An overview of VHF lightning observations by digital interferometry from ISS/JEM-GLIMS
© The Author(s) 2016
Received: 7 March 2016
Accepted: 4 August 2016
Published: 15 August 2016
The efficacy of global environmental monitoring from space has been demonstrated since the 1960s. Many earth observing satellites have been launched, producing outstanding contributions including satellites observations associated with lightning observations. The Optical Transient Detector (OTD) on Microlab-1 satellite was launched in 1995 and the Lightning Imaging Sensor (LIS) on the Tropical Rainfall Measuring Mission (TRMM) satellite in 1997 revealed the global distribution of lightning activities with optical observations (Christian et al. 2003; Boccippio et al. 2000). The array of Low-Energy X-Ray Imaging Sensors (ALEXIS) satellite and the Fast On-orbit Recording of Transient Events (FORTE) satellite were launched in 1993 and 1997, respectively. They recorded many transionospheric pulse pair (TIPP) waveforms by electromagnetic (EM) radio observations (Jacobson et al. 1999; Tierney et al. 2002). The ISS-b satellite, which was launched in 1978 and measured short-wave EM noise, can be considered as the first space-borne EM observation satellite for lightning (Kotaki et al. 1983). The Maido-1 satellite (Nakamura and Hashimoto 2005; Kikuchi et al. 2010, 2013) is the world’s first specialized satellite for lightning observation that is the predecessor project of this study. A microsatellite, Chibis-M of the Space Research Institute of the Russian Academy of Sciences which was launched in 2012, has also conducted EM lightning observations in VHF band (Zelenyi et al. 2014).
The authors have been developing the ground-based very high frequency (VHF) broadband Digital InTerFerometer (DITF) to image precise lightning channels and to extensively monitor lightning activities. The remarkable feature of DITF is its ultra-wide bandwidth (from 25 to 100 MHz) and implicit redundancy for estimating VHF source locations (Mardiana et al. 2000; Morimoto et al. 2004). We applied DITF to the space-borne measurement system and joined Maido-1 satellite and Global Lightning and sprIte MeasurementS (GLIMS) projects as mission scientists. GLIMS is a mission on the Exposed Facility (EF) of Japanese Experiment Module (JEM) on the International Space Station (ISS). Both missions intend to implement lightning observation from space by detecting VHF broadband EM signals associated with lightning discharges as a gradual approach to space-borne lightning monitoring by means of EM observations (Morimoto et al. 2011). Observations of the JEM-GLIMS mission were conducted for 32 months and terminated on August 24, 2015. This paper describes the instrument of an EM payload on the JEM-GLIMS mission, called the VHF broadband digital InTerFerometer (VITF), and provides a preliminary report of its observation results. It also introduces the world’s first lightning observations by means of space-borne VHF interferometry.
Project outline and objectives
Instrument of the VITF
Specifications of the VITF antenna
Antenna (one unit)
200 mm × 200 mm × 106 mm
Bandwidth (S11 > −3 dB)
Omni-directionality to the zenith
It is known that thousands of impulsive EM pulses are radiated intermittently in association with lightning leader progression. The typical width of each radiation pulse is several hundred nanoseconds. The AD converter is designed to record a maximum of 130 waveforms with a duration of 2.5 μs with 200 MS/s as one dataset. When the input signal exceeds the threshold voltage, a waveform for 2.5 μs is stored in the onboard buffer of the AD converter with 25 % of pre-triggering. The input signal to the master channel A is used for detection of the triggering, and the waveform of the slave channel B is recorded in synchronization with channel A. Since the size of the onboard buffer is for 130 waveforms per channel, up to the last 130 waveforms are saved with their time stamp. The accuracy of the time stamp is maintained at a sufficient level using GPS signal, in order to compare the observations to other sensors. The AD converter is connected to the Science Instrument Handling Unit (SHU) of the JEM-GLIMS mission with a 2 Gbps RS-422 interface. The AD converter is controlled by the commands from the SHU, and the VITF captured data are transferred and temporarily stored in the SHU. The VITF operation commands are start/stop recording, waveform/status data output, data clear, threshold setting, and forced triggering to respective channels. All other sensors on the JEM-GLIMS mission are also connected to the SHU, and the timing of the data recording is managed by the SHU, which interfaces with the MCE bus system (Kikuchi et al. 2011).
Specifications of the VITF electronics unit
180 mm × 210 mm × 60 mm
2 Gbps RS-422
8.1 W (+12, +5 V)
3 dB pass band
30 to 100 MHz
−1 dB (at center frequency)
−20 dB/20, 110 MHz
Input and output impedance
−85 to −35 dBm
Input and output impedance
Data sampling speed
20 to 100 MHz
Level trigger (event trigger)
50 to 500 mV (10 steps variable, positive only)
Ring buffer for 130 × 2.5 µs waveforms/channel
After designing and manufacturing, various environmental tests were conducted as well as electrical tests. Vibration, impact, and thermal vacuum tests for the VITF antennas were conducted individually. The electronics unit was tested integrated with the JEM-GLIMS box unit once. The JEM-GLIMS hardware had completed its development and environmental testing, and then it was delivered to the integrator of the MCE.
