Atmospheric Turbulence Statistics and Profile Modeling. Local to DLR Oberpfaffenhofen

Abstract

Laser communication (lasercom) is influenced by atmospheric turbulence, a quality measured by the refractive index structure parameter Cn². This paper quantifies the degree of improvement to lasercom link budgets afforded by using ground-level measurements of turbulence in vertical turbulence models. Ground-level Cn² is measured with an off-the-shelf scintillometer for a path adjacent to DLR's optical ground station (OGS). Measurements are in agreement with literature on turbulence; nighttime Cn² is well represented by a log-normal distribution. Comparisons are drawn between profiles by comparing link budget parameter estimates generated by four turbulence profile models: HV-5/7, HV with Cn²(h0), and HV and HAP models with Cn²(h0) and fitting to downlink experiment data. Vertical turbulence profiles are converted to scintillation index sigma²_I by way of theory described in the literature on weak and strong turbulence. Normalised root-mean-squared-error is used to establish goodness-of-fit of modeled sigma²_I to downlink beam parameter measurements . Use of Cn²(h0) in a profile model improves upon the fit beyond HV-5/7 by ~8.3%. Improvements in the mean expectation from specific fits to satellite downlink experiments improve the NRMSE 30%. However, the variability in margin estimation due to changes in Cn²(h0) indicates fitting might not be a consistent improvement over the HV-5/7 model. This paper describes the setup of the scintillometer, six months of measurements, the use of Cn²(h0) measurements in vertical profile models to find the path integrated intensity scintillation index (sigma²_I), and a comparison of modeled-path integrated scintillation index to the scintillation index of downlink ground measurements at DLR's OGS.