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 satellite´s 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 RA_initial = 186.5^°, RMS_RA = 3.1^°, and Declination Decinitial = - 73.0^°, RMS_Dec = 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.