The LARES mission revisited: an alternative scenario

In the original LARES mission the general relativistic Lense–Thirring effect would be detected by using as observable the sum of the residuals of the nodes of the existing passive geodetic laser-ranged LAGEOS satellite and of its proposed twin LARES. The proposed nominal orbital configuration of the latter one would reduce the systematic error due to the mismodelling in the even zonal harmonics of the geopotential, which is the main source of error, to 0.3%, according to the most recent Earth gravity model EGM96. Unfortunately, this observable turns out to be rather sensitive to the unavoidable departures of the LARES orbital parameters from their nominal values due to the orbital injection errors. By adopting a suitable combination of the orbital residuals of the nodes of LAGEOS, LAGEOS II and LARES and the perigees of LAGEOS II and LARES it would be possible to reduce the error due to the geopotential to 0.02%, according to EGM96 model. Moreover, the sensitivity to the orbital injection errors would be greatly reduced. According to a preliminary estimate of the error budget, the total error of the experiment could be reduced to less than 1%. In the near future, when the new data of the terrestrial gravitational field from CHAMP and GRACE missions will be available, a further increasing in the accuracy could be obtained. The proposal of placing LARES in a polar 2,000 km altitude orbit and considering only its nodal rate would present the drawback that even small departures from the polar geometry would yield large errors due to the mismodelled even zonal harmonics of the geopotential, according to EGM96 model.