Satellite Range Finding by Photon Counting against Optical Time Ruler

This work focused on two primary error sources limiting Satellite Laser Ranging accuracy. The detection delay instability spoils ranging data concerning long-term stability. This results in the impossibility of distinguishing if the error has an origin during calibration or ranging; both errors limit the SLR product quality. Another error source arises from threshold detection, where no information about signal amplitude is available for individual laser signal returns. In this contribution, we will introduce a new ranging strategy, where we time-tag photons of interest against so-called clock photons. They are accurately synchronized to a stable atomic clock and generate equidistantly well-defined time intervals as an optical ruler. The clock photons propagate through the same detection chain as ranging photons and effectively remove and calibrate all variable electrical delays during the detection process. In this detection scenario, the biggest drawback may be the increase in noise since the detector noise is added twice. However, the clock photons can be averaged effectively; therefore, the additive clock detection jitter is suppressed. Since the detector response is sampled using 20 GSPS, it allows us to reconstruct a detector response envelope and consequently remove any amplitude to timing modulation from ranging data. Here we will outline the measurement concept and discuss laboratory tests and range measurements obtained from satellite echos.