In-Flight Calibration of the Near Earth Asteroid Rendezvous Laser Rangefinder

Abstract

The Near Earth Asteroid Rendezvous (NEAR) mission began a year-long rendezvous with 433 Eros on 14 February 2000. The NEAR Laser Rangefinder (NLR) will measure ranges from the spacecraft to the surface of Eros with a single shot accuracy of a few meters. The NLR topographic data, when combined with doppler tracking of the spacecraft, will enable determinations of the asteroid's shape, mass, and density and will contribute to understanding of its internal structure and collisional evolution. NLR is the first space-borne laser altimeter with an in-flight calibration capability, achieved by means of an optical fiber which directs a small portion of every outgoing laser pulse back to the receiver with a known, fixed time delay. Key results of groundbased and in-flight calibrations of NLR are presented for: in-flight measurement of receiver noise statistics, confirmation of instrument stability over the 4-year cruise to Eros, absolute calibration of range measurements for ideal targets (flat, uniform surfaces normal to the boresight), and a prediction of single-pulse detection probability and range errors in the presence of pulse dilation from nonideal target surfaces, based on a Webb's approximation model of receiver performance. We find that pulse dilation is the major source of uncertainty in the single-shot range measurements from NLR, and that this uncertainty is consistent with the 6-m range measurement requirement for NEAR.