In‐orbit geometric calibration and experimental verification of the ZY3‐02 laser altimeter

Abstract

ZY3-02 is the first Chinese earth-observation satellite equipped with a laser altimeter, designed to assist stereo-image mapping elevations. Launch vibrations, the space environment and other factors, both expected and unexpected, cause bias in the designed ranging and pointing of the laser altimeter, influencing its height measurement accuracy. This paper proposes an in-orbit geometric calibration model which considers this bias as unknown systematic errors. The unknown parameters were calibrated by the minimum ranging error principle using laser spot geolocation data captured by a ground-based laser detector array. Based on precise orbit and attitude data, the precise geolocation information of the laser footprints could be obtained. Validation experiments show that the elevation precision of the laser altimeter can reach 1 m.

Overview of the GF‐7 Laser Altimeter System Mission

“…According to the basic characteristics of the GF‐7 laser altimeter, we constructed the GF‐7 laser geometry altimeter calibration model. The model can be showed as (Xie et al, ): where is the location of the laser footprint in the International Terrestrial Reference Frame (ITRF); is the position of the satellite centroid in ITRF; is the rotation matrix of the satellite body coordinate system to the International Celestial Reference Frame (ICRF); is the transition matrix from ICRF to ITRF; is the offset between the laser emission reference point and the center of satellite mass in the satellite body coordinate system; …”

“…Under different pointing conditions, the laser ranging value can be calculated by substituting the laser footprint centroid coordinates, precise orbit determination, precise attitude determination, and the actual measured atmospheric and tidal parameters into the formula (1). According to the relationship between ranging and pointing angle, we use the least squares algorithm to determine the optimal pointing angle of the laser through multiple iterations based on the principle of minimum ranging residual (Xie et al, ).…”

Iterative Pointing Angle Calibration Method for the Spaceborne Photon-Counting Laser Altimeter Based on Small-Range Terrain Matching

“…A geometric model of the spaceborne laser altimeter [25,35] is shown in Figure 5. Within this model, (O − XYZ) BOD is the coordinate system of the satellite body, ∆G is the offset of the GPS antenna in the satellite body coordinate, ∆L is the laser fire point in the satellite coordinate, (O − XYZ) ICRF represents the international celestial reference frame (ICRF) coordinate system, The relationship between the laser boresight and the satellite body coordinate system is defined in Figure 6, where θ 0 is the angle between the laser boresight and −Z BOD direction, and β 0 is the angle between the projection direction of the laser boresight on plane (O − XY) BOD and the Y BOD axis.…”

“…is the range measured by the laser system and is the laser boresight vector in the satellite coordinate. Due to the existence of systemic errors, the estimated footprint geolocation is expressed as Formula (2) [25]. is the geolocation of the laser footprint in the international terrestrial reference frame (ITRF), which can be expressed by Formula (1).…”

“…Some methods have used known topography to calibrate the spaceborne laser altimeter [16,[20][21][22][23][24], and there have also been some methods to perform the calibration by analyzing the laser echo waveform [18]. A more direct kind of calibration method was performed by using the ground calibration site; a large number of sensor arrays were arranged in the site to measure the laser footprints, and the light spot center position was comprehensively analyzed to calibrate the pointing angle [25,26]. Another direct measurement method used an airborne infrared camera to take photos of the laser footprints on the ground as the satellite passed by, and then footprint location was estimated [24].…”