GPS World, August 2013

originated by a UNE jointly observed by both GPS and the VLA For this case study we selected one of the 1992 U S UNEs for which simultaneous GPS and VLA data were available Experimental Results We summarize here the test studies performed by the OSU group in collaboration with Miami University and the U S Naval Research Laboratory on detection and discrimination of TIDs resulting from UNEs using the GNSSbased and VLA based techniques TABLE 1 lists the UNE events that have been analyzed to date As of March 2013 the results of the 2013 North Korean UNE were not fully completed so they are not included here In the 2006 and 2009 North Korean UNE experiments STEC data from six and 11 nearby GNSS stations respectively were used Within about 23 minutes to a few hours since the explosion the GNSS stations detected the TIDs whose arrival time for each station formulated the linear model with respect to the distance to the station TIDs were observed to propagate with speeds of roughly 150 400 m s at stations about 365 km to 1330 km from the explosion site Considering the ionospheric wind effect the wind adjusted TIDs located the UNE to within about 27 km of its seismically determined epicenter for the 2009 event no epicenter location was performed for the 2006 event due to insufficient data The coordinates estimated by our algorithm are comparable to the seismically determined epicenter with the accuracy close to the seismic method itself It is important to note that the accuracy of the proposed method is likely to improve if the stations in better geometry are used and more signals affected by a TID can be observed An example geometry of UNE detection is shown in FIGURE 5 For the Hunters Trophy and the Divider UNE tests the array signature of TIDs at the vicinity of GPS stations was observed for each event By applying the first order polynomial model to Remote Sensing GOVERNMENT compute the approximate velocity of TID propagation for each UNE the data points that is the TID observations were fit to the model within the 95 percent confidence interval resulting in the propagation velocities of 570 m s and 740 m s for the Hunters Trophy and the Divider respectively The VLA has observing bands between 1 and 50 GHz and prior to 2008 had a separate VHF system with two bands centered at 74 and 330 MHz A new wider band VHF system is currently being commissioned The VHF bands and L band 14 GHz are significantly affected by the ionosphere in a similar way as the GPS signal In this study we used VLA observations at L band of ionospheric fluctuations as an independent verification of the earlier developed method based on the GNSS TID detection for UNE location and discrimination from TIDs generated by other types of point sources The VLA operated as an interferometer measures the correlation of complex voltages from each unique pair of antennas baselines to produce what are referred to as visibilities Each antenna is pointed at the same cosmic source however due to spatial separation each antennas line of sight passes through a different part of the ionosphere Consequently the measured visibilities include an extra phase term due to the difference in ionospheric delays which translates to distortions in the image made with the visibilities This extra phase term is proportional to the difference in STEC along the lines of sight of the two telescopes that form a baseline Thus the interferometer is sensitive to the STEC gradient rather than STEC itself which renders it capable of sensing both temporal and spatial fluctuations in STEC The spectral analysis was performed on the STEC gradients recovered from each baseline that observed the Hunters Trophy event Briefly a time series of the two dimensional STEC gradient is computed at each antenna Then a three dimensional Fourier transform is performed one temporal For accuracy and durability the AsteRx2eH has proven to be an incredibly stable system Raven Aerostars fl ight operations team You need precise heading in navigation You want Septentrio Find us at booth 3328 Versatile OEM Receivers for Demanding Applications www gpsworld com August 2013 GPS World 39