Effects of pointing errors on intensity losses in the optical LEO uplink

Abstract

Different pointing errors from different sources cause an angular deviation in the uplink beam transmitted from an optical ground station (OGS) to a satellite. In optical link-budget calculations, the beam intensity loss due to pointing errors, "pointing loss", is usually given a constant value regardless of the satellite elevation. In this paper, elevation-dependent intensity losses are calculated, considering a transmitted uplink beam with a Gaussian profile. The elevation of the satellite and the divergence of the uplink beam are considered to assess the impact of the following sources of tracking and pointing errors: OGS static pointing misalignment, uncorrected or fixed-corrected point-ahead angle (PAA), satellite orbital data uncertainties (specifically the along-track error), and mechanical jitter at the OGS. Each source of error is first evaluated separately and then the combination of their effects on the intensity loss in the LEO uplink is determined. It is demonstrated that the elevation-dependent pointing errors analyzed in this work have a greater impact on the intensity loss for satellites at lower altitudes and higher elevations. Therefore, not considering that the value of the "pointing loss" varies with elevation -especially for LEO satellites-, would result in lower link performance. Values of intensity loss are provided for LEO satellites at different altitudes and elevations, and uplink beam divergences. The results provided can be used for link-budget calculations in the optical LEO uplink in the presence of elevation-dependent pointing errors, and for system improvements in the design of future ground and space optical terminals.