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以中国科学院上海天文台激光测距系统为平台,根据参考文献[23]中激光测距基本框架,100 kHz重复频率激光测距系统框图如图6所示。
图6中,相对于参考文献[23]中常规千赫兹重复率激光测距系统,脉冲群发生器基于低频SLR的卫星轨道预报值确定给出对应预报的低频SLR激光与单光子探测器开启触发信号,低频SLR激光开启触发信号触发脉冲群生成器1 (Pulse group generator 1,型号为DG645)产生脉冲间距5 μs (200 kHz)的点火脉冲群信号,占空比50%,激光器经点火脉冲群触发,发射脉冲群激光脉冲;同样,低频SLR单光子探测器开启触发信号给脉冲群生成器2 (Pulse group generator 2,型号为DG645)产生脉冲群信号触发,再触发单光子探测器。由此,对应单位时间内总脉冲数为100 k, 此时频率为100 kHz。采用华东师范大学提供的低暗噪声APD单光子探测器,同时对接收到的卫星激光回波进行光谱滤波及空间滤波,降低背景光噪声,提高激光回波信号探测成功率。激光脉冲发射时,单光子探测器关闭;单光子探测器开启时,激光脉冲停止发射,由此实现脉冲群收发交替模式,避免单光子探测器在开启时,激光在大气中后向散射光被单光子探测器探测接收,实现后向散射激光的规避。事件计时器 (产地:拉脱维亚,型号USB,A033)记录激光脉冲的主波与单光子探测器的回波时刻,并传输到计算终端,计算终端进行信号识别后,使用多缓冲区数据储存模式存储数据。
经过低频SLR触发产生的脉冲群触发信号如图7所示,其中波形1为低频SLR产生的低频SLR周期信号,波形2为触发脉冲群生成器1生成的脉冲激光点火脉冲群信号,触发激光器输出激光脉冲,波形3为触发脉冲群生成器2生成的单光子探测器触发信号,触发单光子探测器工作。波形2与波形3中的脉冲群内脉冲之间的间距为5 μs。采用重复频率200 kHz (脉冲间距5 μs)的激光器,激光发射、回波探测交替工作模式 (100 kHz)下,激光平均输出功率为8 W,单脉冲能量80 μJ,各脉冲之间幅值相等。为了降低回波损失率,根据卫星预报距离的平均值,对近地星(LEO)采用40 Hz低频,群内脉冲数为2500个,远地星(HEO)采用4 Hz,群内脉冲数为25000个,即对应单位时间的脉冲数为100 k。
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基于中国科学院上海天文台60 cm口径SLR系统,由图6建立100 kHz测距系统,对近地轨道、远地轨道合作目标开展了100 kHz重复率激光测量,数据测量及处理结果如表1所示。
Sate name Number of laser echoes RMS/cm Number of normal point Normal point RMS/μm Starlette 358812 0.97 4 32.39 Ajisai 203465 1.59 2 49.85 Beaconc 393709 1.85 6 72.22 Lageos2 6402 1 2 176.75 Stella 225466 1.1 1 23.17 Glonass105 9736 2.03 1 205.73 Galileo203 8401 1.27 1 138.56 Galileo210 4747 1.1 1 159.66 Galileo218 5154 0.98 1 136.51 Glonass133 6706 2.01 1 245.45 Hy2b 363051 0.86 4 28.55 Table 1. Data result from 100 kHz satellite laser ranging system
图8为Ajisai卫星的脉冲群超高重复频率激光测距激光回波探测实时软件测量显示(图8(a))与测量数据处理(图8(b)),图9为远地星Glonass105卫星的测量数据处理,图8和图9的数据显示及处理呈现周期性状态,表明单光子探测器在工作时是启停交替方式,体现脉冲群的收发交替模式。
Figure 8. Real-time display (a) and data processing (b) for 100 kHz SLR laser echoes from the Ajisai satellite
根据参考文献[21]中标准点时长以及表1中测距点数可以推算出每圈观测数据能够生成的标准点个数及每个标准点内测距数据点数,由公式(8)可以计算出每圈观测数据的标准点精度。对近地星的测量,精度最好的结果为Hy2b卫星的28.55 μm;对远地星的测量,Galileo218卫星的精度达到136.51 μm,相比德国Daniel Hampf等人采用光纤纳秒激光器测量标准点精度[13],实现了数量级的提高,体现了皮秒激光在超高重复百千赫兹卫星激光测距上的优势。
Research on satellite laser ranging at pulse repetition frequency of 100 kHz
doi: 10.3788/IRLA20220121
- Received Date: 2022-02-24
- Rev Recd Date: 2022-03-23
- Accepted Date: 2022-05-16
- Publish Date: 2022-11-30
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Key words:
- satellite laser ranging /
- repetition rate /
- back-scatter avoidance /
- laser emission and reception alternation
Abstract: The relationship between the number of satellite laser ranging (SLR) echoes and the repetition rate, laser pulse energy and power are analyzed, it shows that under the same laser echoes, the higher the repetition rate, the lower the laser pulse energy and laser average power are required. Meanwhile, single-shot accuracy and normal point (NP) accuracy of SLR are analyzed, it shows that the more measuring points within the NP time are, the higher the NP accuracy would be. The working mode of firing pulse trains and receiving pulse trains triggered by a fixed period range gate is proposed to solve the interference problem of laser echo caused by back-scattered laser noise with ultra-high pulse repetition frequency (PRF). The multi-buffer storage mode is developed to improve the real-time processing and storage efficiency of data in measurement software by 4-6 times. Based on the telescope aperture of 60 cm SLR in Shanghai Astronomical Observatory, CAS, the PRF of 100 kHz SLR is realized by using fast event timing, ultra-high pulse trains generator, low noise single photon detector and picosecond laser with pulse spacing of 5 μs, single pulse energy of 80 μJ. Measured data of satellites by the PRF of 100 kHz SLR is from low earth orbit (LEO) and high earth orbit (HEO). The NP accuracy for Hy2b satellite in LEO is 28.55 μm, and that of Galileo218 satellite in HEO is 136.51 μm. This study provides an effective method for higher frequency and high-accuracy space target laser ranging.