Issue-Space-borne laser altimetry technology
2020, 49(11): 20201047.
doi: 10.3788/IRLA20201047
Space-based laser altimetry, which measures the planetary surface topography based on time-of-flight ranging, is an important technology for earth and deep space exploration. The development history and current status were sorted out systematically and development in three directions was proposed, including multi-beam observation, underwater terrain observation of offshore and islands and laser active multispectral detection. It was suggested that some key technologies should be developed, including infrared single photon detection, ultra-narrowband optical filtering, multiphoton resolution detection, single-photon data on-board preprocessing and high conversion efficiency laser and nonlinear optical frequency conversion, to promote the further development of Chinese space laser altimetry technology.
2020, 49(11): 20201044.
doi: 10.3788/IRLA20201044
The rapid development of laser altimetry satellite has a great impact on the traditional satellite earth observation field. The accurate elevation data can effectively compensate the lack of optical satellites, while laser altimetry satellite data processing and mapping applications are worthy of attention. Firstly, the principle and characteristics of laser altimetry satellite was introduced, and the its development history was presented. Then, the current on-orbit LiDAR satellite ICESat-2 was focued on, which had a high degree of attention. The satellite configuration characteristics and data processing methods were discussed, and its surveying and mapping capabilities in many fields, such as image joint adjustment, multi-source terrain fusion, global vegetation survey, and shallow water bathymetry et al. were analyzed and revealed. Finally, some thoughts about the development and construction of laser altimetry satellite in China were shared.
2020, 49(11): 20201045.
doi: 10.3788/IRLA20201045
Started with needs of the national marine strategy, the necessity of China’s oceanographic lidar developing was analyzed, the characteristics and domestic overseas’ development of the oceanographic lidar were summarized; the oceanographic lidar current construction and planning at home and abroad and the launched spaceborne lidar’s situation were analyzed, the future spaceborne ocean lidar’s development direction was put forward and on-orbit data application product was expected, the key technology of the spaceborne ocean lidar was discussed, the application prospect of developing the oceanographic lidar during the 14th five year plan oceanographic was given.
2020, 49(11): 20200233.
doi: 10.3788/IRLA20200233
Antarctica, Arctic and Qinghai Tibet Plateau are known as the three poles of the earth. The " tri-polar" regions, which are undergoing great changes, are not only the key areas affecting the study of global climate change, but also the important strategic special areas to protect national security interests. Compared with other polar satellites, it has obvious advantages in obtaining high-precision 3D earth surface information. In order to meet the needs of remote sensing observation and research in the "tri-polar" region under the new situation, based on the laser altimetry satellite, the status of the existing laser altimetry satellites in polar observation was summarized. The observation requirements in polar region were analyzed, and the observable elements of laser altimetry satellite in polar region were discussed. Moreover, the development of laser altimetry satellite for polar observation in China was proposed, the preliminary ideas and prospects for the future construction of polar remote sensing satellite observation system were discussed.
2020, 49(11): 20200247.
doi: 10.3788/IRLA20200247
The photon events acquired by the single-photon lidar have the characteristics of random distribution, which makes the laser ranging value be uncertain and reduces the ranging accuracy of the single-photon lidar. Under the circumstance that the sampling resolution was not decreased, an accumulative histogram of photon was constructed by accumulating photon events from the multiple contiguous spots. The range was determined on the basis of the time centroid of the derived target response function. As for the ICESat-2 single-photon lidar, a comprehensive evaluation approach of the range error was put forward by considering the effects of the terrestrial relief, the accumulative spot numbers and returned photons on the range error according to the root mean square error (RMSE) and mean absolute error (MAE). The random terrain on a strip of ICESat-2 in the West Valley City of American Utah was selected to validate the proposed method. The results demonstrate that the RMSE value is reduced from 114.25 cm to 63.84 cm and the MAE value is reduced from 70.97 cm to 48.52 cm. They outweigh the RMSE of 137.96 cm and MAE of 97.24 cm from the ICESat-2 products. The conclusions might provide some guidance for improving the ranging precision on the terrain.
2020, 49(11): 20201046.
doi: 10.3788/IRLA20201046
As a typical advanced laser, spaceborne solid state laser is one of the key components used in spaceborne laser altimetry systems. The requirement and working condition of spaceborne laser altimetry determine the technical characteristics of this kind laser source. The spaceborne solid state laser integrates the advanced, high reliable, light weight technologies and mature process, thus can meet the specific requirements of laser altimetry used in space environment condition. The development of high reliable laser resonator technology, high repetition rate narrow pulse width micro-pulse multi-beam laser technology, LD/fiber/solid-state integration technology for spaceborne laser altimetry and the current challenges were reviewed and analyzed, the significant breakthrough of multi-beam laser for spaceborne single photon scanning ladar system was proposed. At last, the development trend of spaceborne laser were summaried.
