Sub-Cm Subsidence Measurements With The Wide-Angle Airborne Laser Ranging System

The Wide-Angle Airborne Laser Ranging System (WA-ALRS) is aninstrument developed at IGN with the aim of measuring landsubsidence with sub-cm accuracy. The instrument provides rangemeasurements that are used to estimate the co-ordinates of aground-based network of cube-corner retroreflectors (CCRs).Millimeter accuracy on relative radial co-ordinates has beendemonstrated in a ground-based experiment and centimeter accuracyhas been obtained in a recent airborne experiment. The mainlimitation in the latter experiment came from the low link budgetat longer distances. In this paper we analyze the performanceexpected with two different detector modules (avalanchephotodiodes with amplifiers). Laboratory measurements are used tocharacterize detector parameters and estimate ranging accuracy.Numerical simulations predict a 2–3 mm accuracy in the verticalco-ordinate with these detectors combined to a 2-km aircraftaltitude, a 20 laser beam divergence, a 250-m CCR separation andsim 103 measurements per CCR.