As the development lead-time for HAYABUSA-2 is tight, we tried to expedite development by introducing an HIC (Hybrid IC) that puts the APD and amp (transimpedance amp) in one package. The HIC performance had been proven in the lunar orbiting satellite KAGUYA. Its one drawback, however, is that it cannot change amp gain. Accordingly, circuit-gain adjustment is made by the two levels of APD bias voltage only, which cannot meet a gain range of up to one million times. To overcome the problem, we decided to prepare a small telescope (absent on HAYABUSA) to be used only in the short distance range. By switching to the telescope at a distance of about 1km, the light intensity can be reduced to approx. 1/1,000. Fig. 3b shows the estimated receiving light power. It can be seen that the characteristics of HAYABUSA (Fig. 3a) and HAYABUSA-2 (Fig. 3b) differ considerably.
Following the design, an Engineering Model (EM) of the HAYABUSA-2 laser altimeter was fabricated and tested in the runway at the Taiki Multipurpose Air Park, Hokkaido, in May 2013. The model is shown in the foreground of the photo of the tent built at the extension of the runway (Fig. 4a). Test team members looking at ground test equipment can be seen in the background. The gray, square board in the center of Fig. 4b is the target of the laser emission while the antenna resembling a mushroom is the GPS surveying instrument. The HAYABUSA-2 landing event was simulated by moving the target: it was mounted on a carriage and manually pulled along the 1km runway. The test was smoothly carried out in the idyllic scene where dandelions blossomed, cuckoos sang in the forest, and Ezo red foxes sometimes appeared. We were able to verify that the laser altimeter performed and functioned as designed.
Distance measurement using lasers is widely utilized in our daily lives, including measurement of golf drive lengths and automobile collision avoidance. This article introduced a laser altimeter for unique application on the planetary explorer HAYABUSA-2, including problems in its design.
Finally, the YAG rod used for altimeters laser is a garnet-structure crystal made of yttrium-aluminum composite oxide. It is said that garnet, the birthstone for January, implies reunionEand fulfillment of effortsEas a power stone. It is hoped that all the readers of this article will support HAYABUSA-2 when it sets out on its long, lonely space journey in keeping with its power stone.