A.

Space-Based Laser

JAXA has been investigating a laser transmitting system for active light ranging missions in near future. In this study, we assume that this “space-based laser” is mounted to the GLS. Main specifications of the space-based laser are described below.

B.

Platform

Three candidates of platform are considered as below.

C.

Eye Safety

In considering a future mission using space-based laser, eye-safety of people on the ground must be considered. The safety standards of laser irradiation are prescribed by the International Electrotechnical Commission (IEC) [5].

These standards define the maximum permissible exposure (MPE) of a single laser pulese to the eyes is 50 mJ/m² at λ =1064 nm.

In the case of space-borne laser sccaner, the situation that observer on the ground is viewing through a telescope (30 cm diameter) and all the laser light which enter the telescope aperture passes through his/her pupil (7 mm in diameter) is considered. In this case, the MPE is 27 μJ/m² at λ =1064 nm, which is (30 / 0.7)² times more strict criteria than the standard of the IEC.

D.

Scanning Mechanism

Two candidates of scanning mechanisim, galvanometer scanner and polygon mirror scanner, are considered, and schematic diagrams of footprints distributed by galvanometer scanner and polygon mirror scanner are shown in Fig. 3. As shown in Fig.3, Density of footprints put by the galvanometer scanner is relatively high at the edges of the observation swath and low at the nadir. On the other hand, the polygon mirror scanner put footprint with uniform density. In this paper, the polygon mirror scanner is assumed to be a scanining mechanism of the GLS. More detiled trade-off study is needed until the final decision of the scanning mechanisim is made.

Fig. 3

Schematic diagram of footprints by galvanometer scanner (left) and polygon mirror scanner (right).

A.

Observable Areas

Assuming that the space-based laser is the light source of the GLS, maximum laser power is 15 W. According to limitation of eye safety, a laser pulse with 1 mJ must be expanded to 37 m² area at the ground (atmospheric extinction is not considered). In this case, the total area of footprint per one second is about 56 ha (= 5.6 × 10⁵ m²). Thus, about 1.7 Gha is observable per year, and it is corresponding to 3.3 % of all surface area of the Earth (overlaps between footprints are ignored.).

TABLE 1.

Required scanning rate frequency, maximum numbers of footprints per a scaning line, maximum observable swath, maximum observable frequency per year, and full scan angle for each interval of footprints.

B.

Placement of the Footprints

As a result of simple simlation for the GLS signal, we consider that about 15 mJ of laser pulse energy can be needed to obtain the data from which canopy height of tree can be estimated. Thus, the laser pule energy is assumed to be 15 mJ in this paper. According to limitation of eye safety, this laser pulse of 15 mJ is expanded to about 560 m² area at the ground and the diameter of the footprint is about 27 m. And, maximum pulse rate frequency of the GLS is supposed to be 1000 Hz since the maximum laser power is 15 W, at this time.

As a function of interval between next footprints, required scanning rate frequency, maximum numbers of footprints per a scaning line, maximum observable swath, maximum observable frequency per year, and full scan angle are estimated, and they are summarized in Talbe 1. With smaller interval of footprints, higher frequency of the scanning rate are required and maximum observable swath is narrower, hence maximum observable frequency per year is lower than those of larger interval between next footprints.

In terms of the observation for terrestrial vegetation, 4 observations per a year is preferable [1]. To make 4 observations per year, about 15 km observation swath is necessary (probability of cloud is not considered). The estimation is below.

Length of the Equator: 40075 km

Cycle number: 15 day^-1

Recurrent days: 365/4 days

This swath is corresponding to full scan angles of 4.2, 2.1, and 1.4 degrees at 200, 400, and 600 km, respectively. Since required scan angles are relatively low, the waveform can be observed at the edge of the 15 km swath without much difficulty.

Thus when the interval of footprints is larger than 330 m, 4 observations per year can be achieved with the space-based laser which has been studied by JAXA.

For the optimazation of the footprints placement, we have disscussed with assumed users and further study will be required.

C.

Simulation of Waveform from Vegetation Area to Estimate Canopy Height of Tree

To develop analysis algorithm for waveform of the GLS and to estimate observation accuracy of tree canopy height by the GLS, waveforms from some vegetation areas are simulated. Fig. 4 shows two examples of the waveform simulations, (a) from cedars, and (b) a zelkova (they are usual trees in Japan). Canopy height of the cedars and the zelkova are about 27 m and 11 m, respectively. 3D illustrations of the trees are in the charts. Thin cylinders above the trees represent laser footprints and diameters of the both cylinders are same. While a number of cedars were used for the waveform simulation, only one zelkova was generated in the fields of simulation since the zelkova has a large canopy area corresponding to the footprint.

Fig. 4.

Waveform simulations (a) from cedars and (b) a zelkova.

Noise of the signals have not been simulated, thus the waveforms only represent intensity and observed time of photons reflected from the trees and the ground. In the case of cedars, there are interspaces between each cedars and intensity of return pulse from the ground is relatively high. On the other hand, a large part of photons are refracted by the zelkova and intensity of return pulse from the ground is low.

In the case of cedars which have acuminate shape, rising of the waveform is relatively slow. On the other hand, waveform of the zelkova shows relatively sharp initial rise. It is indicated that the ideal waveform represents vertical profile of projected area of trees. The initial rise of waveforms are fitted by the Gaussian function, and the half width at half maximum (HWHM) of the initial rise are 6.6 ns and 5.2 ns for the cedars and the zelkova, respectively. In these simulations, HWHM of the laser was 2 ns. Thus, contributions of the tree-canopy shapes to the HWHM are roughly estimated to be 6.3 and 4.8 for the cedars and the zelkova, respectively. Through the simulation and analysis as above, appropriate algorithm for the analysis of the GLS waveform has been studied.

There are various methods of analysis to estimate canopy height of tree from the waveforms obtained by the ICESat/GLAS [6], and a large part of the tree height error is likely to be caused by waveform analysis algorithm. Thus, the further study of the analysis algorithm of GLS waveform is important to improve the accuracy of the canopy height.

D.

Other Application of the GLS

Space-borne laser scanner will provide other information on a wide range of Earth surface characteristics and processes. For example, the GLS will provide the global elevation map with high resolution which is potentially useful to understand and forecast damage that a stricken area may suffer. Observation of ice-sheets is also a candidate of the GLS mission, and the thickness of the ice-sheets in the polar region is considered to be one of the important observation objects to understand the global warming.