) of Shanghai Astronomical Observatory and Purple Mountain Observatory. Tiangong-1 has not been tracked by international laser ranging service (ILRS) and the orbit prediction of consolidated prediction format (CPF) cannot be obtained from ILRS. In general, the two line elements (TLE) of space objects can be downloaded from internet ( https://www.space- track.org) which is released by north American air defense command (NORAD). However, the TLE prediction error of Tiangong-1 (NORAD ID: 37820) is about several kilometers that is not precise enough for SLR. During the experiment, the actual prediction is provided by Purple Mountain Observatory. By the end of the experiment, data of 23 passes was obtained. Precise orbit determination is made by using the SLR data and the root mean squares (RMS) of the SLR observation residuals is about 10 cm. For one pass of scattered data, the orbit determination program is not convergent. Then combined orbit determination is made using one pass of SLR data and optical celestial position data. The relative weight of SLR data to optical data is 100:1 and the RMS of the SLR observation residuals is about 6.9 cm. The new orbit prediction is propagated from the orbit determination of SLR data and its precision is about 200–300 m. The radial precision is better than 100 m that is accurate enough for later laser ranging. In general, the experiment of satellite laser ranging to defunct Tiangong-1 spacecraft will have positive effects on precise orbit determination of both Tiangong-2 and other low earth orbit space objects.