Cloud Detection and Centroid Extraction of Laser Footprint Image of GF-7 Satellite Laser Altimetry

doi:10.11947/j.JGGS.2021.0301

Abstract

Abstract:

The laser altimeter loaded on the GaoFen-7(GF-7) satellite is designed to record the full waveform data and footprint image, which can obtain high-precision elevation control points for stereo image. The footprint camera equipped on the GF-7 laser altimetry system can capture the energy distribution at the time of laser emission and the image of the ground object where the laser falls, which can be used to judge whether the laser is affected by the cloud. At the same time, the centroid of laser spot on the footprint image can be extracted to monitor the change of laser pointing stability. In this manuscript, a data quality analysis scheme of laser altimetry based on footprint image is presented. Firstly, the cloud detection of footprint image is realized based on deep learning. The fusion result of the model is about 5% better than that of the traditional cloud detection algorithm, which can quickly and accurately determine whether the laser spot is affected by cloud. Secondly, according to the characteristics of footprint image, a threshold constrained ellipse fitting method for extracting the centroid of laser spot is proposed to monitor the pointing stability of long-period lasers. Based on the above method, the change of laser spot centroid since GF-7 satellite was put into operation is analyzed, and the conclusions obtained have certain reference significance for the quality control of satellite laser altimetry data and the analysis of pointing angle stability.

Key words: GF-7; quality control; satellite laser altimetry; laser footprint image; cloud detection; stability analysis of laser pointing angle

Jiaqi YAO,Guoyuan LI,Jiyi CHEN,Genghua HUANG,Xiongdan YANG,Shuaitai ZHANG. Cloud Detection and Centroid Extraction of Laser Footprint Image of GF-7 Satellite Laser Altimetry[J]. Journal of Geodesy and Geoinformation Science, 2021, 4(3): 1-12.

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Data information	Parameters
Image type	Panchromatic
Image size	550×550
Spatial resolution	3.2m
Operating spectral range	400~800nm and 1064nm
Instantaneous field of view	6.4μrad
System focal length	2578.125mm
Image quantization bit number	14bits

Experiment	True value of cloud cover	OTSU results	SegNet results
1	0.941	0.474	0.932
2	0.970	0.488	0.961

Index	Otsu	SegNet	U-Net	SegNet+U-Net
OA	86.39	90.43	89.32	91.84
Hausdorff_ Dist	70.97	65.33	61.55	59.33
Jaccard_ Index	85.18	90.43	92.43	92.43
Avg_PerpenDist	78.22	86.34	84.11	88.34
Conformity Coefficient	87.33	91.54	90.18	92.54
Precision	89.18	93.54	92.43	94.88

Time		Laser	X	Y	Absolute deviation
2020	Mar.	1	120.836	263.714	0.379
	Mar.	2	216.329	162.509	0.451
	Apr.	1	120.504	264.513	0.595
	Apr.	2	216.889	162.639	0.316
	May	1	120.357	263.884	0.483
	May	2	216.853	162.569	0.334
	June	1	120.372	263.795	0.466
	June	2	216.876	162.368	0.363
	July	1	120.527	263.902	0.461
	July	2	216.935	162.109	0.403
	Aug.	1	120.635	263.285	0.327
	Aug.	2	216.811	161.655	0.514