5:15 PM - 7:15 PM
[PEM14-P02] Fundamental development of the time-delayed multi-beam method for an upgrade of the Tromsø sodium lidar
Keywords:Sodium lidar, Technical development, Tromsø, EISCAT
The Earth's upper atmosphere is partially ionized, and the ion-neutral coupling has been an important topic. To investigate the ion-neutral coupling, an effective way would be simultaneous observations of the atmosphere and ionosphere. For this purpose, the Tromsø sodium (Na) lidar, for observations of the atmosphere, was installed in 2010 at Tromsø, Norway (69.6oN, 19.2oE), where there are the European Incoherent Scatter (EISCAT) radars. The EISCAT radars are well known as powerful tools for observations in the ionosphere, and the next generation EISCAT project, EISCAT_3D, has been in progress for recent years. The Tromsø Na lidar is equipped with a 1 kHz laser diode (LD)-pumped laser system that is a highly stable system with a longer lifetime. Based on these advantages, the Tromsø Na lidar has been in operation for many years, in collaboration with the current EISCAT radars. Then, in recent years, an upgrade of the Tromsø Na lidar is highly expected for future collaboration with EISCAT_3D.
In this study, we propose a time-delayed multi-beam method for an upgrade of Tromsø Na lidar and develop the key techniques. The current Tromsø Na lidar is capable of a five-direction observation (vertical, north, south, east, and west) in which the simultaneous multi-beam method is applied. In this conventional method, a laser pulse is split into five, and the five pulses are sent to the five directions simultaneously. As a result, the energy of each pulse is one-fifth of that of the laser pulse before splitting. Because the laser repetition rate is 1 kHz, corresponding to the inter-pulse period (IPP) of 1 ms, the observation height range is limited up to 150 km. On the other hand, in the proposed time-delayed multi-beam method, the laser pulses are transmitted in different directions with a time delay. Thus, we do not have to split the laser pulse, which would improve the signal-to-noise ratio (SNR) in the observations. In addition, a longer IPP for each direction is obtained, which would provide a wider height coverage in the observations.
For the time-delayed multi-beam method, we have been working on the fundamental development of two key techniques: (1) time-delayed multi-channel measurements and (2) high-speed switching in the pulse direction. For the key technique (1), we have been developing a self-built FPGA-based system, including a function for time-delayed multi-channel measurements. In the initial stage, we developed a simple FPGA-based data acquisition system. As a performance test of the system, test observations were conducted by using the Na lidar at Tromsø in February 2023. As a result, the developed system showed the same (or a bit better) performance compared with the conventional system. After that, a time-delayed 2-channel system was developed, and there was no problem in its performance tests in Japan. Then, we are now working on a time-delayed 5-channel system. For the key technique (2), we have been developing a galvanometer scanner-based system, which is capable of high-speed switching with high-accuracy pointing. In the initial stage, we developed an experimental system for performance evaluations in Japan, in which a commercially available galvanometer scanner was applied. As a result of the experiments, the switching time of the beam position was less than 0.80 ms, which was fast enough compared with the IPP of 1 ms. In addition, the 1σ accuracy of the pointing was determined as approximately 10 μrad. Our simulations, which can estimate a limitation of the pointing accuracy, indicated that the determined 1σ pointing accuracy was well enough for the Tromsø Na lidar observations. Then, we are now working on long-term operation experiments. In the presentation, we will show these experiments. Furthermore, we will discuss plans toward the future upgrade of the Tromsø Na lidar based on the current status.
In this study, we propose a time-delayed multi-beam method for an upgrade of Tromsø Na lidar and develop the key techniques. The current Tromsø Na lidar is capable of a five-direction observation (vertical, north, south, east, and west) in which the simultaneous multi-beam method is applied. In this conventional method, a laser pulse is split into five, and the five pulses are sent to the five directions simultaneously. As a result, the energy of each pulse is one-fifth of that of the laser pulse before splitting. Because the laser repetition rate is 1 kHz, corresponding to the inter-pulse period (IPP) of 1 ms, the observation height range is limited up to 150 km. On the other hand, in the proposed time-delayed multi-beam method, the laser pulses are transmitted in different directions with a time delay. Thus, we do not have to split the laser pulse, which would improve the signal-to-noise ratio (SNR) in the observations. In addition, a longer IPP for each direction is obtained, which would provide a wider height coverage in the observations.
For the time-delayed multi-beam method, we have been working on the fundamental development of two key techniques: (1) time-delayed multi-channel measurements and (2) high-speed switching in the pulse direction. For the key technique (1), we have been developing a self-built FPGA-based system, including a function for time-delayed multi-channel measurements. In the initial stage, we developed a simple FPGA-based data acquisition system. As a performance test of the system, test observations were conducted by using the Na lidar at Tromsø in February 2023. As a result, the developed system showed the same (or a bit better) performance compared with the conventional system. After that, a time-delayed 2-channel system was developed, and there was no problem in its performance tests in Japan. Then, we are now working on a time-delayed 5-channel system. For the key technique (2), we have been developing a galvanometer scanner-based system, which is capable of high-speed switching with high-accuracy pointing. In the initial stage, we developed an experimental system for performance evaluations in Japan, in which a commercially available galvanometer scanner was applied. As a result of the experiments, the switching time of the beam position was less than 0.80 ms, which was fast enough compared with the IPP of 1 ms. In addition, the 1σ accuracy of the pointing was determined as approximately 10 μrad. Our simulations, which can estimate a limitation of the pointing accuracy, indicated that the determined 1σ pointing accuracy was well enough for the Tromsø Na lidar observations. Then, we are now working on long-term operation experiments. In the presentation, we will show these experiments. Furthermore, we will discuss plans toward the future upgrade of the Tromsø Na lidar based on the current status.