The large dynamic range of the satellite-borne laser altimeter receiver is a necessary condition for obtaining high-precision ranging values. The upper and lower limits of the dynamic range reflect the ecological adaptability of the altimeter. The power of the echo signal is given in the performance of the altimeter, atmospheric reflection, and target surface reflection. If the waveform of the strong echo signal with high ground reflectivity and specular reflection appears saturated, broadened and distorted, it will interfere with the extraction of helpful information during waveform inversion and affect the accuracy of ranging. Although the waveform saturation can be corrected by the mid-to-late GLAS correction method, this paper proposes a high-sensitivity signal detection technology for a space borne laser altimeter with a large dynamic range, which is suitable for laser altimeters with any linear detection system without waveform saturation. The correction can obtain a larger dynamic range, and the mechanism analysis does not require the dual-channel gain method to obtain a larger dynamic range. In this paper, the method of combining Bessel filter and adjustable gain amplifier is utilized to optimize the design bandwidth to suppress noise, and the sensitivity is not less than 5.1nW. If the parameters are not adjusted, the dynamic range is not less than 33.7dB; if the parameters are adjusted, the sensitivity is not Below 1nW, the dynamic range is about 73.9dB. Through the ground vacuum environment test, under the condition of high signal-to-noise ratio, the ranging accuracy is better than 0.08m; when the signal-to-noise ratio is not less than 8, the ranging accuracy is not less than 0.4m.
The laser rangefinder and the mapping camera have strong complementarity in the positioning of the ground target, and the combination of the two can obtain a better three-dimensional positioning accuracy of the ground target.In order to achieve highly accurate positioning of ground targets, meet the needs of large-scale surveying and mapping, and provide the required elevation control points for the stereo surveying and mapping camera, it is necessary to accurately measure the position of the laser footprint.A method for determining the position of the laser footprint based on a footprint camera is proposed. First, a low-resolution footprint camera is used to directly record the laser emission direction, and then the low-resolution footprint camera is recorded by a method based on image feature point matching. The position of the laser footprint is matched to the image of the high-resolution stereo surveying and mapping camera, and finally the direction of the emitted laser is accurately determined. This method is not related to the attitude determination error of the laser rangefinder and the internal orientation element determination of the footprint camera. Through error analysis, it can meet the 1:10 000 scale positioning accuracy requirement.
The receiver dead time is determined by the photon detector’s dead time and the response time interval of its related processing circuit in photon counting lidar. In this paper, the influence of dead time of single pixel and multi-pixel detector on the aperture of receiving lens, ranging error and ranging accuracy is analyzed in detail. From the simulation results, different sizes of receiving lens aperture were selected for detectors with different dead time effects to match the final measurement accuracy. The multi-detector array and multi-channel signal preprocessing circuit can be used to realize independent preprocessing of each detection channel to obtain time mark data of each photon event, and then all data can be processed uniformly based on histogram statistics. With the above two methods, the noise counts of each detected pixel can be significantly reduced, and the dead time effect of each channel can be suppressed. Most importantly, he time-domain drift and ranging error of echo photons can be effectively reduced. Compared with the single pixel, the multi-pixel detector can greatly increase the maximum allowable receiving area, reduce the demand for single pulse energy of the transmitter, and facilitate the engineering implementation.
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