Time flies. It has been 10 years since the launch of the asteroid explorer HAYABUSA. In co-operation with many associates, the development team is proceeding steadily toward the launch of the second asteroid explorer HAYABUSA-2.
This article introduces the laser altimeter onboard HAYABUSA-2 in comparison to the former model onboard HAYABUSA.
Technology to measure long distances is indispensable to decide the position and orbit of rockets and satellites in space. Although GPS has been employed recently for distance measurement between the earth and rockets/satellites, RADAR (RAdio Detection And Ranging) technology using microwaves (1m to 1cm wavelength) is still dominant. For example, the distance between a rocket and the earth is calculated based on the round-trip time of the microwaves pulses, while the distance to a satellite is determined based on the round-trip time of the microwave to which a special sequence code or periodically changing modulation frequency has been added.
A special case among distance measurement in outer space is that of lunar or planetary explorers (i.e., vehicles not orbiting the earth) which measure the distance to a target body by themselves. This is performed for the purpose, for example, of terrain investigation and distance measurement to a celestial bodys surface during a soft-landing. Distance measurement using microwaves has the advantages of no-air attenuation and relatively easy velocity determination in addition to distance mesurement. Since interplanetary explorers are unable to bear large antennas because of strict weight limits, however, the antennas beam directionality (i.e., ability to focus energy in a specific direction) becomes wide. Thus, the measurement range is usually limited to several kilometers. On the other hand, a laser altimeter allows the measurement of great distances up to several hundred kilometers. It uses laser light with good directionality while making possible downsizing and weight reduction compared to the microwave radar.
With these features, laser altimeters play a significant role in important explorer navigation/guidance events such as rendezvous, landing and terrain survey of celestial bodies. As a scientific instrument, the altimeter can also measure a celestial bodys gravity and investigate the distribution of surface characteristics by measuring the intensity of returned light.
The laser altimeter to be installed on HAYABUSA-2 is called LIDAR (LIght Detection And Ranging), and is capable of measuring distances of 30m to 25km based on the round-trip time of a laser pulse. A laser pulse is laser light emitted for a short time. In the case of HAYABUSA-2, the pulse is approx. 10 nano sec. (1/100,000,000 sec.).
The laser altimeters mechanism is explained with reference to the block diagram in Fig. 1. The main functions of the controllerEto the right are: to measure the elapsed time from light emission to target, reflection, and return to the explorer; to command the laser emission; and to communicate with the main explorer system. The laser emitted on command from the controller goes toward the target. Its directionality is intensified via a lens system called the beam expander.ESimultaneously, part of the light is forwarded to the PIN Photo DiodeE(PIN PD) to accurately notify the emission time to the controller. The minute amount of light returned after reflection on the planet surface is converged onto the Avalanche Photo DiodeE(APD) through the Cassegrain-type telescope and converted to minute electric current. The current is regulated to an appropriate voltage level by the amp. It is then digitized by the timing detectorEto notify the controller of the time of return. At a distance of 30km, for example, the altimeter detects returned light that has been reduced to an intensity of 1/several tens of trillions and measures the round-trip time of 200 micro sec. (2/10,000 sec). Since light intensity is inversely proportional to the square of distance, however, at a distance of 30m, light one million times stronger in intensity than that of 30km would return. Thus, we need to make gain adjustment (adjustment of voltage level) to cover a wide distance range in the receiving circuit as discussed below.