Laser Rangefinder

The NEAR Laser Rangefinder (NLR) is an altimeter that uses a solid-state pulsed laser to measure the distance between the spacecraft and the surface of the asteroid 433 Eros. It is one of the five facility instruments onboard the NEAR spacecraft and will make highly accurate measurements of the asteroid's shape and detailed surface structure.

The Laser Rangefinder is a bistatic system, meaning that it consists of separate transmitter and receiver systems. The transmitter uses a diode-pumped neodymium-yttrium-aluminum-garnet (Nd-YAG) laser built by the McDonnell-Douglas Corporation, and the receiver uses a compact reflecting telescope with baffle/door assembly, enhanced hybrid silicon avalanche photodiode detector, microprocessor controller with Mil-Std 1553 bus standard interface, and fast gallium arsenide time-of-flight electronics built by the Johns Hopkins Applied Physics Laboratory.

Specifications:

Laser Altimetry

The NLR directly measures the range between the spacecraft and the asteroid. The transmitter generates a brief laser pulse, and the instrument measures the time required for the light to reach the asteroid and return. When the asteroid is 50 km away, light requires 334 millionths of a second to make the round trip. The time-of-flight measurements will be used to make accurate determinations of the asteroid's shape and will help to determine its rotational dynamics.

These shape and kinematics measurements, when combined with analyses of the very precisely tracked spacecraft orbit around the asteroid, will yield information on its internal density structure. These data may address the fundamental question of whether the asteroid is a homogeneous body or an aggregate of distinctly different bodies.

Laser altimetry complements in several ways the visible and near-infrared imaging that will also be performed on NEAR. Unlike the imager, the NLR does not rely on solar illumination and can make measurements over the entire asteroid surface, including the dark side. The NLR and the NEAR multispectral imager are boresighted, and the imager can detect the laser spot when NLR is operated in its 8-Hz mode over the dark side of the asteroid. Laser altimetry further complements imaging because the direct range measurements enable unambiguous determinations of topography that will improve the interpretation of images.

Laser altimetry will shed new light on geological processes on the surfaces of small bodies by allowing study of surface features such as craters, as well as grooves and vents if present.

NLR Description

The Near Laser Rangefinder is an incoherent, direct-detection laser altimeter. It is designed for high probability of detection in single-pulse operation out to beyond a 50-km range for low albedo, diffusely reflecting surfaces. The instrument uses leading-edge detection with commandable thresholds and range gates, but does not perform waveform analysis.

The NLR is the first spaceborne laser altimeter to have continuous inflight range calibration capability. This is implemented with an optical fiber delay coil of known length, through which a small fraction of every emitted laser pulse is sent. The time at which the light from the calibration coil is detected after each shot then serves to calibrate the timing circuit end-to-end.

Science Objectives

NLR Management

Science Team Leader:
Maria Zuber (MIT, NASA GSFC)
Lead Engineer:
Timothy D. Cole (JHU/APL)
Instrument Scientist:
Andrew F. Cheng (JHU/APL)
NEAR Payload Manager:
Robert E. Gold (JHU/APL)

Links to the other NEAR instruments:


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