2:30 PM - 2:45 PM
[SCG45-16] Semi-realtime transmission of GNSS-A data using a Wave Glider
Keywords:GNSS-A, Wave Glider, Realtime, Autonomous survey, Marine Geodesy
Seafloor crustal deformation obtained by GNSS-Acoustic observation is now essential to evaluate the long-term risk for huge subduction earthquakes and to estimate the fault extent of a huge event. A network of GNSS-A sites was rapidly constructed since the 2011 Tohoku earthquake. However, the increase in the number of sites prevents frequent shipboard surveys since it takes nearly a day for a single site. To get over the situation, JAMSTEC and Tohoku Univ. jointly started automatic GNSS-A surveys using a Wave Glider (WG) capable of long-period and autonomous navigation (Iinuma et al., 2021, Frontiers in Earth Science). Obtaining data of an equivalent quality to usual postprocessing during the automatic survey, we can immediately assess a devastating earthquake, and even in an inter-seismic period, we can flexibly modify the subsequent survey plan according to the realtime data. We report the improvement in our WG system regarding the on-demand data transmission as well as automatic observation sequence.
WG to onshore communication is via Iridium Satellite system for WG navigation and Thuraya Satellite system for the payload including the GNSS-A part. When the WG arrived around a particular site and the main power is turned on from onshore, GNSS positioning is started. If the WG is found close enough to the site, GNSS and attitude data recording are automatically started. Simultaneously the acoustic unit sends wake-up and other commands to individual transponders until the commands certainly functioned. After that, it started simultaneous periodic acoustic ranging to the transponders and storing acoustic signal with a 1-minute interval. When the WG leaves the site, measurement and data recording are stopped and then the system is halted.
The GNSS-A essential data consists of GNSS antenna position, attitude of the WG, and traveltimes of acoustic signals. GNSS receiver outputs adequate precision at 10 Hz rate through realtime processing of kinematic Precise Point Positioning using the Trimble CenterPoint RTX service. Attitude is obtained 9-DOF MEMS device at 20 Hz as well as GNSS dual antenna system at 10 Hz rate. An acoustic signal is recorded for 10 sec. for every shot, in which the reply signals from all the transponder are included.
Antenna positions and attitude data at a time of transmission and predicted times of reception of acoustic signals are read from stored data for every shot. These data at arbitrary timings are interpolated with neighboring three epochs. Phase Only Correlation (POC: Honsho et al., 2021, Frontiers in Earth Science) of the acoustic signal with five wavelengths before and after the main peak is to be sent to onshore rather than traveltime, because complicated processing to detect true peak is needed due to the distortion of a signal. The introduction of the POC significantly reduces the scatter in traveltime detection. All the relevant data above are archived each time of ranging in a file of 570 bytes with gzip-compression, ready to transmit upon request, which only amounts to 34 KB per hour. Further data reduction is possible by transmitting only transducer position, however, including raw attitude data would help to recognize in case of a possible problem in the data.
In the urgent WG survey off Aomori in December 2020, we analyzed the data immediately after the transmission, which found to perform comparable quality of result with postprocessing of full data set or even shipboard survey data. We are now working on the further refinement of the WG operation algorithm, especially in the intelligent automatic handling against a case for unexpected behavior of seafloor transponder and in the fully automatic onshore processing immediately after reception of the WG data. In the presentation, we also introduce the detail of the system and observed data quality in individual processing steps.
WG to onshore communication is via Iridium Satellite system for WG navigation and Thuraya Satellite system for the payload including the GNSS-A part. When the WG arrived around a particular site and the main power is turned on from onshore, GNSS positioning is started. If the WG is found close enough to the site, GNSS and attitude data recording are automatically started. Simultaneously the acoustic unit sends wake-up and other commands to individual transponders until the commands certainly functioned. After that, it started simultaneous periodic acoustic ranging to the transponders and storing acoustic signal with a 1-minute interval. When the WG leaves the site, measurement and data recording are stopped and then the system is halted.
The GNSS-A essential data consists of GNSS antenna position, attitude of the WG, and traveltimes of acoustic signals. GNSS receiver outputs adequate precision at 10 Hz rate through realtime processing of kinematic Precise Point Positioning using the Trimble CenterPoint RTX service. Attitude is obtained 9-DOF MEMS device at 20 Hz as well as GNSS dual antenna system at 10 Hz rate. An acoustic signal is recorded for 10 sec. for every shot, in which the reply signals from all the transponder are included.
Antenna positions and attitude data at a time of transmission and predicted times of reception of acoustic signals are read from stored data for every shot. These data at arbitrary timings are interpolated with neighboring three epochs. Phase Only Correlation (POC: Honsho et al., 2021, Frontiers in Earth Science) of the acoustic signal with five wavelengths before and after the main peak is to be sent to onshore rather than traveltime, because complicated processing to detect true peak is needed due to the distortion of a signal. The introduction of the POC significantly reduces the scatter in traveltime detection. All the relevant data above are archived each time of ranging in a file of 570 bytes with gzip-compression, ready to transmit upon request, which only amounts to 34 KB per hour. Further data reduction is possible by transmitting only transducer position, however, including raw attitude data would help to recognize in case of a possible problem in the data.
In the urgent WG survey off Aomori in December 2020, we analyzed the data immediately after the transmission, which found to perform comparable quality of result with postprocessing of full data set or even shipboard survey data. We are now working on the further refinement of the WG operation algorithm, especially in the intelligent automatic handling against a case for unexpected behavior of seafloor transponder and in the fully automatic onshore processing immediately after reception of the WG data. In the presentation, we also introduce the detail of the system and observed data quality in individual processing steps.