GPS World, August 2013

GOVERNMENT Remote Sensing FIGURE 4 Geometry of point source detection based on TID signals detected at the IPP of GPS station i with GPS satellite k Unknown coordinates of the point source ф λ three components of TID velocity v v and v Observations coordinates of IPP x k T N E i y i k z i k and the corresponding time epoch to TID arrival at IPP t i k Related terms slant distance between IPP and UNE s i k horizontal distance between the point source epicenter and the GPS station coordinates d azimuth and the elevation angle of IPP as seen i from the UNE α k and ε k respectively i i and two spatial over the array and within a given time period here 15 minutes The resulting power spectrum then yields information about the size direction and speed of any detected wavelike disturbances within the STEC gradient data Roughly 20 to 25 minutes after the UNE total fluctuation power increased dramatically by a factor of about 5x103 At this time the signature of waves moving nearly perpendicular to the direction from Hunters Trophy toward the northeast and southwest was observed using the threedimensional spectral analysis technique These fluctuations had wavelengths of about 2 km and inferred speeds of 2 8 m s 1 This implies that they are likely due to small scale distortions moving along the wavefront not visible with GPS Assuming that these waves are associated with the arrival of disturbances associated with the Hunters Trophy event a propagation speed of 570 710 m s was calculated which is consistent with the GPS results detailed above In addition a TID possibly induced by the February 12 2013 North Korean UNE was also detected using the nearby IGS stations by the detection algorithm referred to earlier Eleven TID waves were found from ten IGS stations which were located in South Korea Japan and Russia Due to the weakness of the geometry the epicenter and the ionospheric wind velocity were not determined at this point The apparent velocity of TID was roughly about 330 800 m s and was calculated using the arrival time of the TID after the UNE epoch and the slant distance between the corresponding IPP and the epicenter The reported explosion yield was bigger compared to the 2009 North Korean UNE which possibly affected the propagation velocity by releasing a stronger energy However more in depth investigation of this event and the corresponding GPS data is required FIGURE 5 Locations of the underground nuclear explosion UNE in 2009 and GNSS stations C1 CHAN C2 CHLW D1 DAEJ D2 DOND I1 INJE S1 SUWN S2 SHAO S3 SOUL U1 USUD Y1 YANP Y2 YSSK on the coastline map around Korea China and Japan The TID waves are highlighted for stations C1 D1 D2 I1 The bold dashed line indicates the ground track for satellite PRN 26 with dots that indicating the arrival times of the TIDs at their IPPs All time labels in the figure are in UTC Conclusions Research shows that UNEs disturb the ionosphere which results in TIDs that can be detected by GNSS permanent tracking stations as well as the VLA We have summarized several GNSS based TID detections induced by various UNEs and verified the GNSS based technique independently by a VLA based method using the 1992 U S UNE Hunters Trophy It should be noted that VLA observation was not available during the time of the Divider UNE test hence only the Hunters Trophy was jointly detected by GPS and the VLA Our studies performed to date suggest that the global availability of GNSS tracking networks may offer a future UNE detection method which could complement the International Monitoring System IMS We have also shown that radio frequency arrays like the VLA may also be a useful asset for not only detecting UNEs but for obtaining a better understanding of the structure of the ionospheric waves generated by these explosions The next generation of HV VHF telescopes being developed such as the Lower Frequency Array in the Netherlands the Long Wavelength Array in New Mexico the Murchison Widefield Array in Australia utilize arrays of dipole antennas which are much cheaper to build and operate and are potentially portable It is conceivable that a series of relatively economical and relocatable arrays consisting of these types of dipoles could provide another valuable supplement to the current IMS in the future particularly for low yield UNEs that may not be detectable with GPS Acknowledgment This article is based on a paper presented at the Institute of GPS World August 2013 www gpsworld com 40