2020, 49(11): 20200234.
doi: 10.3788/IRLA20200234
Laser altimetry satellite can obtain the surface elevation information of sub-meter or even centimeter-level in a wide range, but it is inevitably affected by the scattering caused by particles such as clouds and aerosols. The laser ranging and final height measurement errors caused by forward scattering of the cloud or fog can not be ignored. In this paper, the atmospheric scattering error correction technology of laser altimetry satellite was systematically reviewed, and the satellite laser altimeter system parameters, atmospheric detection and scattering correction algorithm at home and abroad were introduced. Different from the Monte Carlo simulation correction method theory, an atmospheric scattering correction algorithm based on exponential function model was proposed. The data of GLAS (Geo-science Laser Altimeter System) on the ICESat (Ice, Cloud and land Elevation Satellite) in Qinghai Lake and other regions was selected for the experiment, and the experimental results show that the algorithm can effectively improve the accuracy of the altimetry data affected by atmospheric scattering when the optical thickness is less than 2, and the data availability rate can be improved by about 9%. The algorithm is easier to realize operation application. Finally, according to the necessity of synchronous detection of atmospheric parameters, some suggestions for atmospheric scattering correction of domestic laser altimetry satellites were put forward in combination with onboard atmospheric parameter detection equipment.
2020, 49(11): 20200259.
doi: 10.3788/IRLA20200259
The ICESat-2/ATLAS employed a micro-pulse multi-beam photon-counting laser altimeter system for the first time. ICESat-2/ALTAS was significantly different from ICESat-1/GLAS in terms of detection mechanism, data processing methods and the breadth and depth of data applications. Firstly, the key configuration parameters, data and products of ICESat-2/ATLAS were introduced. Secondly, the key technologies of noise removal and classification algorithms for ICESat-2/ATLAS data as well as the adaptability and problems of each algorithm were elaborated. Thirdly, the applications of ICESat-2/ATLAS data in ice sheet and sea ice elevation measurement and change monitoring, ground elevation and forest height extraction, forest biomass estimation, lake level and storage change monitoring were summarized. Finally, the development trend and prospect of photon-counting LiDAR data processing methods and applications were expected.
2020, 49(11): 20200214.
doi: 10.3788/IRLA20200214
On-orbit calibration technique is a key factor which affects the photon geolocation accuracy of spaceborne LiDAR. The current status of spaceborne LiDAR on-orbit calibration technique was introduced, and the characteristics of various spaceborne LiDAR on-orbit calibration technique were analyzed. Aiming at the characteristics of the photon counting mode spaceborne LiDAR, a new on-orbit calibration method based on the natural surface was derived, simulated point cloud was used to verify the correctness of the calibration algorithm, and a cross validation experiment was made with the surface data of the Antarctic McMudro Dry Valleys and China Lianyungang areas and ICESat-2 point cloud data, the experimental results show that the plane offset between the point cloud calibrated by proposed algorithm and point cloud provided by National Aeronautics and Space Administration is about 3 m, elevation offset is in centimeter scale. The differences between the point cloud calibrated by the algorithm and the point cloud provided by National Aeronautics and Space Administration were also compared by using the feature points of artificial construction on the ground. Finally, the accuracy of the on-orbit calibration method based on natural surface and the influence of the calibration field topography were discussed.
2020, 49(11): 20200261.
doi: 10.3788/IRLA20200261
The complex full waveforms from laser altimetry, mixed with high noise, are usually reflected by the object with multiple height elevations. To accurately analyze the decomposition, vertical structure and characteristic parameters from these waveforms, a noise reduction method based on empirical mode decomposition (EMD) was investigated and tested with the full waveform of nonlinear and nonstationary signals obtained by GaoFen-7 space-borne laser altimetry. The reconstruction of an effective waveform signal was implemented through reverse superimposition of its intrinsic mode functions (IMFs) and the residual. And then different selection methods for these IMFs were compared, such as removed high frequency, threshold, wavelet and detrended fluctuation analysis (DFA). The results show that EMD-DFA1 and EMD-1 IMF have a higher noise reduction effect on these full waveforms, followed by EMD-Wavelet and EMD-Threshold. Finally, EMD-DFA1 was performed on the full waveforms with single peak, mixed peaks and multiple peaks. And the results show that EMD-DFA1 does well adaptability.
