In‐orbit geometric calibration approach and positioning accuracy analysis for the Gaofen‐7 laser footprint camera

Abstract

The Gaofen‐7 (GF‐7) satellite is China's first submeter high‐resolution optical stereo‐mapping satellite. The satellite is equipped with two area‐array laser footprint cameras to capture the laser spots emitted by laser altimeters. To accurately identify and locate the geographical positions of laser spots, a stepwise geometric calibration method with ground laboratory information for high‐resolution area‐array camera is proposed in this paper. It includes the internal and external calibration. First, the initial values of the camera's internal parameters are determined based on the laboratory calibration results. Then, based on a high‐precision reference ortho‐image and a digital elevation model, the ground control points (GCPs) for multi‐scene footprint images are obtained by image matching. Finally, the external and internal calibration methods for the laser footprint camera are implemented successively. The experimental results indicate that this method can restore geometric imaging relationships with high precision. After the internal and external calibration, the systematic residual error of the image point is eliminated, the remaining residual achieves 0.38 pixel, and the accuracy of direct positioning on the ground is better than 5.0 m, and the elevation accuracy is within 1.5 m.

Rigorous Sensor Model of Gaofen-7 Satellite Laser Altimeter Based on Coupled Footprint Camera

“…The optical axis pointing of laser split-beam was obtained using the spot on image of the footprint camera. The traditional centre extraction methods of laser spot include the Gaussian fitting algorithm, ellipse fitting algorithm, intensity centroid method, and so on [25]- [26]. The accuracy of laser spot's centre will be greatly affected by the strong reflection intensity of ground objects on the footprint camera image.…”

“…From the simulation results, can be seen that the relative pointing deviation between the laser optical axis and the optical axis of the footprint camera is approximately 1.5″ after the geometric calibration. The mean square error (MSE) of the plane positioning of the laser spot can reach within 4.0 m. The results of the current study show that the positioning accuracy of the satellite laser spots on the calibration fields is better than 5.0 m, and the vertical accuracy of laser altimeter can reach 0.10 m [20], [25]. According to the simulation conditions, the laser spot positioning error caused by the orbit and attitude measurement error of the satellite platform is approximately 2.0-3.0 m. The positioning accuracy of the GCPs on the GF-7 calibration field can reach approximately 4.0 m. This positioning accuracy is essentially consistent with the simulation results of the present study.…”

“…The successive launches of ZY3-02 and ZY3-03 satellites have confirmed the feasibility of a laser combined with a three-line array camera for stereo mapping [13]- [18]. The Gaofen-7 (GF-7) satellite was launched in November 2019 and carries two laser altimeters added full waveform for global stereo mapping [19]- [25]. From the above researches, the laser altimetry platform of Chinese satellite mainly adopts the large-spot measurement mode, which are combined with the optical camera on the same platform to obtain the data by combination of active and passive remote sensing payloads.…”

Integrating Stereo Images and Laser Altimetry Points Derived from the Same Satellite for High-Accuracy Stereo Mapping

“…The geometric accuracy of LAPs is primarily affected by attitude measurement errors, orbital measurement errors, laser pointing angle errors, atmospheric delay, the Earth tide, ranging hardware errors, topographic relief, and ground surface roughness [31]. During the generation of LAP products, on-orbit geometric calibration with a reasonable frequency is carried out [34,38]; thus, ranging hardware and laser pointing angle errors can be ignored. The atmospheric delay correction [39] and tidal correction [40] models are also used to correct the errors in the laser ranging value, and any LAPs with topographic relief and a rough ground surface within the footprint area are eliminated.…”

Georeferencing of Satellite Images with Geocoded Image Features

“…The most commonly used form is based on DOM and DEM (Chen et al 2017), which automatically extracts a large amount of control points from DOMs through image matching (Zhang et al 2011). Currently, this form has gained wide applications in sensor geometric calibration and satellite image geometric positioning (Xie et al 2021). Compared to the traditional in-orbit geometric calibration (Chen et al 2015) and satellite image geometric positioning techniques using ground controls as reference, the cloud-controlbased method offers advantages in terms of accuracy, cost, and timeliness (Cao et al 2019).…”