Mission target of the VITF
The main objective of the VITF is lightning observations by recording VHF broadband EM waveforms received by the antennas and estimating their source locations. The VITF is expected to provide location and time information for lightning leader developments. The expected accuracy of the VITF estimation for the direction-of-arrival (DOA) of the VHF EM source is 1.5° based on the experience of the ground-based DITF (Nakamura et al. 2009; Stock et al. 2014). When the ISS altitude is 400 km, the VITF with two antennas is able to identify the location of the EM source as a doughnut-shaped ring with a width of 10 km. This resolution at the Earth’s surface is equivalent to the typical size of a thundercloud. The VITF localizes the lightning activity on the scale of a thundercloud by combining with the observations of the optical sensors on the JEM-GLIMS payload as well as other observations such as satellite imaging, and ground-based lightning, and/or weather radar observations. The range of the lightning leader extension is also given. The information for horizontal distance and temporal relationship between VHF EM sources corresponding to the lightning processes and TLEs might help clarify the mechanism of TLE initiation.
Global lightning observations of JEM-GLIMS
The MCE equipped with the JEM-GLIMS mission payload was successfully launched on July 21, 2012, and transported and installed to the ISS. After the initial check and maintenance, its nominal operation was conducted from November 2012 through December 2014. An extended operation followed until August 2015. Through the operation period, the VITF collected numerous VHF EM observations synchronized with optical signals. In the former operation period from January 2013 to July 2014, 394,407 VITF waveforms were obtained as 4071 datasets. Additional 1,777,106 waveforms were captured as 14,624 datasets in the following 13 months from August 2014 to August 2015. Much more frequent operations of the VITF alone were conducted in the latter period than in the former period to collect many additional observations to bolster the data required for the statistical studies such as producing a global map or regional distribution by means of VHF observations. The data acquisition was triggered when the VITF recorded 100 EM pulses exceeded the threshold level within 100 ms in this mode. Optical signals were not recorded in this mode, but the frequency of VHF EM radiation was obtained continuously.
DOA estimation by digital interferometry
Summary and future work
The JEM-GLIMS mission was conducted on the ISS to observe global distributions of lightning and lightning-associated TLEs by combining observations with radio and optical sensors. Though the early results of each sensor are reported in Sato et al. (2015), this paper focuses on the EM payload of the JEM-GLIMS mission (i.e., VITF) and serves as an initial overview of its observational results after the termination of the mission. The VITF consists of a pair of VHF broadband antennas and electronics to record VHF EM waveforms from lightning discharges. It is designed to estimate the DOA with about 10-km resolution, which is equivalent to the scale of a thundercloud. This means that the VITF is able to monitor thunderclouds with global lightning activity. Comprehensive analyses on the VITF and optical observations of lightning and TLEs during the JEM-GLIMS mission are expected to provide us with new scientific insights and understanding.
The JEM-GLIMS mission payload was successfully launched, transported, and installed to the ISS in July and August 2012. After the initial check and maintenance, its nominal operation continued from November 2012 to December 2014. The extended operation followed for 8 months. Through the entire operation period, VITF collected over two million VHF EM waveforms. Focusing on the synchronized observations with optical sensors, 5538 out of 8049 optical events, about 70 % are accompanied by active VHF radiations. DOA estimates of the received VHF pulses are attempted using the broadband digital interferometry. The results for high SNR and isolated VHF pulses agree with the optical observations, even though DOA estimation is problematic because of the very short antenna baseline. The JEM-GLIMS mission represents the world’s first space-borne lightning location by EM DOA estimation.
The JEM-GLIMS also has an operation mode that acquires only the VITF observations. Optical observations were not recorded in this mode; however, about 1.5 million waveforms in 1300 datasets of VHF EM radiation were obtained across the globe. Kikuchi et al. (2016) propose a new location method of VHF radiation sources by combining the interferometry in this study with measurements of the ionospheric propagation delay. A global lighting map by means of space-borne VHF observation is expected to be provided. Statistical studies involving regional, seasonal, and local time dependencies will be implemented in our future work. Sato et al. (2016) establish a method to distinguish weak optical emissions of sprites from incomparably intense lightning emissions and succeed in identifying 672 TLE events in the JEM-GLIMS observations. Detailed comparison between characteristics of VHF and optical emissions, especially for the TLE events, has begun. A comprehensive analysis of VHF and optical observations on lightning introduced in this paper is also in progress.
TM is responsible for the VITF development and observation. TM and HK analyzed VITF data, and MS analyzed optical data. TM, HK, SM, TU, and MS contributed to the study and discussion associated with the data interpretation. RI, YS, KY, and ZIK contributed significantly to the design, development, manufacture, and test of the VITF. All authors worked for the JEM-GLIMS mission throughout, from instrumentation to operation and analysis. All authors read and approved the final manuscript.
The authors cordially acknowledge the engineers of companies that contributed toward development, especially Advanced Engineering Services Co., Ltd., Dainichi Denshi Co., Ltd., Japan Communication Equipment Co., Ltd., AD Co., Ltd., and Meisei Electric Co., Ltd. The authors also thank JAXA’s full support for the JEM-GLIMS continuous operation and data acquisition. We also acknowledge the editor and reviewers considerably improved the quality of the paper. This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (B) 24340117 and 16H04055.
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