2020, 49(11): 20200235.
doi: 10.3788/IRLA20200235
Forestry observation is an important direction of the development of space-borne LiDAR. So far, foreign space laser remote sensing payloads have been developed. The experimental LiDAR carried by Chinese ZY-3(02) satellite is a big breakthrough in earth observation of space-borne LiDAR in China, and the oncoming multi-beam LiDAR of Chinese CE-1 satellite will be applied to forestry remote sensing. The vertical structure of forest was acquired by ICESAT-1/GLAS waveform, while point cloud of forest was obtained by ICESAT-2/ATLAS photons. The space-borne LiDAR forestry remote sensing gradually entered the practical stage, beyond the experimental stage. During this period, technical system, configuration, and techniques had improved. In this paper, the applications and technique developments of main forestry remote sensing space-borne LiDAR in use were analyzed, and the characteristics and development trend were summarized, based on the study of detection principle, technical system, observation condition, etc. Space-borne forestry observation LiDAR payloads should be designed according to scientific mission, application requirements and technical characteristics, using the advantages of space-borne LiDAR, and based on which the development trend and research focus of space-borne forestry observation technology and application were prospected.
2020, 49(11): 20200292.
doi: 10.3788/IRLA20200292
ICESat-2 data was considered as a new land cover classification data source, and a method was proposed to classify land cover using ICESat-2 data with random forest, to explore the application potential of the space-borne photon counting lidar in the land cover classification. The method used the photon number, the proportion of horizontal and vertical distribution of different types of photons, signal-to-noise ratio, solar conditions and atmospheric conditions as the input of classification, and was verified by the experiment of multi-category land cover in China's Yangtze River Delta. For four categories of water, forest, low vegetation and urban/barren, the classification results show that the overall accuracy of strong beam and weak beam is better than 85%. For three categories of water, forest, and low vegetation/urban/barren, the classification results show that the overall accuracy of strong beam and weak beam is better than 90%.
2020, 49(11): 20200237.
doi: 10.3788/IRLA20200237
In view of the difficulties in retrieving Digital Terrain Model (DTM) with the spaceborne lidar data, the terrain elevation estimation accuracy of Ice, Cloud, and land Elevation Satellite-2 (ICESat-2)/Advanced Terrain Advanced topographic laser altimeter system (ATLAS) strong and weak beam data under the forest was studied, and the effect of canopy height and vegetation coverage on ICESat-2/ATLAS estimation accuracy of DTM was explored. The results show that the accuracy of DTM under the forest with strong beam estimation accuracy is R2=1, RMSE=0.74 m, and that with weak beam estimation accuracy is R2=1, RMSE=0.76 m. The performance of the strong beam estimation accuracy performed better than that of the weak beam, but both the photon cloud data of the strong beam and the weak beam can provide scientific data for estimating the DTM under the forest. In the study area, with the increase of the canopy height and vegetation coverage, the error of different laser types data gradually increases.
2020, 49(11): 20200251.
doi: 10.3788/IRLA20200251
The full-waveform satellite laser altimeter is able to record the complete backscattered echo waveform from the target surface, which can be used for range measurement and geophysical parameters inversion of the target surface. Some received raw full waveforms of the GF-7 satellite laser altimeter contain significant noise, skewed wave peak, and unsaturated flat peak, causing interferences to the information extraction from waveform. Firstly, an iterative method for waveform background noise removal was proposed, then, the characteristics and geometric structure of the waveform noise were analyzed and quantitatively described, and finally, an adaptive Gaussian filter considering the noise and heterogeneity structure of the waveform was designed. Comparison between the proposed method and other existing classical filtering methods was made in the experiment, and the superior performance of proposed method was proved in noise removal, signal retention, and unsaturated flat peak waveform processing. The higher signal-to-noise ratio (SNR) of the filtered waveform was achieved using the proposed method, and the amplitude decline of the waveform is within 3 times of the noise standard deviation. After the filtering of the waveform with unsaturated flat peak, the decomposition precision of the waveform Gaussian parameter amplitude, mean value and standard deviation are (0.69 ± 2.34) mV, (0.007 ± 0.024) ns and (0.026 ± 0.069) ns respectively.
2020, 49(11): 20200245.
doi: 10.3788/IRLA20200245
The echo of the all waveform lidar contains the vertical structure information of the ground object. The traditional all waveform data processing methods rely too much on the initial parameters when extracting these information, resulting in the low availability and accuracy of the data in the terrain complex area. To solve this problem, a waveform decomposition method based on genetic algorithm was proposed. The improved algorithm did not need to provide accurate initial parameters, and used probabilistic transfer rules instead of deterministic rules, which had the characteristics of global optimization. The experiment was carried out with the full waveform data of GF-7 satellite laser. The results show that the correlation coefficient between the echo waveform fitted by the improved waveform processing method and the preprocessed waveform is more than 99%. The inversion of the maximum tree height in the forest area was compared with the forest canopy height parameter in the ATL08 data of ICESat-2. The correlation coefficient is 0.85 and the mean square error is 1.1 m, which shows that the method can extract the feature information of complex waveform more accurately.
